Section 25
Materials of Construction
Lindell R. Hurst, Jr., M.S., P.E. Senior Materials and Corrosion Engineer, Shell Global Solutions (US)
Inc. Retired, Registered Professional Metallurgical Engineer (Alabama, Ohio, North Dakota) (Section Coeditor,
Corrosion Prevention)
Edward R. Naylor, B.S., M.S. Senior Materials Engineering Associate, AkzoNobel; Certified API 510,
570, 653 and Fixed Equipment Source Inspector (Section Coeditor, Corrosion Prevention)
Emory A. Ford, Ph.D. Associate Director, Materials Technology Institute, Chief Scientist and Director
of Research, Lyondell/Bassel Retired, Fellow Materials Technology Institute (Section Coeditor, Corrosion
Prevention)
Sheldon W. Dean, Jr., ScD, P.E. President, Dean Corrosion Technology, Inc.; Fellow, Air Products
and Chemicals, Inc., Retired; Fellow, ASTM; Fellow, NACE; Fellow, AIChE; Fellow, Materials Technology Institute
(Corrosion Fundamentals, Corrosion Prevention)
Eugene L. Liening, M.S., P.E. Manufacturing & Engineering Technology Fellow, The Dow Chemical
Company Retired; Fellow, Materials Technology Institute; Registered Professional Metallurgical Engineer
(Michigan) (Corrosion Testing)
Vinay P. Deodeshmukh, Ph.D. Sr. Applications Development Manager – High Temperature and
Corrosion Resistant Alloys, Haynes International Inc. (Corrosion Fundamentals, High-Temperature Corrosion,
Nickel Alloys)
Pradip R. Khaladkar, M.S., P.E. Principal Consultant, Materials Engineering Group, Dupont
Company (Retired), Registered Professional Engineer (Delaware), Fellow, Materials Technology Institute, St. Louis
(Nonmetallic Materials)
Kevin L. Ganschow, B.S., P.E. Senior Staff Materials Engineer, Chevron Corporation; Registered
Professional Mechanical Engineer (California) (Ferritic Steels)
James D. Fritz, Ph.D. Consultant, NACE International certified Material Selection Design Specialist;
Member of the Metallic Materials and Materials Joining Subcommittees of the ASME Bioprocessing Equipment
Standard, the Ferrous Specifications Subcommittee of the ASME Boiler & Pressure Vessel Code, and ASM
International (Stainless Steels)
Richard C. Sutherlin, B.S., P.E. Richard Sutherlin, PE, Consulting, LLC; Registered Professional
Metallurgical Engineer (Oregon) (Reactive Metals)
Paul E. Manning, Ph.D.
(Nickel Alloys)
Director CRA Marketing and Business Development, Haynes International
25-1
25-2
MATERIALS OF CONSTRUCTION
INTRODUCTION
CORROSION FUNDAMENTALS
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cost of Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion by Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrochemistry of Corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrode Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential-pH Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Passivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Localized Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crevice Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Intergranular Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dealloying Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galvanic Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Velocity Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Environmentally Assisted Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stress Corrosion Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydrogen Embrittlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Liquid Metal Embrittlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other Types of Corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Microbial Influenced Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fretting Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Temperature Gaseous Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion in Carbonaceous Environment—Carburization
and Metal Dusting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion in Sulfur-Bearing Environment—Sulfidation
and Hot Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion by Halogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nitridation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion by Molten Salt or Molten Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-4
25-4
25-4
25-4
25-4
25-5
25-6
25-6
25-6
25-6
25-6
25-6
25-8
25-8
25-8
25-8
25-8
25-8
25-8
25-9
25-9
25-9
25-9
25-9
25-9
25-9
25-9
Unique Uses of On-Line Corrosion Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other Types of Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Considerations for Measuring Corrosion Rate with
Coupon Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Heat Flux Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-23
25-23
25-23
25-23
ALLOY DESIGNATIONS
FERRITIC STEELS
Carbon Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cast Irons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low-Alloy Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-24
25-24
25-24
STAINLESS STEELS
Austenitic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
300 Series Austenitic Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
200 Series Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Performance Austenitic Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferritic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lean Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Standard Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Super Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hyper Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Martensitic Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Precipitation Hardening Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Martensitic PH Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Semi-Austenitic PH Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Austenitic PH Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Temperature Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-25
25-25
25-26
25-26
25-26
25-27
25-27
25-27
25-27
25-27
25-27
25-28
25-28
25-28
25-28
25-28
NICKEL ALLOYS
Corrosion-Resistant Nickel Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Commercially Pure Nickels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Cu Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Mo Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Cr Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Cr-Mo Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Cr-Fe Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Fe-Cr Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Temperature Nickel Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Fe-Cr Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Cr-Fe Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-Cr-Mo/W Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other High-Temperature Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Temperature Cast Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-29
25-29
25-30
25-30
25-30
25-30
25-30
25-30
25-30
25-31
25-31
25-31
25-31
25-32
25-32
25-11
25-11
25-12
25-12
25-12
CORROSION PREVENTION
Materials Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Altering Process/Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Passivation Promotors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removal of Oxidizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scale Enhancers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cracking Preventives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removal of Aggressive Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Process Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Barriers to Prevent Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Paints, Coatings, and Linings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galvanizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Metal Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nonmetallic Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-12
25-12
25-13
25-13
25-13
25-14
25-14
25-14
25-14
25-14
25-14
25-14
25-14
25-14
25-14
25-15
25-15
25-15
25-16
25-16
CORROSION-TESTING METHODS
Corrosion Testing: Laboratory Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coupon Immersion Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Piece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Method of Supporting Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Duration of Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cleaning Specimens after Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evaluation of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical Resistance Corrosion Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Linear Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potentiodynamic Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scan Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Slow-Scan Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crevice Corrosion Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Environmental Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Slow Strain-Rate Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conjunctive Use of Slow- and Rapid-Scan Polarization. . . . . . . . . . . . . . . . . . . . .
Electrochemical Impedance Spectroscopy (EIS) and AC Impedance. . . . . . . .
Other Electrochemical Test Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corrosion Testing: Plant Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
On-Line Corrosion Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-17
25-17
25-17
25-17
25-18
25-18
25-18
25-19
25-19
25-19
25-19
25-19
25-20
25-20
25-20
25-21
25-21
25-21
25-21
25-22
25-22
25-22
25-23
25-23
REACTIVE METALS
Corrosion Resistance of Reactive and Refractory Metals . . . . . . . . . . . . . . . . . . . . .
Physical and Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydrogen Embrittlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crevice Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Major Chemical Processing Applications for Titanium . . . . . . . . . . . . . . . . . . . . . . .
Water and Seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxidizing Media (Peroxides, Chlorine) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxidizing Acids (Nitric, Chromic, Perchloric) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reducing Media (Hydrochloric, Sulfuric, Phosphoric) . . . . . . . . . . . . . . . . . . . . . .
Alkaline Media (Sodium Hydroxide, Potassium
Hydroxide, Ammonium Hydroxide) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Organics and Organic Acids (Acetic, Citric, Formic, Lactic) . . . . . . . . . . . . . . . .
Fabrication of Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Zirconium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Major Chemical Processing Applications for Zirconium . . . . . . . . . . . . . . . . . . . . . .
Water and Seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxidizing Media (e.g., Nitric) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reducing Media (Hydrochloric, Sulfuric, Phosphoric) . . . . . . . . . . . . . . . . . . . . .
Organic Solutions (Acetic, Formic) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alkaline Media (Sodium Hydroxide, Potassium
Hydroxide, Aluminum Hydroxide, and Ammonium Hydroxide) . . . . . . . . . . .
Urea (Ammonium Carbamate Media) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fabrication of Zirconium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tantalum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Major Chemical Processing Applications for Tantalum . . . . . . . . . . . . . . . . . . . . . . .
Oxidizing Media (Nitric, Chromic, Perchloric,
Chlorine, Conc. Sulfuric) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reducing Media (Hydrochloric, Sulfuric, Hydrobromic,
Phosphoric, Formic, Oxalic) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fabrication of Tantalum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Niobium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Major Chemical Processing Applications for Niobium . . . . . . . . . . . . . . . . . . . . . . . .
