CATHODIC PROTECTION 2 Technician CP 2│CATHODIC PROTECTION TECHNICIAN Practical Manual IMPORTANT NOTICE: Neither NACE International, its officers, directors, nor members thereof accept any responsibility for the use of the methods and materials discussed herein. No authorization is implied concerning the use of patented or copyrighted material. The information is advisory only and the use of the materials and methods is solely at the risk of the user. Printed in the United States. All rights reserved. Reproduction of contents in whole or part or transfer into electronic or photographic storage without permission of copyright owner is expressly forbidden. Table of Contents Introduction | 1 CP Concepts Review | 2 Field Measurements | 3 Practical Test Stations #1 | Lab Practical Test Stations #2 | Lab Stray Current | 4 Practical Test Stations #3 | Lab Introduction 1 Chapter 1: Introduction Overview Corrosion is one of the most important problems encountered by the owners and operators of underground, offshore, submerged and other metallic structures exposed to an electrolyte. If corrosion is not controlled, it can lead to large costs in repairs or facility replacement. Even greater costs can be incurred from environmental damage, injuries and fatalities. Corrosion control personnel must have a good basic understanding of corrosion mechanisms. They also need to know the various conditions under which corrosion can occur on underground facilities. The CP 2 - Cathodic Protection Technician training course is the second course in the 4-level NACE Cathodic Protection Training and Certification Program. This course will focus on the topics of corrosion theory, cathodic protection principles and systems, advanced field testing and data collection. Throughout the week, there will be in-class experiments to help demonstrate and reinforce principles discussed in the lecture sections. CAUTION: Students should realize that these experiments are conducted under controlled conditions, field conditions will vary. 1.1 Quizzes and Examinations Quizzes will be given at the end of each chapter as knowledge checks. There are two final examinations: one written and one hands-on practical examination covering selected test instruments. The written exam is a computer based test. Students will schedule and attend the written exam through an approved proctoring center and results will be provided at that time. See “Certification Tab” for additional information. The practical exam is given on the last day of the course and is closed book. Under normal circumstances, within 10 to 15 business days following receipt of practical exams at NACE Headquarters, students may access their practical scores online at www.nace.org. Passing the practical examination is required for successful completion of the course. All questions are from the concepts presented in this training manual. 1.2 Code of Conduct Policy While on site at a NACE course, appropriate behavior towards instructors, NACE class location staff, and fellow students is required. If appropriate behavior is not maintained, NACE has the authority to take proper action against the student(s) in violation, which could result in revocation of one or more of the following: NACE Certification, Membership, and current/future classroom attendance. ©NACE International CP 2 | Technician 2 Introduction 1.3 Classroom and Electronic Device Policy To provide the best environment for training, please observe and follow these requirements: • • • • • • • • • No smoking or other tobacco products; including electronic (vapor) cigarette Class starts at designated times Participants are responsible for their own learning and for timekeeping Please turn off mobile ring tones, and do not make or answer calls, text messages or tweets while in the classroom Observe designated times for lunch breaks, coffee breaks and smoke breaks Note location(s) of restrooms and smoking facilities Mobile phones, smart phones, tablets, notebooks, cameras and any other devices containing cameras are not permitted during quizzes and exam. Students are not allowed to be on the Internet or connect with the outside world through their computer. Students are not allowed to recored or take pictures of any portion of the classroom/lab activities (including lectures). All laptops must be kept in silent mode so as not to disturb others in the class. 1.4 Logo Policy NACE International policy regarding the use of its logos, certification numbers, and titles is that the certification number and category title may only be used by the following individuals: • NACE Cathodic Protection Technician Logos, certification numbers, and titles may not be used by any other persons. All active CP card holders are permitted to use the term NACE Cathodic Protection Tester. Individuals may also show their certification number on business cards and follow the below example, which illustrates how this information can be listed on business cards: NACE Cathodic Protection Technician: John Smith NACE Cathodic Protection Technician - Certified, Cert. No. 9650 ABC Company, Knoxville, TN 1.5 About NACE International NACE International is recognized globally as the premier authority for corrosion control solutions and is built upon over 75 years of knowledge and expertise from dedicated members worldwide. NACE has grown into an organization with more than 37,000 members strong, representing every major industry, in over 130 local sections and communities, throughout 140 countries. NACE’s mission is to equip society to protect people, assets, and the environment from the adverse effects of corrosion. Membership NACE International membership provides you access to industry leading tools and resources, the latest research, and networking opportunities from around the world to help you succeed at every step in your career. CP 2 | Technician ©NACE International Introduction 3 NACE offers various membership levels for both individuals and organizations. Membership levels to choose from include: Individual, Corporate, Student, Senior, and Lifetime. To learn more about NACE membership and benefits visit nace.org/membership. Conferences NACE International connects our global audience at the local level by hosting industry and technology-specific events such as conferences, seminars, exhibits, and area events and section meetings so you can stay ahead of the latest technology and emerging industry trends. One of NACE’s biggest events that is hosts annually is its CORROSION Conference & Exposition. This conference is the world’s largest event for the prevention and mitigation of corrosion, it welcomes more than 6,000 corrosion professionals from around the globe each year and over 400 exhibitors. This comprehensive technical conference is packed with infinite opportunities to exchange knowledge and connect with the entire corrosion-fighting continuum: scientists and researchers, engineers and technicians, coatings inspectors and contractors, educators and students, business executives and owners — all focused on the study, prevention, and control of corrosion. Go to nace.org/events for more information about upcoming events. Technical Committees NACE is a leading voice in developing corrosion-related standards and reports that affect different industries. So why not lend your expertise to developing corrosion-related standards and reports with one of NACE’s 300 technical committees and ~4000+ participants. NACE technical committees are a community of end-users, consultants, manufacturers, technologists, researchers, scientists, technicians, military, and governmental bodies united with a common mission: combining industry expertise to establish standard best practices for corrosion control and mitigation. Participation in standards development is easy and open to everyone. Anyone can participate in the process, but to fully engage in standards development, you must be a NACE member. Go to nace.org/jointcc to learn more about lending your expertise. Education NACE is your place to turn to in achieving your career goals and offers a full array of classroom and online training resources. Our courses are the most recognized and widely accepted corrosion training programs in the world and are designed for continued, career-long professional development. With over 40 courses to choose from that have been developed by industry experts, our programs provide the hands-on skills and knowledge necessary to address corrosion in a variety of application-oriented and industry specific areas that focus on key technologies, best practices, regulations, and business management topics. Go to nace.org/edu to learn more about advancing your career through our educational programs. ©NACE International CP 2 | Technician 4 Introduction NACE International Institute (NII) The NACE International Institute (NII) was formed in 2012 to establish an organization focused on certification activities and further advancing the corrosion profession. NACE credentials are the ultimate proof of competence in the corrosion industry and are the most specified in the world. NII has rapidly grown to include other key programs including IMPACT PLUS, NIICAP and MPI. Certification demonstrates commitment to your profession, continued mastery of your field and offers credibility from your peers, within your company or in today’s competitive job market. Position yourself for success by earning one of the more than 20 certifications available in Coating Inspection, General Corrosion, Pipeline, Cathodic Protection, General Coatings, and other specialty areas. Please visit https://www.naceinstitute.org/Certification/ where you can; • • • • • • Browse through current certification offerings by focus area See detailed requirements for each certification Prepare to take an exam by reviewing the Exam Preparation guide Find detailed exam information and policies in our examinee user guides Complete online applications Renew your certification Explore https://www.naceinstitute.org/ to find out more about other programs offered by the NII including, NIICAP, IMPACT PLUS and MPI. Publications Whether you are considering a career in the corrosion industry – or are looking to learn more about the latest technologies and practices to advance your career – NACE offers the world’s most comprehensive technical resources that address dozens of industries affected by corrosion. The NACE Store provides the largest offering of technical resources dedicated to corrosion control and mitigation. From technical standards that set the criteria for corrosion issues worldwide—to the latest technical papers and research presented at our conferences—to an extensive library of books, the NACE Store is THE destination for industry-leading corrosion resources. For more information go to store.nace.org. NACE produces three industry-leading corrosion and protective coatings publications: Materials Performance magazine, CoatingsPro magazine and CORROSION journal. These publications provide insight from global corrosion experts on the latest case studies, technologies and best practices for corrosion control and mitigation, and protective coatings industries. Materials Performance magazine is included with your NACE membership. It is the leading resource on practical corrosion solutions and is read by 37,000 professionals worldwide. Stay on top of the latest technical and industry news with your monthly subscription to Materials Performance Magazine. This award-winning publication also features case studies and editorials from leading experts and is delivered straight to your mailbox or your inbox as part of your membership benefits. For more information go to materialsperformance.com. CP 2 | Technician ©NACE International Introduction 5 CoatingsPro magazine is the leading magazine serving coatings professionals with more than 27,000 readers worldwide, delivering current information on products, services, tips, and techniques. To subscribe for free, go to coatingspromag.com. CORROSION journal is one of the premier sources for corrosion research. It provides the highest quality of innovative research articles that highlight the needs, gaps, and opportunities in wide ranging areas of corrosion mitigation, prevention and control. Subscriptions are available for an individual to site-wide institutional access. For more information go to corrosionjournal.org. ©NACE International CP 2 | Technician CATHODIC PROTECTION 2 Technician ® Chapter 1 Introduction ® Introduction to 1 NACE International Equips society to protect people, assets, and the environment from the adverse effects of corrosion. NACE International built upon over 75 yrs. of experience Available on-line about 10-15 business days following the course with more than 37,000 members representing every major industry in over 130 local sections and communities NACE Headquarters NACE Training Center NACE Office www.nace.org throughout more than 140 countries Invest in Your Future Provides YOU access to TOOLS and RESOURCES, the latest RESEARCH, and NETWORKING OPPORTUNITIES from around the world to help you succeed at every step in your career. nace.org/membership 2 NACE Offers Corrosion Professionals Networking & Volunteer Opportunities Industry Standards & Reports Conferences Magazines, Education & Training Journals & Books Certification Programs NACE Connects YOU Globally Stay ahead of emerging trends by attending any of NACE’s international or local industry and technology-specific conferences, seminars, exhibits, and area/section events. nace.org/events Have a Voice in Industry Standards Lend your expertise to developing corrosionrelated standards and reports with one of NACE’s 300 Technical Committees and ~4000+ participants. Join a community of corrosion professionals united with a common mission of: “Setting the standard for best practices in corrosion control.” nace.org/jointcc 3 Advance YOUR Career through Education Select from over 40 courses, designed by technical experts Online and Instructor-led courses are available Are application oriented and industry-specific courses that focus on: Key technologies Best practices Regulations Standards nace.org/edu Increase YOUR Knowledge Through CTS Stay current on latest trends and best practices in the corrosion industry with the Corrosion Technical Series (CTS). Technical discussions are presented by Industry experts on emerging topics. Network with speakers and peers from all over the country throughout this 1-2 day event. nace.org/edu Standout Among YOUR Peers The NACE International Institute (NII) Validate your skills by earning one of the most recognized corrosion certifications in the world Select from over 20 certifications in 6 different subjects: 1. Coating Inspector 4. Cathodic Protection 2. General Corrosion 5. General Coatings 3. Pipeline 6. Specialty Demonstrate your dedication to staying current in your field of expertise by becoming certified www.naceinstitute.org/certification 4 Subscribe to Industry Publications Keep up with the latest technologies, best practices for corrosion control and mitigation, and protective coatings with these leading publications from NACE. Included with NACE Membership Complimentary Subscription Individual Subscription materialsperformance.com coatingspromag.com corrosionjournal.org Build YOUR Library of Knowledge Choose from over 450 books in 20 categories, and more than 250 technical standards and reports Get the latest informative reference material, learning from real success stories, and detailed guidance on how to apply the latest technologies in your industry www.store.nace.org Classroom Guidelines Classroom Guidelines and Logistics Arrive on time No disturbances – turn your cell phone off Respect one another Actively participate Agree to disagree Students are responsible for their own learning Classroom Logistics Breaks Restrooms Abide by all safety regulations Fire exits, first aid, eye wash, proper protective equipment, etc. 5 Identify Safety Risks in Course Chemical Copper sulfate (Instructor will advise location of MSDS) Electrical Many electrical devices in course. Most safe but others not Consider all electrical tests hazardous Fire Location of Fire Alarm(s) Preferred exit and alternate Muster Points Tripping Obstacles on floor Wet floors Ice in cold climates Other specific to Building? CP2 | Technician General Safety Precautions Personal Protective Equipment (PPE) Eye Protection Remove Contact Lenses and use glasses Gloves - Recommended Lab Coat - Recommended MSDS sheets Know location in course Students are to read Understand First Aid CP2 | Technician Copper Sulfate Poison: if ingested (stomach, lungs or eyes) Read MSDS Nausea, vomiting damage to body cells or organs. Use Eye protection Irritant to Eyes Damage to clothing and carpets Understand First Aid Wear gloves Wash immediately after contact Do not eat before washing CP2 | Technician 6 Course Safety - Electrical Many electrical devices in course: some hazardous - others not. Assume all are hazardous and work accordingly Use insulated probes and clips Do not touch bare probes, clips or terminals! Wear Eye protection Wires can flip into eyes and arcs can occur! Rectifiers Turn off / unplug when connecting, adjusting or testing! Short circuits Cause arcing and failure of equipment Cause toxic fumes from burning wire insulation or electrical components Contact instructor if unsure of connection Arcing Shorting power source Connecting to power source when energized CP2 | Technician Rectifiers and Power Source Safety With Rectifier Circuit Breaker ON: all terminals are “HOT” Make connections with rectifiers off Do no touch terminals with hands or hands holding metal clips, metal probes etc. Use manufactured insulated meter probes for measurements Use one hand method of measuring any voltage (Instructor to demonstrate) With Rectifier Circuit Breaker OFF: only panel terminals de-energized AC supply and breaker terminals still hot Do not attempt to insert tools or metal probes inside rectifier. Turn off and unplug rectifier before working inside. CP2 | Technician Other Precautions Fire Instructor to advise Muster Point locations If a fire: Turn on fire alarm Exit Building Gather at Muster Point Tripping Do not leave articles on the floor in the classroom or in hallways Wet floors should be immediately cleaned or place a hazard sign Advise instructor of any hazards CP2 | Technician 7 Course Schedule CP2 Practical Schedule Chapter 1 Introduction, Welcome, Overview Chapter 2 CP Concepts Review Chapter 3 Field Measurements Lab 1 Practical Test Stations #1 Lab 2 Practical Test Stations #2 Chapter 4 Lab 3 Stray Current Practical Test Stations #3 Practical Exam Note: Schedule is subject to change Accessing Your Digital Course Materials 1. Students must sign into their NACE Profile 2. Click Education History > Online Course Materials 3. Click Library 4. View the course materials Successful Course Completion To “successfully complete” this course and receive a Certificate of Completion and earn 4.5 CEUs, the following criteria must be met: Attend the entire course (Part 1: Theory and Part 2: Practical) Successfully pass each learning assessment with a score of 70% or higher Learning assessments include a practical exam The Training Certificate of Completion denotes successful completion of the NACE course, and should not be considered or represented to Industry as the achievement of a NACE Institute Certification. NACE Institute certifications are only awarded once a candidate successfully completes all set certification requirements, including, but not limited to, exams, work experience and educational requirements. 8 Exams Computer Based Testing (CBT) A multiple choice exam that you schedule and attend at an approved proctoring center, with results provided at the end of the test. Hands on Practical Exam 12 stations, 10 minutes allowed at each work station. The exam is given on the last day of class and is closed book. However, reference sheets are provided. Certification and CBT Resources Computer Based Testing (CBT) Resources Located under the Certification tab in your manual: Becoming Certified Scheduling your exam Located at www.naceinstitute.org/certification: Exam Preparation Guide Certification Requirements Exam Blue Print Types of Questions Exam Preparation Examinee user guides and policies Online certification application Renew your Certification Available on-line about 10-15 business days following the course 1. Go to www.nace.org 2. Select Training & Education 3. Check My Exam Results 4. Log in to your NACE profile Exam Results 9 Course Objectives Perform advanced field tests, including: current requirement test, shorted casing test, IR drop test, soil resistivity, and interference tests, and evaluate the results Perform tests to verify the presence of stray current interference and recommend method(s) to mitigate the interference Conduct and understand the importance of periodic surveys, including IR-Free readings, polarization decay tests, and current measurements Maintain documentation and records, including data plotting and analysis Describe AC voltage and the methods for mitigation Test and troubleshoot rectifier component parts Recognize the purpose and use of corrosion coupon test stations Recall code requirements related to CP 1 CP Tester Beginner NACE CP 2 CP Technician Intermediate www.nace.org/cp 3 CP Technologist Intermediate Advanced 4 CP Specialist Advanced (Engineering Level) Training Courses Online Courses: Specialty Courses: CP Fundamentals Virtual Test Stations Coatings in Conjunction w/ CP CP Interference Certification Path An approved application is required before certification is issued for CP 2, CP 3 and CP 4 certifications: CP 1 Tester CP 2 Technician CP 3 Technologist CP 4 Specialist Applications include work experience, academic/education background, and qualification references. Online application forms can be found at: www.naceinstitute.org 10 Student Introductions Name and Organization Job Function Industry Experience Course Expectations Chapter 1 Introduction ® 11 2-1 Experiment 2.1 - Corrosion Cell and Cathodic Protection Student to perform Part A - Corrosion Cell with Different Environment Changes Preparation To ensure consistent results during class and from class-to-class, the following preparation is advised. A) B) Use simulated seawater: (35g NaCl/litre ~= 1 Cup NaCl/7.8 litre ~= 2 heaping tablespoons NaCl /litre). This will help to obtain potentials similar to those in the Practical Galvanic series. Presoak the samples overnight or for at least 4 hrs in advance: This will depassivate the samples and establish steady-state corrosion potentials. Otherwise the potentials will be constantly shifting during the test often voiding the results. Procedure 1. Place simulated seawater in the tray 2. Place a steel sheet in left side of tray 3. Place a copper sheet in right side of tray 4. Measure potentials as shown in Figure 2.1 and enter results in table below Figure 2.1 Potential Measurement of metals 5. Connect the two metals together as shown in Figure 2.2, measure the corrosion current (Icorr), not polarity and enter results in table below. ©NACE International CP 2 | Technician 2-2 Figure 2.2 Measurement of Corrosion Current 6. Remeasure the potentials as in Figure 2.3 with them connected via the ammeter and enter the results in table below. Figure 2.3 Measurement of Potentials with Corrosion Current 7. Still connected as in Figure 2.3, either blow air on to the copper sheet or rub the copper surface, repeat the potential and current measurements and enter results in table below 8. Allow to stabilize, stir the electrolyte aggressively and repeat the potential and current measurements. Enter results in table below CP 2 | Technician ©NACE International 2-3 9. Disconnect the ammeter and quickly measure potentials while the electrolyte is still agitated and record results Results Test Step 4 - Open Current EFe ECu Icorr Step 5 & Step 6 - Corrosion Cell Step 7 - Depolarizing Cathode Step 8 - Corrosion Cell with moving electrolyte Step 9 - Open circuit with moving electrolyte Conclusions • • • • • • The cathode of the corrosion cell was the ___________ sheet Adding air or rubbing the surface of the copper sheet resulted in a ______________ (positive - negative) shift in potential Adding air or rubbing the surface of the copper sheet resulted in a ____________ (decrease increase) in Icorr Circulating the water resulted in a ____________ (positive - negative) shift in potential Circulating the water resulted in a _____________ (decrease - increase) in Icorr Circulating the water resulted in a _____________ (decrease - increase) in the open circuit potential ©NACE International CP 2 | Technician 2-4 Part B - Corrosion Cell with CP Procedure 10. Using the equipment in Part A, allow EFe, & ECu potentials and Icorr to stabilize and record. 11. Measure pH and potentials of both metals as shown in Figure 2.3 and enter results in table below. 12. Install an impressed current cathodic protection system as shown in Figure 2.4 noting that a sacrificial anode is shown in the figure, but an impressed current system is optional. 13. After connecting to the 10,000 Ohm resistor, measure: A. the potentials of the two metals, B. the current between the two metals, C. either the total CP current or the voltage across the resistor and calculate the total cathodic protection current from the voltage across the known resistor. 14. Repeat step 13 at the different resistor values. Note when the current between metals changes directions. Figure 2.4 CP System Added to Corrosion Cell 15. Measure pH at both metal surfaces and record CP 2 | Technician ©NACE International 2-5 Results Circuit EFe ECu Icorr / IFe,CP Total ICP (mVCSE) (mVCSE) (mA) (mA) pH Cu Fe Step 1 Open Circuit Step 10 Polarized Step 13 CP ON, RLimit 10,000 in series Step 14 CP ON, RLimit = 1,000 Step 14 CP ON, RLimit = 100 Step 14 CP ON, RLimit = 10 Polarization Plot Plot the results on a polarization diagram on graph paper provided Conclusions • Corrosion current __________ (increases or decreases) as cathodic protection current increases • Corrosion current ___________ (increases or decreases) as the cathode polarized potential is made more electronegative. • Corrosion from galvanic couple is stopped at ______________Ohms (what resistor value) • Corrosion on the steel is mitigated at __________ Ohms (what resistor value?) • The pH after cathodic protection was applied ___________ (increased/decreased/no change) ©NACE International CP 2 | Technician 2-6 CP 2 | Technician ©NACE International Chapter 2 Concepts of Cathodic Protection - Review ® Pipeline Corrosion Cell Cathode 2 Anode Microscopic Corrosion Cell on the Surface of a Pipeline CP2 | Technician Cathodic Protection - Definition 3 When the potential of all cathode sites reach the open circuit potential of the most active anode site, corrosion on the structure is eliminated. CP2 | Technician 1 Application of Cathodic Protection Decreasing Corrosion Current 4 C.P. CURRENT ANODE -0.65 volt CATHODE -0.50 volt CATHODE -0.65 volt I corr = 1 mA CATHODE -0.65 volt I corr = 0 mA Reduction In corrosion current BEFORE Cathodic Protection AFTER Cathodic Protection CP2 | Technician Structure Polarization 5 Cathodic protection is a polarization phenomenon The most cathodic sites on the structure polarize first Polarization of the cathode sites to the open circuit potential of the most active anode sites on the structure CP2 | Technician Cathode Polarized to the Open Circuit Potential of the Anode + EC,OC 6 Cathode polarizes to open-circuit potential of anode, resulting in zero corrosion current EC,P ECORR E EA,OC EA,P E’C,P ECPA,P _ ECPA,OC I CORR ICP Log I CP2 | Technician 2 CP Current Requirement 7 Depends upon surface area to be protected Current requirement is directly proportional to surface area Protective coatings reduce the exposed surface area Polarization behavior of the structure in its environment Current requirement is inversely proportional to polarization The more the environment favors an increase in polarization the lower the CP current requirement CP2 | Technician Cathodic Protection Criteria 8 NACE Standard SP0169 Control of External Corrosion on Underground or Submerged Metallic Piping Systems” several criteria for determining when adequate cathodic protection is achieved are found in Section 6, “Criteria and Other Considerations for Cathodic Protection” CP2 | Technician Criteria for Underground or Submerged Iron or Steel Structures (Reverse Ordered Intentionally) 9 6.2.1.3 A structure-to-electrolyte potential of –850 mV or more negative as measured with respect to a saturated copper/copper sulfate (CSE) reference electrode. This potential may be either a direct measurement of the polarized potential or a current-applied potential. Interpretation of a current-applied measurement requires consideration of the significance of voltage drops in the earth and metallic paths. 6.2.1.2 A minimum of 100 mV of cathodic polarization. Either the formation or the decay of polarization must be measured to satisfy this criterion. ∆E100 = EOFF − EDEPOLARIZED OR ∆E100 = EOFF − ECORR 6.2.1.1 Criteria that have been documented through empirical evidence to indicate corrosion control effectiveness on specific piping systems may be used on those piping systems or others with the same characteristics. CP2 | Technician 3 Measured Potential 10 The potential across the structure/electrolyte interface is the sum of the corrosion potential and the cathodic polarization plus a voltage (IR) drop error if current-applied. Emeasured = Ecorr + Polarization + Voltage (IR) Drop CP2 | Technician Measuring Polarization on a Structure Normal Reference Electrode Location ‐0.900 V ‐ + 11 Preferred Reference Electrode Location Structure To Electrolyte Boundary Polarization Film CP2 | Technician Cathodic Protection - Monitoring -1.6 12 Von -1.4 Voltage (IR) drop Potential -1.2 -1.0 -0.8 Instant Off or Voff Polarization Decay -0.6 -0.4 Polarization Decay Time Time CP2 | Technician 4 Criteria for Other Materials 13 Other NACE International Criteria Aluminum 100 mV polarization Copper 100 mV polarization Reinforced Concrete Structures 100 mV polarization E log I CP2 | Technician Instrument SAFETY in Experiments 14 Even lower voltages can be hazardous Know capacity of meter Replace damaged digital multimeter (DMM) test leads Keep fingers behind probe guards Do not touch probe tip Do not press wire/terminal/probe tip with fingers Confirm correct meter dial position for measurement Confirm correct test lead polarity Never connect ammeter leads across a voltage source Low ammeter resistance shorts power supply Never use Ohmmeter in circuit having voltage One-hand Method CP2 | Technician EXPERIMENT 2.1 15 Experiment 2.1 Part A: Corrosion Cell with Different Environment Changes CP2 | Technician 5 Experiment 2.1 – Part A 16 Presoak samples in simulated seawater (35g NaCl / litre of H2O) -0.45 V -0.18 V Same Meter Place Fe and Cu at each end of tray containing seawater Measure potentials of each Record Noble Active CP2 | Technician Experiment 2.1 – Part A 17 Connect metals through an ammeter Measure current (Icorr) and polarity Record 14 mA Corrosion Current (Icorr) Conventional Current Active Noble CP2 | Technician Experiment 2.1 – Part A 18 Measure potentials of metals when connected Record in table Icorr 14 mA -0.31 V Corrosion Current -0.29 V Same Meter Conventional Current Active Noble CP2 | Technician 6 Experiment 2.1 – Part A Table for Readings Test EFe 19 ECu Icorr Step 4 – Open Circuit Step 5 & Step 6 – Corrosion Cell Step 7 – Depolarizing Cathode Step 8 – Corrosion Cell with moving electrolyte Step 9 – Open circuit with moving electrolyte CP2 | Technician Experiment 2.1 – Part A - Conclusions 20 The cathode of the corrosion cell was the ___________ sheet Adding air or rubbing the surface of the copper sheet resulted in a ______________ (positive - negative) shift in potential Adding air or rubbing the surface of the copper sheet resulted in a ____________ (decrease - increase) in Icorr Circulating the water resulted in a ____________ (positive - negative) shift in potential Circulating the water resulted in a _____________ (decrease - increase) in Icorr Circulating the water resulted in a _____________ (decrease - increase) in the open circuit potential CP2 | Technician Experiment 2.1 – Part B 21 Experiment 2.1 Part B: Corrosion Cell with Cathodic Protection CP2 | Technician 7 Experiment 2.1 – Part B 22 Continue from Part A Corrosion Current 14 mA -0.31 V -0.29 V Allow potentials to stabilize Same Meter Measure pH of 2 metals Record Conventional Current Active Noble CP2 | Technician Experiment 2.1 – Part B 23 Install a CP system Connect through 10,000 Ω resistor & measure: 5 mA 10 mA -0.31 V -0.29 V Potentials of 2 metals Current and polarity ICP Total Total current or Voltage across resistor 10,000 Ω 1,000 Ω 100 Ω Repeat with other resistors 10 Ω Anode RLimit Measure pH Record CP2 | Technician Experiment 2.1 – Part B Table for Readings Circuit 24 EFe ECu Icorr / IFe,CP Total ICP (mVCSE) (mVCSE) (mA) (mA) pH Cu Fe Step 1 – Open Circuit Step 10 Polarized Step 13 CP ON, RLimit = 10,000 Ω in series Step 14 CP ON, RLimit = 1,000 Ω Step 14 CP ON, RLimit = 100 Ω Step 14 CP ON, RLimit = 10 Ω CP2 | Technician 8 Experiment 2.1 – Part B - Conclusions 25 Corrosion current __________ (increases or decreases) as cathodic protection current increases Corrosion current ___________ (increases or decreases) as the cathode polarized potential is made more electronegative. Corrosion from galvanic couple is stopped at ______________Ohms (what resistor value) Corrosion on the steel is mitigated at __________ Ohms (what resistor value?) The pH after cathodic protection was applied ___________ (increased/decreased/no change) CP2 | Technician Chapter 2 Concepts of Cathodic Protection - Review ® 9 Field Measurements 3-1 Chapter 3: Field Measurements Knowledge of proper test procedures is an essential skill of a cathodic protection technician. They must understand the instrumentation, the proper assembly of the instrumentation for each test, the theory behind the tests and the possible sources of error that can lead to misinterpretation of results. 3.1 Instruments Analog meters operate on the basis that the magnetic field created by a meter current through a coil that causes the coil to rotate in a permanent magnet field. A needle on the coil points to a dial that is calibrated to voltage current, or resistance. The meter current is proportional to the voltage thus the meter scale can be calibrated into Volts or amperes. The meter coil changes continuously with a changing meter current and thus tracks a changing voltage or current. A cathodic protection technician should be familiar with these instruments as they are used on instrumentation requiring analog indications (e.g. DCVG meters, ‘null’ meters, some locators, etc.) and are commonly used for output indication on DC power sources. Most field instruments employ electronic signal conditioning with digital displays – often termed ‘digital’ meters. Compared to analog meters discussed above, the electronics in these meters generally have much higher input impedance, are more rugged, and have some protection against over voltage. The typical functions of a digital multimeter (DMM) are, at least, an AC/DC voltmeter, AC/DC ammeter and a DC Ohmmeter but can have other functions such as frequency, continuity, diode test circuit, etc. (Figure 3.1) Digital multimeters (DMMs) condition the input signal with analog electronics, then convert that conditioned signal to a digital format, where it may be further conditioned, and then display the measured value digitally. Figure 3.1 Typical Digital Multimeter (left) and Internal Block Diagram (right) DMMs typically use 3½ digit (i.e. 1.999) displays as shown above, though a higher resolution is also available. Displayed values are typically updated 2 to 4 times per second, which can be difficult to read if the input signal is changing rapidly (an advantage for analog displays!). The LCD displays can be damaged if exposed to extreme hot or cold temperatures (e.g. if kept on the dash of a vehicle), so due care is required. ©NACE International CP 2 | Technician 3-2 Field Measurements 3.1.1 DMM Voltage Measurement When used on DC or AC Voltage ranges, the input signal is protected, conditioned and converted as illustrated in Figure 3.2. When taking an instant OFF potential, the DMM displayed value changes rapidly as the measurement is falling from the ON potential to the OFF potential and then starts to depolarize. This can make it difficult for the user to determine which value is the actual ‘instant off’ potential. Taking the displayed value between 0.5 and 1 second after interruption will avoid any spiking that occurs briefly upon current interruption, before any significant depolarization occurs. Figure 3.2 DMM Voltage Measurement Schematic 3.1.2 DMM Current Measurement When used on DC or AC Current ranges, the input signal is protected, conditioned and converted as illustrated in Figure 3.3. With the ammeter in series with the external circuit, the external current goes through a shunt in the meter of a known resistance. The shunt resistor is sized to a current such that the voltage across the resistor can be measured and calibrated to it. The digital electronics calculates and displays the current based on the measured voltage across the known shunt resistance. Figure 3.3 DMM Current Measurement As the current to be measured decreases, the shunt Schematic resistance must increase to provide a sufficiently high voltage for the measurement electronics. Thus, when inserting an ammeter into a circuit, the internal ammeter resistance increases the total circuit resistance, reducing the total current. Therefore, the current measured will always be less than the actual current without the ammeter installed. Further a measurement on two different ammeter scales may be quite different for the same reason. Use the highest scale that will give a reading and be aware of the internal ammeter resistance on each scale. CP 2 | Technician ©NACE International Field Measurements 3-3 3.1.3 DMM Resistance Measurement An ohmmeter measures the electrical resistance of components between the meter terminals. To measure these components, they normally must be taken out of the circuit or the circuit must be open to ensure that no voltage from an external source exists. An external voltage will either oppose or enhance the internal voltage giving false readings, blow an internal fuse, or in extreme cases will damage the instrument. The Ohmmeter circuit in Figure 3.4 employs an internal DC voltage source to force current through a known internal resistance and the resistance under test Figure 3.4 DMM Resistance Measurement Schematic which are connected in series. The digital electronics calculates and displays the test resistance based on the ratio between the measured voltage across the internal resistance and that of the internal DC voltage source. Note that as the DMM Ohmmeter operates with an internal DC source, it cannot be used on any circuit where there is a back-emf (DC voltage), as exists between any two metal structures in an electrolyte, as the emf will add or subtract from the total DC volts in the measurement circuit, so the displayed resistance value will be either higher or lower than actual. For these types of measurements, an AC resistance meter must be used. 3.1.4 Other DMM Features & Functions 3.1.4.1 AC Volts vs. rms A basic digital AC voltmeter records the peak AC voltage and multiplies this value by a factor (1/√2 = 0.7071) to determine the actual AC Volts. This is only accurate if the waveform is a true sine wave. Otherwise, if not a true sine wave, a rms AC voltmeter must be used to determine the correct AC voltage. If in doubt, use a rms AC voltmeter. 