Fabrication of Niobium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-32
25-32
25-32
25-32
25-32
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-33
25-34
25-34
25-34
25-34
25-34
25-34
25-34
25-34
25-34
25-35
25-35
25-35
MATERIALS OF CONSTRUCTION
OTHER METALS AND ALLOYS
Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Copper Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cobalt Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-35
25-35
25-36
25-36
LOW-TEMPERATURE AND CRYOGENIC MATERIALS
NONMETALLIC MATERIALS FOR CORROSION CONTROL
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inorganic Nonmetallics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Glass and Glassed Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Porcelain and Stoneware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brick Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cement and Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Organic Nonnmetallic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Commonly Used Thermoplastic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Polyethylene (PE) (-CH2-CH2-)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Polyvinyl Chloride (PVC) (-CH2-CHCl-)n- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Polypropylene (PP) (-CH2-CHCH3-)n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mid-Performance Partially Fluorinated Polymers . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Performance Fully Fluorinated Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Commonly Used Thermosetting Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BisA Fumerate Polyester Resins for FRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-37
25-38
25-38
25-38
Chlorendic Anhydride–Based Polyester Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Epoxy Vinyl Ester and Novolac Epoxy Vinyl Ester Resins . . . . . . . . . . . . . . . . . . .
Furan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Epoxy Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Commonly Used Elastomers (Thermosetting Rubbers) . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Life Cycle of Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage Tanks, Reactors, Transportation Equipment: FRP . . . . . . . . . . . . . . . . . . . .
Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design, Construction, Inspection, and Transportation . . . . . . . . . . . . . . . . . . . .
Maintenance (In-Service Inspection, Condition Assessment,
Fitness for Service, NDT, and Destructive Techniques) of FRP . . . . . . . . . . . . .
Vessels with Linings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Selection of Lining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evaluation of Screening Testing by Coupon Immersion . . . . . . . . . . . . . . . . . . . . . . .
Design and Fabrication of Vessels to Be Lined . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spray-Applied Thin Coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spray, Brush, or Trowel-Applied Thick Linings . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Elastomeric (Rubber) Sheet Lining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermoplastic Linings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Expansion Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hoses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seals and Gaskets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Column Internals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25-3
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-38
25-39
25-39
25-39
25-39
25-40
25-40
25-40
25-40
25-40
25-40
General references: The following books are general references that are useful to a process engineer in understanding corrosion science and engineering and
in choosing appropriate materials of construction: Baboian, R., ed., NACE Corrosion
Engineer’s Reference Book, 3d ed., NACE International, Houston, Tex., 2002; Birks, N.,
G. H. Meier, and F. S. Pettit, Introduction to the High-Temperature Oxidation of Metals,
2d ed., Cambridge University Press, Cambridge, UK, 2006; Corrosion in the Petrochemical Industry, 2d ed., ASM International, Materials Park, Ohio, 2015; Cramer, S., and B.
Covino, Jr., eds., ASM Handbook, vol. 13A: Corrosion: Fundamentals, Testing, and Protection, ASM International, Materials Park, Ohio, 2003; Cramer, S., and B. Covino, Jr., eds.,
ASM Handbook, vol. 13B: Corrosion: Materials, ASM International, Materials Park, Ohio,
2005; Cramer, S., and B. Covino, Jr., eds., ASM Handbook, vol. 13C: Corrosion: Environments and Industries: Materials, ASM International, Materials Park, Ohio, 2006; Dillon, C.,
Materials Selection for the Chemical Process Industry, 2d ed., McGraw-Hill, New York,
2004; Fontana, M., and P. Greene, Corrosion Engineering, 2d ed., McGraw-Hill, New York,
1978; Jones, D., Principles and Prevention of Corrosion, 2d ed., Prentice Hall, Englewood
Cliffs, N.J., 1992; Lai, G. Y., High-Temperature Corrosion and Materials Applications, ASM
International, Materials Park, Ohio, 2007; Moniz, B. J., and W. I. Pollock, eds., Process
Industries Corrosion—The Theory and Practice, NACE International, Houston, Tex.,
1986; Uhlig, H., and R. Revie, Corrosion and Corrosion Control—An Introduction to
Corrosion Science and Engineering, 3d ed., Wiley, New York, 1985.
INTRODUCTION
The selection of materials of construction for the equipment and facilities
to produce chemicals is a core competency of chemical engineering. The
chemical products desired cannot be manufactured without considering
the selection of the optimum materials of construction for safe, economical manufacture and required product quality. Pressure containment and
leak prevention are important to maintaining public confidence and the
industry’s right to operate. This section will introduce corrosion science and
fundamentals, corrosion prevention, and common materials of construction used in the process industries. The purpose is to introduce the chemical engineer to the materials he or she will need to successfully operate
process facilities safely and economically. Process engineers should engage
with trained corrosion and materials engineering professionals and subject
matter experts to choose the appropriate materials of construction and to
maintain the physical assets of the facilities for which they are responsible.
CORROSION FUNDAMENTALS
INTRODUCTION
Corrosion has been defined as “the deterioration of a material, usually
a metal, that results from a chemical or electrochemical reaction with its
environment” (ASTM G193, “NACE/ASTM Standard Terminology and
Acronyms Related to Corrosion,” Annual Book of ASTM Standards, 2014,
Vol. 03.02, ASTM International, West Conshohocken, Pa., 2014). Corrosion
damage can show up as subsurface attack or cracking, so the traditional
understanding does not cover the contemporary usage. In addition, the
modern definition includes the deterioration of nonmetals, which will be
covered in a later subsection. Most corrosion processes involve the chemical oxidation of metals, that is, the metal involved loses electrons as it is
converted to corrosion products.
Practically all metals in use today are produced by a smelting process that
reduces the ore and purifies the metal so that it can be converted into useful
products. Corrosion is simply a reversal of the smelting process. Most metals are not thermodynamically stable in atmospheric oxygen, so corrosion
returns the metal to a more stable form. The purpose of corrosion control is
therefore to slow this process to the point where it does not interfere with
the useful function of the metal item.
Although metals will react with liquids and gases, most corrosion processes that occur at temperatures below about 350°C result from exposure
to a liquid phase. At higher temperatures, the reaction rate of metals with
gases becomes high enough to cause significant damage. These processes
are known as oxidation reactions rather than corrosion.
space. For example, atmospheric corrosion occurs because of occasional
exposure of metal surfaces to moisture from dew or rain. However, not
all liquids are corrosive. Nonpolar liquids, liquids with a dielectric
constant less than 10, are not usually corrosive to most metals (Dean,
S. W., Materials Selector for Hazardous Chemicals, Organic Solvents,
Publication MS-8, Materials Technology Institute, St. Louis, Mo., 2011,
p. 9). In the case of polar liquids, many metals and alloys resist corrosion because of a layer of oxidized metal on their surfaces that shields
the metals from the liquid. In some cases, the layer is an oxide resulting
from the exposure of the metal to air. In other cases, the layer is a corrosion product from the liquid. If the corrosion product is not soluble
in the liquid, but forms a continuous adherent layer, the corrosion rate
will generally be low or negligible. Polar liquids are much more likely
to dissolve corrosion products and metal oxides, thereby allowing the
corrosion to continue.
ELECTROCHEMISTRY OF CORROSION
Electrochemical corrosion theory is based on the understanding that all
corrosion processes involve two or more separate electrochemical reactions: the anodic reactions and the cathodic reactions. The total of the electrons liberated by the cathodic reactions must exactly equal the electrons
consumed by the anodic reactions. The anodic reactions are oxidations in
which metallic atoms lose electrons and become positively charged ions. An
example is shown in Eq. (25-1):
COST OF CORROSION
There have been many studies over the years to try to determine the
cost of corrosion. The results of these calculations show that this cost is
in the range of 2 to 4 percent of the GDP (gross domestic product). The
use of existing technology could have prevented costs that were about
15 percent of the total costs of corrosion [Materials Performance 5(6):
6 (1995)].
However, these estimates miss a very important effect of corrosion
damage. Failures resulting from corrosion can cause injuries and fatalities, as well as environmental damage from loss of containment. Corrosion
control is a key element in assuring the safety and reliability of plants and
equipment.
CORROSION BY LIQUIDS
Corrosion of metals at lower temperatures usually results from exposure
to liquids. The liquid phase does not have to be continuous in time or
25-4
Fe → Fe++ + 2e
(25-1)
The electrons shown on the right side of Eq. (25-1) are retained in the
metal, while the ferrous ions dissolve into the solution and diffuse away.
This reaction is balanced by a reduction reaction, for example, the reduction of molecular oxygen dissolved in the solution, as shown in Eq. (25-2):
O 2 + 2H 2O + 4e → 4OH −
(25-2)
These two reactions occur simultaneously, so there is no accumulation or
depletion of electrons in the metal.
Electrode Potential The Gibbs free energy associated with an electrochemical reaction is related to the potential of the reaction through
CORROSION FUNDAMENTALS
Eq. (25-3) (Moore, W. J., Physical Chemistry, 2d ed., Prentice-Hall, Englewood
Cliffs, N.J., 1955, p. 73):
ΔG = −nEF
TABLE 25-1 Potentials of Reference Electrodes versus the
Standard Hydrogen Electrode, SHE, 25°C
Name
Electrode
Designation
Potential vs. SHE
(V)
Calomel
Silver/silver chloride, 0.1 M
Silver/silver chloride 1 M
Silver/silver chloride sat.
Copper/copper sulfate
Mercury/mercury sulfate
Ag/AgCl/0.1 M KCl
Ag/AgCl/ 1.0 M KCl
Ag/AgCl/Sat. KCl
Cu/ Sat. CuSO4
Hg/ Hg2SO4/H2SO4
SCE
0.1 SSC
1.0 SSC
Sat SSC
CCS
—
0.241
0.286
0.235
0.198
0.3
0.616
(25-3)
where ΔG is the Gibbs free energy of the reaction,
n is the number of electrons transferred,
E is the potential of the reaction,
F is Faraday’s constant (96,500 coulombs per gram equivalent).
The realization that the potential of a cell in which a metal surface in contact with an ionic solution coupled to a reference electrode could be used
to measure the free energy of the metal surface thus became the basis for
electrochemical corrosion theory. The reference electrode that has been
adopted as the standard is the platinized platinum/hydrogen gas/hydrogen ion electrode at 1 atm gas pressure, 1 M H+ activity (concentration),
and 25°C, SHE. Unfortunately, this reference electrode is difficult to manage in experimental work, so several other reference electrodes are used.
Table 25-1 provides information on converting between the various electrodes currently in use.
It should also be noted that the sign convention used in displaying electrode potentials is the Gibbs Stockholm convention in which increasingly
positive potentials represent increasingly oxidizing conditions (ASTM G3,
“Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing,” in Annual Book of ASTM Standards, 2014,
vol. 03.02, ASTM International, West Conshohocken, Pa., 2014).
ASTM G3, “Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing,” Annual Book of ASTM Standards, 2014, Vol. 03.02
(ASTM International, West Conshohocken, Pa., 2014).
Potential-pH Diagrams Marcel Pourbaix used thermodynamic theory to construct diagrams in which the electrode potentials of the metallic elements in aqueous solutions are plotted against the pH of the solution
showing where various compounds and ions of the metal would be stable
at 25°C (77°) (Veleva, L., and R. D. Kane, “Thermodynamics of Atmospheric Corrosion and the Use of Pourbaix Diagrams,” in ASM Handbook,
vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ASM International, Materials Park, Ohio, 2003). An example of one of these diagrams is
shown in Fig. 25-1. The electrode potential of the metal against the standard hydrogen reference electrode is shown on the ordinate axis, while the
2.2
2
1.8
9'
20
1.6
1.4
b
0 –2 –4 –6
1.2 Fe+++
2'
10'
FeO4—?
1
0.8
FeOH++
3'
4'
5'
11'
Potential, V
0.6
Fe(OH)+2
0.4
28
6'
Fe2O3
0.2
0
a
–0.4
Fe++
7'
8'
–0.2
26
0
–2
23
17
–0.6
Fe3O4
–6
–0.8
–4
27 –6
13
HFeO2
–1
Fe
24
1'
–1.2
–6
–1.4
–1.6
–1.8
–2
–1
0
1
2
3
4
5
25-5
6
7
pH
8
9
10
11
12
13
14
15
16
FIG. 25-1 Potential pH diagram for iron at 25°C (77°F). (ASM Handbook, “Thermodynamics of Atmospheric Corrosion
and the Use of Pourbaix Diagrams,” Volume 13A Corrosion: Fundamentals, Testing, and Protection. Reprinted with
permission of ASM International. All rights reserved. www.asminternational.org.)
25-6
MATERIALS OF CONSTRUCTION
solution pH is shown on the abscissa axis. The horizontal lines on the left
side of the diagram show the potential of iron in equilibrium with the concentration of ferrous ions in solution. The regions where iron oxides and
hydroxides are stable are also shown. These diagrams have been very helpful in enabling us to understand the reactions that occur during corrosion.
However, they must be modified for alloys.
Polarization When current is passed through a metal surface in an
ionic solution, the electrode potential is changed. This change is denoted
as polarization. J. Tafel discovered that the electrode potential of a metal
surface changed as the current density applied to the surface varied [Tafel, J.,
Z. Physik. Chem. 54: 614 (1905)]. In the case where only one electrochemical
reaction was occurring, the relationship between potential, E, and current
density followed the expression shown in Eq. (25-4):
E = a + b log i
(25-4)
where i is the current density, and a and b are constants.
The b constant is known as the Tafel slope, and its value can, in some
cases, be predicted from reaction rate theory (Glasstone, S., K. J. Laidler,
and H. Eyring, The Theory of Rate Processes, McGraw Hill, New York, 1944,
pp. 552–599). This expression is also known as Tafel kinetics, and it applies
to electrochemical reactions where the rate is limited by the charge transfer
process.
In the case of most corrosion processes, both the anodic and cathodic
processes are occurring simultaneously at the same overall electron transfer
rate. In this case, if a small potential change, for example, <10 mV, is applied
to the surface, the current density does not follow the relationship shown in
Eq. (25-4), but instead it appears to be linearly related to the applied potential change. A typical polarization diagram is shown in Fig. 25-2 for carbon
steel in sulfuric acid (ASTM G82, “Standard Guide for Development and
Use of a Galvanic Series for Predicting Galvanic Corrosion Performance,”
Annual Book of ASTM Standards, 2014, vol. 03.02, ASTM International, West
Conshohocken, Pa., 2014). With larger potential changes, such as >50 mV,
the Tafel behavior begins to appear because one of the reactions becomes
dominant. In the low polarization region, the ratio of potential change to
–0.4
n
utio
ol
iss
ic d
od
An
icorr = B /R p
(25-5)
where icorr is the corrosion rate of the metal expressed as a current density.
B is a constant known as the Stern-Geary constant.
The icorr relationship to corrosion rate is given by Eq. (25-6), which is
based on Faraday’s law (ASTM G102, “Standard Practice for Calculation of
Corrosion Rates and Related Information from Electrochemical Measurements,” Annual Book of ASTM Standards, 2014, vol. 03.02, ASTM International, West Conshohocken, Pa., 2014).
CR = ( K 1 ·icorr · EW )/ρ
(25-6)
where CR is the corrosion rate expressed as penetration per unit time,
K1 is a constant,
EW is the equivalent weight of the metal or alloy,
ρ is the density of the metal.
The value of K1 is determined by the units selected for the other terms of
the equation.