3.1.4.2 Auto-Range A basic meter requires that the range be preset by the operator. If the scale is too small for the reading, a symbol such as “OL” will appear. If the scale is too large, the readout will be inaccurate. Thus, it is important to select the correct scale (mV, V etc.). A DMM with an auto-range feature will select the best range automatically. This may be advantageous in some situations but requires the user to pay attention to the scale of the displayed value (e.g. Volts versus mV). If the measured value has large rapid changes in magnitude, then this could cause the meter to continuously change ranges, resulting in display changes that are difficult to follow. For example, this may occur when measuring instant OFF potentials that are less than 1V while the ON potentials are greater than 2V, or measuring potentials that are affected by dynamic stray currents. It is usually possible to disable autoranging to and select a fixed measurement range. ©NACE International CP 2 | Technician 3-4 Field Measurements 3.1.4.3 Diode Test A diode requires a driving voltage greater than its inherent forward voltage before it will conduct. Thus, DMM diode testing circuits impresses a DC voltage and displays this value in Volts when the diode conducts. In this case the diode is conducting from the positive to negative meter leads. A good diode does not conduct in the reverse direction therefore the reading with the meter leads reversed will be “OL”. The combined readings indicate the diode is in good operating condition. A shorted diode is noted by both forward and reverse readings of 0 Volts (no breakdown voltage required) and an open circuit diode is noted by “OL” readings in both directions. 3.1.4.4 Frequency This scale is intended to measure the number of AC cycles per second (hertz). Functionally, DMMs measure the number of times an input signal changes slope direction (i.e. positive to negative) in one second. Consequently, it may also be used on a full wave rectifier that is not filtered as it will detect the peak to peak half cycles that in this case will be 100 or 120 hertz depending on the AC input. If 50 or 60 hertz are recorded, the rectifier is half-waving. 3.1.4.5 Hold Feature Some DMM have a “hold” feature that allows a reading to be frozen on the readout. This is an advantage when taking a reading in conditions where it is difficult to read the meter scale such as poor lighting or to keep the reading on display until it is recorded. 3.2 Measurement of Cathodic Protection Effectiveness Various techniques may be used to determine the degree to which a structure under cathodic protection (CP) is actually protected against corrosion, including: • • • • • • • Structure-to-electrolyte potential Test coupons Current measurements Soil resistivity Direct observation Leak frequency In-Line Inspection 3.3 Structure-to-Electrolyte Potentials The definition of a structure-to-electrolyte potential, otherwise referred to as a pipe-toelectrolyte, pipe-to-soil, tank-to-electrolyte, or structure-to-soil potential is: “The potential difference between the metallic surface of the structure and electrolyte that is measured with reference to an electrode in contact with the electrolyte.” These measurements must include: • Magnitude CP 2 | Technician ©NACE International Field Measurements • • • 3-5 Polarity Units Reference electrode A structure-to-electrolyte potential is a measurement taken with the voltmeter connected in parallel in the circuit. The external circuit resistance of this measurement is high, so a highinput resistance voltmeter is required to avoid “shunting” too much current from the structure and, therefore, obtaining inaccurate measurements. In addition, a reference electrode, often referred to as a reference cell, is used to make contact with the soil. Structure-to-electrolyte potential profile surveys are used to: • • • • • Locate anodic areas on non-cathodically protected pipelines Determine the effectiveness of cathodic protection on cathodically protected structures Locate stray currents Locate electrical shorts and contacts Locate coating holidays Potential measurements are a common means for determining if adequate protection has been achieved. With the application of current from the environment onto a structure, a potential change with respect to the environment will occur. The potential change is a reflection of polarization. Measurements are made to determine whether one of the cathodic protection criteria is met (see Chapter 2 for discussion on CP criteria). According to the NACE standards, voltage drops other than across the structure-to-electrolyte boundary must be considered in order to assess the effectiveness of a CP system using fixed potential measurements. Several methods are used to consider these voltage drops: • • • • Minimizing the distance between the reference electrode and the surface of the structure Measuring the potential when the current is interrupted (instant-off potential) Measuring the formation of polarization or decay of polarization of the structure when the current is energized or disconnected Installing external CP coupons in the vicinity of the structure to replicate a coating holiday Advantages of the structure-to-reference potential method are that it is relatively straightforward, and potential measurements comply with standard criteria. Disadvantages are that all sources of CP current must be interrupted simultaneously, stray currents will affect the readings, polarized potentials of structures with direct-coupled galvanic anodes cannot be obtained, and surface potential measurements actually measure average potentials. The averaging of the actual structure-to-electrolyte potentials means that surface measurements might not detect small corrosion cells that are not being cathodically protected. 3.3.1 The Potential Measurement Circuit and Measurement Error The intent of the potential measurement is to determine the structure potential (Etrue) accurately at the test location. The measurement circuit can be approximated by the electrical circuit in Figure 3.5. ©NACE International CP 2 | Technician 3-6 Field Measurements Figure 3.5 Electrical Schematic of the Structure-to-Electrolyte Measurement Circuit It is the true potential difference (Etrue) between the pipe and reference electrode that ideally should appear across the meter terminals. Because the meter circuit is a series circuit, the magnitude of the voltage drop that appears across the meter will be proportional to the ratio of the meter resistance to the total meter circuit resistance. For the measurement circuit, Kirchhoff’s voltage law applies and the true potential difference is equal to the sum of the voltage drops around the series circuit. Etrue = ImRt [3.1] Etrue = Im [Rtl,1 + Rtl,2 + Rtl,3 + Rp,e + Rr,e + Rm] Etrue = Vtl,1 + Vtl,2 + Vtl,3 + Vp,e + Vr,e + Vm Vm = Etrue – [Vtl,1 + Vtl,2 + Vtl,3 + Vp,e + Vr,e] Let Vcirc equal all voltage drops in the circuit except for the meter voltage drop Vm = Etrue – Vcirc but: Etrue = ImRt and Vcirc = ImRcirc and: then: Rt – Rcirc = Rm [3.2] Hence, the amount of voltage (Vm) that appears across the meter compared to the true potential difference (Etrue) is proportional to the ratio of the meter resistance (Rm) compared to the total resistance. CP 2 | Technician ©NACE International Field Measurements 3-7 For example: Consider a true potential (Etrue) of 1,000 mV, each test lead resistance (Rtl) of 0.01Ω, a pipe-to-earth resistance (Rp,e) of 10Ω, a reference electrode resistance to earth (Rr,e) of 100 kΩ, and a meter resistance (Rm) of 1 MΩ. Calculate the voltage that would appear across the voltmeter. Rt = 3Rtl + Rp,e + Rr,e + Rm = 3(0.01) + 10 + 105 + 106 Rt = 1.1 MΩ = 909 mV This is an error of: If the meter input resistance in the foregoing example is increased to 10 MΩ, the voltmeter would read 990 mV which would reduce the error to 1%. The voltage across the voltmeter approaches the true potential as the meter resistance becomes much greater than the other resistances in the measuring circuit. High resistances in the measuring circuit, other than across the voltmeter, should therefore be avoided. Reference electrode contact resistance can be a source of error when the reference is placed on dry soil, well drained gravel, crushed stone, frozen ground, asphalt, or concrete. To minimize this error, the contact conductance can be improved by wetting the area around the reference. In extreme cases, a hole can be drilled from the surface to a depth of permanent moisture and the reference placed in the hole, or an electrolytic bridge can be created between the reference and earth. 3.3.1.1 Minimizing Reference Electrode Contact Resistance • • • • Wet soil as needed Avoid grass Avoid asphalt, concrete, and gravel surfaces Access ports (Figure 3.6) ©NACE International CP 2 | Technician 3-8 Field Measurements Figure 3.6 (Left) Methods of Minimizing Reference Electrode Contact Resistance (Dry soil or frozen ground) and (Right) Methods of Minimizing Reference Electrode Contact Resistance (Asphalt or concrete) In Figure 3.6 (Right), the depth from grade to clay must be below the frost line in frozen soil and to the depth of permanent moisture in dry soil. For asphalt or concrete, a soapy water solution will usually provide sufficient electrolytic contact even if the water level in the hole drops. High measurement circuit resistance can also occur as a result of broken test leads, high test lead connection resistances, and pipe resistance to earth if the pipeline is short and well coated. When measuring a structure-to-electrolyte potential, it may not be immediately apparent that a high circuit resistance is present. If the voltmeter has an input resistance selector switch, the existence of a high resistance in the measurement circuit can be identified by switching to a lower or higher input resistance. If the potential indicated by the voltmeter differs significantly (i.e., more than 10%) between the two input impedances, then there is a high resistance in the measurement circuit. Further by knowing the two input resistances and their corresponding measured voltage, the true potential can be calculated. [3.3] where: Etrue = true potential (V) K = input resistance ratio Rl/Rh Rl = lowest input resistance Rh = highest input resistance Vl = voltage measured with lowest input resistance Vh = voltage measured with highest input resistance For example: If a potential difference (Vl) of –650 mVcse was measured with an input resistance (Rl) of 1.0 MΩ and a potential difference of –800 mVcse (Vh) was measured with an input resistance (Rh) of 10 MΩ, then the true potential (Etrue) would be calculated as follows: CP 2 | Technician ©NACE International Field Measurements 3-9 In addition, the total circuit resistance (Rt) can be determined: [3.4] Rt = 10.3 MΩ This means that the resistance in the measuring circuit, excluding the meter resistance is: Rcirc = Rt – Rm [3.5] = 10.3 MΩ – 10 MΩ Rcirc = 0.3 MΩ or 300,000 Ω As charges flow in the earth to or from the pipe and with earth’s resistance, voltage drops occur in the earth creating a voltage gradient around the pipe as illustrated for a bare pipe in Figure 3.7. The radial lines denote the current paths while the lines perpendicular to the current lines represent the equipotential surfaces created by the current. The equipotential surfaces, which are perpendicular to the current paths, are not evenly spaced but increase with distance away from the pipe because each successive shell of earth has a larger surface area and hence a lower resistance. Figure 3.7 Voltage and Current Lines around a Bare Pipeline Receiving Cathodic Protection Current If a potential measurement is taken with the reference electrode located at A and the current direction is Source: Parker, Marshall and Peattie, Edward, Pipeline Corrosion and Cathodic Protection, 3rd toward the pipe (as would be the case in cathodic Edition, Gulf Publishing Co., Houston, TX, p.25 protection), then there is a voltage drop (Vs) in the soil between the reference electrode and the pipe surface. The soil at point A is more positive than the soil immediately adjacent to the pipe surface. If the potential difference between adjacent equipotential surfaces is 10 mV, the voltage drop in the soil between the pipe surface and the reference location would be 10 lines 10 mV/ line = 100 mV. The soil at the pipe surface is –100 mV with respect to the soil at the reference electrode. ©NACE International CP 2 | Technician 3-10 Field Measurements For example, if the polarized potential (Ep) of the pipe is –790 mVcse the voltmeter will read: Vm = –790 mVcse + (–100 mV) Vm = –890 mVcse Thus, there is a 100 mV error in the measurement that makes it appear as if the pipe is 100 mV more negative than it really is. For a well coated pipeline, the equipotential field forms in close proximity to the holidays as shown in Figure 3.8 and Figure 3.9. Figure 3.8 Current and Voltage Lines around a Holiday on a Coated Pipeline Figure 3.9 Current and Voltage Lines in Immediate Vicinity of a Holiday On a coated pipeline, most of the voltage drop is concentrated in the immediate vicinity of the holiday. Typically 95% of the total voltage drop between the reference and the steel exposed at the holiday is found within about 10 diameters of the holiday (i.e., 10 d). For a 1 cm diameter holiday, 95% of the voltage occurs within a radius of 10 cm from the holiday.1 3.3.2 Voltage Drop Errors in the Potential Measurement Due to Current in the Pipeline Voltage drops also occur in current-carrying metal paths and if the connection to the pipe is remote from the location of the reference electrode, as shown in Figure 3.10, there will be a voltage (IR drop) error (Vp) in the potential measurement due to the current in the pipe (Ip) and the pipe resistance (Rp). (See Chapter 6 for close interval potential surveys.) 1. Gummow, R.A., The Cathodic Protection Potential Criterion for Underground Steel Structures, NACE International, CORROSION/93, Paper No. 564, p. 5. CP 2 | Technician ©NACE International Field Measurements 3-11 Figure 3.10 Voltage Drop in a Pipeline Carrying Current 3.3.3 Voltage (IR) Drop Determination and Correction Potential measurements can include errors caused by voltage (IR) drops in the soil and/or in the structure itself. The magnitude of these voltage (IR) drop errors must be determined and the readings corrected. Methods that can be employed to minimize these errors include: • • • Interrupt all influencing current sources and measure the potential before significant depolarization occurs (instant OFF potential). Place the reference electrode close to the exposed metal surface. On a coated structure, the reference is placed next to a coating fault (holiday). Use of coupons to replicate a coating fault. If the current and/or the resistance of its path has been determined to be low, this error may be ignored. An example of such a case may be in the low resistivity of seawater. 3.3.3.1 Current Interruption An effective method of eliminating voltage (IR drop) errors is by making the current zero thereby making the voltage (IR drop) error equal to zero. Typically, zero voltage (IR drop) error is achieved by temporarily interrupting the current and instantly reading the structure potential. This potential must be read quickly since the structure will begin to depolarize with time (see Figure 3.11). However, there may be significant spiking of the potential due to inductive and capacitive effects associated with the interruption of the cathodic protection current. The “instant-off” potential should be measured after this spiking has decayed (see Figure 3.12) but before significant depolarization of the structure has occurred. Time (Milliseconds) Figure 3.11 Waveprint Illustrating Depolarization During OFF Cycle ©NACE International CP 2 | Technician 3-12 Field Measurements Figure 3.12 Structure-to-Electrolyte Potential “Waveprint”Illustrating the “Spike” During Current Interruption To measure the “OFF” structure-to-electrolyte potential, all sources of current influencing the area under investigation must be interrupted. This can be accomplished by installing a current interrupter in all current sources influencing the system in the area under investigation. The interrupters are essentially a mechanical or electronic relay connected to a very precise chronometer. Multiple units can be synchronized to cycle in unison, allowing multiple current sources to be interrupted simultaneously, effectively removing all current from the structure at the same instant. With no current, the voltage (IR drops) go to zero and the measured potential is the polarized potential of the pipeline. Potential Measured (Em) = Ecorr + Polarization + voltage (IR Drop) error [3.6] Em = Ecorr if voltage (IR drop) = 0 [3.7] This is illustrated in Figure 3.13. Figure 3.13 Elimination of IR Drop by Current Interruption and Subsequent Depolarization CP 2 | Technician ©NACE International Field Measurements 3-13 This technique has the added advantage that the voltage (IR drop) error in the circuit’s metallic path is also eliminated. On structures having multiple current sources influencing the potential reading it may be difficult to interrupt all the sources or to interrupt them simultaneously. Since voltage (IR drop) error at any given location is the sum of the effects of current applied at all sources, the total voltage (IR drop) error can be calculated by summing the individual effects. Interruption cycles vary based upon the type of structure-to-electrolyte potential survey that is being conducted. Key factors in selecting an interruption cycle include: • • • • Minimizing depolarization during the survey Minimizing depolarization during the OFF period Maintaining polarization over the duration of the survey project Ability to measure accurate OFF potential data after the “spike” has dissipated. Maintaining an 80%/20% or 75%/25% (4:1 or 3:1) “duty cycle” is important to minimize depolarization during the day and over the duration of the survey project. The duty cycle is the percentage of ON time to OFF time. An example would be 3 seconds ON and 1 second OFF. In addition, it is important to turn off the current interrupters at night. This will reduce the amount of time the current sources are being cycled and help rebuild any polarization which may have been lost during the day when the current sources were cycling ON and OFF. Maintaining the synchronization of the current interrupters is accomplished by several means. Current interrupters available today include quartz crystal timing devices which, once synchronized together, will maintain the timing for a period of time. Global Positioning Satellites (GPS) also provide precise timing and is available at no charge to the public. GPSsynchronized interrupters which maintain precise timing indefinitely as they “resynchronize” themselves at predetermined times throughout the day are available. Note that some will fail in the on position if they lose their satellite signal and then restart when it comes back. Unless there is a stationary datalogger monitoring the data, this event can go unnoticed and true polarized potentials will not be taken. One method of verifying interruption synchronization and of checking the magnitude and duration of the inductive/capacitive spike is by recording a “wave print” or waveform. A wave print is a graph of hundreds or thousands of structure-to-electrolyte potentials recorded every second for the duration of an interruption cycle. The data is graphed versus time and can be reviewed in the field to review: • • • • Interrupter synchronization (see Figure 3.14) Depolarization during the OFF period (see Figure 3.11) Spiking magnitude and duration (see Figure 3.12) Dynamic stray currents (see Figure 3.15) ©NACE International CP 2 | Technician 3-14 Field Measurements Figure 3.14 Waveprint Illustrating Non-Synchronized Current Interrupters Figure 3.15 Waveprint Illustrating Dynamic Stray Current Interference 3.3.3.2 Reference Electrode Near the Structure To minimize the voltage (IR drop) error associated with the electrolyte, the reference electrode should be positioned as near to the structure as possible. This may not eliminate all voltage (IR drop) errors. When dealing with underground piping or tanks, the ideal position of the electrode would be at the bare structure surface or at a coating holiday. There are times, however, when the electrode is purposely placed at some distance from the structure; this is discussed later under the section “Remote Earth.” CP 2 | Technician ©NACE International Field Measurements 3-15 Inside water storage tanks, the electrode should be positioned as close to the wall of the tank as possible. The same is true for waterfront and offshore structures; the electrode should be as close to the piling as possible. In moving water, the electrode may swing about, so some structures are equipped with guide wires or perforated plastic ducts to restrict the movement of a portable electrode. For on-grade storage tanks, data is frequently taken around the periphery of the tank. This may not yield accurate data about the potentials under the tank bottom, particularly if the anodes are in a ring around the tank or the tank is large in diameter. Stationary reference electrodes under the tank bottom yield the best data. Alternately, if a perforated plastic tube is installed under the tank and filled with water, a reference electrode can be pulled through it and potentials measured at intervals underneath (see Figure 3.16). Figure 3.16 Reference Cell Under AST Placing the reference electrode close to the structure minimizes the voltage (IR-drop) error in the electrolyte in proportion to the electrode’s closeness of the electrode to the surface. Unfortunately, for underground structures, this is not a practical technique except at points of structure-to-soil entry and exit. Also, on coated structures, the electrode cannot normally be placed any closer to the structure than a point immediately outside the coating, and it is across the coating where much of the voltage (IR) drop exists. ©NACE International CP 2 | Technician 3-16 Field Measurements 3.3.3.3 External CP Coupons and Probes Coupons Coupons can be used judiciously, particularly when accompanied by other engineering tools and data, to evaluate whether cathodic protection (CP) at a test site complies with a given criterion. Coupons may be used on a wide variety of structures and offer an advantage in the following conditions. • • • • • Multiple influencing rectifiers Multiple pipelines or complex underground structures Direct connected sacrificial anodes Stray (telluric) current Localized corrosive areas • • Areas shielded from surface measurements AC assessments Figure 3.17 External CP Coupon A CP coupon is a metal sample meeting the following conditions. (Figures 3.17 & 3.18) • • • • • Nominally of the same metal and surface condition as the structure; Small to avoid excessive current drain on the cathodic protection system; Placed at structure depth in the same backfill as the structure; Prepared with all mill scale and foreign materials removed from the surface; and Placed at a known location of an ineffective coating where applicable. (See ANSI/NACE Standard SP0104 for more information on coupons2.) Figure 3.18 Special Test Station for Monitoring Cathodic Protection 2. ANSI/NACE Standard SP0104, “The Use of Coupons for Cathodic Protection Monitoring Applications” (Houston, TX: NACE). CP 2 | Technician ©NACE International Field Measurements 3-17 Measurements on coupons can include: 1. Structure-to-Electrolyte Potentials (reference electrode located close to coupon or in soil tube) • Native (Free Corroding) - before connecting to a CP structure) • Initial - after connecting but before CP • ON - connected to a CP structure - may include a voltage (IR) error • Instant OFF - immediately after disconnecting the coupon from CP • Depolarized - after leaving disconnected 2. Current • Value • Direction • Current density (calculated) 3. Corrosion Rate (measured with Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR), Electrical Resistance (ER)) Caution should be exercised in using a free corroding coupon potential after a long exposure. From an ongoing study of a total 483 free corroding 10 cm2 coupons, a significant potential change was noted over a period of 10 years such that: • • • • 174 (36%) shifted electropositively by more than 100mV; 77 (16%) shifted electronegatively by more than 100mV; 146 (30%) shifted between +/- 100mV; and 86 (18%) had no potential reading. The actual potentials are shown in Figure 3.19. Figure 3.19 Potential Change of Free Corroding Coupon ~10 years after Installation Source: Kuwait Oil Company ©NACE International CP 2 | Technician 3-18 Field Measurements Coupon potentials measured to a nearby reference electrode may not be identical to those measured between the structure and a surface reference electrode as the latter can be affected by many factors such as: • • • Different electrolyte conditions • Soil type, moisture, chemistry, resistivity, temperature, oxygen etc. • Resulting in different reactions and products Number and size of exposed metal surfaces (coating holidays) Coating conductance variations • • • • • Current variations around and along the structure Stray current Circulating/long-line current Bimetallic or different alloy structure composition Permanently attached sacrificial anodes. As a result, the structure potentials are a weighted average of all areas exposed to different conditions. In contrast, the coupon potential represents a single, small area of exposed metal to the electrolyte at a given location. That is, the coupon may be a better indication of the status of CP at that location being independent of these factors. However, neither the coupon or structure potential measurements are necessarily in error, only that the factors affecting the potentials in each case must be understood. As the coupon essentially receives the same CP current density as the structure, it can demonstrate whether the CP system has the capacity to provide CP to a coating holiday of equivalent size of the coupon at that location. A coupon has preferably two insulated test leads brought above ground of which one is connected to a pipeline test lead under operating conditions. The other test lead is used for potential measurements. The coupon receives CP current via the pipeline and represents a coating holiday on the pipeline at the test site. A second coupon may be left exposed without CP for a free corroding potential. The voltmeter used must be of a high impedance as otherwise the polarization of the coupon can be drained via the measuring circuit. The time of measurement should also be limited to 3 seconds to avoid causing depolarization of the coupon. Advantages of a coupon include being able to measure a polarized coupon-to-electrolyte potential, free of voltage drop, with a minimum of specialized equipment (without interruption of the CP system), personnel, and travel; and may provide a more comprehensive evaluation of the polarization at the test site than conventional structure-to-electrolyte potential measurements under the influence of the factors listed above. A disadvantage is that it can have high initial costs to install coupons, especially for existing structures and are limited to measurements at the coupon location. Two test procedures to assess the adequacy of cathodic protection on metallic structures are suggested as guides but others can be employed. CP 2 | Technician ©NACE International Field Measurements 3-19 Coupon Test Method 1 – Polarized Structure-to-Electrolyte Potential A polarized potential of the coupon is measured with respect to either a stationary reference electrode located near the coupon or to a portable reference electrode placed inside the nonmetallic coupon station housing and in contact with the internal soil column near the coupon. The connection to the structure CP system (test lead) is quickly interrupted and an instant OFF potential is recorded to compare to the CP polarized potential criterion (-0.850 VCSE) (NACE SP0169). Depending on the coupon size and current, the coupon current and polarity can be measured with a zero-resistance ammeter, current clip-on ammeter, appropriately sized shunt or resistor permanently placed in series with the coupon lead. Typically, the current to be measured is very small. A discharge of current from the coupon either questions the validity of the test or it may indicate a corresponding discharge of current from the pipeline. Coupon Test Method 2 – Polarization Measurement The amount of cathodic polarization can be determined by comparing the polarized coupon-to-electrolyte potential with a subsequent depolarized potential, after the coupon has been left disconnected from the pipeline. At least 100 mV of cathodic polarization decay will satisfy the CP criterion (NACE SP0169). A high impedance recording voltmeter may be set to provide a record of the polarization decay. Again, note that the instrument itself may cause accelerated depolarization. It is not necessary to allow the coupon to fully depolarize if the 100-mV criterion can be satisfied before this occurs. Errors in the potential measurement can occur from improper installation allowing voids to exist at the coupon surface, allowing greater than normal oxygen to access from the surface, high reference electrode contact (e.g. dry soil in tube if used), broken or high resistance wire contacts and low impedance voltmeters depolarizing the coupon. Electrical Resistance Probes An “Electrical Resistance Probe” monitors metal consumption by measuring the change in electrical resistance (ER) of an element with time. As the element corrodes, its cross-section reduces causing an increase in resistance. Since temperature affects electrical resistivity, temperature compensation is needed. The exposed resistive element is mounted together with a reference resistor, which is isolated from the electrolyte but is at the same temperature and the resistances are compared. Since the resistance ratio between the exposed and reference elements at equal temperatures is the same, temperature is no longer a factor. Real time corrosion rates can be measured allowing the detection of a sudden change in the rate. A limitation is that the ER method allows only the measurement of uniform corrosion but cannot identify localized corrosion or pitting. An assessment of cathodic protection can be made with ER probes installed in the soil in areas of anticipated maximum exposure. Dynamic Stray Current Testing with Coupons/Probes Although the instant-OFF potential of the coupon is not expected to be affected by the dynamic stray current, the ON potential when connected to the structure will be influenced thus the ©NACE International CP 2 | Technician 3-20 Field Measurements voltage (IR) error should be determined under quiet stray current conditions. A monitoring program should, however, include the measurement of the ON potential and coupon current over time. An assessment of CP under dynamic stray current conditions can be made with coupons and/or soil ER corrosion probes if installed in areas of anticipated maximum exposure. When using coupons, it is important to measure instant-OFF potentials with time, so different stray current conditions are monitored. If measuring ON structure-to-electrolyte potentials only, the voltage (IR) error in the ON potential must be removed before using the result as a criterion. Coupons can be designed in such a way that the difference between ON and OFF potentials are very low because reference electrodes are located close to them or a nonmetallic tube is used as a salt bridge. These coupons are known as “IR Free” or concentric. This voltage (IR) drop can be measured by interrupting the coupon connection to the structure. If this value is not significant, the ON potential may be considered as a polarized potential. Long time potential measurements are necessary to assess polarization criteria for different conditions during a typical day. 3.4 Current Measurement Measuring current in the cathodic protection circuit is a necessary procedure in evaluating system performance. Typical current measurements are: • • • • galvanic anode current impressed current system output currents current in the structure bond current Both direct and indirect methods of current measurement are available. A direct measurement involves inserting an ammeter into the cathodic protection circuit as illustrated in Figure 3.20. 3.4.1 Using an Ammeter to Measure Current An electronic ammeter is typically composed of a voltage measuring device that measures the voltage drop across a low-resistance internal shunt. Ideally, an ammeter should have a low input resistance compared to the circuit resistance (i.e., Rm << Rcp) to prevent measurement error. Figure 3.20 Measurement of CP Current Using an Ammeter CP 2 | Technician ©NACE International Field Measurements 3-21 For example, in Figure 3.20 from Ohm’s law: [3.8] but with the ammeter inserted in the series circuit, the current measured on the ammeter (Im) is given by: [3.9] Hence, the measured current (Im) will be less than Icp, depending on the resistance of the ammeter. In many digital multimeters when the mA scale is selected, the ammeter circuit has an input resistance of several ohms. This can lead to significant errors if the ammeter is used to measure the current from a galvanic anode. Even if a 10 A or 20 A scale is chosen, the input resistance, which may be as low as 0.1Ω, may still be too high to produce an accurate current measurement in some circumstances. Figure 3.21 Current Measurement in Parallel For instance, if the ammeter is placed in Drain Conductors series with a negative drain cable in a parallel set of drain cables, as shown in Figure 3.21, an appreciable error can occur. If the shunt resistance inside the ammeter is 0.01Ω and the resistance of the negative return cable is 0.01Ω, insertion of the ammeter has doubled the negative return resistance in that circuit and possibly reduced the return current (I1) by half. In both the above examples, a more accurate method is to install an appropriately rated shunt permanently in each circuit, simply measure the voltage drop across the shunt and calculate the current (see the Shunt Table in the Reference Sheets provided at the back of the course manual). 3.4.2 Using a Shunt to Determine Current Magnitude In the parallel negative drain cable example, a shunt of the same rating, hence the same resistance, should be installed in series with each negative drain cable as illustrated in Figure 3.22. ©NACE International CP 2 | Technician 3-22 Field Measurements Figure 3.22 Use of Shunts for Current Measurements in Parallel Conductors When selecting a shunt, its current rating must exceed the anticipated circuit current, and the millivolt drop at the anticipated operating current should be easily measurable on a standard digital multimeter. For instance, if a shunt rated at 5 A, 50 mV is placed in series with a galvanic anode having an output of 5 mA, the voltage drop across the shunt will be: Vshunt = Icp Rshunt [3.10] Vshunt = 0.05 mV This small shunt voltage drop is below the resolution of most digital voltmeters used in the field. For a 5 mA current, a shunt resistance of at least 1Ω is more appropriate (See the Shunt Table in the Reference Sheets provided at the back of the course manual.) CP 2 | Technician ©NACE International Field Measurements 3-23 3.4.3 Zero Resistance Ammeter Sometimes the currents are so small (e.g., < 0.1 mA) they cannot be measured accurately without using very-high-resistance shunts, which can alter the current magnitude because of their resistance. An example is the measurement of coupon current as illustrated in Figure 3.23. If the coupon has a surface area of 10 cm2 and a current density of 10 A/cm2 the coupon current (Icp) would be: Figure 3.23 Current Measurement Using a Zero Resistance Ammeter (ZRA) Icpn = iA [3.11] where: i = current density (A/cm2) A = area (cm2) Icpn = 10 A/cm2 or 10 cm2 Measurement of such a small current with an ammeter would introduce a high resistance into the circuit as would a shunt since a resistance of 100Ω is required to measure in the 10 mV range. Under these circumstances, a zero resistance ammeter (ZRA) should be used. To avoid altering steady state conditions, connect the ZRA in parallel with the terminals on the test post before opening the link. 3.4.4 Clamp-On Ammeter A relatively noninvasive method of measuring current in a conductor is by using a clamp-on ammeter as illustrated in Figure 3.24. Figure 3.24 Using a Clamp-On Ammeter to Measure Current ©NACE International CP 2 | Technician 3-24 Field Measurements The clamp-on ammeter contains a “Hall effect” device that produces a voltage output proportional to the strength of the magnetic field, which is proportional to the magnitude of the current in the conductor. The clamp on ammeter must be installed around a single conductor and should be centered around that conductor. Accuracy of the clamp-on ammeter diminishes at currents of a few mA. When there are multiple current-carrying conductors in a congested area, the accuracy is reduced if there is magnetic interference from adjacent conductors. 3.4.5 Pipeline Current Measurements 3.4.5.1 Clamp-On Ammeter An illustration of the use of clamp-on ammeters is given in Figure 3.25. Figure 3.25 Pipe Current Measurement Using Sensing Loop and Swain Meter Source: Swain, W.H., Clamp-On Ammeters Can Watch Cathodic Protection Current Flow, Pipe Line & Gas Industry, March 1998, p.38 3.4.5.2 2-Wire Line Current Test Current in a pipeline can be measured by means of a test (current span) station calibrated test station such as shown in Figure 3.26 or by using test stations and aboveground appurtenances as a temporary current span test station. In an IR-drop test, the pipe simulates a low-resistance shunt. When properly calibrated, a voltmeter can be used to measure a voltage drop between the two wires or connection points to the pipe. Using Ohm’s Law, the current in the pipeline steel can be calculated. A 2-wire current span test station can be used where a known length of pipeline and the diameter and wall thickness or the weight per foot are known. The current in the pipe span can be calculated by measuring the voltage drop across the span, determining the resistance of the span from a pipe table, and using Ohm’s Law as you would with a shunt. Figure 3.26 shows the test setup to measure the mV drop in the section of pipeline. Table 3.1 provides some resistance values for common pipe sizes. The section of pipe must be continuous with the same diameter and wall thickness without attachments through the pipe span to be used. CP 2 | Technician ©NACE International Field Measurements 3-25 Figure 3.26 2-Wire Line Current Test Because the voltage drop across a pipe span is relatively small, with earlier instruments it was necessary to correct for the voltage drop in the test leads caused by the current drawn by the meter. However, with the high-input resistance meters available today, this correction is not necessary. For example, if the voltage drop across a 61-m (200-ft) span of 762 mm (30-in) pipe weighing 176.65 kg/m (118.7 lbs/ft) is 0.17 mV, then current is calculated as follows: Pipe resistance/ft from Table 3.1 = 8.01 /m (2.44 /ft) = 0.00000801 /m (0.00000244 /ft) Total resistance = 61 m x 0.00000801 /m = 0.000488 OR = 200 ft x 0.00000244 /ft = 0.000488 Measured voltage drop = 0.17 mV Current (I) = V/R = 0.00017V/0.000488= 0.348 A ©NACE International CP 2 | Technician 3-26 Field Measurements Table 3.1: Table of Pipe Resistances - Steel Pipe Resistance* Pipe Size in. 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Outside Diameter Wall Thickness Weight Resistance in. cm in. cm lb/ft kg/m /ft /m 2.35 4.5 6.62 8.62 10.75 12.75 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 36.00 5.97 11.43 16.81 21.89 27.31 32.38 35.56 40.64 45.72 50.80 55.88 60.96 66.04 71.12 76.20 81.28 86.36 91.44 0.154 0.237 0.280 0.322 0.365 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.39 0.60 0.71 0.82 0.93 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 3.65 10.8 16.0 28.6 40.5 46.6 54.6 62.6 70.6 78.6 86.6 94.6 102.6 110.6 118.7 126.6 134.6 142.6 5.43 16.07 28.28 42.56 60.27 73.81 81.26 93.16 105.07 116.97 128.88 140.78 152.69 164.59 176.65 188.41 200.31 212.22 76.2 26.8 15.2 10.1 7.13 5.82 5.29 4.61 4.09 3.68 3.34 3.06 2.82 2.62 2.44 2.28 2.15 2.03 256.84 87.93 46.87 33.14 23.39 16.09 17.36 15.12 13.42 12.07 10.96 10.04 9.25 8.60 8.01 7.48 7.05 6.66 *Conversions: 1 in. = 2.54 cm 1 ft = 0.3048 m Based on steel density of 489 lbs/ft3 (7832 kg/m3) and steel resistivity of 18 μ-cm. Refer again to Figure 3.26. Note the meter is showing a positive indication. This means the current is entering the meter on the positive terminal. The positive terminal is connected to the west end of the span. Since the meter is in parallel with the span, current on the pipe is from west to east. The accuracy of this test method depends greatly on accurate knowledge of the dimensions of the pipe. Should there be an odd-sized joint within the span, or some appurtenance such as a valve, the calculated resistance will not be correct. The 4-wire test method overcomes these difficulties. 3.4.5.3 4-Wire Line Current Test Pipeline current can also be measured using the four-wire span illustrated in Figure 3.27. For accurate measurement, the span is calibrated by injecting a known DC test current through the pipe using the outside test leads (1) and (4) and measuring the resulting voltage drop across test leads (2) and (3). CP 2 | Technician ©NACE International Field Measurements 3-27 Figure 3.27 Calibrating a Pipeline Current Span The resistance of the pipe between test leads (2) and (3). Is calculated from Ohm’s law: [3.12] The result can be anticipated prior to the test by referring to Table 3.1. Attention to polarity is important in this measurement since there will probably be a residual current during the test, and the test current may cause a reversal in the voltage drop polarity. For example: V2-3 = +21 mV (before test current applied) V2-3 = –19 mV (after test current applied) It =10 A The resistance (Rpipe) of the pipe section being tested is: The calibration factor is calculated as follows: [3.13] ©NACE International CP 2 | Technician 3-28 Field Measurements where: K = calibration factor of pipe section (A/mV) Itest = test current applied to section (A) Etest = E with current applied – E with no current applied (mV) Therefore the current calibration factor is: During the test there was residual current in the pipe from another source other than the test current which magnitude is: with a direction from 2 to 3 in Figure 3.27. Normally, if the pipeline operating temperature is stable, this need only be done once as the calibration factor will remain the same for subsequent tests at the same location. On pipelines where the temperature of the pipe changes considerably (with accompanying changes in resistance), more frequent calibration may be necessary. Once the calibration factor is known, the normal current magnitude can be calculated. First, measure the voltage drop in mV across the measuring span (with the test equipment removed between 1 and 4 in Figure 3.27)using the inside test wires. This voltage drop is due to normal pipeline current. Calculate current by multiplying the calibration factor by the voltage drop measured above: [3.14] where: I = pipeline current (A) K = calibration factor of pipe (A/mV) EmV = voltage drop of pipe section (mV) The polarity of the voltage drop will determine the direction of current. • • If the voltage drop reading is positive, then the direction of current is from the positive to the negative terminal of the voltmeter. If the reading is negative, then the direction of current is from the negative to the positive terminal. Most digital meters will not read below 0.1 mV. If readings below 0.1 mV are anticipated, or if a zero reading is obtained during a test, a more sensitive meter must be used. CP 2 | Technician ©NACE International Field Measurements 3-29 3.4.6 Coating Resistance Calculations Coating resistance measurements are a means of establishing a coating’s ability to provide a dielectric barrier between the structure and the environment. The better barrier a coating provides, the higher the coating resistance. The coating resistance is simply the resistance of the structure to the environment multiplied by the surface area of the structure. Units are therefore Ω-m2 (Ω-ft2). Coating resistance measurements years apart can reveal the long-term performance of a coating and if anything has occurred that detrimentally affected the overall structure coating (e.g., electrical shorts, construction damage, and abnormal soil stress). The resistance of a pipe coating can be determined by impressing a current on to a known pipe surface area and measuring the change in potential due to the current being impressed as illustrated by Figure 3.28. Figure 3.28 Set-Up for Coating Resistance Measurement The current being impressed (IC) is determined by measuring the change in the current in the pipeline at each end of the section (ΔITS1, ΔITS2) due to the test and calculating the difference between these values which is the current pickup in that section of pipeline (Equation 3.15). [3.15] The potential change (ΔEave) due to this current being applied is the average shift between the ON and OFF potentials in the pipeline section under test (ΔETS1, ΔETS2 etc.). Equation 3.16 shows the average potentials if potentials are taken at two locations. If potentials are taken at intervals through the section, then all must be averaged. [3.16] The pipe-to-earth resistance (coating resistance (RC)) is calculated by Equation 3.17. ©NACE International CP 2 | Technician 3-30 Field Measurements [3.17] This is the total resistance of the pipe surface area (Apipe) of the pipe section under test (Equation 3.18). The area of the pipe surface is calculated from the outside diameter (d) and the length of the pipe section (L). [3.18] To be able to compare it with other coatings the specific coating resistance of a unit area (rC) of coating is determined by equation 3.19. [3.19] The coating effectiveness may be defined in terms of conductance (Gpipe) of the pipe section which is the inverse of resistance (Equation 3.20) or specific conductance (gC) in terms of a unit area of coating which is the inverse of specific resistance (Equation 3.21). [3.20] [3.21] See NACE TM0102 “Measurement of Protective Coating Electrical Conductance on Underground Pipelines” for further information. EXAMPLE: Calculate the coating resistance and conductance for a pipeline section as follows. Pipe Outside Diameter (d) = 610 mm (24 inches) Length of Pipe Section (L) = 1,609 m (5280 ft) The data in italics in the table was obtained in the field and the regular font was calculated from the above equations as referenced in the table. CP 2 | Technician ©NACE International Field Measurements 3-31 Factor TS1 TS2 IONTS 3.00 A 2.80 A IOFFTS 0.20 A 0.10 A DITS 2.80 A 2.70 A Pipe Section (metric) Pipe Section (Imperial) = 2.80 - 2.70 = 0.10 A = 2.80 - 2.70 = 0.10 A Apipe (Eq 3.16) rC (Eq 3.17) = 1.10 + 0.85 2 = 0.975 V = 0.975 0.10 = 9.75 = π .610 x 1609 = 3,080 m2 = 9.75 x 3080 = 30,030 Ω-m2 = 1.10 + 0.85 2 = 0.975 V = 0.975 0.10 = 9.75 = π 2 x 5280 = 33,158 ft2 = 9.75 x 33158 = 323,291 Ω-ft2 GC (Eq 3.18) = = 0.103 Siemens (S) = = 0.103 Siemens (S) gC (Eq 3.19) = = 0.0000333 S/m2 (33.3 μS/m2) = = 0.00000309 S/ft2 (3.09 μS/ft2) IC (Eq 3.13) EONTS -2.00 V -1.70 V EOFFTS -0.90 V -0.85 V ETS 1.10 V 0.85 V Eave (Eq 3.14) RC (Eq 3.15) 3.5 Current Requirement Tests When a structure is in place in its final configuration, it is possible to perform current requirement tests to determine the current needed for cathodic protection. Such tests have the advantage of producing data on the actual structure and its environment and do not involve assumptions as to effectiveness of the protective coating and other factors. Structures which have been designed for the application of cathodic protection can be tested in their entirety or broken down into sections. ©NACE International CP 2 | Technician 3-32 Field Measurements Current requirement testing involves installation of either a permanent or test anode system. The purpose is to introduce current into the earth at the site where the permanent cathodic protection system (anode bed) will be located. For large structures, a representative area can be chosen for the initial test phase. A suitable power supply (batteries, test rectifier, or motor generator system) is then connected between the structure and test anode bed (groundbed) as illustrated in Figure 3.29. Test current is applied between the anode bed and the structure and the changes in potential of the structure are measured. Figure 3.29 Test Circuit for Current Requirement Test With this circuit, controlled amounts of current can be applied to the structure from the anode until the desired potentials are obtained. The potential measured includes the following: • • The potential between the structure and the reference electrode A voltage (IR drop) produced between the point in the electrolyte where the reference electrode is located and the structure Calculation: [3.22] where: Itest = Test current applied to structure (A) Ireq = Estimated additional current requirement (A) Eptest = Polarization from test (V) [EOFF – EINITIAL] Epreq = Polarization required (V) = [0.850 V – EINITIAL] for -850 mVCSE polarized potential criterion or 0.100 V (100 mV) for 100 mV polarization criterion The formulas used for these calculations consider: The value of resistance producing a voltage (IR drop) R = (EON - EOFF)/ION CP 2 | Technician [3.23] ©NACE International Field Measurements 3-33 The voltage (IR) drop can be calculated for other intermediate levels of current (I1) using an average value for R: Voltage (IR) drop = I1 x R [3.24] Following is an example of a current requirement calculation: 1. Static structure-to-electrolyte potential = –0.645 VCSE 2. Desired polarized structure-to-electrolyte potential = –0.850 VCSE 3. Test current = 50 mA (0.050 A) 4. Polarized potential due to test current = 0.775 VCSE From Equation 3.25: [3.25] 3.6 Electrical Isolation 3.6.1 Problems Electrical isolation is important to the success of cathodic protection except where the system being protected cannot be electrically isolated and the CP system is designed to take this into account. In an isolated system, the failure of the CP system to maintain a satisfactory level of cathodic protection might be caused by an electrical short to another metallic structure including other pipelines, power grounds, station grounding, etc. An isolation can be rendered ineffective by a fault in the isolating fitting or by a metallic bypass around the isolation. 3.6.2 Locating the Problem There are many methods to locate an electrical isolating device and/or tests to determine if the device is functioning properly: • • • • • • • • Pipeline/cable locators Isolation testing instrument Structure-to-electrolyte potential Interrupted structure-to-electrolyte potential DC line current measurements Fixed cell to moving ground Current response test Isolation resistance test. ©NACE International CP 2 | Technician 3-34 Field Measurements Each technique has advantages and disadvantages including whether or not the particular test can be used under the circumstances and if the test is conclusive under those circumstances. For more information on isolation testing consult NACE SP0286 “Electrical Isolation of Cathodically Protected Pipelines”. 3.7 Corrosivity Evaluations Electrolyte resistivity and pH are two of the factors that determine the corrosivity of an electrolyte. Electrolyte resistivity is a measure of the ability of the electrolyte to support electrochemical corrosion, and electrolyte pH is a measure of the acidity or alkalinity of the electrolyte. Both influence the corrosion rates of metals placed underground or that are submerged. Resistivity and pH are not the only factors affecting corrosion since total acidity, aeration, moisture content, soil type, soil permeability and composition, and heterogeneity play a role in determining the corrosivity of a given soil. Because of the natural heterogeneity of soil, there is no single value of soil resistivity that represents a particular site. Instead, for a given site a range of values are measured and calculations made to determine the “layer” resistivities. 3.8 Soil Resistivity Testing 3.8.1 Purposes Soil resistivity is one factor that can determine the corrosivity of an environment. Resistivity is also essential in the design of cathodic protection systems. 3.8.2 Measurement Techniques Various means of measuring electrolyte resistivities are used. These include the Wenner FourPin Method as described in ASTM Test Method G57, Collins Rod, and electromagnetic induction methods. 3.8.2.1 Wenner Four-Pin Method The four-pin resistivity method developed by Wenner involves the use of 4 pins driven into the ground (Figure 3.30). Current is applied to the outer pins, and the voltage between the inner pins is measured. The resistivity is a function of the current, voltage, and spacing of the electrodes (which is equal to the depth of the test). The current is usually applied using an instrument that supplies alternating current; otherwise, polarization effects occur at the electrodes that can alter the reading. If a DC current is used, “instant” current and potential measurements must be taken. Stray earth currents can also affect the readings if these effects are not separated from the data. CP 2 | Technician Figure 3.30 Wenner Four-Pin Soil Resistivity Measurement ©NACE International Field Measurements 3-35 The first step is to determine the resistance in ohms between the center pair of pins. Using the commercially available equipment detailed above, the resistance is indicated directly by the soil resistance meter. If the pin depth is insignificant with respect to the pin spacing (depth <5% of pin spacing), the resistivity is calculated using Equation 3.26. [3.26] Where: ρ – resistivity (Ω-cm) π – pi constant (=3.1416…) a – Spacing of the pins (cm) b – depth of the pins (cm) R – resistance (Ω) If the spacing is measured in feet, a conversion factor of 191.5 can be used that includes the conversion of feet to cms and 2 as in Equation 3.27. = 191.5 a R [3.27] Where: ρ – resistivity, Ω-cm 191.5 = Factor (= 2 π 30.48 cm/ft), cm/ft a – inside pin spacing, ft R – resistance, Ω For Example: Where the inner and outer pin spacings are not equal, the resistivity can be calculated by Equation 3.22 (ASTM 57). Where: ρ – resistivity (Ω-cm) π – Pi constant (=3.1416…) a – Inner pin spacing (cm) b – outer pin spacing (cm) R – resistance (Ω) Note that if a = b in equation 3.23, it simplifies to equation 3.21. ©NACE International CP 2 | Technician 3-36 Field Measurements For Example: If the resistance reading from the meter is 8Ω at a pin spacing of 3.084 m (304.8 cm), then the average resistivity to an approximate depth of 3 m is: = 2 x x 304.8 cm x 8 Ω = 15,320 Ω-cm or, similarly if the resistance reading is 8Ω at a pin spacing of 10 ft, then the average resistivity to a depth of 10 ft is = 191.5 cm/ft x 10 ft x 8 Ω = 15,320 Ω-cm If the line of soil pins runs closely parallel to a metallic structure, the presence of the structure may cause the indicated soil resistivity values to be lower than is actually the case. This is because a portion of the test current will choose the metallic structure path rather than through the earth. For this reason, this situation should be avoided. When taking soil resistivity measurements along a structure, it is good practice to place the line of pins perpendicular to the structure, with the nearest pin no closer than 3 m (15 ft) from the structure. 3.8.2.2 Layer Resistivity Calculations The Barnes Layer method of predicting the resistivity of layers is based on the principle that each layer is a resistor that is in parallel with the other layer resistors (R1, R2, R3 etc.). Therefore, the parallel resistance equation would apply (Equation 3.28) where RT is the total resistance of all resistors in parallel. [3.28] If only two parallel resistances are involved, the Equation 3.28 becomes Equation 3.29. [3.29] If the measured resistance (not resistivity) of the upper layer (R1) and that of the upper and lower layers combined or the total resistance (RT) is substituted in Equation 3.29, the predicted resistance of the lower layer (R2) can be calculated from which the resistivity is calculated. This approach will only apply if the total measured resistance (RT) values become smaller with depth. In theory this should always occur but because a resistance measurement to deeper depths requires larger pin spacings, the sample of soil changes horizontally as the depth increases. If the sample includes an increased amount of high resistivity soil at larger pin spacings, a higher resistance at a deeper depth might be measured. In this case the measurement location should be moved. CP 2 | Technician ©NACE International Field Measurements 3-37 Figure 3.31 illustrates three different layers of resistance and resistivity. The resistance can be measured to depths of a1, a2 and the resistivity calculated by Equation 3.26 but the resistance and resistivity of layers L1, L2 and L3 are desired. Figure 3.31 Average and Layer Resistivity The depth of Layer 1 (L1 (cm)) is the same as (a1 (cm)) and the measured resistance (Ra1 (Ohms)) is the same as the layer resistance (RL1 (Ohms)). The resistivity (ρL1 (Ohm-cm)) can be determined from Equation 3.30 modified from Equation 3.26. [3.30] From the measured resistance values for Layer 2, the layer resistance can be calculated from Equation 3.29 but using the symbols in Figure 3.31, it becomes Equation 3.31. [3.31] Where: RL2 - Resistance of Layer 2 (L2) (Ohms) R1 - Resistance measured to depth a1 (Ohms) R2 - Resistance measured to depth a2 (Ohms) The resistivity (ρL2 (Ohm-cm)) of Layer 2 (L2 (cm)) in Figure 3.31 can then be calculated by Equation 3.32 modified from Equation 3.26. [3.32] Similarly, the resistance of Layer 3 can be calculated by Equation 3.33. ©NACE International CP 2 | Technician 3-38 Field Measurements [3.33] Where: RL3 - Resistance of Layer 3 (L3) (Ohms) R2 - Resistance measured to depth a2 (Ohms) R3 - Resistance measured to depth a3 (Ohms) The Layer 3 (L3 (cm)) resistivity (ρL3 (Ohm-cm)) in Figure 3.31 can then be calculated by Equation 3.26. [3.34] Example 1 Predict the resistivity of the three layers of soil given the following data using Figure 3.31 as a reference. a1 = 160 cmRa1 = 10.0 Ωρa1 = 10,050 Ω-cm a2 = 320 cmRa2 = 7.4 Ωρa2 = 14,880 Ω-cm a3 = 480 cmRa3 = 3.1 Ωρa3 = 9,350 Ω-cm The calculations are given in the table for each layer. Example 1 Factor Layer Depth (L) Layer 1 L1 = a1 = 160 cm RL1 = Ral = 10.0 Layer 2 L2 = a2 – a1 = 320 – 160 = 160 cm Eq 3.31 Layer 3 L3 = a3 – a2 = 480 – 320 = 160 cm Eq 3.33 = 28.46 Ω Eq 3.32 = 5.33 Ω Eq 3.34 Layer Resistance (RL) Layer Resistivity (L) ρL1 = ρa1 = 10,050 Ω-cm OR Eq 3.30 = 2 π 160 x 28.46 = 28,610 Ω-cm = 2 π 160 x 5.33 = 5,360 Ω-cm = 2 π 160 x 10.0 = 10,050 Ω-cm CP 2 | Technician ©NACE International Field Measurements 3-39 Example 2 Alternately, if using Equation 3.27 and equivalent spacing in feet, predict the resistivity of the three layers of soil in Ohm-cm given the following data using Figure 3.31 as a reference. a1 = 5.25 ft Ra1 = 10.0 Ωρa1 = 10,050 Ω-cm a2 = 10.50 ftRa2 = 7.4 Ωρa2 = 14,880 Ω-cm a3 = 15.75 ftRa3 = 3.1 Ωρa3 = 9,350 Ω-cm The calculations using adaptations from Equation 3.27 are given in the table for each layer. Example 2 Factor Layer Depth (L) Layer 1 L1 = a1 = 5.25 ft RL1 = Ra1 = 10.0 Ω Layer 2 L2 = a2 – a1 =10.50 – 5.25 = 5.25 ft Eq 3.31 Layer 3 L3 = a3 – a2 = 15.75 – 10.50 = 5.25 ft Eq 3.33 = 28.46 Ω Eq 3.27 modified = 5.33 Ω Eq 3.27 modified = 191.5 x 5.25 x 28.46 = 28,610 Ω-cm = 191.5 x 5.25 x 5.33 = 5,360 Ω-cm Layer Resistance (RL) Layer Resistivity (L) ρL1 = ρa1 = 10,050 Ωcm OR Eq 3.27 modified = 191.5 cm/ft x 5.25 ft x 10.0Ω = 10,050 Ω-cm 3.9 Measuring pH Electrolyte pH can be measured in several ways. For liquids, pH (litmus) paper or a pH meter may be used. For soils, a pH meter may be used, or a filtrate may be made from distilled water and a soil sample and the pH measured with litmus paper, a pH meter, or a pH test kit. Note that a pH meter uses a glass electrode with a rather fragile glass bulb on the bottom. Care must be taken when using these instruments not to break the electrode bulb. Soil pH may also be measured using an antimony electrode and a copper-copper sulfate electrode. The antimony electrode consists of a slug of antimony metal in the bottom of a nonmetallic tube. The slug is connected to a terminal on the top of the tube. It is important to keep the antimony shiny and bright. Use fine non-metal bearing sand paper or emery cloth for cleaning. Do not use steel wool or other metallic abrasive since particles of metal may become embedded in the antimony and affect the reading. ©NACE International CP 2 | Technician 3-40 Field Measurements The antimony electrode and copper-sulfate reference electrode are placed close together with the electrode tips in the soil and connected to a voltmeter. It does not matter which cell is connected to which terminal of the meter since it is the potential between the two electrodes that is of interest. Take care not to get any copper sulfate on the antimony slug. There is a scale on the side of the antimony electrode that is calibrated in mV and pH. Once the potential difference between the two electrodes is obtained, the pH can be determined from the scale. 3.10 Concrete Structures The following discussion pertains primarily to atmospherically exposed reinforced concrete structures, such as bridges and buildings. Reinforced concrete structures requiring cathodic protection are found in immersed situations as well, such as prestressed concrete pipes, prestressed concrete pilings, and cement-mortar-coated steel pipes. Since cathodic protection requires a continuous structure, and since reinforcing is usually only connected through tie wires, electrical continuity must be verified. Items such as drains, railings, and expansion joints must also be continuous with reinforcing to avoid interference. Electrical continuity can be evaluated using AC-resistance measurements. Connections to two widely spaced points on the structure are made through the AC instrument. A resistance of less than 0.100Ω indicates continuity. Continuity can also be estimated by measuring the rebar-to-reference cell potential between several locations using a constant reference cell location. rebar-to-structure potentials within 0.001 V of each other suggest that the points are continuous. Measurements greater than 0.001 V but less than 0.003 V indicate uncertain continuity, and measurements greater than 0.003 V indicate that the structure is discontinuous. Depth of concrete cover is important when an impressed current cathodic protection system is to be installed. The anode must not come into contact with the rebar or any other metallic component of the structure, otherwise a short circuit will occur rendering the system useless. Depth of cover is measured using a Pachometer. Metal reinforcing-to-reference electrode potentials are taken on concrete structures to determine the probability of corrosion of the embedded metal. Details of this procedure are provided in ASTM Test Method C876, “Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete.” A high-impedance voltmeter is connected between the reinforcing steel and a reference electrode placed on the surface of the concrete. The most commonly used reference electrode is the copper-copper sulfate half-cell. Excavation of the concrete to expose the reinforcing steel might be required to obtain the structure contact. Because of the high resistance inherent in the measurement circuit, the voltmeter must have sufficient input impedance to accurately measure the half-cell potentials. The minimum voltmeter impedance should be 10 x 106Ω and be selectable to higher values in order to identify significant resistances in the circuit. To reduce the contact resistance between the concrete surface and the half-cell, a moist sponge is placed between the half-cell and the surface. It is also desirable to pre-moisten the entire surface with clean potable water. This will reduce instabilities, but will result in a “leveling” of the measured potentials. CP 2 | Technician ©NACE International Field Measurements 3-41 Half-cell potential measurements can be interpreted as follows: Potential Probability of Corrosion More positive than –200 mV –200 to –350 mV More negative than –350 mV < 5 percent About 50 percent > 95 percent Potential measurements on concrete that is completely saturated, e.g., submerged piling, may be more negative than –350 mV yet have little corrosion activity because the water saturation limits oxygen diffusion which reduces corrosion. Potential measurements are usually taken in a grid pattern, for example, one reading every 0.6 to 1.5 m (2 to 5 ft). The resulting data is plotted on a contour map showing areas that are corroding. 3.11 Direct Inspection A CP technician may be requested to inspect a structure for corrosion. This is an opportunity to gather enough forensic information to determine the cause of the corrosion and to predict the fitness for service. As a minimum the following steps should be followed but each owner can have additional specific requirements such as ultrasonic or magnetic flux leakage (crack) testing that require special training. 1. Before excavation, measure potentials and soil resistivity over the site 2. Photographs are to be taken at each stage of exposure. 3. When the structure is exposed, a pH of the surface should be compared to the soil pH and potentials repeated at the ends of the excavation (a high pH is expected at the surface of a structure with CP). 4. Inspect the coating for signs of deterioration, damage and lack of adhesion. Outline these areas for a photograph. 5. Note any moisture underneath the coating and take a pH of this moisture with litmus paper. 6. Obtain samples of any corrosion product and seal in a plastic bag as they may be used for further analysis and/or bacteria tests. 7. After the surface is cleaned, identify the welds to known geographical points (GPS) or if a pipeline to the pipe chainage and weld numbers. 8. Locate areas of corrosion, outline them, tie them to the clock position and closest weld. Overlay them with a grid of known dimensions. Measure depths of pits and record them on the surface for photographs. 9. Record any repairs made 10. Inspect the repair coating for holidays and adherence. 11. Repeat the potential measurements and adjust the CP system if necessary. 3.12 Leak Frequency Comparing the cumulative frequency of leaks vs. time often identifies the effect of corrosion control. It is typical of corrosion that the cumulative frequency increases logarithmically with ©NACE International CP 2 | Technician 3-42 Field Measurements time. Leaks that occur from purely mechanical causes would not behave with such regularity. The effect of cathodic protection is to slow down or flatten the leak rate curve. Figure 3.32 is an example of a leak vs. time curve before and after the application of cathodic protection. Figure 3.32 comes from actual data on the leak history of a cast iron water main before and after cathodic protection was installed. The leaks occurring before 1984 show the logarithmic increase in the number of leaks with time. The leaks are shown to have essentially stopped after cathodic protection was installed. Maintaining leak record curves is a method that can be used to prove the effectiveness of cathodic protection. It is particularly useful for owners of structures that are not regulated by a government agency. Figure 3.32 Cumulative Leaks vs. Time Before and After Cathodic Protection CP 2 | Technician ©NACE International Chapter 3 Field Measurements ® Measurement of CP Effectiveness 2 ▪ Instruments ▪ Structure-to-electrolyte potential ▪ Test coupons ▪ Current measurements ▪ Soil resistivity layers ▪ Direct inspection ▪ Leak frequency CP2 | Technician Analog Meters 3 ▪ Needle attached to coil rotates through permanent magnetic field as current in coil increases Scale Pointer Permanent Magnet ▪ Needle points to a calibrated scale ▪ Typically relatively low internal resistance Range Resistors & Selector Switch N ▪ Usual type in DC power sources S Im Moving Coil Damping Resistor + ▪ High internal impedance CP2 | Technician 1 Digital Meters 4 Digital Meters ▪ Condition input with analog electronics & convert to digital. Digital Display ▪ Many samples taken of which an average displayed 2 to 4 per sec to minimize variations (e.g. noise) & improve accuracy 1.999 VOLTS AMPS OHMS Analog to Digital Conversion Analog Signal Conditioning ▪ Read out on 3 ½ or 4 ½ digit LED or LCD display Signal Input ▪ Display damaged at extreme temperatures CP2 | Technician Digital Multimeters (DMM) 5 DMM Functions: ▪ Minimum AC/DC voltmeter. DC ammeter, DC Ohmmeter ▪ May also include Hertz, continuity & diode test ▪ May have Auto-ranging & Hold features ▪ All functions eventually derived from a voltage and ADC CP2 | Technician Digital Voltmeter 6 ▪ When on DC/AC Voltage – input protected, conditioned and converted (figure) ▪ ON to OFF - display changes rapidly ▪ Instant OFF potential: record displayed value 0.5 to 1 second after interruption ▪ Avoids error due to spiking CP2 | Technician 2 Digital Ammeter 7 ▪ Ammeter in series ▪ All current through internal resistor (shunt) ▪ Voltage across internal resistance processed & displayed in A, mA, μA. ▪ Ammeter increases total circuit resistance & reduces overall current ▪ Lower current requires higher resistance internal shunt. ▪ Different scales have different internal resistances & change total current ▪ Use highest scale that can be read accurately CP2 | Technician Digital DC Ohmmeter 8 ▪ Impresses a current through known resistance & external resistance under test ▪ Digital electronics calculates and displays test resistance ▪ Based on ratio of two resistances ▪ Resistance affected by temperature but eliminated with ratio of both at same temperature. ▪ Readout calibrated in Ohms (kΩ, Ω etc.) CP2 | Technician Digital DC Ohmmeter Precautions 9 ▪ DMM Ohmmeter uses internal voltage source ▪ External voltage gives reading too low or too high depending on polarity. ▪ Do not use DMM in circuit with external voltage including: ▪ back emf (anodes) or ▪ galvanic potential (road casings) ▪ Use AC Ohmmeter in these cases. CP2 | Technician 3 Other DMM Features 10 AC Volts versus Volts rms ▪ Basic voltmeter assumes a pure sine wave ▪ Reads peak AC x 0.7071 (1/√2) to convert to AC Volts ▪ Rms measures AC wave for true value Auto-range ▪ Basic meter requires selecting correct scale ▪ Auto-range selects best scale while processing data ▪ Convenient but adds time ▪ Must pay attention to scale on readout ▪ Instant OFF potential cycle may be too short (numbers flashing) ▪ Manually set scale if so CP2 | Technician Other Instrument Features 11 Diode Test ▪ Diode requires small voltage before conducting in forward direction ▪ If good, reads forward voltage and OL in reverse ▪ If shorted, reads 0 V in both directions ▪ If open circuit, reads OL in both directions Hertz ▪ Measures number of AC cycles (hz) (50 or 60 hz) ▪ DC output - measures peaks (100 or 120 hz if full wave) Hold ▪ Freezes readout where difficult to read or capture reading CP2 | Technician Structure-to-Electrolyte Potentials 12 “The potential difference between the metallic surface of the structure and electrolyte that is measured with reference to an electrode in contact with the electrolyte.” Measurements must include: ▪ Magnitude ▪ Polarity ▪ Units ▪ Reference Electrodes CP2 | Technician 4 Structure-to-Electrolyte Potentials 13 Used to: ▪ Locate anodic areas on non-cathodically protected pipelines ▪ Determine the effectiveness of cathodic protection ▪ Locate stray currents ▪ Locate electrical shorts and contacts ▪ Locate coating holidays CP2 | Technician Minimizing IR Drop 14 ▪ Reduce distance between reference electrode and structure surface ▪ Measure potential with current interrupted (instant-off potential) ▪ Measure polarization formation or decay (after current ON or after left OFF) ▪ Install external CP coupons to replicate a coating holiday CP2 | Technician Structure-to-Electrolyte Measurement Circuit 15 Where: Rm = voltmeter input resistance Rtl = test lead resistance Rt = total circuit resistance Rp,e = Resistance of the pipe with respect to Remote Earth Rr,e = Resistance of the Reference Electrode with respect to Remote Earth Im = meter current Etrue = Polarized potential of the pipe with respect to the Reference Electrode Vm = Rm Rt Vm = voltmeter reading x Etrue re = remote earth CP2 | Technician 5 Measurement Error (Example) 16 Given: Etrue = 1,000 mV, Rm = 1 MΩ, Rr,e = 100 kΩ, Rp,e = 10 Ω, and Rtl = 0.01 Ω each Calculate voltage across the voltmeter Rt = = = Rt Vm = Rm Rt Rm + Rr,e + Rp,e + 3Rtl 106 + 105 + 10 + 3(0.01) 1.1 MΩ X Et = 909mV Error: 1,000 − 909 100 = 9% 1,000 ▪ If Rm = 10 MΩ Rt = 990 mV reducing error to 1% CP2 | Technician Methods to Minimize Electrode Contact Resistance 17 Wet soil as needed Avoid grass Avoid asphalt/concrete/gravel surfaces Access ports ▪ ▪ ▪ ▪ Small diameter Hole filled with A soapy water PVC tube Dry Soil or Frozen Ground Clay CP2 | Technician Etrue Knowing Two Input Resistances E𝑡𝑟𝑢𝑒 = 18 Vh (1 − K) V 1 − (K h ) Vl where: Etrue = true potential (V) K = input resistance ratio Rl/Rh Rl = lowest input resistance Rh = highest input resistance Vl = voltage measured with lowest input resistance Vh = voltage measured with highest input resistance CP2 | Technician 6 Etrue Knowing Two Input Resistances 19 Example: Vl of –650 mVcse at Rl = 1.0 MΩ Vh = –800 mVcse at Rh = 10.0 MΩ Calculate Etrue: E true = = - 720 mV 1 − 0.123 - 800 mV (1 − 0.1) - 800 mV 1 − 0.1 - 650 mV = - 720 mV 0.877 = - 821 mVcse CP2 | Technician Rtotal & RCircuit 20 Rt = = Rm E true Vm 10 M 821 mV 800 mV Rt = 10.3 MΩ Rcirc = R t – Rm = 10.3 MΩ – 10 MΩ Rcirc = 0.3 MΩ or 300,000 Ω CP2 | Technician Bare Pipe Voltage Gradients Equipotential 21 Vm line (surface) Example: Given: 10 mV per equipotential line If Ep is –790 mVcse Voltmeter will read: Current line Vm = –790 mVcse + (–10 mV x 10) Vm = –890 mVcse CP2 | Technician 7 Coated Pipe Voltage Gradient 22 Voltmeter Holiday d CP2 | Technician Voltage Drop with Current in a Pipeline 23 Voltmeter (1) Ip Ip (2) Ip Vp = Ip Rp Vp = Potential at (2) to trailing wire tied to (1) minus potential to pipe (2) to same reference electrode position CP2 | Technician Voltage (IR-Drop) Error Determination & Correction 24 ▪ Interrupt the current and measure the potential before significant depolarization occurs ("instant off" potential) ▪ Voltage (IR) error = On – instant OFF ▪ Place reference electrode close to exposed metal (coating fault (holiday)) ▪ Coupons ▪ May be neglected if current and/or resistance is small. Before neglecting magnitude must be determined. CP2 | Technician 8 Current Interruption ▪ All sources of current must be interrupted ▪ Must be read quickly as structure depolarizes Potential ▪ If spiking, then read after spike Time P/S Potential (Volts) ▪ Effective as when I= 0 A, then IR = 0 V 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Time Milliseconds CP2 | Technician Current Interruption Showing IR & Depolarization 26 Em = Ecorr + Ep + IR Potential ( - mV) ( ) (–) ON Potential Voltage (IR) Drop 100 mV Polarization OFF Potential 100 mV Depolarization Native (Free Corroding, Static) Potential (+) Time CP2 | Technician Interruption Cycle 27 ▪ Interruption cycles vary based upon survey and equipment ▪ Key factors in interruption cycle: ▪ Maintain polarization during survey ▪ Minimize depolarization during OFF period ▪ Measure true OFF-potential after a “spike” has dissipated (0.3 to 0.6 seconds) ▪ Leave interrupters in ON position when not surveying CP2 | Technician 9 Using a Datalogger Waveprint 28 Used to: ▪ Confirm synchronization (Top figure not synchronized) ▪ Polarization & depolarization ▪ Spiking magnitude & direction ▪ Detect dynamic stray current (bottom figure) ▪ Determine Etrue in stray current CP2 | Technician Reference Electrode Near Structure Aboveground Storage Tank 29 Test / Access Station Grade Reference Cell Monitoring Tube Rim 25' On -1411 -698 Off -902 -664 Center -404 -402 55' Rim -601 -1455 -578 -911 Potentials (mV CSE) CP2 | Technician External Coupons 30 ▪ Used with other tools & data to evaluate CP to a given criterion. ▪ Applicable to variety of structures and conditions such as: ▪ Multiple influencing rectifiers ▪ Multiple pipelines or complex underground structures ▪ Direct connected sacrificial anodes ▪ Stray current including telluric ▪ Localized corrosive areas ▪ Areas shielded from surface measurements ▪ AC assessments CP2 | Technician 10 External Coupons 22 ▪ Coupons - bare metal samples [1] ▪ Represents coating holiday ▪ Same metal and surface as the structure; ▪ Small to avoid excessive current drain on CP; Coupons ▪ At structure depth in the same backfill; ▪ Mill scale and foreign materials removed ▪ Placed at a known location of an ineffective coating where applicable. ▪ Reference at coupon or in soil tube ▪ Measure voltage (IR) drop free potentials by disconnecting coupon. [1] ANSI/NACE Standard SP0104, “The Use of Coupons for Cathodic Protection Monitoring Applications” CP2 | Technician External Coupon Measurements 32 Structure-to-electrolyte potentials: ▪ Native (before connecting to CP structure) ▪ Initial (after connecting but before CP) ▪ ON (connected to CP & may include voltage (IR) drop) ▪ Instant OFF (immediately after disconnecting coupon from CP) ▪ Depolarized (after leaving disconnected) Current ▪ Value ▪ Direction ▪ Current density (calculated) Corrosion Rate (Special - EIS, LPR, ER) CP2 | Technician External Coupon Measurements 33 ▪ Caution with Native (Free Corroding) coupon potentials after exposure ▪ Ongoing study of 483 10 cm2 native coupons - significant change in ~10 years ▪ 174 (36%) more (+) than 100 mV ▪ 77 (16%) more (-) than 100 mV ▪ 146 (30%) +/- 100 mV ▪ 86 (18%) no reading ▪ Actual potentials on graph CP2 | Technician 11 Potentials - Coupons vs Structure 34 ▪ Coupon potentials may not be identical to structure ▪ Structure potential is subject to: ▪ Electrolyte variations (soil, moisture, chemistry, temperature, O2) - different reactions and products ▪ Number & size of holidays ▪ Coating conductance variations ▪ Current variations around/along structure ▪ Stray current ▪ Circulating / long-line current ▪ Bimetallic or different alloys ▪ Permanently attached sacrificial anodes CP2 | Technician Coupons vs. Structure Potentials 35 ▪ Structure potential – weighted average of surfaces in different conditions ▪ Coupon potential – single small metal surface in limited conditions ▪ Coupon receives same CP - not subject to all influences ▪ Coupon may be better indication of local status ▪ Neither coupon / structure potential necessarily in error ▪ Important to understand factors affecting each ▪ Coupon can demonstrate if CP system can protect a holiday of that size and location CP2 | Technician External Coupons Test Methods 36 Test Method 1 ▪ Interrupt current (disconnect) coupon lead in test station ▪ Measure instant OFF potential: ▪ High impedance voltmeter ▪ stationary reference near coupon OR ▪ portable reference in plastic tube (electrolytic bridge) to coupon ▪ Measure current (zero resistance ammeter, clip-on ammeter (if large coupon), shunt) ▪ Note polarity for current pickup/discharge Test Method 2 ▪ Measure polarized potential as Test #1 ▪ Leave coupon disconnected and measure depolarized potential ▪ Calculate polarization by “Depolarized potential – polarized potential” CP2 | Technician 12 External Coupon Errors 37 Improper Installation ▪ Soil voids at coupon surface ▪ Oxygen ingress to coupon through unconsolidated backfill ▪ Broken or high resistance wire contacts ▪ Poor stationary reference location Measurements ▪ Low impedance voltmeter depolarizing coupon ▪ Poor reference contact ▪ Extreme dry soil in tube ▪ Especially in soil tube if soil above ground level CP2 | Technician Electrical Resistance (ER) Probe 38 ▪ Exposes a resistive element to electrolyte ▪ As element corrodes, cross section reduces, increasing resistance ▪ Resistance is temperature dependent ▪ Exposed element compared to a nonexposed element ▪ Resistance ratio is same regardless of temperature ▪ Corrosion rate determined by metal loss versus time ▪ Suitable for uniform corrosion, not suitable for pitting corrosion CP2 | Technician External Coupons – Dynamic Stray Current 39 ▪ Coupon not affected by stray current except when connected ▪ Both ON & instant OFF potentials to be recorded ▪ Voltage (IR) error reduced but still considered ▪ Coupons designed to minimize error ▪ Reference near coupon, or ▪ Reference in soil tube close to coupon ▪ ON may be more electropositive than OFF ▪ High impedance datalogger for stray current. CP2 | Technician 13 Current Measurement 40 Typical current measurements in CP ▪ Galvanic current output ▪ Impressed current output ▪ Current in structure ▪ Bond current CP2 | Technician Current Measurement with an Ammeter 41 ▪ Ammeter is inserted in series ▪ Adds resistance to circuit A – Without Ammeter Icp = Vd,cp R cp Vd,cp Rcp – With Ammeter (Possible significant errors) Im = Icp Vd,cp Rcp + Rm CP2 | Technician Shunt to Measure Current 42 Shunt remains in circuit, therefore current stays balanced V equally rated shunts I1 I2 I3 I4 IT CP2 | Technician 14 Shunt Current Calculations Current through shunt resistance produces a voltage drop 43 ▪ Three ways to calculate shunt current ▪ Ratio Vmeas / Vshunt x I shunt Vshunt = Icp x Rshunt Rshunt = Vrating / Irating ▪ Calibration Factor Vmeas x K (factor) Example: 5 A, 50 mV Shunt Icp = (0.05 V) 5A ▪ Resistance Icp = Vshunt / Rshunt Rshunt = 0.050 / 5 = 0.01Ω Vshunt = 5A x 0.01 = 0.05 V (50 mV) [See Tables in CP Technician Reference Sheet at back of manual for table] CP2 | Technician Zero-Resistance Ammeter 44 Zero-resistance Ammeter I cp Soil Tube I cp Steel Coupon CP2 | Technician Clamp-On (Clip-on) Ammeter 45 Magnetic Field I dc Clamp-On Ammeter Clamp Earth Current +0.9 A Clamp +0.4 A Indicator Earth current leaving subject pipeline is 0.9 – 0.4 = 0.5 A CP2 | Technician 15 2-Wire Line Current Test Pipe resistance or size and wall thickness or weight per foot known 46 Wires must be color coded East Current West Pipe span in meters or feet CP2 | Technician 2-Wire Line Current Test (Example) 47 Pipe Span: 762 mm (30-in.) diameter. 