Passivity When some alloys are polarized in strong acid solutions, it is possible to observe the development of passivity. Passivity is a
corrosion-resistant condition that occurs with many metals and alloys in
oxidizing conditions. Stainless steels are typical examples of alloys exhibiting passivity. The polarization diagram of Type 430 (UNS S43000) stainless steel in deaerated 1 N sulfuric acid at 30°C (86°F) is shown in Fig. 25-3
(ASTM G5, “Standard Reference Test Method for Making Potentiodynamic
Anodic Polarization Measurements,” Annual Book of ASTM Standards, 2014,
vol. 03.02, ASTM International, West Conshohocken, Pa., 2014). In this
case the corrosion rate of the metal is very high in the potential range of
−0.3 to −0.5 V versus the SCE reference electrode. However, as the potential increases from this range, the current density, which is proportional to
the corrosion rate, decreases by four decades. Surface examinations of the
steel have shown that this decrease is caused by the development of a thin,
invisible layer of a mixed chrome–iron oxide layer. This mixed oxide layer is
almost insoluble in the acid, and this low solubility results in the corrosion
resistance of the stainless steel. This phenomenon is responsible for the corrosion resistance of many alloys containing chromium.
GENERAL CORROSION
βa = .060 V/decade
Potential (volts) vs SCE
current density is known as the polarization resistance, Rp, and it is related
to the corrosion rate of the metal surface through Eq. (25-5),
–0.5
Ecorr = .060 V/decade
icorr
Corrosion that occurs uniformly over a metal surface is known as general
corrosion. Although the rate can vary from point to point on the surface,
the variations are relatively small, and they do not persist for extended time
periods. As a result, the overall loss tends to be uniform. This type of corrosion is observed in many situations, especially in the corrosion of carbon
steel in the atmosphere. In cases where the corrosion rate is modest, engineers may accommodate for the loss of metal over time by adding a corrosion allowance to the required thickness of metal used.
LOCALIZED CORROSION
βc = .105 V/decade
–0.6
Hy
dr
og
en
re
du
ct
io
n
–0.7
10–1
100
Current density, mA/cm2
101
Polarization diagram for carbon steel in deaerated 0.52 N sulfuric acid
showing Tafel behavior. (Reprinted with permission, from G82-98(2014) Standard
Guide for Development and Use of a Galvanic Series for Predicting Galvanic
Corrosion Performance, copyright ASTM International, 100 Barr Harbor Drive, West
Conshohocken, PA 19428. A copy of the complete standard may be obtained from
ASTM, www.astm.org.)
FIG. 25-2
Pitting Pitting is a form of localized corrosion that occurs in passive metals and metals with thin corrosion product layers (Frankel, G. S.,
“Pitting Corrosion,” in ASM Handbook, vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer and B. S. Covino, Jr., ASM
International, Metals Park, Ohio, 2003, p. 236). It may occur because of
impurities in the metal surface or a variety of other irregularities on the
surface that cause anodic attack to localize. A major problem with pitting
is that the environment tends to stabilize the localization of the attack
so that the pits continue to grow. Figure 25-4 shows common morphologies of pit penetrations as seen in sections through the pit (ASTM G46,
“Standard Guide for Examination and Evaluation of Pitting Corrosion,”
Annual Book of ASTM Standards, 2014, vol. 03.02, ASTM International,
West Conshohocken, Pa., 2014). Pits usually occur randomly and may
result in penetration with loss of containment before any significant loss
of strength. Environments containing chloride or other halide ions are
particularly aggressive in causing pitting.
Crevice Corrosion Another form of localized corrosion is crevice corrosion. This type of attack is similar to pitting in that the crevice permits
the environmental changes that lead to accelerated anodic attack. Chloride
Noble
Noble
CORROSION FUNDAMENTALS
+1.8
+1.4
+1.6
+1.0
+1.2
+.8
+1.0
+.6
+.8
+.4
+.6
+.2
+.4
0
+.2
–.2
0
–.4
Active
–.6
Electrode potential vs SHE (volts)
+1.4
–.2
.001
.01
.1
1.0
Current density (mA/cm2)
10
Active
Electrode potential vs SCE (volts)
+1.2
FIG. 25-3 Anodic polarization diagram for type 430 stainless steel in 1 N sulfuric acid at 30°C. (Reprinted with permission, from G3-14 Standard
Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, copyright ASTM International, 100 Barr Harbor
Drive, West Conshohocken, PA 19428. A copy of the complete standard may be obtained from ASTM, www.astm.org.)
(a) Narrow, deep
(b) Elliptical
(c) Wide, shallow
(e) Undercutting
(d) Subsurface
(Horizontal)
(Vertical)
(f) Microstructural orientation
Typical profiles of pits as seen in sections through pits in metals. [Reprinted with permission from G46-94(2013) Standard Guide for
Examination and Evaluation of Pitting Corrosion, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy
of the complete standard may be obtained from ASTM, www.astm.org.]
FIG. 25-4
25-7
25-8
MATERIALS OF CONSTRUCTION
ions are often involved in the development of the aggressive environment,
which is acidic. Crevice corrosion will develop in conditions that are less
aggressive than those that cause pitting.
Intergranular Corrosion A very detrimental form of localized corrosion is known as intergranular corrosion. Practically all metals are
crystalline, and the crystals that comprise the item are known as grains.
Intergranular corrosion results from compositional variations at the grain
boundaries that cause these areas to be more susceptible to corrosive
attack. The resulting corrosion damage will allow the grains to become
loose and wash away. As a result, the penetration removes a large fraction
of the metal with only a small fraction being oxidized. The problem was
originally observed with higher-carbon stainless steel alloys that had been
improperly heat treated or had been welded (Fritz, J. D., “Effects of Metallurgical Variables on the Corrosion of Stainless Steels,” in ASM Handbook,
vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer
and B. S. Covino, Jr., ASM International, Metals Park, Ohio, 2003, p. 267).
The condition of being susceptible to intergranular corrosion is known as
sensitization.
Dealloying Corrosion Dealloying corrosion is another form of localized attack that occurs with some metals. Copper-based alloys are most
known for this form of attack (Corcoran, S. G., “Effects of Dealloying Corrosion on Dealloying Corrosion,” in ASM Handbook, vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer and B. S. Covino, Jr., ASM
International, Metals Park, Ohio, 2003, p. 287). Copper-zinc alloys (brasses)
are particularly prone to this problem. In that case, the corrosion is termed
dezincification. The mechanism is still being debated, but the result is that
the zinc in the alloy is lost, and the remaining copper retains the same volume, but has no strength. This attack can occur in spots on a surface, or
it can cover the entire surface. Other types of dealloying corrosion include
denickelification and dealuminification of certain copper alloys, and graphitization of gray cast irons.
Galvanic Corrosion This form of attack occurs when two different alloys are in contact with each other in a corrosive environment
(Baboian, R., “Galvanic Corrosion,” in ASM Handbook, vol. 13A, Corrosion:
Fundamentals, Testing, and Protection, ed. S. D. Cramer and B. S. Covino,
Jr., ASM International, Metals Park, Ohio, 2003, p. 210). This problem is
also known as bimetallic corrosion. In this situation, one of the alloys will
corrode at an accelerated rate, while the other suffers little or no damage.
This problem is most prevalent in liquids that have a significant electrical
conductivity such as seawater. The accelerated corrosion is accompanied
by the passage of electrical current through the junction where the two
alloys are connected.
Velocity Effects Accelerated corrosion also occurs in areas exposed
to high liquid velocities (Aylor, D., and B. Phull, “Evaluation Erosion
Corrosion, Cavitation and Impingement,” in ASM Handbook, vol. 13A,
Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer and
B. S. Covino, Jr., ASM International, Metals Park, Ohio, 2003, p. 639). This
problem has also been known as erosion corrosion, flow-assisted corrosion, flow-sensitive corrosion, and a variety of similar terms. There
are three different classes of flow-accelerated corrosion. The first class
occurs when only the velocity of the liquid and the turbulence it produces
cause the damage. The second class covers cases where solid particles
entrained in the liquid strike the metal surface and damage the protective corrosion product layer, thereby accelerating the corrosion. The third
class involves bubbles or voids in the liquid that strike the surface. In the
case of voids, the collapse of the voids causes localized damage and disrupts the corrosion product layer. This case is known as cavitation corrosion. Rising bubbles can also cause enhanced corrosion. This problem
has been observed with carbon steel in concentrated sulfuric acid and is
known as hydrogen grooving [Dean, S. W., Jr., and G. D. Grab, Materials
Performance 25(7): 48–52 (1986)].