61-m (200-ft) long Weight 176.65 kg/m (118.7 lbs/ft) Span voltage = 0.17 mV, Calculate pipe current Pipe resistance/ft from Table = 8.01 /m (2.44 /ft ) = 0.00000801 /m (0.00000244 /ft) Total resistance = 61 m x 0.00000801 /m = 0.000488 OR = 200 ft x 0.00000244 /ft = 0.000488 Current (I) = 𝑉 𝑅 = .00017𝑉 .000488𝛺 = .348A or 348 mA CP2 | Technician 4-Wire Line Current Test 48 Calibrating Current Span 𝐑𝐩𝐢𝐩𝐞 = ∆𝐕𝟐 − 𝟑 ∆𝐈𝐭 K = Itest / E test + A It V 1 2 3 4 R pipe CP2 | Technician 16 Resistance of 4-Wire Span (Example) 49 V2-3 = +21 mV (before test current applied) (residual current) V2-3 = –19 mV (after test current applied) It = 10 A Calculate Resistance (Rp) of the pipe section & residual current: Rpipe = +21 mV - (-19 mV) 10 A R pipe = +40 mV 10 A I residual = = 0.021V .004 4mW = 5.25 A CP2 | Technician Calibration Factor of 4-Wire Span (Example Continued) V2-3 = +21 mV (before test current applied) V2-3 = –19 mV (after test current applied) ΔE = 40 mV It = 10 A 50 Calculate Calibration Factor (K) of the pipe section: K = Itest / E test = 10 A 0.25 A = 40 mV mV Calculate residual current from factor: Iresidual = 21 mV 0.25 A mV = 5.25 A CP2 | Technician Coating Resistance Calculations 51 = I1 – I2 CP2 | Technician 17 Earth Current Measurement 52 Voltmeter With Reading + - Reference Electrode Reference Electrode Current CP2 | Technician Coating Resistance Calculations (Example) Factor Pipe OD (d) = 610 mm (24”) TS1 TS2 IONTS 3.00 A 2.80 A IOFFTS 0.20 A 0.10 A ΔITS 2.80 A 2.70 A EONTS -2.00 V -1.70 V EOFFTS -0.90 V -0.85 V ΔETS 1.10 V 0.85 V IC Pipe Section Pipe Section (metric) (Imperial) = 2.80 – 2.70 = 2.80 – 2.70 = 0.10 A = 0.10 A (Eq 3.13) Length (L) = 1,609 m (5,280 ft) ΔEave = (Eq 3.14) RC 1.10 + 0.85 2 = 0.975 V = 0.975 = 1.10 + 0.85 2 = 0.975 V = 0.10 0.975 0.10 (Eq 3.15) = 9.75 Ω = 9.75 Ω Apipe = π .610 x 1609 = π 2 x 5280 (Eq 3.16) = 3,080 m 2 = 33,158 ft2 rC = 9.75 x 3080 = 9.75 x 33158 (Eq 3.17) = 30,030 Ω-m2 = 323,291 Ω-ft2 GC = (Eq 3.18) 53 1 9.75 = = 0.103 Siemens (S) 1 1 9.75 = 0.103 Siemens (S) 1 gC = 30030 = 323291 (Eq 3.19) = 0.0000333 S/m2 = 0.00000309 S/ft2 (33.3 µS/m2) (3.09 µS/ft2) CP2 | Technician Current Requirement Test 54 Adjustable Current Source Reference Electrode A - - V + + Anode Test Anode bed Soil Electrolyte Pipe CP2 | Technician 18 Current Requirement if Test Sub-Criterion Example: 55 Ireq = Epreq*Itest Eptest Itest = Test current applied to structure (A) Ireq = Estimated additional current required (A) ΔEptest = Polarization from test (V) [EOFF –EINITIAL] ΔEpreq = Polarization required (V) = [-0.850 V – EINITIAL] for -850 mVCSE polarized potential criterion OR = 0.100 (100 mV) polarization criterion Considers resistance producing IR drop: R = (EON - EOFF)/ION CP2 | Technician Example of Current Requirement Calculation 56 Example: Native (Static) pipe-to-soil potential = – 0.645 VCSE Desired polarized p/s potential = – 0.850 VCSE Test current = 0.050 A (50 mA) Polarized potential from test current = – 0.775 VCSE Ireq = Epreq*Itest Eptest I req = [-0.850 V - (-0.645 V)] 0.050 A [−0.775 V − (− 0.645V)] = 0.0788 A or 78.8 mA CP2 | Technician Coating Resistance Calculations (1) Pipe Surface Area = 57 d x x L = 0.61m x x 1609 m = 3080 m2 = 24 in x x 5280 ft = 33,158 ft 2 12 in / ft CP2 | Technician 19 Coating Resistance Calculations (2) Eave = ETS 1 + ETS 2 2 Eav e = 58 1.10 V + .85 V = 0.975 Volt 2 If more than two potentials are taken, you will need to average all potentials to determine Eave CP2 | Technician Coating Resistance Calculations (3) 59 Ic = ITS1 − ITS 2 = 2.80 A − 2.70 A = 0.10 A CP2 | Technician Coating Resistance Calculations (4) Rc = 60 Eave 0.975 V = = 9.75 IC 0.1 A CP2 | Technician 20 Coating Resistance Calculations 61 ▪ Resistance (RC) is the resistance to earth of the overall pipe ▪ Specific resistance (rCE) is resistance of one unit of area ▪ Each unit of area is in parallel ▪ Therefore the total resistance is multiplied by the unit area If RC = 9.75 Ω Specific coating resistance = rCE = Apipe x RC = 3,080 m2 x 9.75 Ω (30,030 Ω-m2 ) = 33,158 ft2 x 9.75 Ω (323,291 Ω-ft2 ) CP2 | Technician Coating Conductance 62 Conductance is the reciprocal of resistance (units Siemens [S] or microsiemens [ S]) g = 1 = 1 = 0.0000333 S = 33.3 μS rCE 30,030 m2 m2 g = 1 = 1 = 0.00000309 S = 3.09 μS rCE 30,030 ft2 ft2 where: g = conductance (S) RCE = specific coating resistance (Ω) CP2 | Technician Electrical Isolation 63 Indication of Problems ▪ Short (metal to metal contact): ▪ Significant electropositive change in normal structure-toelectrolyte potential ▪ Decrease in resistance (short to another structure) ▪ Discontinuity (open circuit): ▪ Sudden change in structure-to-electrolyte potential along a structure CP2 | Technician 21 Electrical Isolation 64 Indication of Problems ▪ Significant electropositive change in normal (expected) structure-to-electrolyte potential or decrease in resistance indicates possible short ▪ A sudden electropositive shift in the structure-to-electrolyte potential along a structure may indicate discontinuity ▪ A lower than expected structure-to-electrolyte resistance may indicate a short to another large structure CP2 | Technician Locating the Problem ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ 65 Structure-to-electrolyte potential measurement Interrupted structure-to-electrolyte potential measurement Pipeline/cable locators Isolation testing instrument DC line current measurements Fixed cell to moving ground test Current response test Isolation resistance test Tests not discussed but see … NACE SP0286 Electrical Isolation of Cathodically Protected Pipelines CP2 | Technician Fixed Cell to Moving Ground Example 66 Site 4 Site 1 Site 3 Reference Electrode Site 2 Voltmeter Site No. Electrically Continuous Electrically Discontinuous 1 –1.560 VCSE –1.560 VCSE 2 –1.560 VCSE –1.456 VCSE 3 –1.560 VCSE –1.652 VCSE 4 –1.560 VCSE –1.488 VCSE CP2 | Technician 22 Corrosivity of Electrolyte (Soil) 67 Soil resistivity: ▪ measure of ability of electrolyte to support electrochemical corrosion ▪ Used in CP design pH: ▪ measure of electrolyte acidity or alkalinity Both influence underground/ submerged metal corrosion CP2 | Technician pH Testing 68 Measurement of Electrolyte pH: ▪ For liquids, pH (litmus) paper or a pH meter ▪ For soils, ▪ Filtrate made from distilled water and a soil sample ▪ Measure with litmus paper, pH meter, or a pH test kit ▪ Note that a pH meter uses a fragile glass electrode ▪ Potential between antimony electrode and CSE (being careful to not contaminate antimony). ▪ See conversion scale on side of antimony electrode for pH. CP2 | Technician Measuring Electrolyte Resistivity 69 ▪ Wenner 4-Pin Method ▪ Other methods not discussed ▪ Uneven pin spacing method ▪ Single-Probe method ▪ Soil Box ▪ Electromagnetic Induction Method CP2 | Technician 23 4-Pin Soil Resistivity Pin Location 70 ▪ 4 pins in a straight line at equal spacing ▪ Avoid placing pins over underground structures ▪ Either metallic or non-metallic ▪ If unavoidable, place pins perpendicular (not parallel) to pipelines or cables CP2 | Technician Resistivity by Wenner 4-Pin Method 71 ▪ Current (I) is applied to the outer pins ▪ (C1 & C2) Soil Resistivity Meter C1 ▪ (P1 & P2) ▪ Resistance (R): R = V/I a OR ▪ AC resistance meter measures R directly ▪ Reversed wires gives a negative resistance so instrument will not balance C2 P2 P1 ▪ Voltage (V) measured between inner pins a a =2aR * a is in cm ▪ Approximate depth to spacing of inner electrodes (a) ▪ Average soil resistivity (ρ) to depth (a) calculated from resistance (R) CP2 | Technician Resistivity Calculations (metric) 72 =2aR = resistivity in Ω-cm = pi (3.14) a = pin Spacing in cm R = resistance in Ω CP2 | Technician 24 Resistivity Calculations (modified to feet) 73 If spacing is measured in feet but resistivity still in Ω-cm: = 191.5aR = resistivity in Ω–cm 191.5 = factor in cm/ft a = pin spacing in feet R = resistance in Ω Factor = 2* 𝛑 X 2.54𝑐𝑚 1 𝑖𝑛. 𝑋 12 𝑖𝑛 1 𝑓𝑡 = 191.5 𝑐𝑚/𝑓𝑡 CP2 | Technician Barnes Layer Analysis – Parallel Resistance Formula 74 ▪ Barnes Layer predicts resistivity of layers ▪ Principle: ▪ each layer is a parallel resistance (R), and ▪ parallel resistance equation applies; 1/RT = 1/R1 + 1/R2 + 1/R3 + … + 1/RN ▪ If only two resistances (layers) then: R2 = R1RT (R1 - RT) ▪ Barnes Layer only applies if RT reduces with increased depth (a) ▪ May not due to larger surface sample with depth CP2 | Technician Average and Layer Resistivity Symbols 1 avg a1 2 avg a2 a3 3 avg 75 L1 L1 L2 L2 L3 L3 CP2 | Technician 25 Layer Resistivity Equations for Barnes Layer Analysis Resistances (R1, R2, & R3) measured at spacings (a1, a2, & a3) Resistivity of Layers (L1, L2 and L3) are desired 1 avg L1 RL1 L1 = 2a1R1 = a1 = R1 = 2L1 RL1 2 avg L2 RL2 L2 = 2a2R2 = a2 – a1 = (R1 R2)/(R1 – R2) = 2L2 RL2 3 avg L3 layer RL3 L3 = 2a3R3 = a3 – a2 = (R2 R3)/(R2 – R3) = 2L3 RL3 76 CP2 | Technician Example Calculations for Barnes Layer Resistivity (metric) a1 = 160 cm a2 = 320 cm a3 = 480 cm Ra1 = 10.0 Ω Ra2 = 7.4 Ω Ra3 = 3.1 Ω ρa1 = 10,050 Ω-cm ρa2 = 14,880 Ω-cm ρa3 = 9,350 Ω-cm Example 1 Layer 1 Layer 2 L1 = a 1 L2 = a2 – a1 =160 cm =320 – 160 = 160 cm Factor Layer depth (L) Layer resistance (RL) RL1 = Ra1 = 10.0 Ω Layer resistivity (ρL) ρL1 = ρa1 = 10,050 Ω-cm OR 𝜌𝐿1 = 2 𝜋𝐿1 𝑅𝐿1 = 2 π 160 x 10.0 = 10,050 Ω-cm 77 𝑅𝐿2 = 𝑅1 𝑅2 (𝑅1 − 𝑅2 ) 10 𝑥 7.4 = (10 − 7.4) = 28.46 Ω 𝜌𝐿2 = 2 𝜋𝐿2 𝑅𝐿2 = 2 π 160 x 28.46 = 28,610 Ω-cm Layer 3 L3 = a3 – a2 = 480 – 320 = 160 cm 𝑅𝐿3 = 𝑅3 𝑅3 (𝑅2 − 𝑅3 ) 7.4 𝑥 3.1 = (7.4 − 3.1) = 5.33 Ω 𝜌𝐿3 = 2 𝜋𝐿3 𝑅𝐿3 = 2 π 160 x 5.33 = 5,360 Ω-cm CP2 | Technician Example Calculations for Barnes Layer Resistivity (Spacing in feet) a1 = 5.25 ft a2 = 10.50 ft a3 = 15.75 ft Ra1 = 10.0 Ω Ra2 = 7.4 Ω Ra3 = 3.1 Ω 78 ρa1 = 10,050 Ω-cm ρa2 = 14,880 Ω-cm ρa3 = 9,350 Ω-cm Example 2 Factor Layer depth (L) Layer 1 L1 = a 1 =5.25 ft Layer resistance (RL) RL1 = Ra1 = 10.0 Ω Layer resistivity (ρL) ρL1 = ρa1 = 10,050 Ω-cm OR 𝜌𝐿1 = 191.5𝐿1 𝑅𝐿1 = 191.5 cm/ft x 5.25 ft x 10.0Ω = 10,050 Ω-cm Layer 2 L2 = a2 – a1 =10.50 – 5.25 = 5.25 ft 𝑅𝐿2 = 𝑅1 𝑅2 (𝑅1 − 𝑅2 ) 10 𝑥 7.4 = (10 − 7.4) = 28.46 Ω 𝜌𝐿2 = 191.5𝜋𝐿2 𝑅𝐿2 = 191.5 x 5.25 x 28.46 = 28,610 Ω-cm Layer 3 L3 = a3 – a2 = 15.75 – 10.50 = 5.25 ft 𝑅𝐿3 = 𝑅3 𝑅3 (𝑅2 − 𝑅3 ) 7.4 𝑥 3.1 = (7.4 − 3.1) = 5.33 Ω 𝜌𝐿3 = 191.5𝐿3 𝑅𝐿3 = 191.5 x 5.25 x 5.33 = 5,360 Ω-cm CP2 | Technician 26 Reinforced Concrete Structures 79 Atmospherically exposed structures ▪ Bridge decks ▪ Bridge substructures ▪ Buildings Buried or immersed structures ▪ Reinforced and prestressed pipelines ▪ Prestressed pilings ▪ Cement-mortar coated steel pipelines CP2 | Technician Concrete Structures Continuity 80 ▪ Cathodic protection requires a continuous structure ▪ Drains, railings, reinforcing bars, and expansion joints must all be verified for electrical continuity ▪ Electrical continuity can be verified using AC resistance measurements ▪ Resistance <0.100 Ω (excluding test wires) between widely spaced points of contact indicates electrical continuity ▪ Fixed electrode to moving ground techniques: ▪ <1mV difference indicates continuity; and ▪ >3 mV indicates discontinuity between contacts CP2 | Technician Concrete Structures Depth of Cover 81 ▪ Depth of concrete cover is important with an impressed current CP system ▪ Anode system must not contact reinforcing or other metal component of the structure ▪ Otherwise, a short circuit will occur ▪ Depth of cover is measured using a Pachometer ▪ Depth of cover unimportant with a galvanic anode CP system CP2 | Technician 27 Concrete Structures Potential Surveys 82 ▪ A high-impedance voltmeter and a CSE most common for potentials ▪ Removal of some concrete might be required for metal contact ▪ To reduce the CSE contact resistance, ▪ Place a moist sponge between the concrete & CSE, or ▪ Completely pre-wet the entire concrete surface CP2 | Technician Concrete Structures Potential Surveys 83 ▪ Typically potential measurements taken in a grid pattern (approximately every 0.6 m to 1.5 m (2 ft to 5 ft)) and ▪ Plot on a contour map ▪ Interpretation of potential measurements by ASTM Test Method C876 for atmospherically-exposed concrete: Potential wrt CSE Probability of Corrosion more pos. than −200 mV −200 to −350 mV more neg. than −350 mV < 5% about 50% > 95% CP2 | Technician Direct Inspection of Structure 84 Gather forensic information to determine cause of corrosion ▪ Before excavation, measure potentials & soil resistivity over site ▪ Photograph each stage of exposure ▪ Repeat potentials during excavation ▪ Immediately on exposure, measure pH of metal surface and soil ▪ Inspect coating for deterioration, damage and lack of adhesion ▪ Take a pH of moisture underneath coating with litmus paper ▪ High pH is expected at the surface of a structure with CP ▪ Sample corrosion product and seal in a plastic bag ▪ Used for further analysis and/or bacteria tests. CP2 | Technician 28 Direct Inspection (cont’d) 85 ▪ After surface is cleaned, identify welds to known geographical points (GPS) ▪ if a pipeline, to pipe chainage and weld numbers ▪ Locate & outline areas of corrosion, measure clock position and distance to closest weld or ID location ▪ Overlay metal loss features with a grid of known dimensions ▪ Measure depths of pits and record them on the surface for photographs ▪ Record and photograph any repairs made ▪ Inspect the repair coating for holidays and adherence ▪ After backfill, repeat the potential measurements ▪ Adjust the CP system if necessary CP2 | Technician Leak Frequency 86 1000 636 Cumulative Breaks 100 53 10 1 1970 1975 1980 1985 1990 Year CP Installed 1984 1995 2000 Actual Projected (W ithout CP) CP2 | Technician Knowledge Check | 1 87 A structure-to-electrolyte potential measures the potential difference between A. two different structures acting as electrodes. B. the structure and a standard reference electrode. C. the structure and the anode. D. two different points on the structure. CP2 | Technician 29 Knowledge Check | 2 88 If 5.0 A of battery current passes through the outside leads of a 4-wire current span spaced at 30 m(100 ft), resulting in a voltage drop of 5.0 mV with the test current applied and of 1 mV without the test current, what is the calibration factor of the current span? A. 0.00025 A/mV B. 4.0 A/mV C. 1.25 A/mV D. 5.0 A/mV CP2 | Technician Knowledge Check | 3 89 Polarized potentials are measured A. when the rectifiers are ON. B. just after the rectifiers are turned OFF. C. after the rectifiers are left off for several days. D. just after the rectifiers are turned ON. CP2 | Technician Knowledge Check | 4 90 Polarization is determined by A. subtracting the ON from the instant OFF potential. B. measuring the instant OFF potential. C. subtracting the instant OFF from the Native (free corroding) potential. D. subtracting the IR Drop from the ON potential. CP2 | Technician 30 Knowledge Check | 5 91 Current pickup on a pipeline is indicated when A. The structure-to-electrolyte potential over the pipe is more electronegative than to each side. B. The structure-to-electrolyte potential over the pipe is more electropositive than to each side. C. The two electrode method (side drain) shows the electrode over the pipe is more electropositive. D. A CIS shows a more electropositive potential with the rectifier ON than OFF. CP2 | Technician Knowledge Check | 6 92 A 61 cm diameter (24-in) pipe has a current span at each end of a 2 km (1.24 miles) section with the following information determined: EONTSI = -1200 mVCSE EOFFTSI = -1000 mVCSE IONTSI = 3.0 A IOFFTSI = 0.20 A EONTS2 = -1140 mVCSE EOFFTS2 = -980 mVCSE IONTS2 = 2.8 A IOFFTS2 = 0.20 A What is the pipe-to-earth resistance in Ω? A. 0.9 B. 9 C. 66.7 D. 900 CP2 | Technician Knowledge Check | 7 93 A 61 cm diameter (24-in) pipe which has a current span 2 km (1.24 miles) with the following information determined: EONTSI = -1200 mVCSE EOFFTSI = -1000 mVCSE IONTSI = 3.0 A IOFFTSI = 0.20 A EONTS2 = -1140 mVCSE EOFFTS2 = -980 mVCSE IONTS2 = 2.8 A IOFFTS2 = 0.20 A What is the specific coating resistance Ω-m2 A. 0.00023 B. 0.023 C. 3,450 D. 345,000 CP2 | Technician 31 Knowledge Check | 8 94 A 61 cm diameter (24-in) pipe has a current span at each end 2 km (1.24 miles) section with the following information determined: EONTSI = -1200 mVCSE EONTS2 = -1140 mVCSE EOFFTSI = -1000 mVCSE EOFFTS2 = -980 mVCSE IONTSI = 3.0 A IONTS2 = 2.8 A IOFFTSI = 0.20 A IOFFTS2 = 0.20 A What is the specific coating conductance in siemens/m2? A. 0.0000029 B. 0.00029 C. 43.5 D. 4,347 CP2 | Technician Knowledge Check | 9 95 Structure-to-electrolyte potentials can be used for which of the following purpose(s)? A. Confirm if a CP potential criterion is met. B. Locate anodic areas on non-CP pipelines. C. Determine resistivity of the soil. D. Locate stray current. CP2 | Technician Knowledge Check | 10 96 Which of the following potential measurements would clearly indicate that a tank is isolated? A. A structure-to-electrolyte potential of -950 mVCSE with the electrode at the pipe and a tank-to-electrolyte potential of -800 mVCSE with the electrode at the tank. B. A structure-to-electrolyte potential of -950 mVCSE with the electrode at the pipe and a tank-to-electrolyte potential of -940 mVCSE with the electrode at the tank. C. A structure-to-electrolyte potential of -950 mVCSE with the electrode at the pipe and tank-to-electrolyte potential of -800 mVCSE with the electrode at the pipe . D. A structure-to-electrolyte potential of -650 mVCSE with the electrode at the tank and a tank-to-electrolyte potential of -640 mVCSE with the electrode at the tank. CP2 | Technician 32 Knowledge Check | 11 97 When using the Wenner four-pin method, what is the resistivity when 10 Ω was measured with a pin spacing of 5 m? A. 314 Ω-cm B. 31,416 Ω-cm C. 50,000 Ω-cm D. 314,160 Ω-cm CP2 | Technician Knowledge Check | 12 98 What is the resistivity of a block of material that is 2 cm by 3 cm and 10 cm long has a resistance along its length of 1000 Ω? A. 100 Ω-cm B. 600 Ω-cm C. 1667 Ω-cm D. 15,000 Ω-cm CP2 | Technician Knowledge Check | 13 99 What is the true potential if the voltmeter reads 1000 mV with a test lead resistance of 0.01Ω, a pipe-toearth resistance (Rp,e) of 10 Ω, a reference electrode resistance to earth (Rr,e) of 100 kΩ, and a meter resistance of 1 MΩ? A. 800 mV B. 909 mV C. 1010 mV D. 1100 mV CP2 | Technician 33 Knowledge Check | 14 100 What is the percent error if the voltmeter read 1000 mV with a test lead resistance of 0.01Ω, a pipe-toearth resistance (Rp,e) of 10 Ω, a reference electrode resistance to earth (Rr,e) of 100 kΩ, and a meter resistance of 10 MΩ? A. 1% B. 2% C. 5% D. 10% CP2 | Technician Knowledge Check | 15 101 If a potential difference (Vl) of -750 mVCSE was measured with an input resistance (Rl) of 1.0 MΩ and a potential difference of -850 mVCSE (Vh) was measured with an input resistance (Rh) of 10 MΩ, what is the true potential (Etrue)? A. -399 mV B. -821 mV C. -862 mV D. -1128 mV CP2 | Technician Knowledge Check | 16 102 If current being impressed on to a structure is causing a potential gradient of 50 mV per meter (average) and the reference electrode is 3 m from the structure, what is the potential being measured if the true potential at the structure is -800 mV? A. -150 mV B. -650 mV C. -950 mV D. +950 mV CP2 | Technician 34 Knowledge Check | 17 103 With proper instrumentation, a dual soil coupon can be used to predict which of the following? A. The potential of the pipe surface at the location of the coupon. B. The polarized potential of a coating holiday of a given size at the location. C. The amount of polarization of a coating holiday of a given size at the location. D. The amount of CP current pick up on a coating holiday of a given size at the location. CP2 | Technician Knowledge Check | 18 104 When an ammeter with an internal resistance of 0.1Ω is inserted into a circuit that is normally operating at 10 V and 20 A, it will read a current of A. 9.5 A B. 16.7 A C. 20.1 A D. 24.0 A CP2 | Technician Knowledge Check | 19 105 If the voltage drop across a 61 m (200 ft) span of 762 mm (30 in.) pipe weighing 176.65 kg/m (118.7 lbs/ft) is 0.34 mV, calculate the current. A. 0.309 A B. 0.348 A C. 0.464 A D. 0.696 A CP2 | Technician 35 Chapter 3 Field Measurements ® 36 Stray Current 4-1 Chapter 4: Stray Current Stray currents are currents through electrical paths other than the intended circuit. Stray current is not the galvanic corrosion current between anodes and cathodes on the same structure. Stray currents, or interference currents, can be classified as being either static or dynamic. Stray current can refer to either alternating current or direct current. AC stray current can be both a safety hazard and a corrosion problem. DC stray current causes significant corrosion of most metals. Stray current is especially damaging because large currents, often involving many amperes, might be involved. Even small amounts of stray currents can be highly damaging if discharged over a small surface area. In some areas, particularly around older rail rapid-transit systems or in the vicinity of underground mine railroads, pipelines may carry hundreds of amperes of stray current. For every ampere discharged from a structure, a certain amount of metal is lost. Faraday’s Law allows us to relate the corrosion lost to the amount of current discharged. For ferrous metals (cast iron, ductile iron, steel), copper, and lead the loss rates are: • • • Ferrous 9.1 Kg / A-yr (20 pounds / A-yr) Copper 10.4 Kg / A-yr (23 pounds / A-yr) Lead 33.85 Kg / A-yr (74.64 pounds/ A-yr) It is apparent that, if not controlled, stray current can destroy a structure very rapidly or at least to a potentially catastrophic situation. Structures in environments that normally would not be considered extremely corrosive will be subjected to accelerated corrosion when exposed to stray current conditions. Further consideration must be given to the current density for total penetration of the metal. Stray current enters the pipe or other structure through the soil (or other electrolyte) but does not cause corrosion at that point. Some amphoteric metals such as aluminum and lead may suffer “cathodic corrosion” in stray current “pick up” areas. Many structures, in fact, receive “free” cathodic protection from stray currents at the point of entry. The damaging effect of stray current typically occurs if the current leaves the structure through the electrolyte where corrosion will be accelerated. The objective of stray current analysis is to determine: • • • • • The source of the stray current. Where and over what area does the stray current enter the structure? Where and over what area does the stray current leave the structure? What is the magnitude of the stray current? How can the stray current be mitigated? 4.1 Dynamic Currents Dynamic stray currents are those currents that vary in amplitude and/or change in direction. These currents can be man made or natural in origin. ©NACE International CP 2 | Technician 4-2 Stray Current 4.1.1 Sources of Dynamic Stray Currents Dynamic stray current can originate from any of the following sources: • • • • • • DC transit systems Welding Mining operations Electrical power transmission Industrial plants Telluric Examples of man-made sources are DC welding equipment, mining operations, and DC transit systems. Telluric currents are naturally occurring stray currents that are caused by disturbances in the Earth’s magnetic field by sun spot activity. Figure 4.1 shows stray currents from a DC railway. These are commonly called stray traction currents. Figure 4.1 Illustration of Dynamic Stray Currents from a Transit System 4.1.1.1 Natural (Telluric) Currents Telluric current is generated by the interaction of the solar wind (high-energy particles given off by the sun), the Earth’s magnetic field, and metal structures at the surface of the Earth. The current generated shifts in magnitude and direction with time. It generally occurs during times of increased sun spot activity. Telluric currents create significant monitoring difficulties to confirm a cathodic protection criterion is met. 4.1.2 Detecting Dynamic Stray Currents Dynamic stray currents can be readily detected from structure-to-electrolyte potentials and/or line current measurements. Dynamic stray currents can be indicated by a structure-toelectrolyte potential that is changing with time with the reference electrode in a stationary position in contact with the electrolyte. These potential changes are a direct result of current changes at the source of the interference. CP 2 | Technician ©NACE International Stray Current 4-3 Figure 4.2 Pipe Potential Fluctuating Due to Dynamic Stray Current Varying voltage gradients in the earth can be detected using two reference electrodes space 3 to 6 m (10 to 20 ft) apart. Reference electrodes placed on the soil over the existing pipeline or proposed pipeline route will show the voltage gradients that the pipe is exposed to between the two cells Figure 4.3 and Figure 4.4. Figure 4.4 shows an example of current pickup, current passing by and current discharge on a pipeline. Figure 4.3 Potential Measurement Between Two Electrodes ©NACE International CP 2 | Technician 4-4 Stray Current Figure 4.4 Potential Measurement Between Two Electrodes The analysis of dynamic stray currents may require the use of correlations, or comparisons, of currents and voltages from one point on the structure to another. It is necessary to have sufficient test facilities on the structure to measure structure-to-soil potentials and current on the structure. If the structure has few or no test facilities and stray currents are detected, then install additional test facilities to permit a meaningful analysis. 4.1.2.1 Continuous Data Recording The techniques from section 4.1.2 “Detecting Dynamic Stray Currents” on page 4-2 provide valuable information about where stray current enters and discharges from a structure. Section 4.1.2 does not: • • • provide information on the value of the maximum stray current affecting the structure show whether the stray current voltages observed during the test period are significant document the predominant direction of voltage or current. To perform this function, data about the stray current magnitude over time is needed. This data is obtained using a long term (24 hours or more) voltage or current recording. A recorder or data logger should be used for this purpose. CP 2 | Technician ©NACE International Stray Current 4-5 Figure 4.5 Data Logger Plot Illustrating Dynamic Stray Currents Select the location to be tested using the results of the correlations, the best location being the one of most exposure. The location of the recording should obviously be one of the test stations used in the correlations as long as the voltages in the long term recording can be correlated back to the other test stations using the correlations. Select a location where the recording device will be safe and out of the way of traffic, pedestrians, and vandals. Note: The recording device should be identified with the company name, address, and telephone number. More than one recording device might be needed. 4.1.2.2 Finding the Source of the Stray Current Engineering solutions to stray current problems can be complex. Frequently, more than one source of current is involved. Transit systems, for example, do not operate from a single substation; rather they are supplied by a number of substations operating in parallel and feeding many different lines simultaneously. Such transit systems operate with many thousands of amperes of current, and the effects can be seen for many miles on underground structures. If stray current is to be successfully drained from the structure of interest to the source, two conditions must be met. 1. The source must tend to pick up stray current at the point where the mitigation bond is to be installed. This is point determined using the correlations described above. 2. The structure must tend to discharge stray current at the point where the mitigation bond is to be installed. This point is also determined using the correlations described above. Sufficient current must be drained to mitigate the most severe stray current exposure. This is determined by measuring or calculating the maximum current through the proposed bond or other mitigation system. ©NACE International CP 2 | Technician 4-6 Stray Current 4.2 Static (Steady State) Currents Static, or steady state, interference current can be defined as those that maintain a constant amplitude and constant geographical paths with time. The best example of steady state interference is from a cathodic protection (CP) system. Figure 4.6 shows how interference can occur from a CP system on a foreign pipeline. Although interference is often shown where pipelines cross, this may not always be the case depending on the polarity and strength of the voltage gradients to remote earth that extend from Figure 4.6 Static Stray Current Interference from a CP System one structure or source to the other structure. That is, tests should not be limited to only pipeline crossings. Interference can be from one pipeline’s cathodic gradient causing discharge from the other pipeline or from an anodic gradient causing a current pickup that must leave the other pipeline by some means to return to its source. 4.2.1 Current Discharge Interference When a voltage gradient that is negative with respect to remote earth overlaps a foreign structure, it promotes a detrimental current discharge from the foreign structure in the area of influence. This cathodic gradient could be considered as the controlling factor in this situation. If current discharges from a structure, then it must pick up current outside of the area of influence. This is illustrated in Figure 4.7. Figure 4.7 Cathodic Interference CP 2 | Technician ©NACE International Stray Current 4-7 4.2.2 Current Pickup Interference If a foreign structure crosses a voltage gradient that is positive with respect to remote earth, it will promote a current pickup on the foreign structure within the area of influence. The anodic gradient could be considered as the controlling factor in this case. Because there is an unintended current pick up on the structure, the current must discharge outside the area of influence to return to its source. This may be through the soil that is detrimental or may occur through the foreign CP system. A current pickup is illustrated in Figure 4.8. Figure 4.8 Current Pickup Interference In certain conditions, a foreign structure may cross voltage gradients that promote anodic and cathodic interference from a single current source. For example, a foreign structure could cross the cathodically protected structure at one location and its impressed current groundbed at another location. 4.2.3 Detection of Static Interference Currents Certain changes and/or conditions in the system may indicate the presence of interfering currents, including: • • • • Structure-to-electrolyte potential changes on the affected structure caused by the foreign cathodic protection current source Changes in line current magnitude or direction caused by the foreign cathodic protection current source Localized pitting in areas near or immediately adjacent to a foreign structure Breakdown of protective coatings in a localized area near a foreign anode bed or near any other source of stray direct current Static interference is detected by analysis of structure-to-electrolyte potential surveys. This can sometimes misinterpreted as a structure-to-electrolyte potential survey because of static stray currents. In an unprotected pipeline, the corroding (anodic) areas exhibit a potential that is more negative when measured against a reference electrode, while the non-corroding (cathodic) areas yield a more positive (less negative) potential. Consider the case where stray current is towards the pipe in a protective direction. In this case, the stray current makes the soil more ©NACE International CP 2 | Technician 4-8 Stray Current positive than the pipe, and a more negative area appears on the structure-to-electrolyte potential profile, giving a false indication of a corroding area. The reverse is true in areas where stray current leaves the structure. Surface potential profiles may be misleading, as they may represent a composite of current leaving the structure being surveyed and close to a crossing structure. Static interference effects are detected similarly on protected pipelines. In this instance, however, the interpretation of the potentials is opposite to that described above for an unprotected pipeline. Figure 4.9 and Figure 4.10 show examples of close-interval surveys (CIS) and the effect of static stray current on structure-to-electrolyte potentials for coated and uncoated cathodically protected structures. The key points to consider in detecting static interference effects on a pipeline are: • • • • • Potential profiles show abnormal variation from previous surveys. Large negative values are noted remote from any cathodic protection system on the pipeline or are noted on unprotected piping. Unusual currents are measured along the pipeline. Low negative or positive potentials are present. Changes in the current output of a nearby cathodic protection system may cause changes in the structure-to-electrolyte potential of the pipeline. Figure 4.9 Effect of Static Stray Current on Pipeto-Electrolyte Potentials for Uncoated Cathodically Protected Structure (Interfered-with line) (Line 2) Figure 4.10 Effect of Static Stray Current on Pipe-toElectrolyte Potentials for Coated Cathodically Protected Structure (Interfered-with line) (Line 2) 4.3 Sample Field Data 4.3.1 Structure-to-Electrolyte Potentials When testing for stray current interference with an interrupter installed in the suspected source of interference it is critical that the ON and OFF structure-to-electrolyte potentials be identified CP 2 | Technician ©NACE International Stray Current 4-9 by the length of the interrupter cycles and the assumption that the ON potential is the most negative value must NOT be made. Consider an example where two structures are protected by independent cathodic protection (CP) systems and the structure designated as the “Foreign” Structure is afforded a new system that adversely affects the other structure (Affected Structure). The data from three test locations along the Affected Structure are shown in Table 4.1. Table 4.1: Sample of Data Indicating Current Discharge and Pickup Interference LOCATION Structure-to-Electrolyte Potential (mVCSE) X Y Z ON OFF -1600 -400 -1000 -1100 -1050 -1000 A negative change in potentials as the “foreign” current comes ON indicates possible current pick-up. In some cases, a negative shift on a structure when the foreign CP system is energized can be attributed to voltage gradients in the environment that don’t result in current pick-up. A potential shift in the positive direction (less negative) with the foreign structure CP system energized can be indicative of an overall reduction in CP current collected on the affected structure, or can indicate current discharge. In the example data shown in Table 4.1, a current pickup is likely indicated at Point “X” on the affected structure and suggests that it is in an anodic gradient of the foreign structures CP anodes. A current discharge is indicated at Point “Y” on the affected structure as the potentials become less negative than the free corrosion potential when the foreign structure’s CP system comes ON. There is no current pickup or discharge at Point “Z” on the affected discharge. 4.3.2 Basic Stray Current Testing The test procedure for stray current interference will vary depending on the structures and conditions. With a sound understanding of the basic concepts the sample procedure given below can be adapted to a specific situation. 4.3.2.1 Interruption The first concept is to determine the effect of the interfering current on the “interfered-with” pipeline or structure. This is done by interrupting only the “interfering” source. This is not to be confused with determining a polarized potential used for criterion purposes. The “off” potentials so obtained are not polarized potentials as the interfered-with pipeline CP current is still being applied. Interrupting the interfered-with source does not show the effect of the interfering power source and interrupting them together is the combined effect which does not distinguish between the “interfering” versus the “interfered-with” sources. An option is to interrupt the two power sources on different cycles which overlap (Example: 4 seconds on/2seconds off and the other at 8 seconds on/4 seconds off). ©NACE International CP 2 | Technician 4-10 Stray Current 4.3.2.2 Identification of “on” and “off” Potentials and Polarity The “on” potential may be either more electronegative than the “off” potential or it can also be more “electropositive”. One must not assume the “on” potential will always be the most electronegative potential. Instead the “on” potential is identified by the length of the cycle time. If using a long “on” cycle, either use a watch or count to determine which reading stays for the longest period of time and this is the “on” potential regardless of being more or less than the other potential. The polarity of the potential can switch from the “on” cycle to the “off” cycle or from one location to another. It is imperative to observe the polarity on the meter during each cycle at all reading locations. The examples in Table 4.2, taken with the interfering power source(s) interrupting, illustrate how fatal errors can result in missing serious interference effects by always assuming the “on” is more electronegative than the “off” and/or all readings are electronegative. The readings in Table 4.2 Example 1 indicate that there is no current discharge at either Location A or B however, the actual readings in Example 2 show there is indeed significant current discharge at these locations. The condition at Location A was missed because the surveyor assumed incorrectly that the “on” reading was the most electronegative. The condition at Location B which is very severe was missed because the polarity was not noted. Table 4.2: Examples of Incorrect Potential Records 4.3.2.3 Reference Electrode and Structure Contact The reference electrode must be near the structure but in the cathodic or anode gradient caused by the interfering source that is, the expected point of maximum exposure. CP 2 | Technician ©NACE International Stray Current 4-11 For example, there is a pipeline crossing in the middle of a field but the test station is located 20 meters (66 feet) away at a fence, the test station may be outside of the cathodic gradient at the crossing. A reference electrode located at the test station is not expected to determine the condition at the crossing. The reference electrode must be placed at the crossing and the preferred location would be between the pipelines. The voltmeter wires must be extended to the test station. Should there be a resistance bond between the pipelines, the wires carrying current must not be used for structure-to-electrolyte potential measurements. The voltage (IR) drop in the wire due to the current and wire resistance will increase the electronegative potential value on the positive side of the resistor (interfered-with) and reduce the electronegative potential on the other side (interfering). The following example illustrates this error: Assume: Ibond = 2 A If bond wire is #12 AWG and 20 meters (66 feet) long Rwire = 0.16 Ohms Verror = IR = 2A x 0.16 Ohms = +/- 0.32V = +/- 320 mV … a very large error! 4.3.2.4 Test Data The data in Table 4.3 is taken with the interfering source interrupted, that is, the “off” potentials is with the interfering structure source “off” but the interfered-with structure source is still “on”. Therefore the “off” potential is not a true potential. The potentials recorded are only on the interfered-with pipeline. Locations B and C both show cathodic interference effects with the potentials becoming less electronegative (more electropositive) with the interfering current coming on. The point of maximum exposure is the combined largest shift in potentials with the least electronegative potential. Table 4.3: Potentials Showing Current Discharge Interference Table 4.4 are potentials on the “interfered-with” pipeline but with the “interfering” source being interrupted. Locations B and C both indicate anodic interference with Location B the point of maximum exposure with the largest shift and the most electronegative potential. ©NACE International CP 2 | Technician 4-12 Stray Current Table 4.4: Potentials Showing Current Pickup Interference 4.3.3 Current Measurements Current measurements within a structure such as a pipeline can be used to predict current pickup and discharge areas using the principle of Kirchoff’s Current Law as illustrated by Figure 4.11. Figure 4.11 Current Pickup and Discharge Using Pipeline Current Current was measured at locations A, B, and C and the current discharge or pickup was determined using Kirchoff’s Current Law, that is, the difference in current between the points measured represents a current pickup or discharge. In Figure 4.11, there must be a current discharge between A and B equal to the difference of current (1.0 – (-2.0) = 3.0 A) while there must be a current pickup between B and C equal to the current difference between these points (-2.0 - 4.0 = -6.0 A). 4.4 Resolving Interference Problems Interference problems are individual in nature, and the resolution should be agreeable to all parties involved. Resolving interference problems generally involves one or more of the following: • • • • Removal of the detrimental effects of interfering current by installing a metallic return path Counteracting the effect of interfering current by applying cathodic protection Consultation with utility coordinating committee Removing or relocating the interfering current source CP 2 | Technician ©NACE International Stray Current • 4-13 Preventing the pick up or limitation of the interfering current These general approaches can be translated into some typical specific techniques: • • • • • • • • • Adjust current output from the interfering systems. Reduce the stray current at the source (e.g., isolation from ground, better conductivity of the negative return paths, or lower voltages). Apply coating to current pick-up area(s). Install a mitigation bond, or electrical connection, between the structures to drain the stray current back to its source through an electrical conductor rather than the earth. Relocate existing structures or re-route proposed structures. Properly locate isolating fittings. Apply cathodic protection to the affected structure at the interfering current’s discharge site. Relocate anodes Break up the structure of interest into smaller electrically isolated segments to reduce the stray current voltage gradients being traversed by the structure. 4.4.1 Tests with Bonds The potentials taken in testing for and mitigating stray current interference are not intended to duplicate a polarized potential survey to confirm a cathodic protection criterion. If a potential is taken with a bond in place, the system is still tied to a cathodic protection system. If the bond current is interrupted, the resulting potential will be subject to the effect of interference. A true polarized potential must be taken with all DC power sources and any bond interrupted. Do not be confused with the “off” potentials used in interference as being a polarized potential. Because of the above problems, the “on” potential is often used as a “target” potential. This “on” potential is related to a polarized potential obtained with all sources interrupted. The intent of any mitigation device is to return the pipeline back to the target “on” potential. There is no obligation to meet a cathodic protection criterion if it was not meeting it without interference. A possible procedure to test and install a mitigation bond is given below. • • • • • • • Interrupt “interfering” and “interfered-with” power sources at different cycles or separately Determine the point of “maximum electropositive shift exposure” with all power sources “on”. This will be the largest electropositive potential shift and/or the least electronegative potential. Determine the interfered-with “on” and “off” potentials when interfering power source off. The true polarized potential is obtained with all power sources “off”. Relate the “on” to the “off” potential for future reference. “Target” potential is the interfered-with “on” potential at maximum electropositive shift exposure with interfering power sources “off”. Remove interrupter(s) from interfering power sources. Install bond and measure potentials on both systems. Adjust resistance until interfered-with potential meets the target “on” potential. ©NACE International CP 2 | Technician 4-14 Stray Current Tests using cathodic protection mitigation such as sacrificial anodes are more difficult as readings may see the current discharge from the anodes. The bonds for cathodic interference are to be installed at the point(s) of maximum electropositive shift exposure. However, it may not always be practical and a nearby location is chosen. Regardless, the bond must be installed at a location where the interfering pipe potential is more electronegative than the interfered-with pipe potential. This will ensure the stray current goes through the bond back to its source as shown in Figure 4.12, Case 1. If the potentials are reversed as shown in Figure 4.12 Case 2, the current will go in the wrong direction and can increase the stray current problem on the interfered-with pipeline. In addition to measuring potentials on each side of the bond (using noncurrent carrying wires) it is very important to record not only the current but the direction of current through the bond. Figure 4.12 Current Direction Through Bond Class Exercise Complete the exercise at the end of the chapter. CP 2 | Technician ©NACE International Stray Current 4-15 4.4.2 Installation of Metallic Bonds to Control Interference Figure 4.13 illustrates a metallic bond between pipelines. The resistance of the bond must be lower than the parallel path through the soil. Figure 4.13 Electrical Bonding Note: Protected structure must be more negative than foreign structure at the bond location to return current safely. Figure 4.14 illustrates the use of a metallic bond to control stray current interference from a transit system. Figure 4.14 Installation of Metallic Bond When installing metallic bonds, the following should be considered: • • Uni-directional control devices, such as diodes or reverse current switches, may be required in conjunction with the metallic bond if currents are changing magnitude and direction present. These devices prevent reversal of current. A resistor may be needed in the metallic bond circuit to control the electrical current from the affected structure to the interfering structure. At the proper bond resistance, the discharge of interfering current from the structure to electrolyte is stopped. ©NACE International CP 2 | Technician 4-16 • • Stray Current If cathodic protection exists on the interfering structure, attaching metallic bonds can reduce the magnitude of protection. Supplementary cathodic protection may then be required on the interfering structure to compensate for this effect. A metallic bond may not perform properly in the case of a cathodically protected bare or poorly coated pipeline that is causing interference on a coated pipeline. A metallic bond can increase the current discharge. Coating the bare pipe at current pickup points or installing local galvanic anodes on the coated pipe may reduce interference effects. 4.4.2.1 Single Bond Problem If after doing all that is practical to minimize the accumulation of stray currents on structures it is necessary to control the stray current, a bond may be required. If there is a definite location (i.e., structure such as the substation on the trolley system) of current discharge, it is frequently possible to design a metallic bond through which this current can be returned to the traction system or, in the case of an industrial operation, to the power source (Figure 4.15). This requires accumulating certain data and calculating the needed bond resistance. Mitigation of dynamic interference is not part of this course but is covered in an separate “Cathodic Protection Interference” course. Figure 4.15 Solution of a Single Bond Problem In relatively simple cases where a single source of stray current is involved, a trial-and-error solution may be possible. If you are only interested in eliminating the corrosive changes in potential caused by the source using a bond connection, then a variable resistor of adequate current-carrying capacity is inserted at the proposed bond location. The value of resistance is slowly reduced while the structure-to-electrolyte at the most critical location is observed. When the structure-to-electrolyte potential of the interfered-with structure at the most critical location is returned to normal, the correct value of resistance for the bond has been set. Typically, it is desirable to use a resistance bond instead of a solid bond for the following reasons: • • • The resistance bond limits the amount of current for the structure being protected. The resistance bond must be of low enough resistance to ensure that the stray current return is using this metallic path rather than the electrolyte. Resistance bonds are subject to damage by high current surges and therefore must be inspected frequently (see Chapter 7) CP 2 | Technician ©NACE International Stray Current 4-17 4.4.2.2 Controlling the Direction of Stray Current Through the Bond Sometimes the direction of stray current through the bond is in the desired direction for much of the time but not all of the time. In these cases, stray current is toward the source when the structure to be protected is discharging current (corroding) and the reverse when the structure is receiving stray current. Stray current onto the structure is not desirable since it causes a discharge point at some other location(s). Diodes or reverse-current switches are used to prevent a reverse current. The resistance to the forward current created by these devices must be included in the bond calculations. 4.4.2.3 Reverse-Current Switch With dynamic stray current sources the direction of current through a bond could reverse causing accelerated corrosion. A diode can be installed to prevent this but it may be slow to respond. A relay can be installed that will open or reverse current. A combination of a diode and a relay reduces the disadvantages of each. Another solution may be a potentially controlled rectifier that controls the current in a forced drainage bond as shown in Figure 4.16. 4.4.2.4 Controlling Stray Current Through Cathodic Protection Figure 4.16 Forced Drainage Bond Stray current can safely be returned to its source through a cathodic protection system. An example using a carefully located sacrificial (galvanic) anode is shown in Figure 4.17. Figure 4.17 Galvanic Anodes ©NACE International CP 2 | Technician 4-18 Stray Current 4.4.2.5 Coating Coating of a known current pickup area can reduce the amount of stray current but the major portion of the anodic gradient should be coated (see Figure 4.18). Do not coat areas of current discharge as this will cause accelerated pitting at the small coating holidays. Figure 4.18 Protective Coatings 4.5 AC Testing and Mitigation Increased difficulty in obtaining utility rights-of-way and the concept of utility corridors have brought many underground structures, and pipelines in particular, into close proximity with electric power transmission and distribution systems. The electromagnetic field created by the alternating current expands and collapses and changes direction 100 to 120 times per second (Figure 4.19). Figure 4.19 AC Interference on Pipeline from Changing Electromagnetic Field Any metallic object subjected to an alternating electromagnetic field will exhibit an induced voltage. In addition, power conductor faults to ground can cause substantial fault currents in the underground structure. CP 2 | Technician ©NACE International Stray Current 4-19 There are three basic methods by which AC currents and voltages appear on metallic structures near AC power lines. These methods are as follows: • • • Electrostatic coupling, where the structure acts as one side of a capacitor with respect to ground. This is only of concern when the structure is abovegrade (e.g., pipeline supported on skids). Electromagnetic induction, where the structure acts as the single-turn secondary of an aircore transformer in which the overhead power line is the primary. This type of induction may occur when the structure is either above or below ground. Resistive coupling, where AC power is transmitted to ground the underground structure. Stray alternating currents can cause corrosion on metallic structures, although the amount of metal loss is less than an equivalent amount of DC current discharge would produce. For instance, 1 A DC discharge results in a loss of approximately 20 lbs of steel in one year, while one ampere of AC would consume less than 1 lb. The corrosion weight loss varies depending on the metal and the alternating current density. For instance, aluminum can exhibit a weight loss of approximately 40% of the DC equivalent at AC densities greater than about 40 mA/cm2. Even though the corrosion weight loss for AC currents is less than for equivalent DC currents, the magnitude of AC stray current is often large–hundreds of amperes under electromagnetic induction and thousands of amperes during power line faults. These high current levels can produce a shock hazard for personnel and can damage the structure and related equipment, such as cathodic protection facilities. 4.5.1 Electrostatic or Capacitive Coupling Any two materials separated by a dielectric material can be considered as a capacitor. Capacitance is the ability to store electrical charge between two conductors relative to the voltage between the conductors. In this case the air is the dielectric and the power line is one conductor while the pipe or equipment is the other conductor. Injury is more likely from an uncontrolled reaction to a shock rather than electrocution due to this couple. This coupling is of most concern on aboveground equipment or pipelines such as during construction or maintenance of the pipeline. During these operations a qualified person should be on site to monitor the AC voltage and install grounds or bonds around open sections of pipe as necessary. Figure 4.20 Electrostatic Coupling 4.5.2 Electromagnetic Induction The most important AC-induced interference current occurs as a result of electromagnetic induction as illustrated in Figure 4.19. Charges in the pipeline are alternately separated toward opposite ends of the pipeline. The resulting stray current magnitude is directly proportional to the phase currents (I) and their relative magnitudes and to the length (L) of mutual exposure; it is inversely proportional to the relative distance (d) between the structure and the power line conductors. ©NACE International CP 2 | Technician 4-20 Stray Current Figure 4.21 Electromagnetically-Induced AC Voltages Figure 4.22 Electromagnetically-Induced Voltage Analogy The induced voltage does not directly depend on the power line voltage; therefore, relatively low voltage AC power systems can produce electromagnetically-induced currents. In particular, single-phase heavily loaded AC power distribution lines can produce significant AC straycurrent activity. The pipe acts as a single-turn secondary of an air-core transformer, and the overhead AC power lines are primary (see Figures 4.21 and 4.22). The induced voltage (Vinduced) appears across the ends of the pipe. The voltage to ground (Vground) at each end of the structure is one-half the total induced voltage. Figure 4.23 Electromagnetically-Induced Voltages on a Parallel Pipeline CP 2 | Technician ©NACE International Stray Current 4-21 For a short, well-coated structure that is not electrically lossy (i.e., causing attenuation or dissipation of electrical energy), the induced voltage profile with distance is linear. However, for poorly coated or very long, well-coated structures the voltage profile is nonlinear and the induced voltage peaks are of a lower magnitude. The induced voltage peaks appear at any electromagnetic field or pipeline discontinuity. For example, where a pipeline closely parallels a power line for some distance, induced voltage peaks would be expected where the power line and pipeline separate. Under such circumstances, the induced voltage effect can extend some distance along the pipeline from the power line right-of-way. Measuring the structure’s AC voltage with respect to earth can easily identify interference currents of an AC origin. The reference electrode used for cathodic protection measurements can be used as the ground contact. It is prudent to measure both AC voltage and DC potentials with respect to earth on all structures in close proximity to power lines. Typically, the DC potentials will be more positive at the AC voltage peaks than in the absence of AC interference. 4.5.3 Resistance or Conductive Coupling During power line faults to ground, large AC currents can be transmitted to the earth through resistance coupling and, subsequently, into nearby underground structures. These currents, which can be several thousand amperes, can cause substantial physical damage to structure coatings; in extreme cases where the AC density is high, steel piping has been known to melt. Normally, these faults occur infrequently and are of short duration; therefore, they do not represent a serious risk to operating personnel. 4.5.4 AC Voltage on Pipelines Hazardous AC voltages can occur on a structure as a result of induction, ground return currents, or faulted power circuits. In cases where a structure or test leads parallel power transmission circuits, significant AC voltages may be encountered. If an AC voltage is near or is in excess of 15 V, the structure is considered hazardous and personnel working on the structure must be advised that a hazardous situation exits. The owner or supervisor must also be advised that steps need to be taken to reduce the hazardous voltage. If the AC voltage is determined to be less than 15 V, no specific action is necessary. However, be aware that unlike DC potentials the AC voltage changes with the load on the power line so that it can increase from when measured to later in the day. Therefore contact the power company to find out what percent of load they were operating at when the readings were taken. This will indicate the AC voltage that can be expected at full load conditions otherwise continue to measure it during the day. Whenever an AC voltage is close to 15 V (e.g., 12 VAC to 14 VAC) the same action should be taken as if it were 15 VAC or greater. 4.5.5 Measurement of AC Voltage-to-Ground The measurement of an AC voltage-to-ground is similar to a DC structure-to-electrolyte potential in that the voltage is measured between the structure and a reference electrode in the ground. There are significant differences including: • The voltage measured may be hazardous and safety is of great concern. Do not touch any metal directly or through the meter leads/clips until it is established that there is no hazardous voltage. ©NACE International CP 2 | Technician 4-22 • • • • • • Stray Current The reference electrode can be any bare metal but if a structure-to-electrolyte potential is also to be taken then a standard reference electrode can be used. The exposed portions of the reference electrode must not be touched and the electrode is to be installed first. If using a CSE then the top should be taped to prevent accidental contact. Induced AC voltage changes with the power line load and/or power line fault conditions. AC voltages change on aboveground pipelines under construction as the pipe length increases during the welding operation. Induced AC voltages can be expected to be higher at the discontinuities between the pipeline and the power line, that is, where they come together, separate or there is a change in the pipe or power line electrical characteristic (Figure 4.24). The measurement should duplicate step voltages. Therefore the electrode should be about one (1) meter away from the Figure 4.24 Examples of a Discontinuity closest contact point(s) above ground on the structure. (Unlike structure-to-electrolyte potentials where the reference electrode is to be placed close to the structure where it enters the ground.) Measure the AC structure-to-ground voltage first to make certain that it is safe to continue to work. First, place the electrode into the ground approximately one (1) meter from the structure contact point then contact the structure with an insulated probe and turn the meter to AC Volts. If the AC voltage is 15 V or greater, cease work until the AC voltage has been mitigated or, if qualified, proceed with further investigation into the cause and best means of mitigation. At no time should the structure be contacted under these conditions. Qualified personnel are to install temporary grounds, grids and/or nonmetallic fences at the work area or where the public can contact the structure until permanent protective equipment is installed. 4.5.6 Mitigation of AC Interference The close proximity of structures to AC power lines and their sharing parallel paths for relatively long distances is the principal cause of stray AC. If the structure is remote from the power line(s), the interference can be virtually eliminated. Obviously, this method of mitigation is practical only at the preconstruction stage of either the power line(s) or the structure. Otherwise, the mitigation must be accomplished by alternative methods. AC mitigation methods include: • • • Significant separation between pipe and HVAC system Ground pipe using distributed galvanic anodes Grounding pipe using a metal such as zinc, magnesium, steel, or copper. DC decouplers such as a capacitor, polarization cells, or polarization cell replacements (PCR) connected between pipe and a separate “grounding” structure will reduce the CP requirements when a more noble ground is tied directly to the structure. A DC decoupler allows AC current to pass but blocks DC current. CP 2 | Technician ©NACE International Stray Current • • • 4-23 Note: These devices may interfere with close interval surveys. Protective devices for electrical isolating devices such as flange kits or joints. Step-and-touch protection systems (gradient mats or grids) Figure 4.23 shows that the induced voltage is reduced if the structure becomes electrically conductive to earth. A well-coated structure would normally result in higher induced voltages. On short lines, this effect can be remedied by using distributed sacrificial anodes on the structure; the anodes will not only be sufficient to provide cathodic protection current but will also simultaneously lower the resistance of the structure with respect to earth. Another approach is to use a zinc strip or ribbon anode parallel to the structure as the grounding method. On longer lines, the approach is to install grounds at the higher voltage peaks, normally at the discontinuities. In Figure 4.25 a similar effect could likely be achieved by installing larger grounds at the two voltage peaks. Even with the grounds, hazardous voltages may still exist at other separation points and require more extensive grounds to reduce the AC voltage to 15 V or less. Corrosion personnel must be aware of the possible shock hazard that can appear on a structure Figure 4.25 Mitigation of AC Interference Using subjected to AC interference. Precautions such as Distributed Galvanic Anodes nonmetallic dead-front test stations should be incorporated into the cathodic protection systems, and nearby cathodic protection rectifiers should be protected from AC fault currents with devices such as capacitors, polarization cells, and zinc grounding cells. Bare casings interconnected through a capacitor bank, polarization cell replacement, or a polarization cell to the structure also are effective in mitigating AC interference. 4.5.6.1 Polarization Cells Figure 4.26 shows a polarization cell used as a DC decoupler that allows AC current to pass to a ground while blocking the DC current intended for the cathodic protection of the structure. The cell consists of a container filled with a potassium hydroxide solution (the electrolyte) into which stainless steel plates are immersed and alternately connected to the cell terminals. One of the cell terminals is connected to the structure and the other to ground (or if used to protect an insulating joint from AC, the terminals are connected across the insulating joint). The cell acts as an electrochemical switch to shunt voltage to ground. Under normal conditions, the plates of the cell polarize and allow normal cathodic protection ©NACE International Figure 4.26 Polarization Cell CP 2 | Technician 4-24 Stray Current potentials to exist. As the applied voltage across the cell increases from either AC or DC, the polarization film on the plates breaks down and the cell shunts current. The cells must be properly vented (to allow gases generated during operation to escape) and periodically inspected to ensure adequate electrolyte levels. 4.5.6.2 Semiconductor Devices Solid state devices are available that block the low-potential DC cathodic protection current from leaking across an insulating device while still providing instant protection from highvoltage spikes and induced AC. These devices do not involve caustic liquid electrolytes and are a low-maintenance device. Figure 4.27 shows a semiconductor-type device. Figure 4.27 Semiconductor Device for AC and High-Voltage Fault Control (courtesy of Dairy land Electrical Industries) 4.5.6.3 Ground Mats Ground mats may be required to protect personnel from electric shock while working on well coated pipelines. Induced voltage from AC power lines and lightning strikes can create large voltage gradients between a pipeline and earth. Ground mats are metal conductors placed in the soil around locations where a person may come in contact with the pipeline. Test station wires, line valves, and other fittings are examples of where a hazard may exist. The ground mat is connected to the pipe thus assuring that the pipe and the ground in the immediate area are at the same electrical voltage. They are not intended to be an electrical ground as they are installed close to the surface often in high-resistivity soil or frost. Ground mats can be made from any metal. A more active metal such as zinc or magnesium is used to have minimum impact on the cathodic protection system. Other metal such as copper or steel can be used but will add significantly to the current required for cathodic protection due to their large area of earth contact. If the mats are constructed of copper or other noble alloy or metal, adequate cathodic protection must be applied to eliminate the corrosive galvanic couple or isolated using a DC decoupler. Figure 4.28 illustrates how ground mats are applied with the rings becoming consecutively deeper at about 15° away from the structure. CP 2 | Technician ©NACE International Stray Current 4-25 It should be noted that if zinc or magnesium is used as a gradient mat around a test station, then the instant OFF potential is no longer valid as all sources of DC current have not been interrupted. Figure 4.28 Typical Ground Mat Used to Protect Personnel from Electric Shock Figure 4.29 Spiral Gradient Ground Map (Top View) 4.5.7 AC Corrosion AC current has long been known to cause corrosion but at a rate much below that for an equivalent amount of DC current. It was also largely believed that AC corrosion could be overcome by cathodic protection. However, in the 1990’s corrosion failures occurred on cathodically protected pipelines that were attributed to AC. Research is ongoing with some recent understanding summarized in NACE SP21424-20181 and ISO 18086:20152. A very simplified description of the AC corrosion process is during the anodic half AC cycle, the bare metal surface is oxidized with the formation of an oxide film while during the cathodic cycle, this oxide film is reduced and converted to a non-protective rust layer. In the following anodic cycle, a new oxide film grows, and the oxide film is increased and so the cycles continue. It is assumed that the oxide film never fully forms in field conditions otherwise the corrosion rate would be much higher. However, subsequent research has shown that AC corrosion is much more complex. AC corrosion is a result of the AC current density that in turn is dependent on the AC voltage and the spread resistance of the steel surface at the coating holiday. Spread resistance refers to the resistance from the pipe to remote earth through a specific coating defect, or of a coupon or probe of known area1. In practice, the current density is difficult to predict as the holiday size is not known, and the spread resistance is not only dependent on the local soil resistivity but on the presence of earth alkaline ions (Ca2+ & Mg2+) and/or alkaline ions (Na+, K+, Li+) in the soil. Calcium ions form products that increase the spread resistance, but sodium can form hygroscopic products (attract water) that reduce the spread resistance. With a CP polarized potential more electropositive than ~ -1.10 VSCE in an environment containing large quantities of earth alkaline ions, calcium deposits form on the surface of the 1. NACE SP21424-2018 Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation and Monitoring 2. ISO 18086 Corrosion of metals and alloys – Determination of AC corrosion – Protection criteria ©NACE International CP 2 | Technician 4-26 Stray Current steel resulting in an increase of spread resistance. However, when the CP polarized potential is more electronegative than ~ -1.10 VSCE with a hydrogen evolution reaction, the deposits formed on the steel surface are destroyed and the spread resistance decreases3,4. AC corrosion can be mitigated by CP at low AC and DC current densities but cannot be mitigated by cathodic protection at a high AC or DC current densities. Further the AC current density can be greater at the edges of the coating holiday or coupon than in the middle thus an average current density may not yield an accurate prediction. The prediction of AC corrosion is best completed using coupons or probes recognizing that these results give an average current density. Since conditions vary, a single voltage threshold value to control AC corrosion is not applicable. NACE SP21424-2018 provides the following current density criteria with effective CP: “Unless effective AC corrosion control has been otherwise documented, the AC current density should not exceed a time weighted average of: • 30 A/m2 (2.79 A/ft2)if DC current density exceeds 1 A/m2 (0.093 A/ft2) • 100 A/m2 (9.3 A/ft2) if DC current density is less than 1 A/m2” ISO 18086:2015 provides the following criteria. “ …effective AC corrosion mitigation can be achieved by meeting the cathodic protection potentials defined in ISO 15589-1 :2015 Table 1 and • maintaining the AC current density (rms) over a representative period of time (e.g. 24 h) to be lower than 30 A/m2 on a 1 cm2 coupon or probe, or • maintaining the average cathodic current density over a representative period of time (e.g. 24 h) lower than 1 A/m2 on a 1 cm2 coupon or probe if AC current density (rms) is more than 30 A/m2, or • maintaining the ratio between AC current density (Ja.c.) and DC current density (Jd.c.) less than 5 over a representative period of time (e.g. 24 h). Note: Current density ratios between 3 and 5 indicate a small risk of AC corrosion. However, to reduce the corrosion risk to a minimum value, smaller ratios than 3 are preferable.” Some notable differences in criteria between these standards are given below thus judgement is necessary with their application. • NACE states that the “… AC current density should not exceed 30 A/m2 if the DC current density exceeds 1 A/m2” while ISO states the “…AC current density be lower than 30 A/m2” (regardless of DC current density) 3. Yanxia Du, Sili Xie, Yingwu Xiao etc. Research on the Effects of Environmental Parameters on AC corrosion Behavior. NACE CORROSION 2018. Paper No. 10676. 4. Andreas Junker, Louise J. Belmonte. Nick Kioupis. Investigation of stone-hard-soil formation from AC corrosion of cathodically protected pipeline. Materials and Corrosion.2018,69:1170-1179. CP 2 | Technician ©NACE International Stray Current • • 4-27 NACE infers an upper AC current density limit of 100 A/m2 if the DC current density is less than 1 A/m2 while ISO does not indicate an upper limit if the AC current density is greater than 30 A/m2. Current density ratios in the ISO criteria are not addressed in the NACE criteria. A low DC current density yields a limited increase in pH, does not significantly change the spread resistance and has less reductive effect on the metal oxides. A high DC current density can result in either a high or low AC corrosion rate. The ratio of AC to DC current densities may be used to assess the possibility of corrosion, where the precise coupon area is not critical. If the ratio is below a certain threshold (see ISO 18086:2015 criteria third bullet), no AC corrosion occurs since metal oxidation in the anodic half wave is prevented. The ratio is used only as an indicator of possible corrosion. Research has also demonstrated that a polarized potential of -850 mVCSE may not be effective in controlling AC corrosion, rather an increased electronegative potential may be necessary to a limit (e.g. of limit is -1.15 VCSE). In summary, AC corrosion mechanisms are still being researched and debated. At low AC current densities, CP can be successful in controlling AC corrosion, but caution against high DC current densities or potentials more electronegative than -1.15 VCSE is advised. The present consensus is that coupons installed where AC current densities are likely to be high are the preferred method to predict AC corrosion or its successful mitigation. Lowering the AC voltage by grounding will in turn reduce the AC current density5. 5. NACE SP0177, Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems ©NACE International CP 2 | Technician 4-28 Stray Current Experiment 4.1 - Demonstration of Stray Current Interference Preparation • • • Use fresh water (no salt), as the higher the resistivity, the larger the effect Keep the water shallow (e.g. ~1cm above the structures), to increase the resistance of the ionic path through the water relative to that through the ‘foreign structure’ In step 4 below, add a resistance in series with 9V battery to limit & regulate the current to ~5-10 mA during the experiment and ~2-5 mA if left overnight. Procedure 1. Place tap water in tray 2. Place steel sheet or steel rod across left side of tray (see Figure 4.30 A to C) 3. Measure the pH with litmus paper and record in the table below and measure potentials at A. B, C 4. Connect the steel sheet or rod to a CP system (a battery, resistor and anode installed to the extreme right side of the tray) (See Figure 4.30) 5. Install an ammeter or shunt in the circuit and measure or calculate current 6. Measure the potentials to the steel at locations A, B, C and record the results in the table below Figure 4.30 Set up of Steel Rod (pipeline) and isolated (foreign pipeline) demonstrating current pickup and current discharge interference 7. Measure the pH at A, B & C after CP applied 8. Disconnect the CP system 9. Place a steel rod lengthwise along tray (Figure 4.30 D to F) CP 2 | Technician ©NACE International Stray Current 4-29 10. Measure potentials and pH along this foreign (isolated) steel rod at locations D, E & F and record the results in the table below 11. Connect the CP system 12. Repeat the potentials at A, B, C, D, E, & F and current and record the results in the table below. 