In the case of a liquid flowing into a tube or pipe, the corrosion damage tends to be greatest at the inlet end, bends, elbows, or obstacles in the
flow path. In these cases, the damage often appears as horseshoe-shaped
grooves in the metal. Generally, the cause of the enhanced corrosion rate
is the higher mass transfer rate that results from the higher velocity and
turbulence it produces.
Experimental studies showed that the cracking processes were related
to corrosion. When corrosion was prevented through cathodic protection, cracking did not happen. The cracks followed specific directions
in the metal related to the metallurgy of the alloy. For example, cracking in brass tends to be intergranular, that is, it follows grain boundaries
in the metal. Figure 25-5 shows a section through an SCC crack in brass
(ASTM G186, “Standard Test Method for Determining Whether Gas-LeakDetector Fluid Solutions Can Cause Stress Corrosion Cracking of Brass
Alloys,” Annual Book of ASTM Standards, 2014, vol. 03.02, ASTM International, West Conshohocken, Pa., 2014). In austenitic stainless steels in
chloride environments, the cracks are transgranular, but they follow
specific crystallographic directions through the grain. This requires the
cracks to reinitiate every time they encounter a grain boundary. In some
alloys, both transgranular and intergranular cracks are found. In all of
these situations, the cracks tend to be highly branched, propagating in
many directions through the metal.
In structures, the most likely locations for cracks are around welds and
cold-formed areas. The stresses in these areas are termed residual stresses,
and they are generally much higher than externally applied stresses.
SCC requires four components (Dean, S. W., Laboratory Corrosion Testing of Metals and Alloys, Materials Technology Institute, St. Louis, Mo., 2015,
p.129). The alloy must be susceptible to SCC in a corrosive environment. The
environment must have the composition and temperature that will allow
SCC to proceed. The tensile stress must be high enough for SCC to initiate.
The metal item must be in the environment long enough for the cracking
process to initiate. As with pitting and crevice corrosion, an induction time
is usually observed before cracking begins.
Hydrogen Embrittlement Hydrogen embrittlement (HE) is caused by
the presence of atomic hydrogen in the metal, resulting in the development
of cracks, or a reduction of ductility and toughness. Although corrosion can
generate atomic hydrogen as a corrosion product, it is not the only source
of atomic hydrogen that can cause HE. Electroplating, cathodic protection,
and high-pressure hydrogen gas are other sources. HE cracks are intergranular and exhibit branching. The cracks are believed to be caused by decohesion at these grain boundaries. As a general rule, alloys become susceptible
to HE when their tensile strength exceeds a critical level. Because hardness
and tensile strength are correlated, the critical value is often reported as a
hardness value.
ENVIRONMENTALLY ASSISTED CRACKING
Corrosion can cause or accelerate the cracking of metals when they are
under tensile stress. Four different mechanisms have been observed: stress
corrosion cracking (SCC), hydrogen embrittlement (HE), liquid metal
embrittlement (LME), and corrosion fatigue (CF).
Stress Corrosion Cracking This type of cracking occurs with alloys
in specific environments and limited conditions. It occurs locally in regions
where high tensile stresses exist, and it results in penetration or rupture of
the metal part.
Section through an SCC crack in brass showing intergranular cracking.
[Reprinted with permission from G186-05(2011) Standard Test Method for Determining Whether Gas-Leak Detector Fluid Solutions Can Cause Stress Corrosion
Cracking of Brass Alloys, copyright ASTM International, 100 Barr Harbor Drive, West
Conshohocken, PA 19428. A copy of the complete standard may be obtained from
ASTM, www.astm.org.]
FIG. 25-5
CORROSION FUNDAMENTALS
In petroleum production and refining, and in chemical processing, HE
has been a particular concern when hydrogen sulfide, H2S, hydrogen cyanide, HCN, or anhydrous hydrogen fluoride, HF, is present. In the case of
iron- and nickel-based alloys, the critical hardness is Rockwell C 22 (NACE
MR01-75, “Sulfide Stress Cracking Resistant Metallic Materials for Oilfield
Equipment,” NACE International, Houston, Tex.). This value was determined for H2S-containing environments, but in less aggressive environments a higher critical hardness may apply. For example, in atmospheric
exposures, the value for steel is about Rockwell C 35. It has been found that
in such service there is a threshold stress value that is necessary to initiate HE. Stress-relieving and postweld heat treatments are regularly used
to minimize the probability of HE for vessels and equipment, especially in
H2S service.
Liquid Metal Embrittlement This form of cracking does not
require corrosion for the cracks to develop (Kohlman, D. G., “Liquid Metal
Induced Embrittlement,” in ASM Handbook, vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer and B. S. Covino, Jr., ASM
International, Metals Park, Ohio, 2003, p. 381). At temperatures below
350°C (662°F) mercury is the most common agent causing this type of
cracking. Copper, nickel, and aluminum-based alloys are susceptible to
failures from mercury.
Austenitic stainless steels are susceptible to LME in molten zinc and molten copper. Failures have occurred in fires when molten zinc dripped from
galvanized steel onto the stainless steel surface. Molten copper has also
caused failures when the copper contacted the hot stainless during welding
operations.
Corrosion Fatigue This form of cracking requires a stress that varies or cycles (Phull, B., “Evaluation Corrosion Fatigue,” in ASM Handbook,
vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer
and B. S. Covino, Jr., ASM International, Metals Park, Ohio, 2003, p. 625).
Crack growth occurs only while the stress is increasing. What distinguishes
CF from conventional fatigue is that corrosion is necessary for it to progress, and the stress level at which it occurs is lower than the threshold for
conventional fatigue.
OTHER TYPES OF CORROSION
Microbial Influenced Corrosion Metals in exposed waters—
including seawater, cooling water, and natural freshwater—and soils will
often be covered with biological growth. In the case of natural waters, this is
known as slime or fouling. In some cases, the biological layer that develops
will cause accelerated corrosion (Dexter, S. C., “Microbiologically Influenced
Corrosion,” in ASM Handbook, vol. 13A, Corrosion: Fundamentals, Testing,
and Protection, ed. S. D. Cramer and B. S. Covino, Jr., ASM International,
Metals Park, Ohio, 2003, p. 398). The corrosion often takes the form of deep
pits in the metal. Carbon and stainless steels are susceptible to this form of
attack. Sulfate-reducing bacteria are often present in these cases, and the
pits often contain iron sulfide in the corrosion products within the pits if
these bacteria are present.
Fretting Corrosion Fretting corrosion occurs when metal surfaces
rub against each other (Glaeser, W., and I. G. Wright, “Forms of Mechanically Assisted Degradation,” in ASM Handbook, vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer and B. S. Covino, Jr.,
ASM International, Metals Park, Ohio, 2003, p. 324). The mechanism is
believed to be a result of many regions of the contacting surfaces being
welded together, and then the welds are broken off to create tiny balls
of rolled-up metal and oxide. This type of attack is not corrosion in the
traditional sense, but it is usually included in discussions of corrosion
damage.
HIGH-TEMPERATURE GASEOUS CORROSION
Overview All high-temperature alloys and coatings corrode to various
extents when exposed to a variety of high-temperature engineering environments. Oxidation is the most commonly observed high-temperature corrosion reaction, and it forms the basis for other high-temperature degradation
processes. In mixed gaseous environments containing gases such as H2S, CO,
Cl2, and NH3, or in the presence of molten deposits, high-temperature degradation is initiated from more than one type of oxidant. As a consequence,
high-temperature corrosion reactions, such as sulfidation, carburization,
and nitridation, occur in combination with oxidation. However, the nature
of corrosion products and the extent of corrosion varies significantly from
alloy to alloy. Ideally, field testing is recommended to understand corrosion
behavior of the alloys in mixed gaseous environments and when molten
deposits are present on the alloy surface. In most cases, high-temperature
corrosion data generated in the lab is used to compare and rank the alloy
performances. In this section, different forms of high-temperature corrosion reactions that occur in industries are briefly explained. A special
25-9
emphasis is given to the corrosion mechanisms found in the chemical and
petrochemical process industries.