13. Install a resistance bond between the CP protected line to the foreign structure (Figure 4.31). Figure 4.31 Resistance Bond and Ammeter Installed Between Structures 14. Repeat step 12 plus the bond current/resistance and record the results in the table below. 15. Remove the resistance bond. 16. Leave CP operating overnight. Note corrosion occurring due to interference and record pH ©NACE International CP 2 | Technician 4-30 Stray Current Results Step CP Structure-to-Electrolyte Potentials (mVCSE) or pH A B C Foreign Structure-to-Electrolyte Potentials (mVCSE) D E F ICCP Current (mA) Step 3 Step 3 pH Steps 5, 6&7 Step 7 pH Step 10 Step 10 pH Step 12 Step 14 Step 15 pH • Resistance bond current ________ mA • Bond resistance ______ Ohms Conclusions • • • • • • • Stray current pickup on the foreign structure occurs at ______ (name all) Stray current discharge on the foreign structure occurs at ______ (name all) An increase in current on the CP structure occurs at ________ (name all) A decrease in current on the CP structure occurs at ________ (name all) Effect on potential of bonding the foreign structure to the CP structure ________________ Effect on current of bonding the foreign structure to the CP structure __________________ The effect of pH was _________________________________________ (at each location) CP 2 | Technician ©NACE International Chapter 4 Stray Current Interference ® Stray Current - Definition 2 Stray current: Current through an electrical path(s) other than the intended circuit Interference: Any detectable electrical disturbance on a structure caused by a stray current ▪ Stray current not galvanic corrosion current between anodes and cathodes on same structure ▪ Stray current or interference, can be classified as being either: ▪ Static ▪ Dynamic CP2 | Technician Stray Current 3 ▪ Stray current can be either DC or AC ▪ AC can be both a safety and a corrosion hazard CP2 | Technician 1 DC Stray Current Corrosion 4 Foreign Structure Corrosion occurs at Current discharge point DC Current Source CP2 | Technician DC Stray Current Corrosion 5 At current discharge, accelerated corrosion possible ▪ large magnitude of current or ▪ high current density Consumption rates are: ▪ Ferrous 9.1 Kg / A-yr (20 pounds / A-yr) ▪ Copper 10.4 Kg / A-yr (23 pounds / A-yr) ▪ Lead 33.85 Kg / A-yr (74.64 pounds / A-yr) CP2 | Technician Stray Current Analysis 6 ▪ Source of the stray current? ▪ Area stray current enters the structure? ▪ Area stray current leaves the structure? ▪ Magnitude of the stray current? ▪ How can stray current be mitigated? CP2 | Technician 2 Dynamic Stray Current 7 ▪ Dynamic stray current changes in amplitude and/or direction ▪ Dynamic stray currents are ▪ man-made or ▪ geomagnetic (telluric current) CP2 | Technician Dynamic Stray Current Sources 8 ▪ DC transit systems ▪ Welding ▪ Mining operations ▪ Electrical power transmission ▪ Industrial plants ▪ Telluric CP2 | Technician Stray Current 9 Transit Power Line DC Generator Protection (Cathodic Area) Corrosion (Anodic Area) CP2 | Technician 3 Telluric Currents 10 ▪ Natural Earth Currents ▪ Interaction of solar particles with Earth’s magnetic field ▪ Highest near magnetic poles ▪ Dynamic changes ▪ Affect Cathodic Protection Measurements CP2 | Technician Detection of Dynamic Stray Currents 11 ▪ Structure-to-electrolyte potential changes ▪ Usually during reading ▪ Varying earth gradient voltages ▪ Changes in line current magnitude or direction CP2 | Technician Potential Fluctuating Due to Dynamic Stray Current 12 CP2 | Technician 4 Earth Current (Potential) Measurement Between Two Reference Electrodes 13 Voltmeter With + Reading + - Reference Electrode Reference Electrode Current CP2 | Technician Earth Current (Potential) Measurement Between Two Reference Electrodes (+) (+) V V V V (+) 14 (–) V V (–) (–) CP2 | Technician Mitigating Stray Current - General 15 ▪ Stray current from one or more sources ▪ To successfully drain stray current from structure: ▪ Structure must tend to discharge stray current at bond ▪ Source must tend to pick up stray current at bond ▪ Interfering structure must be more electronegative than interfered-with structure ▪ Sufficient current must be drained CP2 | Technician 5 Static (Steady State) 16 Static or steady-state interference currents: ▪ those that maintain constant amplitude and constant geographical paths Example: ▪ CP systems CP2 | Technician Static Stray Current Interference from a CP System 17 Stray Current CP2 | Technician Types of Interference 18 ▪ Current Discharge Interference ▪ Current Pickup Interference With current pickup there may be an associated current discharge CP2 | Technician 6 Cathodic Interference 19 ▪ Cathodic voltage gradient promoting current discharge from foreign structure when it: ▪ overlaps a foreign structure’s gradient ▪ is electronegative with respect to remote earth, ▪ A corresponding current pick up current exists elsewhere CP2 | Technician Current Discharge Interference Interference Current Pickup Foreign Pipe 20 Interference Current Pickup Interference Current Discharge Voltage Gradients Around the Protected Pipe Impressed Current Anode bed Remote from Foreign Pipe Rectifier Protected Pipe PROFILE VIEW CP2 | Technician Current Pickup Interference 21 ▪ Anode gradient promoting current pick up on foreign structure when it: ▪ overlaps the foreign structure’s gradient and ▪ is electropositive with respect to remote earth ▪ Current discharge elsewhere is concern for accelerated corrosion ▪ Possible coating damage in anodic gradient CP2 | Technician 7 Current Pickup Interference Interference Current Discharge Protected Pipeline Remote from Foreign Pipe Foreign Pipe 22 Interference Current Pickup Interference Current Discharge Voltage Gradients Around the Anode bed Rectifier Impressed Current Anode bed TOP VIEW CP2 | Technician Detection of Static Interference Currents 23 ▪ Structure-to-electrolyte potential changes ▪ Changes in the structure current magnitude or direction ▪ Localized pitting near or at foreign structure ▪ Coating breakdown in localized area near stray current source CP2 | Technician Static Interference Detection by Potentials 24 Current discharge indicated by electropositive potential change ▪ Energy source is external Mixed metal system: ▪ Most electronegative metal discharges most current and corrodes ▪ Energy source is between metals CP2 | Technician 8 Current Pickup from Interfering Anode bed Effect of Static Stray Current on Potentials for Uncoated CP Structure 25 1400 Struccture-to-Electrolye Potential (-mVcse) 1200 Current pickup from Interfering Anodes 1000 800 600 400 A B C D Current Discharge To Interfering Line 200 (+) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Distance Interfering Rectifier Interfering Line Line 1 B Interfered-with Line A Line 2 D CP2 | Technician Current Pickup from Interfering Anode bed Struccture-to-Electrolye Potential (-mVcse) Effect of Static Stray Current on Potentials for Coated CP Structure 1400 Current pickup from Interfering Anode Bed 1200 1000 A 800 B C 600 400 Current Discharge To Interfering Line 200 (+) 26 1600 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Distance Interfering Rectifier Line 1 B Interfering Line Interfered-with Line A Line 2 D CP2 | Technician Combined Current Discharge and Pickup Interference 27 Potential Profile On Foreign Pipeline Current Pickup – Near Anode Bed – E + Current Discharge CP2 | Technician 9 Interference Shown by Potential Field Data 28 ▪ (X) More electronegative with current ON - current pickup ▪ (Y) More electropositive with current ON - current discharge ▪ (Z) No effect Structure-to-Electrolyte Potential (mV CSE) Location ON OFF X -1600 -1100 Y -400 -1050 Z -1000 -1000 CP2 | Technician Steady State Interference Testing with Potentials 29 Interrupt “interfering” power source ▪ NOT power source on “interfered-with” structure ▪ NOT both rectifiers interrupted synchronously Use different cycle length to determine ON or OFF potential ▪ DO NOT assume the most electronegative is ON potential! CP2 | Technician Missed Interference ▪ Interference missed by incorrectly identifying on/off potentials and/or polarity ▪ Example 1 incorrectly indicates no current discharge at Location A or B ▪ Example 2 correct readings showing current discharge especially at B ▪ Location A was missed by assuming the “on” was the highest number ▪ Location B was missed by not reading the polarity 30 Assumed – mV with cycle and Polarity Ignored Potentials (mVcse) Location Remarks Incorrect ON OFF A 1000 800 B 1000 850 Actual measurements Potentials (mVcse) Location ON OFF A -800 -1000 B +1000 +850 Remarks CP2 | Technician 10 Structure-to-Electrolyte Potential Measurements Reference electrode ▪ Must be at point of exposure ▪ Example: ▪ Test station remote from pipe crossing ▪ Outside cathodic gradient ▪ Reference must be placed at crossing ▪ Voltmeter wires extended to nearest test station ▪ Preferred reference location between the pipelines 31 Structure Contact for Potentials ▪ Must NOT use a current carrying wire ▪ If current bond wires used for potential measurements: ▪ Current increases negative potential on positive (interferedwith) side ▪ Reduces negative potential on the negative (interfering) side ▪ Example: ▪ Ibond = 2 A ▪ Bond Wire #12 AWG - 20 meters ▪ Rwire = 0.16 Ohms ▪ Eerror = IR = 2A x 0.16 Ohms ▪ = +/- 0.32V = +/- 320 mV CP2 | Technician Interference Test Data 32 Current Discharge Interference Current Pickup Interference ▪ Interfering source interrupted (OFF potential is with Interfered-with on) ▪ Interfering source interrupted (OFF potential is with Interfered-with on) ▪ Potentials taken on interfered-with pipe ▪ Potentials taken on interfered-with pipe ▪ OFF is not a polarized potential ▪ OFF is not a polarized potential ▪ Table 1 Location B & C experiencing current discharge interference but C is maximum exposure ▪ Table 2 Location B & C experiencing current pickup interference but B is maximum exposure Table 1 Potentials (mVcse) Location ON OFF A -1000 -1000 B -800 -870 C -600 -840 D -900 -900 Remarks Table 2 Potentials (mVcse) Location Max exposure ON OFF A -1100 -1100 B -1800 -1200 C -1400 -1100 D -1000 -1000 Remarks Max exposure CP2 | Technician Interference from Current Data 33 Current reducing or in opposite directions indicates current discharge Measured Calculated A B C 1.0 A 2.0 A 4.0 A 3.0 A 6.0 A CP2 | Technician 11 Resolving Interference Problems 34 ▪ Provide metallic return path for interfering current ▪ Counteract interfering current by CP ▪ Install sacrificial anode by interfering pipe and ▪ Connect to interfered-with pipe ▪ Consult with owners and/or Electrolysis Coordinating Committee to readjust source ▪ Removal or relocation of interfering current source ▪ Prevention of pick up or limit interfering current CP2 | Technician Resolving Interference Problems 35 ▪ Adjust current output from interfering systems ▪ Reduce stray current at the source ▪ Apply coatings to strategic areas (current pickup not discharge areas) ▪ Relocate structure ▪ Install isolation fittings CP2 | Technician Tests with Bond Connected: Potentials 36 True Polarized Potentials Target Potential ▪ Not with bond installed ▪ Now tied to another CP system ▪ Target ON potentials used with interfering sources interrupting ▪ If bond current also interrupted, OFF potential is an “interfered-with” potential ▪ Do not confuse this OFF with a polarized potential ▪ Structure now affected with stray current ▪ One Mitigation Method: ▪ Bond returns ON potential to that before interference ▪ Bond not intended to meet criterion if not before ▪ Interfering company not obliged to provide CP CP2 | Technician 12 Interference Mitigation Bond Test ▪ Interrupt “interfering” and “interferedwith” sources separately or at different cycles 37 ▪ “Target” is ON potential at ▪ maximum exposure ▪ interfering power sources off ▪ Determine point of “maximum ▪ Remove interrupter exposure” with both cycles on ▪ Largest electropositive shift and/or ▪ Install bond ▪ Least electronegative potential ▪ Adjust resistance until interferedwith potential meets target ON ▪ Determine interfered-with ON and OFF potential potentials with interfering source off ▪ True polarized potential is obtained Tests using CP mitigation are more with all power sources “off” difficult as reference may pick up current discharge from anode CP2 | Technician Bonds 38 Bond Location to return stray current safely: ▪ Locate at point of maximum exposure CASE 1 Bond provides safe return path -700 mVcse (+) ▪ Always measure: ▪ Potential on each side ▪ Current through bond ▪ Polarity ▪ Describe current direction -900 mVcse (-) Resistor ▪ Interfering potential must be more electronegative than interfered-with potential (Case 1) ▪ Otherwise bond current is reversed ▪ stray current discharge increased on interfered-with pipe (Case 2) wrt Interfering Pipe Interfered-with Pipe CASE 2 Bond makes condition worse -700 mVcse (+) wrt -600 mVcse (-) Resistor Interfered-with Pipe Interfering Pipe CP2 | Technician Class Exercise 39 Answer the right four (4) columns for each case where stray current interference is occurring. Write reasons for your decision for each case. Interfered-with and interfering power sources are assumed before the test but may not be true. Class Exercise at the end of chapter 5 Case Interfered with Pipe-to-Electrolyte Potential (mVcse) Interfering Pipe-toElectrolyte Potential (mVcse) DC Power Source Interrupting ON OFF ON OFF A -600 -800 -1200 -850 B -800 -600 -1500 -1500 Interferedwith -1100 Interfering & interferedwith synchronized -1500 Interference Cathodic Will Res Bond Mitigate? Target Potentia l for Bond Anodic Interfering C -850 -750 D -1200 -900 -850 -600 Interfering E -400 -900 -1200 -1200 Interferedwith F -400 -400 -1400 -1000 Interfering CP2 | Technician 13 Cathodic Protection Interference Mitigation Bond 40 Mitigation Bond Cathodic Protection Current Interference (Corrosive) Current Interfering Pipeline Foreign or Interfered with Pipeline Interference control current in bond CP2 | Technician Installation of Metallic Bond Load Current Required to Operate Train Overhead Positive Feeder 41 DC Substation + Tracks Negative Return _ Bond Cable Parallel soil return path Bond across High Resistance Joints CP2 | Technician Installation of Metallic Bond 42 Considerations: ▪ Uni-directional control devices (diodes or reverse current switches) ▪ Resistor ▪ Interfering structure must be more electronegative ▪ otherwise bond current reversed - increasing interference ▪ Instead of Bond: ▪ Reduce interfering CP source (supplementary CP may be required elsewhere) ▪ Coat pickup structure instead of bond ▪ Increase CP on interfered-with structure ▪ Add galvanic anodes at discharge CP2 | Technician 14 Resistance Bond vs. Solid Bond 43 ▪ Resistance bond limits amount of current lost to interfered-with structure ▪ Resistance must be lower than parallel path through electrolyte ▪ Resistance bond subject to failure from current surges ▪ Solid bonds are lower in resistance but do not control current CP2 | Technician Controlling the Direction of Stray Current through the Bond 44 ▪ Diodes or reverse-current switches prevent reverse current ▪ Diode or reverse-current switch part of bond circuit resistance CP2 | Technician Interference Mitigation Forced Drainage Bond 45 Rectifier + Controller Pipeline Stationary Reference Electrode CP2 | Technician 15 Interference Mitigation Galvanic Anodes 46 Protected Structure Current Discharge from Anode Galvanic Anode Current Pickup From ICCP Anode ICCP Anode CP2 | Technician Interference Mitigation Protective Coatings 47 Recoated Area Foreign Structure Reduced Current Pickup in Recoated Section Protected Structure CP2 | Technician AC Interference – Pipeline & Power Line 48 Electromagnetic Induction Coupling ▪ Structure acts as secondary winding ▪ Applies to above or below-ground structures Electrostatic (Capacitive) Coupling ▪ Charge on aboveground structures only Conductive (Resistive) Coupling ▪ AC fault or lightning surges from electrical ground to structure through electrolyte CP2 | Technician 16 Electromagnetic Induction 49 ▪ AC-induced voltage from parallel power line ▪ Charges in pipeline alternately separated toward opposite ends of section under influence ▪ Voltage peaks at end and zero in middle ▪ Exposure length between physical discontinuities ▪ Pipe and power line - join, cross, depart, or power line transformation ▪ Induced voltage directly proportional to ▪ phase currents (IΦ) and ▪ length (L) of mutual exposure ▪ Induced voltage inversely proportional to distance (d) between pipe and power line conductors CP2 | Technician AC Induced Voltage on Pipeline from Changing Electromagnetic Field 50 Magnetic Field Produced By Overhead Lines Pipeline Soil CP2 | Technician Electromagnetic Induction Overhead Primary 51 1 VINDUCED= f ( I F, ,L ) d Air Core VINDUCED V Induced V0 = 2 Underground Pipeline Secondary CP2 | Technician 17 Electromagnetic Induction 52 Poorly Coated Short Pipeline Or Well Coated Long Pipeline + E Well Coated Short Pipeline Distance CP2 | Technician Electromagnetic Induction 53 Overhead Conductor - IF d = distance between overhead conductor and pipeline Length, L Vo Vo Soil (Voltage to Ground) CP2 | Technician Electrostatic Coupling 54 Vehicles or above ground pipe Pipe Soil CP2 | Technician 18 Conductive (Resistive) Coupling 55 Power Line Faults to Ground ▪ Very High Currents ▪ Infrequent ▪ Short Duration ▪ Possible pipe damage (arc burns) ▪ Possible CP damage CP2 | Technician Conductive (Resistive) Coupling 56 Fault current Pipeline Soil CP2 | Technician AC Conditions 57 ▪ Hazardous AC voltage = 15 VoltsAC or greater ▪ More frequent with common utility corridors or right-of-way ▪ AC electromagnetic field changes ▪ 100 times/second at 50 Hertz or ▪ 120 times/second at 60 Hertz ▪ Changing electromagnetic field induces voltage on parallel pipeline ▪ Single turn air core transformer (AC is primary & pipe secondary) ▪ Phase-to-ground faults expose an underground structure to high currents CP2 | Technician 19 AC Structure-to-Earth Voltage Testing 58 AC structure-to-earth voltage ▪ AC voltmeter ▪ Any independent ground can be used as a reference ▪ Connect to reference in ground first Do not touch any metal directly or through tools, meter leads/clips ▪ If CSE, tape top terminal Induced AC voltage changes during day ▪ with power line load and/or power line fault conditions CP2 | Technician AC Structure-to-Earth Voltage Testing (cont’d) 59 ▪ VoltsAC increases on aboveground pipe as length increases during welding ▪ Higher induced AC voltages at ends or discontinuities CP2 | Technician AC Structure-to-Earth Voltage Testing (cont’d) 60 ▪ Measurement duplicates “touch voltage”. ▪ Place electrode ~one (1) meter (~3 ft) from closest contact point(s) ▪ Contact electrode first and pipe next ▪ Unlike structure-to-electrolyte potentials where reference electrode is placed close to structure CP2 | Technician 20 AC Interference Mitigation 61 ▪ Install structure remote from power lines where practical ▪ Install surge protection across isolating features ▪ Install grids at above-ground pipe appurtenances (valves etc.) ▪ Shallow ribbon spiral or grid ▪ Grid not a substitute for a ground being shallow CP2 | Technician AC Interference Mitigation (cont’d) 62 Install electrical grounds (galvanic anodes or metal grounds): ▪ Connect to ground first and pipe next ▪ Use DC decoupler in series with metal grounds ▪ Capacitors, polarization cells, or polarization cell replacements (PCR) ▪ Allows AC to pass but blocks DC ▪ Not to be confused with Surge Protectors ▪ Pass high fault current only ▪ Devices may interfere with CP surveys CP2 | Technician Mitigation of AC Interference Using Distributed Galvanic Anodes 63 Overhead AC Transmission Line Underground Pipeline Distributed Sacrificial Anodes Without Anodes Induced Voltage With Anodes Distance CP2 | Technician 21 Polarization Cell 64 Fill Hole Cell Terminals Potassium Hydroxide Solution Stainless Steel Plates CP2 | Technician Semiconductor Polarization Cell Replacement (PCR) 65 CP2 | Technician Spiral Gradient Control Mat Washed Stone Zinc Ribbon Mat Sign (Typical) Earth Surface 300mm Extend Gravel Pipeline 66 Valve Stranded Copper Cable With Insulation Typical Thermite Weld Connection Copper Cable to Zinc Ribbon Core Wire Connection CP2 | Technician 22 Spiral Gradient Control Mat – Top View Aboveground appurtenance 67 Underground Mat Sign 300mm (1 ft) (Typical) Stranded Copper Cable Connection Zinc Ribbon 2m Minimum Grid being shallow is not intended to act as a ground CP2 | Technician AC Corrosion 68 Simple version: ▪ Metal oxidized during anodic half cycle ▪ During cathodic half cycle, oxide film reduced to non-protective rust layer ▪ Repeated during each cycle ▪ Assumed oxide film never fully forms or corrosion rate more But is more complex … SP0169-2013 expects AC corrosion >100 A/m2 with CP - recommends investigation >30 A/m2 CP2 | Technician AC Corrosion 69 ▪ Depends on AC current density ▪ In turn depends on: ▪ AC Voltage ▪ Spread resistance (Varies with CP reactions) Reference ISO 18086:2015 and NACE SP21424-2018 CP2 | Technician 23 AC Corrosion – Spread Resistance 70 ▪ Spread resistance is: ▪ Resistance of coating defect, coupon or probe to remote earth ▪ Dependent on earth alkaline (Ca+, Mg+) & alkaline (Na+, K+, Li+) ions in soil ▪ Ca+ products increase & hygroscopic Na+ products reduce SR ▪ With CP more (+) than ~-1.10 VSCE Ca+ products form on Fe to increase spread resistance ▪ With CP less (+) than ~-1.10 VSCE, with H2 evolution, ▪ products destroyed and ▪ spread resistance decreases CP2 | Technician AC Corrosion – Current Density 71 ▪ AC corrosion mitigated at low AC & DC current densities ▪ Not mitigated at high AC / DC current densities ▪ Predict current density from coupons ▪ Not accurate to calculate due to unknown defect size and variable spread resistance ▪ AC can be greater at edges of holiday or coupon ▪ average may be conservative prediction CP2 | Technician AC Corrosion Criteria 72 ▪ NACE SP0169-2013 expects AC corrosion >100 A/m2 with CP - recommends investigation >30 A/m2 ▪ NACE SP21424-2018: Meet CP criteria and not to exceed ▪ 30 A/m2 if DC exceeds 30 A/m2 ▪ 100 A/m2 if DC less than 1 A/m2 ▪ ISO 18086:2015: CP criteria & ▪ AC < 30 A/m2 over 24 hr on 1 cm2 coupon/probe ▪ AC > 30 A/m2 over 24 hr on 1 cm2 coupon/probe – maintain CP < 1 A/m2 ▪ Ratio over 24 hrs; Jac/Jdc < 5 preferably < 3. (J – current density) CP2 | Technician 24 Notable Differences in Criteria NACE 1 2 3 73 ISO Jac < 30 A/m2 if Jdc > 1 A/m2 Suggests upper Jac limit of 100 A/m2 if Jdc < 1 A/m2 Jac < 30 A/m2 (regardless of DC) No upper limit if Jac > 30 A/m2 Jac/Jdc not addressed Jac/Jdc < 5 preferably <3 recommended Precise coupon area not critical with ratio CP2 | Technician AC Corrosion – Jac & Jdc 74 ▪ Low Jdc ▪ Limited increase in pH ▪ Less reductive effect on metal oxides ▪ High Jdc ▪ Results in a high or low spread resistance ▪ With a low or high Jac ▪ Low Jac/Jdc ratio metal oxidation in anodic half cycle prevented CP2 | Technician AC Corrosion – Summary 75 ▪ AC Corrosion still researched and debated ▪ Low Jac, CP can be effective ▪ -850 mVCSE may not be sufficient ▪ Caution against high potential ▪ Less electronegative than -1.15 VCSE ▪ Use coupons/probes to predict Jac. ▪ Lowering VAC by grounding will reduce Jac and AC corrosion CP2 | Technician 25 Knowledge Check | 1 76 A mitigation control bond should be designed to A. Carry the necessary current. B. Restrict the current to that required to mitigate the problem. C. Keep the current going in one direction if necessary. D. Return the potential on the interfered with structure to its pre-interference state. CP2 | Technician Knowledge Check | 2 77 A mitigation bond may cause more problems under which of the following conditions? A. Where the interfering structure bond point is more electropositive than the interfered with structure. B. Where the interfering structure bond point is more electronegative than the interfered with structure. C. Where there are more than one, or changing, current discharge points. D. Where the interfering structure bond point potential varies from being more electronegative to more electropositive than the interfered with structure. CP2 | Technician Knowledge Check | 3 78 AC Voltages on pipelines do NOT result from A. resistive (conductive) coupling B. electrostatic (capacitive) coupling C. electromagnetic induction coupling D. thermodynamic coupling CP2 | Technician 26 Knowledge Check | 4 79 On a cathodically protected pipeline, AC corrosion can be expected at which current densities? A. 30 mA/m2 B. 10 A/m2 C. 110 A/m2 D. 200 A/m2 CP2 | Technician Knowledge Check | 5 80 The AC voltage-to-ground on a section of pipeline near a power line tends to be A. Highest where the power line and pipeline cross B. Highest where the power line and pipeline separate after a parallel section C. Highest at the ends of the overall pipeline with the parallel section in the middle D. The same along the pipeline/power line parallel section CP2 | Technician Knowledge Check | 6 81 Methods of making hazardous AC voltages safe includes installing which of the following? A. grounds at the discontinuities between power line and pipeline B. a bond to the steel power line tower C. a gradient mat or a grid around aboveground structures D. a remote ground away from the parallel section CP2 | Technician 27 Knowledge Check | 7 82 A polarization cell is used to A. polarize the pipeline B. polarize the electrical ground C. provide DC coupling but block AC D. provide AC coupling but block DC CP2 | Technician Knowledge Check | 8 83 Tests for interference can be indicated by which of the following? A. Structure-to-electrolyte potential changes B. Changes in line current magnitude or direction C. Localized pitting in areas near or immediately adjacent to a foreign structure D. Breakdown of protective coatings in a localized area CP2 | Technician Knowledge Check | 9 84 When AC voltage of 13 VAC is measured on a pipeline, what course of action should be taken? A. Wait until next year or take an AC voltage to confirm it has not changed. B. Contact the power company to determine what percent of load the power line is operating. C. Advise other personnel that a near hazardous situation exists on the pipeline. D. Advise the owner or supervisor that an AC mitigation program should investigated. CP2 | Technician 28 Knowledge Check | 10 85 The purpose of a gradient mat is to A. Electrically ground the pipe to the earth to eliminate the AC voltage. B. Bring the voltage difference between the pipe and earth close to 0 VAC. C. Provide cathodic protection to large structures such as tank bottoms. D. To eliminate the earth gradient to obtain a true polarized potential. CP2 | Technician Knowledge Check | 11 86 Which of the following is NOT a source of dynamic stray current? A. Cathodic protection B. Mining equipment C. Transit systems D. Telluric current CP2 | Technician Knowledge Check | 12 87 Stray current CANNOT be detected by A. structure-to-electrolyte potentials. B. reference electrode to electrode measurements. C. pipeline current span measurements. D. soil resistivity measurements. CP2 | Technician 29 Knowledge Check | 13 88 Cathodic interference occurs A. in a voltage gradient that is a positive to remote earth. B. in a voltage gradient that is negative to remote earth. C. at the cathode in a corrosion cell. D. at the structure’s cathodic protection system. CP2 | Technician Knowledge Check | 14 89 An electronegative peak in potentials along a pipeline between CP power sources may be indicative of A. Current pick up interference B. Current discharge interference C. Shorted casing D. Shorted isolation CP2 | Technician Knowledge Check | 15 90 Analysis of dynamic stray current does NOT typically include the evaluation of A. soil resistivity. B. the interaction of voltages between different structures. C. measuring of current on the structure. D. the amount of CP current pick up on a coating holiday of a given size at the location. CP2 | Technician 30 Knowledge Check | 16 91 Recommended methods of mitigating cathodic interference from another cathodic protection system include which of the following? A. Drainage control bond B. Cathodic protection C. Coating the current discharge area D. Reduce source of stray current CP2 | Technician Knowledge Check | 17 92 Which location(s) indicates a stray current discharge on an interfered-with pipeline? Location Pipe-to-Electrolyte Potential (mVcse) Power Source Interrupting ON OFF A -1500 -900 Interfering B -1100 -900 Interfered-with C -1500 -900 D -700 -900 Interfering & Interfered-with synchronized Interfering A. A B. B C. C D. D CP2 | Technician Knowledge Check | 18 93 Which location(s) indicates a stray current pickup on an interfered-with pipeline? Location Pipe-to-Electrolyte Potential (mVcse) Power Source Interrupting ON OFF A -1500 -1000 Interfering B -1100 -900 Interfered-with C -1500 -900 D -800 -1000 Interfering & Interfered-with synchronized Interfering A. A B. B C. C D. D CP2 | Technician 31 Knowledge Check | 19 94 Where would be the best location to install a resistance bond to mitigate interference? Location Interfered-with Pipe-toelectrolyte Potential (mVcse) Interfering Pipe-toelectrolyte Potential (mVcse) Power Source Interrupting ON OFF ON OFF A -700 -820 -1050 -850 Interfering B -800 -700 -1100 -900 Interfered-with C -850 -750 -1500 -1100 D -700 -850 -650 -600 Interfering & Interfered-with synchronized Interfering A. A B. B C. C D. D CP2 | Technician Experiment 4.1 95 Experiment 4.1 Demonstration of Stray Current Interference – Instructor Demo CP2 | Technician Experiment 4.1 96 To Demonstrate DC Interference and Its Mitigation C 1 cm 1 cm steel rod B mag anode D E 9V 10 ohm F A A CP2 | Technician 32 Experiment 4.1 97 resistance bond A 3 Steel rod Mag anode 2 6 5 4 1 A 9V 10 ohm CP2 | Technician Experiment 4.1 – Table for Readings Step CP Structure-to-Electrolyte Potentials (mVCSE) or pH A B C Foreign Structure-to-Electrolyte Potentials (mVCSE) D E 98 ICCP Current (mA) F Step 3 Step 3 pH Step 5, 6 & 7 Step 7 pH Step 10 Step 10 pH Step 12 Step 14 Step 15 pH CP2 | Technician Experiment 4.1 – Conclusions 99 ▪ Stray current pickup on the foreign structure occurs at ______ (name all) ▪ Stray current discharge on the foreign structure occurs at ______ (name all) ▪ An increase in current on the CP structure occurs at ________ (name all) ▪ A decrease in current on the CP structure occurs at ________ (name all) ▪ Effect on potential of bonding the foreign structure to the CP structure ___________ ▪ Effect on current of bonding the foreign structure to the CP structure _____________ ▪ The effect of pH was ___________________________________ (at each location) CP2 | Technician 33 Chapter 4 Stray Current Interference ® 34 Chapter 4 | Exercise Handout Sheet Answer the right four (4) columns for each case where stray current interference is occurring. Write reasons for your decision for each case. Case Interfered-with Pipe-toElectrolyte Potential (mVcse) ON OFF Interfering Pipe-toElectrolyte Potential (mVcse) ON OFF DC Power Source Interrupting -600 -800 -1200 B -800 -600 -1500 -850 -750 -1500 -1200 -900 -850 -600 d Interfering -400 -900 -1200 -1200 Interferedwith -400 -400 -1400 C D E F NACE International Will Res Bond Mitigate? Target Potential for Bond Cathodic Anodic A -850 Interference Interfering -1500 Interferedwith Interfering & Interfered-1100 with synchronize -1000 Interfering CP2|Page 1 CP 2│CATHODIC PROTECTION TECHNICIAN Practical Exercises Data Sheets SET #1 Version: March2020 NACE International | CP2 Page | 1 NACE International | CP2 Page | 2 Station 1 RECTIFIER TROUBLESHOOTING (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 3 Practical Exercise Station 1 - Rectifier Troubleshooting There are a number of rectifiers set up at this station. Troubleshoot each rectifier to determine its fault. RECTIFIER STATION 1 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ TAP SETTINGS _________________________________ DC VOLTAGE ON PANEL METER ____________________________ DC AMPS ON PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ___________________________ DC mV ACROSS SHUNT ______________________________________ DC AMPS FROM SHUNT READING ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? ___________________________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? ___________________________________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? ____________________________________________________________________________________________ NACE International | CP2 Page | 4 RECTIFIER STATION 2 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ TAP SETTINGS _________________________________ DC VOLTAGE ON PANEL METER ____________________________ DC AMPS ON PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ___________________________ DC mV ACROSS SHUNT _______________________________________ DC AMPS FROM SHUNT READING ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? ___________________________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? ___________________________________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? ____________________________________________________________________________________________ NACE International | CP2 Page | 5 RECTIFIER STATION 3 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ TAP SETTINGS _________________________________ DC VOLTAGE FROM PANEL METER ____________________________ DC CURRENT FROM PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ______________ DC mV ACROSS SHUNT_______________________ DC CURRENT FROM SHUNT ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? ___________________________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? ___________________________________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? ____________________________________________________________________________________________ NACE International | CP2 Page | 6 RECTIFIER STATION 4 Confirm that there is no power to the rectifier. DIODE TEST FROM RECTIFIER PANEL Remove taps and one load connection Diode: Coarse Tap to Positive Forward _____ Reverse _____Status? _______ Diode: Fine Tap to Positive Forward _____ Reverse _____Status? _______ Diode: Negative to Coarse Tap Forward _____ Reverse _____Status? _______ Diode: Negative to Fine Tap Forward _____ Reverse _____Status? _______ WHICH DIODE NEEDS REPAIR AND HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? _____________________________________________________________________________________________ NACE International | CP2 Page | 7 NACE International | CP2 Page | 8 Station 2 IMPRESSED CURRENT INTERRUPTION (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 9 Practical Exercise Station 2 - Impressed Current Interruption Current interrupters are used to “interrupt” an electrical circuit in order to obtain “on “ and “interrupted” structure-to-electrolyte potential measurements. During this exercise you will practice installing the interrupter and obtaining measurements using the interrupter. Part A. Installing the current interrupter Turn off the rectifier using the rectifier circuit breaker. (Note in some jurisdictions this breaker will need to be locked out.) 1. Connect the rectifier negative terminal to the structure. 2. To install the interrupter in the anode cable as follows. 3. Connect one lead wire of the current interrupter to the anode cable and the other connection to the positive rectifier terminal. Watch for polarity on certain interrupters and if polarity is indicated, connect rectifier positive to interrupter positive. 4. Set the interrupter for the proper time you wish the interrupter to turn “on” and “off”. Part B. Test Procedure 1. Connect meter to measure a structure-to-electrolyte potential to the reference electrodes. 2. Take native structure-to-electrolyte potentials to reference electrodes at locations marked A, B, and C. 3. Turn on the interrupter. 4. Turn on the rectifier using the rectifier circuit breaker. 5. Confirm that the current output is being interrupted. 6. Determine the length of the ON cycle and the OFF cycle. 7. Take structure-to-electrolyte potential measurements at locations marked A, B, and C noting the length of time each reading is displayed and comparing it to the length of each cycle on the interrupter. 8. Determine the ON and OFF potentials by the length of time of the cycle. DO NOT ASSUME THE MOST ELECTRONEGATIVE POTENTIAL IS ALWAYS THE “ON” POTENTIAL. IF THE “ON” POTENTIAL IS LESS ELECTRONEGATIVE THAN THE “OFF”, THE RECTIFIER IS CONNECTED WITH REVERSE POLARITY. 9. If necessary, change the leads to achieve a more electronegative potential when the rectifier comes on. NACE International | CP2 Page | 10 Record data in the table below. Note: Potentials measured must be converted to the equivalent values of other reference electrodes as noted. Reference Electrode Location Native structure-toelectrolyte potential VZN Structure-toelectrolyte potential measured "ON" VCSE VZN VCSE Structure-toelectrolyte potential measured "INTERRUPTED" (INSTANT OFF) VZN Voltage (IR) Drop VCSE A B C Do the potential measurements meet a cathodic protection criterion? Yes No If so, which criterion was met? _____________________________ Which location is the conclusion based on? A ____ B ____ C ____ Why?_________________________________________________________________ Which reference electrode location is closest to the anode? A B C Which reference electrode location is closest to the structure? A B C NACE International | CP2 Page | 11 NACE International | CP2 Page | 12 Station 3 CURRENT REQUIREMENT TEST (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 13 Practical Exercise Station 3 - Current Requirement Test A temporary anode, a current interrupter and a power supply are used to apply current to the structure. The objective of the test is to determine by measurement and calculations the current required to achieve a -0.85 VoltCSE polarized potential on the structure. Part A. Measure and record native structure-to-electrolyte potentials at the locations identified or supplied by instructor. Part B. Install the current interrupter in the DC power supply. To install in the D.C. Circuit of a rectifier, turn off breaker or disconnect the D.C. fuse and connect the interrupter in the anode cable as in Part A of Station 2. 1. Connect the power supply negative to the structure. 2. Connect one side of the current interrupter to the anode cable and the other connection to the positive terminal of the DC power supply. 3. Set the interrupter for the proper time you wish the interrupter to turn “on” and “off” if not preset by instructor. 4. Turn on the interrupter. 5. Turn on the power source and confirm that it is being interrupted. 