Oxidation In most industrial processes, oxidation takes place in air
or combustion environments. The combustion environment often consists of free molecular oxygen that dictates the oxidation behavior of the
alloys; however, sometimes stoichiometric and substoichiometric conditions prevail. In such “reducing” environments, partial pressure of oxygen
PO2 is rather low, and the oxidation reaction is rather sluggish and dependent on CO /CO2 or H 2 /H 2O ratios. In the former case, if secondary oxidant (e.g., sulfur) is present, then it often dictates the corrosion kinetics.
The thermodynamic consideration for the formation of oxide is described
using the following reaction:
M + 1 2 O 2 = MO
(25-7)
The oxidation takes place when PO2 in the environment is greater than
PO2 in equilibrium with the oxide. The PO2 in equilibrium with oxide for
Eq. (25-7) is determined from the standard free energy of formation, as
shown in Eq. (25-8), where activities of M and MO are considered to be unity.
⎛ ΔG ° ⎞
1
K p = exp⎜−
⎟=
1
⎝ RT ⎠ PO2 2
(25-8)
Kp is a reaction rate constant in equilibrium. The plots for free energy
of formation for various oxides as a function of temperature (i.e., the
Ellingham diagram) shown in Fig. 25-6 are quite useful in determining the
stabilities of various oxides and PO2 in equilibrium with oxide (Shifler, D.,
“Factors Affecting High-Temperature Corrosion and Materials Properties,”
in Metals Handbook, vol. 13A, Corrosion: Fundamentals, Testing, and Protection, ed. S. D. Cramer and B. S. Covino, Jr., ASM International, Metals
Park, Ohio, 2003). In reducing environments when PO2 is controlled by
either CO /CO2 or H 2 /H 2O ratios, the reactions rate constant is related to
ΔG° in Eq. (25-9):
⎛ ΔG ° ⎞ PCO PO2 2
PH 2 PO2 2
K p = exp⎜−
or
⎟=
PCO2
PH 2O
⎝ RT ⎠
1
1
(25-9)
If equilibrium ratios PCO /PCO2 or PH 2 /PH 2O are known, then PO2 can
be found from Eq. (25-9). The Ellingham diagram also reports CO /CO2 or
H 2 /H 2O ratios for the corresponding oxidation equilibria, which could be
used to determine equilibrium PO2.
The amount of oxidation attack in the alloys can be measured by oxidation kinetics and the amount of metal consumed. The oxidation kinetics
follows three principal laws—linear, parabolic, and logarithmic. The linear kinetics shown in Eq. (25-10) occurs when the scale is porous and the
reaction is controlled by a gas phase mass transfer. The oxidation reactions
follow parabolic kinetics for compact and adherent scales, which are commonly controlled by diffusion of mass transfer through scales. Most of the
oxides follow parabolic oxidation rate laws at higher temperatures, and
absolute rates shown in Fig. 25-7 are useful in comparing the oxidation
kinetics of metals and alloys. The oxides with the highest defect concentrations indicate high rate constants. High-temperature alloys, when exposed
to elevated temperatures, follow linear kinetics due to rapid scale growth
during transient oxidation, but once a protective scale is established, the
oxidation reaction becomes diffusion controlled and follows parabolic
behavior. In the case of thin film oxidation, particularly at lower temperatures, scale formation follows inverse or cubic logarithmic rate laws, as
shown in Eqs. (25-10) and (25-11).
x = kl t
(25-10)
x 2 = 2 k pt
(25-11)
Here kl and kp are linear and parabolic rate constants, respectively, and x is
oxide thickness. The engineering alloys and coatings are designed such that
they can establish thin, slow-growing, and adherent oxide scales for hightemperature corrosion protection. It is seen from Fig. 25-8 that the alloys
are designed to form either alumina, chromia, or silica scales due to rather
low rate constants. It is also seen that even within these scales, depending
25-10
MATERIALS OF CONSTRUCTION
10–8
H2/H2O ratio
CO/CO2 ratio
O
390
0
10–6
10–8
10–6
Fe
–100
+
O4
Fe 3
O2
=6
O3
2
M
M
4
4Cu
+ O2
Po2
10–2
Temperature, °F
1110 1470 1830 2190 2550 2910 3270 3630 3990 4350
750
–200
10–4
10–4
iO
Cu 2O
=2
O2
i+
2N
N
=2
10–2
1
1
CO 2
CoO
=2
=2
O2
M
2
O
+
+
2CO
2Co
H 2O
=2
O2
+
M
2H 2
10–2
1
10–4
102
–300
10–6
C + O2 = CO2
102
ΔG° – RT ln Po2 (kJ/mol O2)
–400
H
C
O
Zn
–500
n
2Z
M
–600
2
+O
=2
Cr
+ O2
M
B
CrO 3
2C
2
=
10–8
+O
2=
104
2C
O
M
B
104
iO 2
Si
–700
2
+O
=S
Al
+O
O2
=2
a
2C
2M
M
106
O
Mg
M
g+
2
O3
Al 2
=
200
400
600
2
+O
CO/CO2 ratio
B
Change of state
M
800
O
Ca
=2
–1100
0
10–14
B
–900
–1200
10–12
106
–800
–1000
10–10
10–16
108
Element
Oxide
Melting point
M
M
Boiling point
B
B
108
1010
1000 1200 1400 1600 1800 2000 2200 2400
Temperature, °C
1014
1012
10–18
10–20
1010
10–22
H2/H2O ratio
0K
Po2, atm
10–200
10–100
10–70 10–60
10–50
10–42 10–38
10–34
10–30 10–28
10–26 10–24
Ellingham/Richardson diagram showing standard free energy of formation of selected oxides as a function of temperature. (“Factors
Affecting High-Temperature Corrosion and Materials Properties,” Metals Handbook Volume 13A Corrosion: Fundamentals. Testing, and Protection.
Reprinted with permission of ASM International. All rights reserved. www.asminternational.org.)
FIG. 25-6
upon the alloy, parabolic rate constants vary, but they typically fall within
the bands represented.
The scale developed on the alloy surface acts as a diffusion barrier;
thus adherent scale formation is extremely important for long-term oxidation or corrosion protection. The adherence of the oxide scale can be
determined by the ratio of the molar volumes of the metal to the oxide
formed on it, known as the Pilling-Bedford ratio (PBR). When there is no
large difference in the molar volumes (PBR ratio is equal or nearly equal
to 1), the scale adheres to the metal. Under cyclic conditions, however,
oxides still spall due to differences in the coefficients of thermal expansion between the oxide and the underlying alloy substrate. Thus, when
the bare metal is exposed to an oxidizing environment, if the alloy cannot
reestablish protective or semiprotective scale, nonprotective scale formation occurs, eventually resulting in increased oxidation rates. The reactive elements, such as La, Zr, Y, and Hf, are added to further improve the
scale adhesion characteristics of the alloy. A number of external factors
affect the oxidation behavior of high-temperature alloys and coatings;
these are primarily thermal cyclic conditions, long-term exposures, flowing atmospheres, and scale volatilization in the presence of water vapor.
In general, alumina-forming alloys offer better oxidation resistance than
chromia-forming alloys, while Ni-base alloys perform significantly better
than Fe- and Co-base alloys. A number of references are available that
report oxidation behavior and performance characteristics of engineering alloys and coatings used in the industries (Young, D. J., “High Temperature Oxidation and Corrosion of Metals,” Elsevier Corrosion Series,
2008, p. 185; Birks, N., G. H. Meier, and F. S. Pettit, Introduction to the
High-Temperature Oxidation of Metals, 2d ed., Cambridge University Press,
Cambridge, UK, 2006, p. 101; Lai, G. Y., High Temperature Corrosion and
Materials Application, 2d ed., Cambridge University Press, Cambridge,
UK, 2006, p. 15).
CORROSION FUNDAMENTALS
For thermodynamic consideration, when carbon activity is ≤1, but high
enough to stabilize carbides, carburization prevails, while if carbon activity is supersaturated >>1, carbon becomes stable as deposits known as
coking. The catalysis process of carbon deposition by the alloy often leads
to catastrophic disintegration of the metal surface, resulting in continued
metal wastage rates.
Depending on the process conditions, a combination of the following
reactions occurs:
T (°C)
1200 1100
10–5
1000
900
800
700
600
Parabolic scaling constant, kp (g2cm–4s–1)
10–7
FeO/Fe3O4/Fe2O3
10–9
CoO
10–11
Cr2O3
10–13
Al2O3
10–15
10–17
(25-12)
2CO = CO 2 + C (Boudouard reaction)
(25-13)
CH 4 = 2H 2 + C (Hydrocarbon thermal cracking)
(25-14)
ac =
6
7
8
9
10
104/T (K–1)
11
12
13
FIG. 25-7 Parabolic rate constants for selected oxides showing scatter bands
reported in literature for chromia- and alumina-forming alloys. (Gleeson, B., HighTemperature Corrosion of Metallic Alloys and Coatings. Corrosion and Environmental
Degradation, Vol. II, p. 179, ed. Schutze, M., Reprinted with permission from WileyVCH, Weinheim, Germany, 2000.)
Corrosion in Carbonaceous Environment—Carburization and
Metal Dusting In many industrial processes, the combustion of fossil fuels creates exhaust gases containing H2, CHx, CO, and CO2 mixtures, while a number of processes in the chemical and petrochemical
industry involve reducing conditions to produce mixture of CO and H2
via steam methane re-forming of natural gas. The presence of stable
carbon as a solid over a wide temperature range from various oxidants
CO, CO2, and CHx becomes the source of carburization or metal dusting.
Transient oxidation (linear
growth region; x = kl t)
Oxidation rate
CO + H 2 = H 2O + C (CO reduction reaction)
The hydrocarbon cracking reaction produces high carbon activity with
increased temperature and often controls the carburization reaction at high
temperatures [above about 750°C (1382°F)]; however, CO reduction and
Boudouard reactions generate high carbon activity even at relatively low
temperatures [∼450°C to 700°C (1292°F)] and takes time to equilibrate in
this temperature range. Consequently, supersaturated gas produces high ac,
resulting in metal dusting reactions. The carbon activity for the preceding
reactions is calculated as in Eq. (25-15):
NiO
SiO2 on MoSi2
Diffusion controlled
(parabolic rate kinetics;
x2 = 2kpt)
Formation of protective TGO
Time, h
FIG. 25-8 Oxidation kinetics showing linear and parabolic behavior of thermal
grown oxide formation in high-temperature alloys.
25-11
k 12
p PH 2 PCO
PH 2O
; ac =
2
k 13
p pCO
PCO2
; ac =
k 14
p PCH 4
P 2H2
(25-15)
where kp12, kp13, kp14 are reaction rate constants for Eqs. (25-12), (25-13), and
(25-14), respectively, and they are usually calculated from the Gibbs free
energy of formation, ΔG°.
Carburization results in the formation of intergranular and intragranular carbides throughout the matrix. The carburization front is dictated by
ac, the local carbon activity, and if ac is high enough, such that carbides of
the elements present in the alloy can be formed, the front moves internally
within the alloy matrix. The carbide formation often results in embrittlement and can severely affect the mechanical properties of the alloy. In
general, Fe-base alloys are more susceptible to carburization than Ni-base
alloys, and the addition of nickel improves the carburization resistance of
the alloys by lowering the diffusivity and solubility of carbon (Young, D. J.,
High Temperature Corrosion and Materials Application, Elsevier Corrosion
Series, 2008, p. 397). Moreover, minor additions of elements, such as molybdenum, niobium, tungsten, and reactive elements, help improve carburization resistance (Young 2008, p. 397; Lai, G. Y., High Temperature Corrosion
and Materials Application, 2d ed., Cambridge University Press, Cambridge,
UK, 2006, p. 54). The additions of Si and Al also help improve carburization
and metal dusting resistance by forming dense scale that slows down carbon ingress through the scale. Additionally, elements such as copper and tin
that are noncatalytic to carbon also minimize metal dusting attack in the
alloys (Young 2008, p. 397).
Corrosion in Sulfur-Bearing Environment—Sulfidation and Hot
Corrosion Sulfur is a strong corrodent and can cause catastrophic corrosion when it is present as an impurity in fuel and feedstocks. In certain
industrial processes containing excess air or free molecular oxygen, fuel or
feedstock reacts completely to convert sulfur into SO2 or SO3 gas mixtures.
Such an environment is considered oxidizing-sulfidizing in nature. It is less
aggressive for the most high-temperature alloys up to approximately 900°C
(1652°F) compared to other sulfidation reactions unless molten deposits are also present. Sulfidation in an O2–SO2/SO3 environment is not dictated by the formation of low-temperature molten sulfide eutectics such as
Ni–Ni3S2 [Tmelt = 635°C (1175°F)] unlike in other forms of sulfidation attack
(Birks, N., G. H. Meier, F. S. Pettit, Introduction to the High-Temperature Oxidation of Metals, 2d ed., Cambridge University Press, Cambridge, UK, 2006,
p. 165). Notwithstanding, oxidizing-sulfidizing corrosion can become a catastrophic degradation mechanism in the presence of small amounts of salt
vapors. Sulfidizing-oxidizing environment in the presence of salt vapors of
alkali sulfates and chlorides can cause the destruction of protective thermally grown oxide (TGO). This form of corrosion attack is defined as “hot
corrosion/coal-ash corrosion/oil-ash corrosion,” depending on the types
of salt deposits and the mixed gaseous environment. There are two types
of hot corrosion observed in industrial environments: type I and type II.
Typically, type I hot corrosion occurs above the melting temperature of salt
deposits [800°C to 1000°C (1472°F to 1832°F)], while type II hot corrosion
occurs below the melting temperature of external salt deposits [600°C to
750°C (1112°F to 1382°F)] (Birks, N., G. H. Meier, F. S. Pettit, Introduction
MATERIALS OF CONSTRUCTION
to the High-Temperature Oxidation of Metals, 2d ed., Cambridge University
Press, Cambridge, UK, 2006, p. 205). The molten deposits in type II hot corrosion are established due to sufficient PSO3 present in the system from the
reaction between salt deposits and reaction product (e.g., Na2SO4–NiSO4 or
Na2SO4–CoSO4) (Birks et al. 2006, p. 205).
In many environmental conditions, sulfur in the system is converted to
H2S due to a lack of free molecular oxygen in gaseous mixtures containing
CO, CH4, and H2. Such conditions are typically called reducing or nonoxidizing conditions. In such environments, oxidation and sulfidation occur
simultaneously, and the types of scales and precipitates formed depend on
the partial pressures of oxygen and sulfur. The phase stability diagrams as
shown in Fig. 25-9 for an Ni-S-O system are quite helpful in understanding
degradation modes and resulting corrosion products. Sulfidation occurs
because of the formation of internal sulfides beneath external oxidation/
corrosion products. The common alloying elements that react to form various sulfides are Cr, Fe, Ni, Co, and Mn. While considering the thermodynamic stabilities of various sulfides, it is also important to compare their
rates of formation. For instance, the growth rate for chromium sulfides
is four to five orders of magnitude faster than that for chromium oxide.
Another important consideration unlike oxidation is the formation of molten eutectics (e.g., Ni–Ni3S2), which further results in catastrophic degradation of the alloys.
Ni-base alloys are in general resistant to oxidizing-sulfidizing and hot corrosive environments, but they are susceptible to catastrophic sulfidation in
reducing environments due to the formation of low-melting-temperature
sulfides, while Co-base alloys offer superior performance in reducing environments. Co-base alloys offer rather inferior type II hot corrosion resistance compared to Ni-base alloys. Higher amounts of chromium additions
help in lowering sulfidation attack in the alloys along with additions of Al
and Si (Young, D. J., High Temperature Oxidation and Corrosion of Metals,
Elsevier Corrosion Series, 2008, p. 361; Birks, N., G. H. Meier, and F. S. Pettit,
Introduction to the High-Temperature Oxidation of Metals, 2d ed., Cambridge
University Press, Cambridge, UK, 2006, p. 163; Lai, G. Y., High Temperature
Corrosion and Materials Application, 2d ed., Cambridge University Press,
Cambridge, UK, 2006, p. 130). Minor elemental additions of Nb and Zr, and
Ti also lower sulfidation kinetics. (Lai 2006, p. 141).