6. Determine the length of the ON and OFF cycle. Measure and record ON and OFF structure-to-electrolyte potentials at the location identified by noting the length of time for each cycle. Potentials must be converted to Volts and current to amperes in the following equations. Measure and record the power source current output. STRUCTURE-TO-ELECTROLYTE POTENTIAL (mVcse) Native NACE International | CP2 On Off POWER SOURCE CURRENT (mA) Page | 14 CALCULATE Ireq Ireq = _ Polarization Required x Itest Polarization Achieved During Test I req = Ireq = (−0.850V − E Nativ e ) * I test ( EOff − E Native ) ____ OR I req = Ireq = − 0.100V * I test (E Off − E Native ) ____ In this case, which criterion would be achieved first (at the least current)? ___________________________ NACE International | CP2 Page | 15 NACE International | CP2 Page | 16 Station 4 CATHODIC PROTECTION COMMISSIONING (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 17 Practical Exercise Station 4 - Cathodic Protection Commissioning Commissioning (energizing and adjusting) a new Cathodic Protection system is an important task and must be done properly. Adequate levels of cathodic protection must be maintained without causing overvoltage situations, stray current interference or premature failure of the anode system. Your task is to energize a new Cathodic Protection system however stray current interference will be demonstrated in a different station. Unfortunately, the construction crew that installed the Cathodic Protection system forgot to label which wire is the anode wire and which wire is the pipe wire but the test lead that would normally be at a nearby point is known to be on the pipeline. Identify which wire(s) is/are connected to the pipe and to the anode (explain why), connect the rectifier up properly, install an interrupter, take ON and OFF structure-to-electrolyte (S/E) potentials and adjust the output of the rectifier until the pipeline meets the –850 VCSE polarized potential criterion for steel. Start with rectifier on minimum output setting. STRUCTURE-TO-ELECTROLYTE POTENTIAL WIRE NO. Anode wires: _______________ _______ Pipe wires: _______ _______________ How did you come to the above conclusion? _________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ NACE International | CP2 Page | 18 Attempt No. Tap Setting Rect. Voltage Output Rect. Amperage Output S/E On S/E Off 1 2 3 4 5 At what tap setting and current output does the pipe meet a CP criterion and which criterion is met? Tap Setting:___________________________________________________________________ DC Current output:_____________________________________________________________ Which criterion was met and why do you say the pipe meets this criterion? _____________________________________________________________________________ _____________________________________________________________________________ NACE International | CP2 Page | 19 NACE International | CP2 Page | 20 Station 5 CURRENT SPAN (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 21 Practical Exercise Station 5 - Current Span Students are to become competent in conducting tests to calibrate pipeline current spans. Calibrating the current span is similar to establishing the shunt value or shunt factor in a current shunt. Once calibrated, the calibration factor can be used to determine the residual current flowing from CP current sources and/or stray current influences in the pipe span. PROCEDURE Part A. Test Span Calibration 1. Calibrate the current spans #1 and #2 by impressing the provided current from the CS-10 power source through the outside wires (top terminals) of the respective test station and measuring the voltage drop across the inside wires (bottom terminals) at each tests station. TEST CONNECTIONS + CS-10 mV - + Test Station Panel PIPE Note: There are two identical test heads. Test Station #1 represents Current Span #1. Test Station #2 represents Current Span #2. Span #1 and Span #2 are spaced along a segment of pipe and may be a mile or more apart from each other. The test circuit is essentially a series electrical circuit as shown in the following schematic drawing. NACE International | CP2 Page | 22 Controlled Power Supply Current Interrupter - + A ITEST CS-10 mV - + IRESIDUAL VTEST ITOTAL (IT) Resistor (RSPAN) 2. Test Station #1 Measurements (Note polarity and thus direction) ITEST1 ON (CS-10 ON) __________mA ITEST1 OFF (CS-10 OFF) __________mA ITEST1 = ITEST1 ON - ITEST1 OFF __________mA VTEST1 ON (CS-10 ON) __________mV VTEST1 OFF (CS-10 OFF) __________mV 3. Test Station #2 Measurements (Note polarity and thus direction) ITEST1 ON (CS-10 ON) __________mA ITEST1 OFF (CS-10 OFF) __________mA ITEST1 = ITEST1 ON - ITEST1 OFF __________mA VTEST1 ON (CS-10 ON) __________mV VTEST1 OFF (CS-10 OFF) __________mV 4. Pipe Span Resistance and Calibration Factor NACE International | CP2 Page | 23 Calculate the current span resistance [Ω] and the calibration factor (A/mV) for each current span. Note: potentials must be in Volts to calculate resistance and mV to calculate CF. RSPAN = (VTESTON − VTESTOFF ) ( I TEST ) CFspan = ( I TEST ) (mVTESTON − mVTESTOFF ) Test Station #1 R1span = _______ Ω CFspan = _______ A/mV Test Station #2 R2span = _______ Ω CFspan = _______ A/mV 5. Residual Current Was there a residual current on the pipe section; that is, was there a VTEST reading with no Test Current from CS-10? Yes? ______ No?________ If so, calculate the current. I RESIDUAL = (VTESTOFF ) ( RSPAN ) I RESIDUAL = mVOFF CFSPAN Test Station #1 Residual Current = ________A ________mA Test Station #2 Residual Current = ________ A ________mA NACE International | CP2 Page | 24 Station 6 COATING CONDUCTANCE (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 25 Practical Exercise Station 6 - Coating Conductance Students are to become competent in completing tests to measure and calculate the pipe coating resistance and conductance. The instructor will connect the station such that the pipe is receiving an interrupted supply of current from an anode. The instructor will give the following information: RSPAN1 = _____________ Ω or CFSPAN1 = _____________ A/mV RSPAN2 = _____________ Ω or CFSPAN2 = _____________ A/mV Pipe Diameter _____________ m Pipe Length _____________ m Student: □ □ 1. Confirm that the power supply is on and interrupting. Confirm that the switch behind Test Station #1 is in the “on” position as it controls the structure-to-electrolyte potentials Test Connections Test Station #1 + + mV V or mV V or mV mV - Test Station #2 - - Reference Electrode + - + Reference Electrode Panel Top PIPE PIPE The test is intended to determine the pipe to earth resistance through the coating (Rc) between the test stations as represented by the darker colored pipe. NACE International | CP2 Page | 26 This can be represented by an electrical circuit as shown below. - mVp1 + - Rp1 mVp2 + Rp2 Ip1 Ip2 Rc r r r r ΔEave Ic The objective is to calculate Rc by: RC = ∆E AVE IC 2. Test Station #1 Measurements mVp1 ON __________mV (Pipe Span with current ON) mVp1 OFF __________mV (Pipe Span with Current OFF) E1ON __________mV (ON Structure-to-electrolyte Potential) E1OFF __________mV (OFF Structure-to-electrolyte Potential) 3. Test Station #2 Measurements mVp2 ON __________mV (Pipe Span with current ON) mVp2 OFF __________mV (Pipe Span with Current OFF) E2ON __________mV (ON Structure-to-electrolyte Potential) E2OFF __________mV (OFF Structure-to-electrolyte Potential) NACE International | CP2 Page | 27 4. Calculate Current in Each Pipe Span Equation Summary IC ΔIP1 = - ΔIP2 IP1ON ΔIP1 = IP1ON - IP1OFF IP2ON ΔIP2 = IP2ON - IP2OFF IP1OFF IP2OFF V (in Volts) = mV / 1000 = VP1ON / RP1 = VP2ON / RP2 IP1ON = VP1OFF / RP1 IP1OFF = VP2OFF / RP2 IP2OFF OR IP2ON = mVP1ON x CF1 = mVP2ON x CF2 = mVP1OFF x CF1 = mVP2OFF X CF2 mV = V x 1000 Test Station #1 IP1 ON __________A IP1 OFF __________A ΔIP1 __________A Test Station #2 IP2 ON __________A IP2 OFF __________A ΔIP2 __________A 5. Current Pick Up in Coating Section (IC) IC = ΔIP1 - ΔIP2 IC = __________A 6. Average Potential Across Coating Due to Current Applied ∆E1 = E1ON – E1OFF. ∆E2 = E2ON – E2OFF. ∆E1 = _______ mV ∆E2 = _______ mV Calculate the average ∆E [∆Eave =(∆E1 + ∆E2 )/ 2]. Average ∆Eave ________ mV NACE International | CP2 Page | 28 7. Pipe Coating Resistance Calculate the total resistance of the pipe section to earth between test stations. RC = ∆E AVE IC Total pipe coating resistance (Rc) = ____________ Ω 8. Pipe Surface Area Calculate surface area of pipe based on diameter and length of pipe given by: A = πdL Where: A - Surface area of pipe D – Outside Pipe Diameter L – Length of coated pipe section π – 3.1416 Pipe Diameter = ____________ m (given) Pipe Length = ____________ m (given) Surface area = ____________ m2 9. Specific Coating Resistance Calculate the specific unit coating resistance (rc )in Ω-m2 . rC = Rc * A Specific unit coating resistance (rc) = ____________ Ω-m2 10. Specific Coating Conductance Calculate the specific unit coating conductance (gc ) in S/m2 . gC = 1 rC Specific unit coating conductance (gc) = ____________ S/m2 NACE International | CP2 Page | 29 NACE International | CP2 Page | 30 Station 7 MITIGATION OF DC INTERFERENCE USING A RESISTANCE BOND (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 31 Practical Exercise Station 7 - Mitigation of DC Interference Using a Resistance Bond This exercise will demonstrate DC stray current interference and how it can be mitigated using a resistance bond. The test station is shown in the photograph above with instructions for each part of the exercise as follows: Part A 1. Measure the AS FOUND potential of Line 1 and Line 2 at each of the locations. Location A Connected to (VCSE) Line 1 B Line 1 C Line 1 B Line 2 D Line 2 E Line 2 A/F Potential (VCSE) Part B 1. Connect the impressed current system to Line 2. 2. Measure the potential of Line 1 and Line 2 while interrupting the impressed current CP System. Location Connected to A Line 1 B Line 1 C Line 1 B Line 2 D Line 2 E Line 2 NACE International | CP2 ON Potential (VCSE) OFF Potential (VCSE) Page | 32 Part C 1. What is the location of maximum exposure to interference on the line not connected to the interrupting source? ________________________________ 2. Starting at the highest resistance value available, install different resistors between Line 1 and Line 2 at the point of maximum exposure. 3. Record the potentials at the location of maximum exposure and the value of the resistance in the table below. 4. Repeat using lower resistance values until there is no longer a stray current interference situation (“on” potential of the interfered-with structure has been returned to its original value). 5. Once the ideal resistance is obtained (potential returned to original value before interference current) at the crossing, measure the voltage drop across this fixed resistance and record in the table below 6. Calculate the interference control current drain. 7. Turn off and disconnect the apparatus. Control Resistance Ohms Potential Line 1 at maximum exposure ON OFF Potential Line 2 at maximum exposure ON OFF Voltage Across Bond Resistor Interference Control Current R= R= R= R= Current in resistance bond with DC interference mitigated: ______ mA NACE International | CP2 Page | 33 NACE International | CP2 Page | 34 Station 8 MEASUREMENTS IN ELECTRICAL CIRCUITS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 35 Practical Exercise Station 8 – Measurements in Electrical Circuits This exercise gives students practice connecting resistors in both series and parallel circuits. Using the resistor board provided, follow the instructions noted in Part A and Part B of this exercise. CAUTION: Remember to: — measure resistance with the power off — return the meter from Resistance to the DC Volts setting and — turn the power back on after the connections are complete. Part A: Parallel Circuit 1. Measure and record the resistance of Resistors A, B, C and D. 2. Measure source voltage (before connecting resistors): ___________________ 3. Connect resistors A, B, C and D in parallel and complete the following table using Ohm’s Law where necessary. Include polarity, numerical value and units when recording the measurement. 4. Explain difference (if any) with source voltage after connecting resistors: ____________________________________________________________________________________ ____________________________________________________________________________________ RESISTOR RESISTANCE BATTERY EMF (VOLTS) VOLTAGE DROP (VOLTS) CURRENT (AMPERES) A B C D TOTAL NACE International | CP2 Page | 36 5. Calculate the total resistance of the circuit. (Show your work.) RESISTANCE (OHMS) 6. Which of Kirchoff’s Laws applies to this circuit? Kirchoff’s Law Voltage Current NACE International | CP2 Page | 37 Practical Exercise Station 8 - Measurements in Electrical Circuits Part B: Series Circuit 1. Measure and record the resistance of Resistors A, B, C and D. 2. Connect resistors A, B, C and D in series 3. Measure voltage across resistors 4. Complete the following table using Ohm’s Law where necessary. Include polarity, numerical value and units when recording the measurement. 5. Measure source voltage. RESISTOR RESISTANCE BATTERY EMF (VOLTS) VOLTAGE DROP (VOLTS) CURRENT (AMPERES) A B C D TOTAL 5. Calculate the total resistance of the circuit. (Show your work.) RESISTANCE (OHMS) 7. Which of Kirchoff’s Laws applies to this circuit? Kirchoff’s Law Voltage Current NACE International | CP2 Page | 38 Station 9 UNDERGROUND STORAGE TANK (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 39 Practical Exercise Station 9 - Underground Storage Tank This test station simulates conditions for an underground fuel storage tank installation. You will obtain the measurements indicated in the table that follows and record data obtained. PART A Continuity Test (Fixed Cell Reference Electrode Position) Structure Being Tested Tank Fill Pipe Product Pipe Vent Pipe Station Ground Survey Comments Fill Pipe Product Pipe Vent Pipe Station Ground Survey Comments Potentials PART B Potential Survey Structure Being Tested Reference Electrode Location Product end of tank Tank ON OFF ON OFF ON OFF ON OFF ON OFF Center Vent end of tank At product riser At vent riser At station ground CONCLUSIONS ON TEST RESULTS 1. What structures are electrically continuous? ___________________________________________________ 2. What structures meet a CP (NACE) criterion? __________________________________________________ NACE International | CP2 Page | 40 Station 10 CALCULATION STATION (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 41 Practical Exercise Station 10 – Calculation Station 1. Make the following conversions -0.050 VSCE to VSSC +0.750 VZn to VSCE +0.075 VSSC to VCSE 2. What is the efficiency of a rectifier operating as follows: DC output of 20 Volts 10 Amperes KWH meter revolutions of 4 per minute with a factor of Kh = 2. NACE International | CP2 Page | 42 Station 11 ROAD CASING AND CARRIER PIPE MEASUREMENTS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 43 Practical Exercise Station 11 – Road Casing and Carrier Pipe Measurements OBJECTIVE The object of this station is to demonstrate the difference in an electronic and a electrolytic shorted road casing and to demonstrate the fact that the pipeline inside a shorted casing is not receiving CP even though the potential outside the casing meets criterion. PROCEDURE 1. Student to measure the on/off potential of pipe and casing with the reference outside of each casing and decide if it is shorted. 2. Student to measure the on/off pipe and casing potentials to a reference inside each casing and decide if the pipe meets the criterion. Test Wire #1 CASING A Reference Electrode Outside Casing On Off Reference Electrode Inside Casing On Off CASING B Reference Electrode Outside Casing On Off Reference Electrode Inside Casing On Off #2 Test Wire #3 #4 1. What is the casing status? a. Casing A i. Electronic short ii. Electrolytic couple b. Casing B i. Electronic short ii. Electrolytic couple NACE International | CP2 Yes No Yes No Yes No Yes No Page | 44 2. Does the carrier pipe inside the casing meet the polarized potential criterion? a. Casing A i. Meets criterion Yes N No b. Casing B i. Meets criterion NACE International | CP2 Yes No Page | 45 NACE International | CP2 Page | 46 Station 12 SOIL RESISTIVITY MEASUREMENTS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 47 Practical Exercise Station 12 - Soil Resistivity Measurements During this exercise, you will measure resistance and calculate resistivity using the 4-pin Wenner method and then calculate layer resistivity using the Barnes Layer formula. Part A 1. Using the equipment provided, connect the 4 wires from the meter to the station electrodes at the spacing in the table below. 2. Enter the results and calculate the resistivity based on a pin depth of 0.1 cm. Pin Spacing (cm) Resistance (Ohms) Resistivity (Ohm-cm) 2.5 _____________ _____________ 5.0 _____________ _____________ 7.5 ___________ ___________ Part B Using the Barnes Layer formula and the following data, calculate the resistivity of each layer. Pin Spacing (m) Resistance (Ohms) 1.00 25.0 2.00 7.2 3.00 4.6 Resistivity (Ohm-cm) Using the Barnes Layer formula and the data above, calculate the resistivity of each layer in the table below. Layer (cm) Layer Resistance (Ohms) Layer Resistivity (Ohm-cm) 0 to 100 100 to 200 200 to 300 NACE International | CP2 Page | 48 Note: Care should be taken in the resolution of the answers. The significant number of digits in the data determines the significant number of digits in the answer. Basic rules: All non-zero digits are significant: 1, 2, 3, 4, 5, 6, 7, 8, 9. • Zeros between non-zero digits are significant: 102, 3004, 50006. • Leading zeros are never significant: 0.01, 002.345, 0.000678. • In a number with or without a decimal point, trailing zeros (those to the right of the last non-zero digit) are significant provided they are justified by the precision of the data: 345,000; 6.0600; 7.8000; 13.5700. NACE International | CP2 Page | 49 NACE International | CP2 Page | 50 Station 13 POLARITY BOARD (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 51 Practical Exercise Station 13 – Polarity Board 1. Measure and record the potential differences between each pair of banana jacks listed below. Record magnitude, polarity and units for each measurement using the second color in each case as the reference. 2. After obtaining and recording the potential differences, use the data to order the colored banana jacks by most negative polarity to most positive polarity. PAIR POTENTIAL White to Blue Red to Yellow Green to Red Blue to Yellow Order the banana jacks by color from most negative to least negative. Colored Banana Jack MOST NEGATIVE LEAST NEGATIVE NACE International | CP2 Page | 52 CP 2│CATHODIC PROTECTION TECHNICIAN Practical Exercises Data Sheets SET #2 Version: March2020 NACE International | CP2 Page | 1 NACE International | CP2 Page | 2 Station 1 RECTIFIER TROUBLESHOOTING (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 3 Practical Exercise Station 1 - Rectifier Troubleshooting There are a number of rectifiers set up at this station. Troubleshoot each rectifier to determine its fault. RECTIFIER STATION 1 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ IDENTIFY COMPONENTS LABELED A ________________________ IDENTIFY COMPONENTS LABELED B ________________________ TAP SETTINGS _________________________________ DC VOLTAGE ON PANEL METER ____________________________ DC AMPS ON PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ___________________________ DC mV ACROSS SHUNT ______________________________________ DC AMPS FROM SHUNT READING ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? __________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? __________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? ____________________________________________________________________________ NACE International | CP2 Page | 4 RECTIFIER STATION 2 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ IDENTIFY COMPONENTS LABELED C __________________________ IDENTIFY COMPONENTS LABELED D __________________________ TAP SETTINGS _________________________________ DC VOLTAGE ON PANEL METER ____________________________ DC AMPS ON PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ___________________________ DC mV ACROSS SHUNT _______________________________________ DC AMPS FROM SHUNT READING ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? __________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? __________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? __________________________________________________________________ NACE International | CP2 Page | 5 RECTIFIER STATION 3 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ TAP SETTINGS _________________________________ DC VOLTAGE FROM PANEL METER ____________________________ DC CURRENT FROM PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ______________ DC mV ACROSS SHUNT_______________________ DC CURRENT FROM SHUNT ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? __________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? __________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? __________________________________________________________________ NACE International | CP2 Page | 6 RECTIFIER STATION 4 Confirm that there is no power to the rectifier. DIODE TEST FROM RECTIFIER PANEL Remove taps and one load connection Diode: Coarse Tap to Positive Forward _____ Reverse _____Status? _______ Diode: Fine Tap to Positive Forward _____ Reverse _____Status? _______ Diode: Negative to Coarse Tap Forward _____ Reverse _____Status? _______ Diode: Negative to Fine Tap Forward _____ Reverse _____Status? _______ WHICH DIODE NEEDS REPAIR AND HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? _____________________________________________________________________________________________ NACE International | CP2 Page | 7 NACE International | CP2 Page | 8 Station 2 IMPRESSED CURRENT INTERRUPTION (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 9 Practical Exercise Station 2 - Impressed Current Interruption Current interrupters are used to “interrupt” an electrical circuit in order to obtain “on “ and “interrupted” structure-to-electrolyte potential measurements. During this exercise you will practice installing the interrupter and obtaining measurements using the interrupter. Part A. Installing the current interrupter Turn off the rectifier using the rectifier circuit breaker. (Note in some jurisdictions this breaker will need to be locked out.) 1. Connect the rectifier negative terminal to the structure. 2. To install the interrupter in the anode cable as follows. 3. Connect one lead wire of the current interrupter to the anode cable and the other connection to the positive rectifier terminal. Watch for polarity on certain interrupters and if polarity is indicated, connect rectifier positive to interrupter positive. 4. Set the interrupter for the proper time you wish the interrupter to turn “on” and “off”. Part B. Test Procedure 1. Connect meter to measure a structure-to-electrolyte potential to the reference electrodes. 2. Take native structure-to-electrolyte potentials to reference electrodes at locations marked A, B, and C. 3. Turn on the interrupter. 4. Turn on the rectifier using the rectifier circuit breaker. 5. Confirm that the current output is being interrupted. 6. Determine the length of the ON cycle and the OFF cycle. 7. Take structure-to-electrolyte potential measurements at locations marked A, B, and C noting the length of time each reading is displayed and comparing it to the length of each cycle on the interrupter. 8. Determine the ON and OFF potentials by the length of time of the cycle. DO NOT ASSUME THE MOST ELECTRONEGATIVE POTENTIAL IS ALWAYS THE “ON” POTENTIAL. IF THE “ON” POTENTIAL IS LESS ELECTRONEGATIVE THAN THE “OFF”, THE RECTIFIER IS CONNECTED WITH REVERSE POLARITY. 9. If necessary, change the leads to achieve a more electronegative potential when the rectifier comes on. NACE International | CP2 Page | 10 Record data in the table below. Note: Potentials measured must be converted to the equivalent values of other reference electrodes as noted. Reference Electrode Location Native structure-toelectrolyte potential VZN Structure-toelectrolyte potential measured "ON" VCSE VZN VCSE Structure-toelectrolyte potential measured "INTERRUPTED" (INSTANT OFF) VZN Voltage (IR) Drop VCSE A B C Do the potential measurements meet a cathodic protection criterion? Yes No If so, which criterion was met? _____________________________ Which location is the conclusion based on? A ____ B ____ C ____ Why?_________________________________________________________________ Which reference electrode location is closest to the anode? A B C Which reference electrode location is closest to the structure? A B C NACE International | CP2 Page | 11 NACE International | CP2 Page | 12 Station 3 CURRENT REQUIREMENT TEST (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 13 Practical Exercise Station 3 - Current Requirement Test A temporary anode, a current interrupter and a power supply are used to apply current to the structure. The objective of the test is to determine by measurement and calculations the current required to achieve a -0.85 VoltCSE polarized potential on the structure. Part A. Measure and record native structure-to-electrolyte potentials at the locations identified or supplied by instructor. Part B. Install the current interrupter in the DC power supply. To install in the D.C. Circuit of a rectifier, turn off breaker or disconnect the D.C. fuse and connect the interrupter in the anode cable as in Part A of Station 2. 1. Connect the power supply negative to the structure. 2. Connect one side of the current interrupter to the anode cable and the other connection to the positive terminal of the DC power supply. 3. Set the interrupter for the proper time you wish the interrupter to turn “on” and “off” if not preset by instructor. 4. Turn on the interrupter. 5. Turn on the power source and confirm that it is being interrupted. 6. Determine the length of the ON and OFF cycle. Measure and record ON and OFF structure-to-electrolyte potentials at the location identified by noting the length of time for each cycle. Potentials must be converted to Volts and current to amperes in the following equations. NACE International | CP2 Page | 14 Measure and record the power source current output. STRUCTURE-TO-ELECTROLYTE POTENTIAL (mVcse) Native On Off POWER SOURCE CURRENT (mA) CALCULATE Ireq Ireq = _ Polarization Required x Itest Polarization Achieved During Test I req = Ireq = (−0.850V − E Nativ e ) * I test ( EOff − E Native ) ____ OR I req = Ireq = − 0.100V * I test (E Off − E Native ) ____ In this case, which criterion would be achieved first (at the least current)? ___________________________ NACE International | CP2 Page | 15 NACE International | CP2 Page | 16 Station 4 CATHODIC PROTECTION COMMISSIONING (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 17 Practical Exercise Station 4 - Cathodic Protection Commissioning Commissioning (energizing and adjusting) a new Cathodic Protection system is an important task and must be done properly. Adequate levels of cathodic protection must be maintained without causing overvoltage situations, stray current interference or premature failure of the anode system. Your task is to energize a new Cathodic Protection system however stray current interference will be demonstrated in a different station. Unfortunately, the construction crew that installed the Cathodic Protection system forgot to label which wire is the anode wire and which wire is the pipe wire but the test lead that would normally be at a nearby point is known to be on the pipeline. Identify which wire(s) is/are connected to the pipe and to the anode (explain why), connect the rectifier up properly, install an interrupter, take ON and OFF structure-to-electrolyte (S/E) potentials and adjust the output of the rectifier until the pipeline meets the –850 VCSE polarized potential criterion for steel. Start with rectifier on minimum output setting. STRUCTURE-TO-ELECTROLYTE POTENTIAL WIRE NO. Anode wires: _______________ _______ Pipe wires: _______ _______________ How did you come to the above conclusion? _________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ NACE International | CP2 Page | 18 Attempt No. Tap Setting Rect. Voltage Output Rect. Amperage Output S/E On S/E Off 1 2 3 4 5 At what tap setting and current output does the pipe meet a CP criterion and which criterion is met? Tap Setting: ___________________________________________________________________ DC Current output: _____________________________________________________________ Which criterion was met and why do you say the pipe meets this criterion? _____________________________________________________________________________ _____________________________________________________________________________ NACE International | CP2 Page | 19 NACE International | CP2 Page | 20 Station 5 CURRENT SPAN (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 21 Practical Exercise Station 5 - Current Span Students are to become competent in conducting tests to calibrate pipeline current spans. Calibrating the current span is similar to establishing the shunt value or shunt factor in a current shunt. Once calibrated, the calibration factor can be used to determine the residual current flowing from CP current sources and/or stray current influences in the pipe span. PROCEDURE Part A. Test Span Calibration 1. Calibrate the current spans #1 and #2 by impressing the provided current from the CS-10 power source through the outside wires (top terminals) of the respective test station and measuring the voltage drop across the inside wires (bottom terminals) at each tests station. TEST CONNECTIONS + CS-10 mV - + Test Station Panel PIPE Note: There are two identical test heads. Test Station #1 represents Current Span #1. Test Station #2 represents Current Span #2. Span #1 and Span #2 are spaced along a segment of pipe and may be a mile or more apart from each other. The test circuit is essentially a series electrical circuit as shown in the following schematic drawing. NACE International | CP2 Page | 22 Controlled Power Supply Current Interrupter - + A ITEST CS-10 mV - + IRESIDUAL VTEST ITOTAL (IT) Resistor (RSPAN) 2. Test Station #1 Measurements (Note polarity and thus direction) ITEST1 ON (CS-10 ON) __________mA ITEST1 OFF (CS-10 OFF) __________mA ITEST1 = ITEST1 ON - ITEST1 OFF __________mA VTEST1 ON (CS-10 ON) __________mV VTEST1 OFF (CS-10 OFF) __________mV 3. Test Station #2 Measurements (Note polarity and thus direction) ITEST1 ON (CS-10 ON) __________mA ITEST1 OFF (CS-10 OFF) __________mA ITEST1 = ITEST1 ON - ITEST1 OFF __________mA VTEST1 ON (CS-10 ON) __________mV VTEST1 OFF (CS-10 OFF) __________mV 4. Pipe Span Resistance and Calibration Factor NACE International | CP2 Page | 23 Calculate the current span resistance [Ω] and the calibration factor (A/mV) for each current span. Note: potentials must be in Volts to calculate resistance and mV to calculate CF. RSPAN = (VTESTON − VTESTOFF ) ( I TEST ) CFspan = ( I TEST ) (mVTESTON − mVTESTOFF ) Test Station #1 R1span = _______ Ω CFspan = _______ A/mV Test Station #2 R2span = _______ Ω CFspan = _______ A/mV 5. Residual Current Was there a residual current on the pipe section; that is, was there a VTEST reading with no Test Current from CS-10? Yes? ______ No?________ If so, calculate the current. I RESIDUAL = (VTESTOFF ) ( RSPAN ) I RESIDUAL = mVOFF CFSPAN Test Station #1 Residual Current = ________A ________mA Test Station #2 Residual Current = ________ A ________mA NACE International | CP2 Page | 24 Station 6 COATING CONDUCTANCE (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 25 Practical Exercise Station 6 - Coating Conductance Students are to become competent in completing tests to measure and calculate the pipe coating resistance and conductance. The instructor will connect the station such that the pipe is receiving an interrupted supply of current from an anode. The instructor will give the following information: RSPAN1 = _____________ Ω or CFSPAN1 = _____________ A/mV RSPAN2 = _____________ Ω or CFSPAN2 = _____________ A/mV Pipe Diameter _____________ m Pipe Length _____________ m Student: □ □ 1. Confirm that the power supply is on and interrupting. Confirm that the switch behind Test Station #1 is in the “on” position as it controls the structure-to-electrolyte potentials. Test Connections Test Station #1 + + mV V or mV V or mV mV - Test Station #2 - - Reference Electrode + - + Reference Electrode Panel Top PIPE PIPE The test is intended to determine the pipe to earth resistance through the coating (Rc) between the test stations as represented by the darker colored pipe. NACE International | CP2 Page | 26 This can be represented by an electrical circuit as shown below. - mVp1 + mVp2 - Rp1 + Rp2 Ip1 Ip2 Rc r r r r ΔEave Ic The objective is to calculate Rc by: RC = ∆E AVE IC 2. Test Station #1 Measurements mVp1 ON __________mV (Pipe Span with current ON) mVp1 OFF __________mV (Pipe Span with Current OFF) E1ON __________mV (ON Structure-to-electrolyte Potential) E1OFF __________mV (OFF Structure-to-electrolyte Potential) 3. Test Station #2 Measurements mVp2 ON __________mV (Pipe Span with current ON) mVp2 OFF __________mV (Pipe Span with Current OFF) E2ON __________mV (ON Structure-to-electrolyte Potential) E2OFF __________mV (OFF Structure-to-electrolyte Potential) 4. Calculate Current in Each Pipe Span Equation Summary IC = ΔIP1 - ΔIP2 IP1ON = VP1ON / RP1 = mVP1ON x CF1 x CF2 ΔIP1 = IP1ON - IP1OFF IP2ON = VP2ON / RP2 IP2ON = mVP2ON ΔIP2 = IP2ON - IP2OFF IP1OFF = VP1OFF / RP1 IP1OFF = mVP1OFF x CF1 IP2OFF = VP2OFF / RP2 IP2OFF = mVP2OFF X CF2 V (in Volts) = mV / 1000 NACE International | CP2 IP1ON OR mV = V x 1000 Page | 27 Test Station #1 IP1 ON __________A IP1 OFF __________A ΔIP1 __________A Test Station #2 IP2 ON __________A IP2 OFF __________A ΔIP2 __________A 5. Current Pick Up in Coating Section (IC) IC = ΔIP1 - ΔIP2 IC = __________A 6. Average Potential Across Coating Due to Current Applied ∆E1 = E1ON – E1OFF. ∆E2 = E2ON – E2OFF. ∆E1 = _______ mV ∆E2 = _______ mV Calculate the average ∆E [∆Eave =(∆E1 + ∆E2 )/ 2]. Average ∆Eave ________ mV 7. Pipe Coating Resistance Calculate the total resistance of the pipe section to earth between test stations. RC = ∆E AVE IC Total pipe coating resistance (Rc) = ____________ Ω NACE International | CP2 Page | 28 8. Pipe Surface Area Calculate surface area of pipe based on diameter and length of pipe given by: A = πdL Where: A - Surface area of pipe D – Outside Pipe Diameter L – Length of coated pipe section π – 3.1416 Pipe Diameter = ____________ m (given) Pipe Length = ____________ m (given) Surface area = ____________ m2 9. Specific Coating Resistance Calculate the specific unit coating resistance (rc )in Ω-m2 . rC = Rc * A Specific unit coating resistance (rc) = ____________ Ω-m2 10. Specific Coating Conductance Calculate the specific unit coating conductance (gc ) in S/m2 . gC = 1 rC Specific unit coating conductance (gc) = ____________ S/m2 NACE International | CP2 Page | 29 NACE International | CP2 Page | 30 Station 7 MITIGATION OF DC INTERFERENCE USING A RESISTANCE BOND (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 31 Practical Exercise Station 7 - Mitigation of DC Interference Using a Resistance Bond This exercise will demonstrate DC stray current interference and how it can be mitigated using a resistance bond. The test station is shown in the photograph above with instructions for each part of the exercise as follows: Part A 1. Measure the AS FOUND potential of Line 1 and Line 2 at each of the locations. Location A Connected to (VCSE) Line 1 B Line 1 C Line 1 B Line 2 D Line 2 E Line 2 A/F Potential (VCSE) Part B 1. Connect the impressed current system to Line 2. 2. Measure the potential of Line 1 and Line 2 while interrupting the impressed current CP System. Location Connected to A Line 1 B Line 1 B Line 2 C Line 1 D Line 2 E Line 2 NACE International | CP2 ON Potential (VCSE) OFF Potential (VCSE) Page | 32 Part C 1. What is the location of maximum exposure to interference on the line not connected to the interrupting source? ________________________________ 2. Starting at the highest resistance value available, install different resistors between Line 1 and Line 2 at the point of maximum exposure. 3. Record the potentials at the location of maximum exposure and the value of the resistance in the table below. 4. Repeat using lower resistance values until there is no longer a stray current interference situation (“on” potential of the interfered-with structure has been returned to its original value). 5. Once the ideal resistance is obtained (potential returned to original value before interference current) at the crossing, measure the voltage drop across this fixed resistance and record in the table below 6. Calculate the interference control current drain. 7. Turn off and disconnect the apparatus. Control Resistance Ohms Potential Line 1 at maximum exposure ON OFF Potential Line 2 at maximum exposure ON OFF Voltage Across Bond Resistor Interference Control Current R= R= R= R= Current in resistance bond with DC interference mitigated: ______ mA NACE International | CP2 Page | 33 NACE International | CP2 Page | 34 Station 8 MEASUREMENTS IN ELECTRICAL CIRCUITS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 35 Practical Exercise Station 8 – Measurements in Electrical Circuits This exercise gives students practice connecting resistors in both series and parallel circuits. Using the resistor board provided, follow the instructions noted in Part A and Part B of this exercise. CAUTION: Remember to: — measure resistance with the power off — return the meter from Resistance to the DC Volts setting and — turn the power back on after the connections are complete. Part A: Parallel Circuit 1. Measure and record the resistance of Resistors A, B, C and D. 2. Measure source voltage (before connecting resistors): ___________________ 3. Connect resistors A, B, C and D in parallel and complete the following table using Ohm’s Law where necessary. Include polarity, numerical value and units when recording the measurement.. 4. Explain difference (if any) with source voltage after connecting resistors. ____________________________________________________________________________________ ____________________________________________________________________________________ RESISTOR RESISTANCE BATTERY EMF (VOLTS) VOLTAGE DROP (VOLTS) CURRENT (AMPERES) A B C D TOTAL NACE International | CP2 Page | 36 5. Calculate the total resistance of the circuit. (Show your work.) RESISTANCE (OHMS) 6. Which of Kirchoff’s Laws applies to this circuit? Kirchoff’s Law Voltage Current NACE International | CP2 Page | 37 Practical Exercise Station 8 - Measurements in Electrical Circuits Part B: Series Circuit 1. Measure and record the resistance of Resistors A, B, C and D. 2. Connect resistors A, B, C and D in series 3. Measure voltage across resistors 4. Complete the following table using Ohm’s Law where necessary. Include polarity, numerical value and units when recording the measurement. 5. Measure source voltage. RESISTOR RESISTANCE BATTERY EMF (VOLTS) VOLTAGE DROP (VOLTS) CURRENT (AMPERES) A B C D TOTAL 5. Calculate the total resistance of the circuit. (Show your work.) RESISTANCE (OHMS) 7. Which of Kirchoff’s Laws applies to this circuit? Kirchoff’s Law Voltage Current NACE International | CP2 Page | 38 Station 9 UNDERGROUND STORAGE TANK (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 39 Practical Exercise Station 9 - Underground Storage Tank This test station simulates conditions for an underground fuel storage tank installation. You will obtain the measurements indicated in the table that follows and record data obtained. PART A Continuity Test (Fixed Cell Reference Electrode Position) Structure Being Tested Tank Fill Pipe Product Pipe Vent Pipe Station Ground Survey Comments Fill Pipe Product Pipe Vent Pipe Station Ground Survey Comments Potentials PART B Potential Survey Structure Being Tested Reference Electrode Location Product end of tank Tank ON OFF ON OFF ON OFF ON OFF ON OFF Center Vent end of tank At product riser At vent riser At station ground CONCLUSIONS ON TEST RESULTS 1. What structures are electrically continuous? ___________________________________________________ 2. What structures meet a CP (NACE) criterion? __________________________________________________ NACE International | CP2 Page | 40 Station 10 CALCULATION STATION (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 41 Practical Exercise Station 10 – Calculation Station Et R1 R3 R2 NACE International | CP2 R1 = 4 Ohms I2 = 2 Amperes I3 = 1 Amperes Et = 20 Volts It = ____ Amperes Rt = ____ Ohms I1 = _____ Amperes V1 = ___ Volts V2 = ___ Volts V3 = ___ Volts R2 = ____ Ohms R3 = ____ Ohms Page | 42 Station 11 ROAD CASING AND CARRIER PIPE MEASUREMENTS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 43 Practical Exercise Station 11 – Road Casing and Carrier Pipe Measurements OBJECTIVE The object of this station is to demonstrate the difference in an electronic and a electrolytic shorted road casing and to demonstrate the fact that the pipeline inside a shorted casing is not receiving CP even though the potential outside the casing meets criterion. PROCEDURE 1. Student to measure the on/off potential of pipe and casing with the reference outside of each casing and decide if it is shorted. 2. Student to measure the on/off pipe and casing potentials to a reference inside each casing and decide if the pipe meets the criterion. Test Wire #1 CASING A Reference Electrode Outside Casing On Off Reference Electrode Inside Casing On Off CASING B Reference Electrode Outside Casing On Off Reference Electrode Inside Casing On Off #2 Test Wire #3 #4 NACE International | CP2 Page | 44 1. What is the casing status? a. Casing A i. Electronic short ii. Electrolytic couple b. Casing B i. Electronic short ii. Electrolytic couple Yes No Yes No Yes No Yes No 2. Does the carrier pipe inside the casing meet the polarized potential criterion? a. Casing A i. Meets criterion Yes N No b. Casing B i. Meets criterion NACE International | CP2 Yes No Page | 45 NACE International | CP2 Page | 46 Station 12 SOIL RESISTIVITY MEASUREMENTS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 47 Practical Exercise Station 12 - Soil Resistivity Measurements During this exercise, you will measure resistance and calculate resistivity using the 4-pin Wenner method and then calculate layer resistivity using the Barnes Layer formula. Part A 1. Using the equipment provided, connect the 4 wires from the meter to the station electrodes at the spacing in the table below. 2. Enter the results and calculate the resistivity based on a pin depth of 0.4 cm. Pin Spacing (cm) Resistance (ohms) Resistivity (Ohm-cm) 2.5 5.0 7.5 Part B Using the data provided, calculate the resistivity Pin Spacing (m) Resistance (Ohms) 1.60 10.0 3.20 6.8 5.60 3.2 Resistivity (Ohm-cm) Using the Barnes Layer formula and the data above, calculate the resistivity of each layer in the table below. Layer (cm) Layer Resistance (Ohms) Layer Resistivity (Ohm-cm) 0 to 160 160 to 320 320 to 560 NACE International | CP2 Page | 48 Note: Care should be taken in the resolution of the answers. The significant number of digits in the data determines the significant number of digits in the answer. Basic rules: All non-zero digits are significant: 1, 2, 3, 4, 5, 6, 7, 8, 9. • Zeros between non-zero digits are significant: 102, 3004, 50006. • Leading zeros are never significant: 0.01, 002.345, 0.000678. In a number with or without a decimal point, trailing zeros (those to the right of the last non-zero digit) are significant provided they are justified by the precision of the data: 345,000; 6.0600; 7.8000; 13.5700. NACE International | CP2 Page | 49 NACE International | CP2 Page | 50 Station 13 POLARITY BOARD (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 51 Practical Exercise Station 13 – Polarity Board 1. Measure and record the potential differences between each pair of banana jacks listed below. Record magnitude, polarity and units for each measurement using the second color in each case as the reference. 2. After obtaining and recording the potential differences, use the data to order the colored banana jacks by most negative polarity to most positive polarity. PAIR POTENTIAL White to Blue Red to Yellow Green to Red Blue to Yellow Order the banana jacks by color from most negative to least negative. Colored Banana Jack MOST NEGATIVE LEAST NEGATIVE NACE International | CP2 Page | 52 CP 2│CATHODIC PROTECTION TECHNICIAN Practical Exercises Data Sheets SET #3 Version: March2020 NACE International | CP2 Page | 1 NACE International | CP2 Page | 2 Station 1 RECTIFIER TROUBLESHOOTING (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 3 Practical Exercise Station 1 - Rectifier Troubleshooting RECTIFIER STATION 1 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ IDENTIFY COMPONENTS LABELED A ________________________ IDENTIFY COMPONENTS LABELED B ________________________ TAP SETTINGS _________________________________ DC VOLTAGE ON PANEL METER ____________________________ DC AMPS ON PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ___________________________ DC mV ACROSS SHUNT ______________________________________ DC AMPS FROM SHUNT READING ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? __________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? __________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? ____________________________________________________________________________ NACE International | CP2 Page | 4 RECTIFIER STATION 2 AC INPUT VOLTAGE ______________________________________ AC STEP-DOWN VOLTAGE _________________________________ IDENTIFY COMPONENTS LABELED C __________________________ IDENTIFY COMPONENTS LABELED D __________________________ TAP SETTINGS _________________________________ DC VOLTAGE ON PANEL METER ____________________________ DC AMPS ON PANEL METER ________________________________ DC VOLTAGE ON RECTIFIER OUTPUT TERMINAL USING MULTIMETER ___________________________ DC mV ACROSS SHUNT _______________________________________ DC AMPS FROM SHUNT READING ______________________________ CYCLES Secondary Taps ___________Hz DC Output ___________Hz WHAT PROBLEM DOES THIS RECTIFIER HAVE, IF ANY? __________________________________________________________________ HOW COULD THIS PROBLEM BE CORRECTED? __________________________________________________________________ HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? __________________________________________________________________ NACE International | CP2 Page | 5 RECTIFIER STATION 3 Confirm that there is no power to the rectifier. DIODE TEST FROM RECTIFIER PANEL Remove taps and one load connection Diode: Coarse Tap to Positive Forward _____ Reverse _____Status? _______ Diode: Fine Tap to Positive Forward _____ Reverse _____Status? _______ Diode: Negative to Coarse Tap Forward _____ Reverse _____Status? _______ Diode: Negative to Fine Tap Forward _____ Reverse _____Status? _______ WHICH DIODE NEEDS REPAIR AND HOW WOULD YOU DETERMINE CORRECT PARTS (SIZE, RATING, TYPE, ETC.) NEEDED FOR THE REPAIR? _________________________________________________________________________________ ____________ NACE International | CP2 Page | 6 Station 2 IMPRESSED CURRENT INTERRUPTION (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 7 Practical Exercise Station 2 - Impressed Current Interruption Record data in the table below. Potentials measured must be converted to the equivalent values of other reference electrodes as noted. Reference Electrode Location Native structure-toelectrolyte potential VZN VCSE Structure-toelectrolyte potential measured "ON" VZN VCSE Structure-to-electrolyte potential measured "INTERRUPTED" (INSTANT OFF) VZN Voltage (IR) Drop VCSE A B C Do the potential measurements meet a cathodic protection criterion? Yes No If so, which criterion was met? _____________________________ Which location is the conclusion based on? A ____ B ____ C ____ Why? _________________________________________________________________ Which reference electrode location is closest to the anode? A B C Which reference electrode location is closest to the structure? A B C NACE International | CP2 Page | 8 Station 3 CURRENT REQUIREMENT TEST (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 9 Practical Exercise Station 3 - Current Requirement Test STRUCTURE-TO-ELECTROLYTE POTENTIAL (mVcse) Native On Off POWER SOURCE CURRENT (mA) CALCULATE Ireq Ireq for –0.850 VCSE ________ OR Ireq for 100 mV _______ In this case, which criterion would be achieved first? __________________________________________ ______________________________________________________________________________________ NACE International | CP2 Page | 10 Station 4 CATHODIC PROTECTION COMMISSIONING (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 11 Practical Exercise Station 4 - Cathodic Protection Commissioning Identify which wire(s) is/are connected to the pipe and to the anode (explain why), connect the rectifier up properly, install an interrupter, take ON and OFF structure-to-electrolyte (S/E) potentials and adjust the output of the rectifier until the pipeline meets the –850 VCSE polarized potential criterion for steel. Start with rectifier on minimum output setting. STRUCTURE-TO-ELECTROLYTE POTENTIAL WIRE NO. Anode wires: _______________ _______ Pipe wires: _______ _______________ How did you come to the above conclusion? ______________________________________ ______________________________________________________________________ ______________________________________________________________________ Attempt No. Tap Setting Rect. Voltage Output Rect. Amperage Output S/E On S/E Off 1 2 3 4 5 At what tap setting and current output does the pipe meet a CP criterion and which criterion is met? Tap Setting: ___________________________________________________ DC Current output: ___________________________________________ Which criterion was met and why do you say the pipe meets this criterion? __________________________________________________________________ __________________________________________________________________ NACE International | CP2 Page | 12 Station 5 CURRENT SPAN (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 13 Practical Exercise Station 5 - Current Span Test Span Calibration 1. Calibrate the current spans at Test Station #1 2. Record Test Station #1 Measurements (Note polarity and thus direction) ITEST1 ON (CS-10 ON) __________mA ITEST1 OFF (CS-10 OFF) __________mA ITEST1 = ITEST1 ON - ITEST1 OFF __________mA VTEST1 ON (CS-10 ON) __________mV VTEST1 OFF (CS-10 OFF) __________mV 3. Pipe Span Resistance and Calibration Factor Calculate the current span resistance [Ω] and the calibration factor (A/mV) for each current span. Test Station #1 R1span = _______Ω CF span = ________ A/mV 4. Residual Current Was there a residual current on the pipe section; that is, was there a VTEST reading with no Test Current from CS-10? Yes? ______ No? ________ If so, calculate the current. Test Station #1 Residual Current = ________A ________mA NACE International | CP2 Page | 14 Station 6 COATING CONDUCTANCE (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 15 Practical Exercise Station 6 - Coating Conductance CALCULATE THE COATING CONDUCTANCE OF THE PIPELINE SECTION. The instructor will give the following information: RSPAN1 = _____________ Ω CFSPAN1 = _____________ A/mV RSPAN2 = _____________ Ω CFSPAN2 = _____________ A/mV Pipe Diameter _____________ m Pipe Length _____________ m 1. Test Station #1 Measurements mVp1 ON __________mV (Pipe Span with current ON) mVp1 OFF __________mV (Pipe Span with Current OFF) E1ON __________mV (ON Structure-to-electrolyte Potential) E1OFF __________mV (OFF Structure-to-electrolyte Potential) 2. Test Station #2 Measurements mVp2 ON __________mV (Pipe Span with current ON) mVp2 OFF __________mV (Pipe Span with Current OFF) E2ON __________mV (ON Structure-to-electrolyte Potential) E2OFF __________mV (OFF Structure-to-electrolyte Potential) 3. Calculate Current in Each Pipe Span Test Station #1 IP1 ON __________A IP1 OFF __________A ΔIP1 __________A NACE International | CP2 Page | 16 Test Station #2 IP2 ON __________A IP2 OFF __________A ΔIP2 __________A 4. Current Pick Up in Coating Section (IC) IC = __________A 5. Average Potential Across Coating Due to Current Applied ∆E1 = _______ mV ∆E2 = _______ mV Calculate the average ∆E Average ∆Eave ________ mV 6. Pipe Coating Resistance Calculate the total resistance of the pipe section to earth between test stations Total pipe coating resistance (Rc) = ____________ Ω 7. Pipe Surface Area Calculate surface area of pipe based on diameter and length of pipe given by Surface area = ____________ m2 8. Specific Coating Resistance Calculate the specific unit coating resistance (rc )in Ω-m2 . Specific unit coating resistance (rc) = ____________ Ω-m2 9. Specific Coating Conductance Calculate the specific unit coating conductance (gc ) in S/m2 . Specific unit coating conductance (gc) = ____________ S/m2 NACE International | CP2 Page | 17 NACE International | CP2 Page | 18 Station 7 MITIGATION OF DC INTERFERENCE USING A RESISTANCE BOND (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 19 Practical Exercise Station 7 - Mitigation of DC Interference Using a Resistance Bond Part A 1. Measure the ON Potential of Line 1 and Line 2 at each of the locations. Location A Connected to (VCSE) Line 1 B Line 1 B Line 2 D Line 2 A/F Potential (VCSE) Part B 1. Connect the impressed current system to Line 2. 2. Measure the potential of Line 1 and Line 2 while interrupting the impressed current CP System. Location Connected to A Line 1 B Line 1 B Line 2 D Line 2 ON Potential (VCSE) OFF Potential (VCSE) Part C 1. What is the location of maximum exposure to interference on the line not connected to the interrupting source? ________________________________ 2. Starting at the highest resistance value available, install different resistors between Line 1 and Line 2 at the point of maximum exposure. 3. Record the data in the table below with the reference at the location of maximum exposure and the value of the resistance in the table below. Control Resistance Ohms Potential Line 1 at maximum exposure ON OFF Potential Line 2 at maximum exposure ON OFF Voltage Across Bond Resistor Interference Control Current R= R= R= R= Current in resistance bond with DC interference mitigated: ___ mA NACE International | CP2 Page | 20 Station 8 MEASUREMENTS IN ELECTRICAL CIRCUITS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 21 Practical Exercise Station 8 – Measurements in Electrical Circuits Part A: Parallel Circuit 1. Measure and record the resistance of Resistors A, B, C and D. 2. Measure source voltage (after connecting resistors): ___________________ 3. Connect resistors A, B, C and D in parallel and complete the following table. 4. Explain difference (if any) with source voltage after connecting resistors. ________________________________________________________________________ ________________________________________________________________________ RESISTANCE RESISTOR BATTERY EMF (VOLTS) VOLTAGE DROP (VOLTS) CURRENT (AMPERES) A B C D TOTAL 5. Calculate the total resistance of the circuit. (Show your work.) RESISTANCE (OHMS) 6. Which of Kirchoff’s Laws applies to this circuit? Kirchoff’s Law Voltage Current NACE International | CP2 Page | 22 Practical Exercise Station 8 - Measurements in Electrical Circuits Part B: Series Circuit 1. 2. 3. 4. Measure and record the resistance of Resistors A, B, C and D. Connect resistors A, B, C and D in series. Complete the following table. Measure source voltage. RESISTOR RESISTANCE VOLTAGE DROP (VOLTS) CURRENT (AMPERES) A B C D TOTAL 5. Calculate the total resistance of the circuit. (Show your work.) RESISTANCE (OHMS) 7. Which of Kirchoff’s Laws applies to this circuit? Kirchoff’s Law Voltage Current NACE International | CP2 Page | 23 NACE International | CP2 Page | 24 Station 9 UNDERGROUND STORAGE TANK (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 25 Practical Exercise Station 9 - Underground Storage Tank PART A Structure Being Tested Continuity Test (Fixed Cell Reference Electrode Position) Tank Fill Pipe Product Pipe Vent Pipe Station Ground Survey Comments Product Pipe Vent Pipe Station Ground Survey Comments Potentials PART B Structure Being Tested Reference Electrode Location Product end of tank Potential Survey Tank ON OFF Fill Pipe ON OFF ON OFF ON OFF ON OFF Center Vent end of tank At product riser At vent riser At station ground CONCLUSIONS ON TEST RESULTS 1. What structures are electrically continuous? 2. What structures meet a CP (NACE) criterion? ______________________________________ NACE International | CP2 Page | 26 Station 10 CALCULATION STATION (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 27 Practical Exercise Station 10 – Calculation Station Given the following, what is the specific coating conductance of the pipe section? 5 km long & 914 mm Diameter 0.5 A On -1000 mVCSE Off -950 mVCSE NACE International | CP2 0.1 A On -980 mVCSE Off -950 mVCSE Page | 28 Station 11 ROAD CASING AND CARRIER PIPE MEASUREMENTS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 29 Practical Exercise Station 11 – Road Casing and Carrier Pipe Measurements Test Wire #1 CASING A Reference Electrode Outside Casing On Off Reference Electrode Inside Casing On Off CASING B Reference Electrode Outside Casing On Off Reference Electrode Inside Casing On Off #2 Test Wire #3 #4 1. What is the casing status? a. Casing A i. Electronic short ii. Electrolytic couple b. Casing B i. Electronic short ii. Electrolytic couple Yes No Yes No Yes No Yes No 2. Does the carrier pipe inside the casing meet the polarized potential criterion? a. Casing A i. Meets criterion Yes N No b. Casing B i. Meets criterion NACE International | CP2 Yes No Page | 30 Station 12 SOIL RESISTIVITY MEASUREMENTS (SEE NEXT PAGE FOR TEST DATA) NACE International | CP2 Page | 31 Practical Exercise Station 12 - Soil Resistivity Measurements During this exercise, you will measure the resistance and calculate the resistivity using the 4-pin Wenner method and then calculate layer resistivity using the Barnes Layer formula. Part A 1. Using the equipment provided, connect the 4 wires from the meter to the station electrodes at the spacing in the table below. 2. Enter the results and calculate the resistivity based on a pin depth of 0.1 cm. Pin Spacing (cm) Resistance (Ohms) Resistivity (Ohm-cm) 2.50 5.00 7.50 Part B Using the data provided, calculate the resistivity. Pin Spacing (m) Resistance (Ohms) 2.50 12.0 5.00 4.2 7.50 0.8 Resistivity (Ohm-cm) Using the Barnes Layer formula and the data above, calculate the resistivity of each layer in the table below. Layer (cm) Layer Resistance (Ohms) Layer Resistivity (Ohm-cm) 0 to 250 250 to 500 500 to 750 Note: NACE International | CP2 Page | 32 Care should be taken in the resolution of the answers. The significant number of digits in the data determines the significant number of digits in the answer. Basic rules: All non-zero digits are significant: 1, 2, 3, 4, 5, 6, 7, 8, 9. Zeros between non-zero digits are significant: 102, 3004, 50006. Leading zeros are never significant: 0.01, 002.345, 0.000678. In a number with or without a decimal point, trailing zeros (those to the right of the last nonzero digit) are significant provided they are justified by the precision of the data: 345,000; 6.0600; 7.8000; 13.5700. NACE International | CP2 Page | 33 NACE International | CP2 Page | 34 CP2 | Reference Sheets RESISTIVITY CONVERSIONS EMF E or e V mV µV I i mA µA R, or Ω electromotive force – any voltage unit any voltage unit volts millivolts microvolts any amperage unit current density (amp/area) milliamperes or milliamps microamperes or microamps Resistance 1,000,000 volts 1,000 volts 1.0 volt 0.100 volt 0.010 volt 0.001 volt 0.000001 volt = 1 megavolt = 1 kilovolt = 1000 millivolts = 100 millivolts = 10 millivolts = 1 millivolt = 1 microvolt 1,000,000 amperes 1,000 amperes 1.0 ampere 0.100 ampere 0.010 ampere 0.001 ampere 0.000001 ampere = 1 mega-ampere = 1 kiloampere = 1000 milliamperes = 100 milliamperes = 10 milliamperes = 1 milliampere = 1 microampere 1,000,000 ohms 1,000 ohms 1.0 ohms 0.100 ohm 0.010 ohm 0.001 ohm 0.000001 ohm = 1 mega-ohm = 1 kilo-ohm = 1000 milliohms = 100 milliohms = 10 milliohms = 1 milliohm = 1 micro-ohm 1 meter = 1000 mm = 100 cm 1 inch = 25.4 mm = 2.54 cm = 0.0254m 1 foot = 304.8mm = 30.48 cm = 0.3048m ρ=R A L R = ρL A ρ, Resistivity in ohm-cm R, Resistance in ohms A, Cross-Sectional Area in cm2* L, Length in cm* *length and area can be in any units as long as they are consistent where WENNER METHOD ρ=2πaR where ρ, resistivity in ohm-cm* a, the spacing of the pins (cm)* R, the resistance measured (ohms) ρ, resistivity (ohm-cm)* *pin spacing can be in any units as long as it is consistent with resistivity OR ρ=191.5aR where ρ, resistivity in ohm-cm a, the spacing of the pins (feet) R, the resistance measured (ohms) ρ, resistivity (ohm-cm) 191.5 = conversion factor from feet to cm RESISTIVITY LAYER CALCULATIONS RL2 = R2 Layer ρ2 Layer (R1 R2) (R1 – R2) ρL2= 2πL2 RL2 R3 Layer RL3 = (R2 R3) / (R2 – R3) ρ3 Layer ρL3 = 2πL3 RL3 OHMS LAW V = IR I=V/R R=V/I POWER P = EI P = I2 R NACE International | Page 1 CP2 | Reference Sheets SERIES CIRCUIT PARALLEL CIRCUIT ET = V1 + V2 + V3 ET = V1 = V2 = V3 IT = I1 = I2 = I3 IT = I1 + I2 + I3 RT = R1 + R2 + R3 1/RT = 1/R1 + 1/R2 + 1/R3 RT = CONSUMPTION RATE (K) FOR VARIOUS METALS(1) Reduced Species Al Cd Be Ca Cr Cu H2 Fe Pb Mg Ni OH Zn 1 1 1 1 + + R1 R2 R3 GEOMETRY Area of circle = π r2 Surface area of a cylinder = π *d * L Oxidized Species Al*** Cd** Be** Ca** Cr*** Cu** H* Fe** Pb** Mg** Ni** O2 Zn** Molecular Weight M (g) 26.98 112.4 9.01 40.08 52.00 63.54 2.00 55.85 207.19 24.31 58.71 32.00 65.37 Electrons Transferred (n) 3 2 2 2 3 2 2 2 2 2 2 4 2 Equivalent Weight M/n (g) 8.99 56.2 4.51 20.04 17.3 31.77 1.00 27.93 103.6 12.16 29.36 8.00 32.69 Theoretical Consumption Rate (Kg/A-y) 2.94 18.4 1.47 6.55 5.65 10.38 0.33 9.13 33.85 3.97 9.59 2.61 10.7 π = Pi approximately = 3.14 STEEL PIPE RESISTANCE(A) FARADAY’S LAW Pipe Size Wt = KIT where Wt K I T in. = = = = weight loss, kg electrochemical equivalent, kg/A-yr Amps years RELATIVE VALUES OF TYPICAL REFERENCE ELECTRODES TO A HYDROGEN ELECTRODE Electrode (Half-Cell)* Potential (volt) Copper-Copper Sulfate (CSE) +0.316 Silver-Silver Chloride (3.5%) (SSC) +0.256 Saturated Calomel (SCE) +0.244 Standard Hydrogen (SHE) 0.000 Zinc (ZN) –0.800 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Outside Diameter in. 2.35 4.5 6.62 8.62 10.75 12.75 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 36.00 Wall Thickness Weight Cm in. cm lb / ft 5.97 11.43 16.81 21.89 27.31 32.38 35.56 40.64 45.72 50.80 55.88 60.96 66.04 71.12 76.20 81.28 86.36 91.44 0.154 0.237 0.280 0.322 0.365 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.39 0.60 0.71 0.82 0.93 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 3.65 10.8 16.0 28.6 40.5 46.6 54.6 62.6 70.6 78.6 86.6 94.6 102.6 110.6 118.7 126.6 134.6 142.6 kg / m 5.43 16.07 28.28 42.56 60.27 73.81 81.26 93.16 105.07 116.97 128.88 140.78 152.69 164.59 176.65 188.41 200.31 212.22 Resistance µohms/ft µohms/m 76.2 26.8 15.2 10.1 7.13 5.82 5.29 4.61 4.09 3.68 3.34 3.06 2.82 2.62 2.44 2.28 2.15 2.03 256.84 87.93 46.87 33.14 23.39 16.09 17.36 15.12 13.42 12.07 10.96 10.04 9.25 8.60 8.01 7.48 7.05 6.66 (A) Based on steel density of 489 lbs/ft3 (7832 kg/m3) and steel resistivity of 18 microhmcm. NACE International | Page 2 CP2 | Reference Sheets 4-WIRE CURRENT SPAN TEST STATION 𝑅𝑅𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = ∆𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑉𝑉 ∆𝐼𝐼𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝐴𝐴 Ir = 𝐶𝐶𝐶𝐶 ∗ 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑚𝑚𝑚𝑚 CFspan = 𝐾𝐾 = OR Ir = ∆𝐼𝐼𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝐴𝐴 ∆𝐸𝐸𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑚𝑚𝑚𝑚 COATING CONDUCTANCE ∆Eave Rc = ∆Ic ∆Eave = 𝑉𝑉 𝑜𝑜𝑜𝑜𝑜𝑜 𝑉𝑉 ∆E1 (∆E 2 ) = ( EON − EOFF ) 𝑅𝑅𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 Where: Rspan – Resistance of pipeline span (Ω) ΔV - Voltage drop due to test current (Von – Voff) ΔI - Net Test Current (Ion – Ioff) CF = K = Calibration Factor (A/mV) Ir - Pipeline or residual pipeline current (A) V - Voltage drop across span (∆E1 + ∆E 2 ) 2 ∆Ic = ∆I 1 − ∆I 2 ∆I 1 (∆I 2 ) = ( I ON − I OFF ) Gc = g= 1 r 𝑟𝑟 = 𝑅𝑅𝑐𝑐 𝑥𝑥 𝐴𝐴 1 Rc g= Gc A A=π d L Where: Rc – Coating resistance of pipe section (Ω) ΔEave - Average pipe-to-electrolyte potential change due to applied current (V) ΔE1 (ΔE2) - Potential change at each end of pipe section due to applied current (V) Eon (Eoff) - On or off pipe-to-electrolyte potential at each end of pipe section ΔIc - Net current pickup in pipe section (A) ΔI1 (ΔI2) - Net pipeline current at each end of pipe section due to applied current (A) Ion (Ioff) - On or off pipeline current at each end of pipe section (A) r - Specific coating resistance (Ω-m2 or Ω-ft2 depending on surface area units) A - Surface area of pipe (m2 or ft2) π - Pi = 3.1416 Gc - Coating conductance of pipe section (Siemens) g - Specific coating conductance (Siemens/ m2 or Siemens/ft2) NACE International | Page 3 CP2 | Reference Sheets RECTIFIER EFFICIENCY FORMULA SHUNT TYPES AND VALUES P Efficiency = DC *100 PAC Shunt Rating PDC = VDC x IDC PDC = DC power (watts) VDC = Output DC Voltage (Volts) Output DC Current (Amperes) IDC = PAC = VAC x I AC PAC = AC power (watts) Input AC Voltage (Volts) VAC = Input AC Current (Amperes) IAC = AC Input Power = 3600 K N T where: K = meter constant (shown on face of meter) N = No. disk revolutions (observe for 60 sec.) T = time in seconds of observation CURRENT REQUIREMENT CALCULATIONS I req = I req (−0.850V − E Nativ e ) * I test ( EOff − E Native ) − 0.100V * I test = (E Off − E Native ) mV Ohms A/mV RS 5 50 .01 .1 SS 25 25 .001 1 SO 50 50 .001 1 SW or CP 1 50 .05 .02 SW or CP 2 50 .025 .04 SW or CP 3 50 .017 .06 SW or CP 4 50 .0125 .08 SW or CP 5 50 .01 .1 SW or CP 10 50 .005 .2 SW 15 50 .0033 .3 SW 20 50 .0025 .4 SW 25 50 .002 .5 SW 30 50 .0017 .6 SW 50 50 .001 1 SW 60 50 .0008 1.2 SW 75 50 0.00067 1.5 SW 100 50 .0005 2 5 50 .01 .1 Holloway Type J.B. Type R Etrue = Vmeter x total Rmeter Agra-Mesa K= Rl Rh Vh (1 − K) ……. Vh 1− K Vl Shunt Factor Amps METER MEASUREMENT ERROR Etrue = Shunt Value Cott or MCM Red 2 200 .1 .01 Yellow 8 80 .01 .1 Orange 25 25 .001 1 NACE International | Page 4 CP2 | Reference Sheets U.S. Customary/Metric Conversion for Units of Measure Commonly Used in Corrosion-Related Publications 1 A/ft2 1 acre 1 A·h/lb 1 bbl (oil, U.S.) 1 bpd (oil) 1 Btu 1 Btu/ft2 1 Btu/h 1 Btu/h·ft2 1 Btu/h·ft2·°F 1 Btu·in/h·ft2·°F 1 cfm 1 cup 1 cycle/s 1 ft 1 ft2 1 ft3 1 ft·lbf (energy) 1 ft·lbf (torque) 1 ft/s 1 gal (Imp.) 1 gal (U.S.) 1 gal (U.S.)/min (gpm) 1 gal/bag (U.S.) 1 grain 1 grain/ft3 1 grain/100 ft3 1 hp 1 microinch (µin) 1 in 1 in2 1 in3 1 in·lbf (torque) 1 inHg NACE International = 10.76 A/m2 = 4,047 m2 = 0.4047 ha = 2.205 A·h/kg = 159 L = 0.159 m3 = 159 L/d = 0.159 m3/d = 1,055 J = 11,360 J/m2 = 0.2931 W = 3.155 W/m2 (K-factor) = 5.678 W/m2·K = 0.1442 W/m·K = 28.32 L/min = 0.02832 m3/min = 40.78 m3/d = 236.6 mL = 0.2366 L = 1 Hz = 0.3048 m = 0.0929 m2 = 929 cm2 = 0.02832 m3 = 28.32 L = 1.356 J = 1.356 N·m = 0.3048 m/s = 4.546 L = 0.004546 m3 = 3.785 L = 0.003785 m3 = 3.785 L/min = 0.2271 m3/h = 89 mL/kg (water/cement ratio) = 0.06480 g = 64.80 mg = 2.288 g/m3 = 22.88 mg/m3 = 0.7457 kW = 0.0254 µm = 25.4 nm = 0.0254 m = 2.54 cm = 25.4 mm = 6.452 cm2 = 645.2 mm2 = 16.387 cm3 = 0.01639 L = 0.113 N·m = 3.386 kPa 1 inH2O 1 knot 1 ksi 1 lb 1 lbf/ft2 1 lb/ft3 1 lb/100 gal (U.S.) 1 lb/1,000 bbl 1 mA/in2 1 mA/ft2 1 Mbpd (oil) 1 mile = 249.1 Pa = 0.5144 m/s = 6.895 MPa = 453.6 g = 0.4536 kg = 47.88 Pa = 16.02 kg/m3 = 1.198 g/L = 2.853 mg/L = 0.155 mA/cm2 = 10.76 mA/m2 = 159 kL/d = 159 m3/d = 1.609 km 1 square mile 1 mile (nautical) 1 mil 1 MMcfd 1 mph 1 mpy 1 oz 1 oz fluid (Imp.) 1 oz fluid (U.S.) 1 oz/ft2 1 oz/gal (U.S.) 1 psi 1 qt (Imp.) 1 qt (U.S.) 1 tablespoon (tbs) 1 teaspoon (tsp) 1 ton (short) 1 U.S. bag cement 1 yd 1 yd2 1 yd3 = 2.590 km2 = 1.852 km = 0.0254 mm = 25.4 µm = 2.832 x 104 m3/d = 1.609 km/h = 0.0254 mm/y = 25.4 µm/y = 28.35 g = 28.41 mL = 29.57 mL = 2.993 Pa = 7.49 g/L = 0.006895 MPa = 6.895 kPa = 1.1365 L = 0.9464 L = 14.79 mL = 4.929 mL = 907.2 kg = 42.63 kg (94 lb) = 0.9144 m = 0.8361 m2 = 0.7646 m3 CP2 | Page 5 CP2 | Reference Sheets Color Code for Resistors A B C D First Significant Figure. If double wide, resistor is wire wound. Tolerance Multiplier Second Significant Figure Band A Black Brown Red Orange Yellow Green Blue Violet Gray White 0 1 2 3 4 5 6 7 8 9 Band B Black Brown Red Orange Yellow Green Blue Violet Gray White Values 0 1 2 3 4 5 6 7 8 9 Black Brown Red Orange Yellow Green Blue Silver Gold Band C 1 10 100 1,000 10,000 100,000 1,000,000 Silver Gold Band D ± 10% ± 5% 0.01 0.1 Example A B C D A = Red = 2 C = Orange = 1,000 B = Blue = 6 D = Silver = ± 10% Resistance Value = 26,000 Ω ± 10%, or between 23,400 Ω and 28,600 Ω. NACE International CP2 | Page 6 BECOMING CERTIFIED All certifications are administered and managed by the NACE International Institute (NII). NACE International continues to set the standard for corrosion education worldwide, as part of its mission to protect people, assets and the environment from corrosion. Built upon decades of knowledge and expertise from industry subject matter experts, the NACE International Institute certifications are the most recognized and widely accepted corrosion certifications in the world. Why should you get a NACE International Institute Certification? • Personnel qualifications are reaching the highest level of standardization ever; a necessity in a global economy. • Worldwide businesses rely on the NACE Institute certifications as a metric for increased standardization of personnel performance. • The value of a certification held by someone increases as new international standards related to that certification unfold. ****************************************************************************** Certification requirements could include but are not limited to: Educational or academic requirements Successful completion of required exam(s) NDA (Non-Disclosure Agreement) - a signed NDA will be required before taking any NII certification exam 4) Submission of a certification application and subsequent approval by the NACE Institute 5) Work experience requirements (amount varies per certification type and level) 6) Qualification references 1) 2) 3) To submit an application for consideration or to find out more information about our certification programs please visit the NACE International Institute website at http://www.naceinstitute.org/Certification/ Houston, Texas USA | Phone: +1 281-228-6223 or +1 800-797-6223 | Fax: +1 281-228-6300 niifirstservice@nace.org Scheduling Your Computer-based Exam with Pearson Vue 1. Go to naceinstitute.org, Click on the Login button Sign in with your credentials 2. Click on the down arrow next to your profile icon 3. Click on the profile button and then on ‘My Certification Portal’ Accessing Your Exam 4. Select ‘Schedule/Manage Exams’ 5. Your pre-paid exam will be listed here (If taking an exam that is not included in a course registration fee, choose an exam here) Terms, Conditions, and Accommodations 6. Agree to the legal Terms and Conditions Enter your digital signature (full name) and Click Agree (if you do not agree, you will not be able to proceed) 7. Special Accommodations If needed in accordance with the ADA for the course or exam, please select ‘Yes’ and complete the application. Please allow 5-10 business days for response. If you do not need special accommodations, select ‘No’ and proceed to checkout Scheduling Your Exam Note: If your exam is pre-paid (included with a course registration fee) a zero balance due will be reflected at checkout. For exam retakes, payment will be required. Once the checkout process is complete, you will be redirected to the Pearson VUE scheduling portal to schedule your CBT (computer-based testing) exam. For non-CBT exams, you will be directed to the NACE Institute for scheduling. 8. On the Pearson VUE page, you will be able to see your pre-approved exams available for scheduling. Select your exam to continue. Select a Test Center 9. Select ‘Schedule this Exam’ 10. Pearson VUE will search for the nearest Pearson Test Centers based on your address. You can also change the address and find others more convenient to your current location. Select a Test Center and click next. Select Your Test Date, Confirm Details 11. Select a date, which will show available times to schedule your exam. 12. Confirm your appointment details before proceeding to check out Personal Information and Policy Acceptance 13. Confirm your personal information and click next. 14. Review the policy information, click the acknowledgement, and click next. Verify Exam Details 15. Verify your exam details and click submit order to finalize your exam registration Submit Exam Order 16. An email confirmation will be sent to the email address on record. You’re registered! CP2 | Reference Sheets Does Breaker Trip? No Temporary short or circuit resistance dropped. output voltage level Restore Yes Disconnect DC Output Cable & Reset Breaker Tripped Input Circuit Breaker Yes Lower Voltage Taps & Reset Breaker Does Breaker Trip? No Look for short in output circuit Yes Remove taps Does Breaker Trip? Fault in transformer or breaker Fault in rectifier between Transformer and output NACE International CP2 | Page 7 CP2 | Reference Sheets NACE International CP2 | Page 8 CP2 | NACE Standards English SP0169-2013 (Formerly RP0169), "Control of external corrosion on underground or submerged metallic piping systems" SP0177-2014 (Formerly RP0177), Mitigation of alternating current and lightning effects on metallic structures and corrosion control systems SP0200-2014, Steel-cased pipeline practices TM0497-2012, Measurement techniques related to criteria for cathodic protection on underground or submerged metallic piping systems TM0102-2002, Measurement of protective coating electrical conductance on underground pipelines SP0290-2007 (Formerly RP0290), Impressed current cathodic protection of reinforcing steel in atmospherically exposed concrete structures SP0285-2011 (Formerly RP0285), “Corrosion control of underground storage tank systems by cathodic protection” SP0196-2015 (Formerly RP0196), “Galvanic anode cathodic protection of internal submerged surfaces of steel water storage tanks” SP0575-2007 (Formerly RP0575), Internal cathodic protection (cp) systems in oiltreating vessels SP0193-2016 (Formerly RP0193) External cathodic protection of on-grade carbon steel storage tank bottoms SP0286-2007 (Formerly RP0286), Electrical isolation of cathodically protected pipelines TM0115-2015, “Cathodic disbondment test for coated steel structures under cathodic protection” NACE SP0104-2014(Formerly RP0104), "The use of coupons for cathodic protection monitoring applications" SP0408-2014, Cathodic protection of reinforcing steel in buried or submerged concrete structures SP0100-2014 (Formerly RP0100), Cathodic protection to control external corrosion of concrete pressure pipelines and mortar-coated steel pipelines for water or waste water service NACE TM0294-2016, “Testing of embeddable impressed current anodes for use in cathodic protection of atmospherically exposed steel-reinforced concrete” CP2 | NACE Standards SP0572-2007 (Formerly RP0572), Design, installation, operation, and maintenance of impressed current deep anode beds NACE STANDARD TM0105-2016, “Evaluation of coatings containing conductive carbon pigmentation for use as an anode on atmospherically exposed reinforced concrete” TM0101-2012 Measurement techniques related to criteria for cathodic protection of underground storage tank systems RP0375-2006, Field-applied underground wax coating systems for underground pipelines: application, performance, and quality control NACE STANDARD TM21423-2017, “Test method for determination of substrate and surface temperature limits for insulative coatings used for personnel protection” SP0176-2007 (Formerly RP0176), Corrosion control of submerged areas of permanently installed steel offshore structures associated with petroleum production MR0174-2007, Selecting inhibitors for use as sucker-rod thread lubricants TM0404-2004, Offshore platform atmospheric and splash zone new c TM0304-2004, Offshore platform atmospheric and splash zone maintenance coating system evaluation SP0107-2017, “Electrochemical realkalization and chloride extraction for reinforced concrete” SP0207-2007, Performing close-interval potential surveys and dc surface potential gradient surveys on buried or submerged metallic pipelines SP0187-2017 (Formerly RP0187), Design considerations for corrosion control of reinforcing steel in concrete. SP0487-2007 (FORMERLY RP0487), Considerations in the selection and evaluation of rust preventives and vapor corrosion inhibitors for interim (temporary) corrosion protection NACE NO. 12/AWS C2.23M/SSPC CS 23, “Specification for the application of thermal spray coatings (metallizing) of aluminum, zinc, and their alloys and composites for the corrosion protection of steel” TM0183-2018, Evaluation of internal plastic coatings for corrosion control of tubular goods in an aqueous flowing environment TM0113-2013, "Evaluating the accuracy of field-grade reference electrodes" CP2 | NACE Standards Chinese RP0285-2002, Corrosion control of underground storage tank systems by cathodic protection SP0177-2007 Mitigation of alternating current and lightning effects on metallic structures and corrosion control systems RP0388-2001, Impressed current cathodic protection of internal submerged surfaces of carbon steel water storage tanks RP0196-2004, Galvanic anode cathodic protection of internal submerged surfaces of steel water storage tanks SP0572-2007 - Design, installation, operation, and maintenance of impressed current deep anode beds RP0169-2002, Control of external corrosion on underground or submerged metallic piping systems SP0206-2006, Internal corrosion direct assessment methodology for pipelines carrying normally dry natural gas (dg-icda) NACE NO. 12/AWS C2.23M/SSPC-CS 23, Specification for the application of thermal spray coatings (metallizing) of aluminum, zinc, and their alloys and composites for the corrosion protection of steel RP0176-2003, Corrosion control of steel fixed offshore structures associated with petroleum production RP0288-2004, Inspection of linings on steel and concrete SP0207-2007 Performing close-interval potential surveys and dc surface potential gradient surveys on buried or submerged metallic pipelines Spanish SP0285-2011 (ANTES RP0285), "Control de la corrosión exterior en sistemas de tanques de almacenamiento enterrados mediante protección catódica"