Corrosion by Halogens Corrosion in environments containing
chlorine and fluorine is commonly encountered, while corrosion by the
presence of bromides and iodides is occasionally found in industrial environments. The elements present in high-temperature alloys react with
halogens to form low-melting-temperature metal halides, while some elements form halides that even vaporize in temperature ranges where hightemperature alloys are used. The vaporization of metal halides due to high
vapor pressures could cause catastrophic corrosion in high-temperature
alloys. Ni-base alloys are generally more resistant in chlorine- and fluorine-bearing environments due to the formation of FeClx volatile vapor
species, and high chromium, aluminum, and silicon contents help in the
corrosion resistance. In reducing fluorine-bearing environments, however, Ni-Mo–based systems seem to offer the best resistance, but the trend
reverses when oxygen is present in the system. In O2 and chlorine/fluorine
environments, Mo and W form highly volatile oxychlorides and oxyfluorides, and the use of high refractory-containing alloys should be avoided
(Lai 2006, p. 94).
Nitridation Nitridation typically occurs in ammonia or nitrogenbearing environments. When alloys are exposed to highly oxidizing environments, nitridation is less severe unless the protective scale cannot
offer resistance, especially in thermal cycling conditions. For instance, in
combustion environments involving thermal cycling, most alloys offer oxidation resistance up to certain point, but after long-term exposures they
suffer internal nitridation. Nitrogen gas, N2, is not as severe as ammonia,
and nitridation in nitrogen environments often occurs at temperatures
Log pso2, atm
–10
–5
0
0
p – 0.2 atm
NiS (I)
–10
NiSO4 (s)
Log ps2, atm
25-12
–20
NiO (s)
Ni (s)
–30
–40
–40
–30
–20
Log po2, atm
–10
0
FIG. 25-9 Phase stability diagram for nickel-sulfur-oxygen system at 1250 K (1790°F).
(“Factors Affecting High-Temperature Corrosion and Materials Properties,” Metals
Handbook Volume 13A Corrosion: Fundamentals, Testing, and Protection. Reprinted with
permission of ASM International. All rights reserved. www.asminternational.org.)
above 1000°C (1832°F), while alloys may suffer nitridation in ammonia at
a relatively low temperature range. Nitridation in industrial atmospheres
takes place due to dissociation of nitrogen or ammonia, subsequent adsorption followed by absorption of [N] into the metal. Consequently, the metal
becomes supersaturated with [N], thus precipitating nitrides of the elements present in the metal. Nitrides of some of the common elements
observed in high-temperature alloys include Fe2N, Cr2N, MoN, TiN, AlN, and
NbN. Ni-based alloys are more resistant than Fe-base alloys, while aluminaforming alloys that can establish alumina scales in reducing environments
are generally more resistant than chromia-formers (Lai, G. Y., High Temperature Corrosion and Materials Application, 2d ed., Cambridge University Press,
Cambridge, UK, 2006).
Corrosion by Molten Salt or Molten Metal This form of degradation is observed when metal is completely immersed in a molten environment, either in molten salt or molten metal. In such an environment, an
external gaseous mixture, even if present, does not directly interact with
metal. Mixed gaseous environments still affect corrosion properties, but
by the reaction with molten medium rather than by its direct interaction
with the alloy surface. In this form of corrosion, molten medium reacts with
elements present in the alloy and results in leaching of the most reactive
elements from the alloy substrate into the molten medium. The resistance
of the alloys depends on whether the molten medium is oxidizing or reducing, and the performance of the alloys varies significantly from one medium
to another. Some commonly found molten media are molten nitrates, chlorides, carbonates, zinc, and sodium.
CORROSION PREVENTION
MATERIALS SELECTION
Introduction The first step in determining the materials of construction for a project is an evaluation of the environments that will be present
to determine if they are corrosive to the materials commonly used. As a
general rule, carbon steel is probably the material of choice for projects if
corrosion is not a concern, and the maximum temperature will not exceed
425°C (800°F). Above this temperature, a variety of alloy steels and other
materials may be required to provide sufficient long-term integrity. Most
organic chemicals are not corrosive to steel, and gases and vapors are also
not corrosive to steels in most cases. However, acids, molten salts, and aqueous liquids are corrosive to steels, and these environments would normally
require resistant materials. Usually the engineering and construction firm
engaged to build the project will have specific materials recommendations
for the various units in the project. However, it is usually helpful to consult
authoritative sources to become familiar with the performance of materials that will be used in the project. Table 25-2 shows several sources that
may be helpful. It must be noted that literature references often do not provide details on materials of construction, and the guidance found in these
sources may not answer many questions that arise.
CORROSION PREVENTION
TABLE 25-2
Sources of Corrosion Resistance Information
Name
Source
Corrosion Survey Database (COR SUR)
MTI Materials Selector (MS) Series
NACE
MTI
Materials Selector for the Chemical
Process Industries
Handbook of Stainless Steels
Corrosion Resistance Handbook
Chemical Resistance of Plastics and
Elastomers
MTI
McGraw-Hill
Wm. Andrew
Wm. Andrew
Environment
Reference
Chemicals
Industrial
chemicals
Industrial
chemicals
Chemicals
Chemicals
Chemicals
1
2
2
3
4
4
note: Vendors usually have good information on materials they supply.
References:
1. NACE International, The Corrosion Society, International, Copyright © 2002.
2. Materials Technology Institute, 1001 Craig Road, Suite 490, St. Louis, MO 63146.
3. Peckner, D., and I. M. Bernstein. Handbook Of Stainless Steels. McGraw-Hill,
New York, 1977.
4. Elsevier B.V. Registered office: Radarweg 29, 1043 NX Amsterdam, The
Netherlands.
Strategy After information has been obtained on the corrosion rates
that can be expected in an environment, it is necessary to determine the
strategy for dealing with the possible corrosion that may occur with various
materials. In order to judge the corrosion rates that may be found for different materials, see Table 25-3. This table provides guidelines for making
decisions on how to deal with the corrosion that may be encountered.
Generally, corrosion-resistant alloys cost more than carbon steel, so the
decision to use a corrosion-resistant material will add cost to the project.
However, there are options that may reduce the cost while still obtaining
the benefit of corrosion-resistant materials. In addition, in some cases, it
may be possible to deal with the corrosion loss by increasing the thickness of
metal. This additional metal is known as a corrosion allowance, and its cost
is minor compared to the cost of many corrosion-resistant alloys. Corrosion
allowances are generally used when the corrosion rate is low or moderate
and there are no issues of pitting or cracking. In addition, when a corrosion
allowance is specified, there must be a program to carry out routine inspections to monitor the loss of metal so that action can be taken before the
corrosion loss exceeds the corrosion allowance. As a general rule, corrosion
allowances are in the range of 3 to 10 mm (0.125 to 0.4 in)
Another possibility is to investigate the possibility of cladding or lining
the vessel or piping with a corrosion-resistant material so that the carbon steel carries the structural load while the layer of corrosion-resistant
material provides the protection from corrosion. This approach is effective and has been widely used. The use of cladding or overlaying adds cost
beyond the base cost of the materials, so it is seldom economical to use
this approach if the total thickness of metal is less than 25 mm (1.0 in). In
these cases, it usually is more economical to use solid corrosion-resistant
material construction. The subject of cladding will be discussed in more
detail later.
The issue of environmentally assisted cracking must be considered and
understood with any project. This form of corrosion cannot be solved with
a corrosion allowance. Cladding and other barrier layers can be effective,
but minor penetrations through the layer can result in catastrophic failures,
so it is very important to set up procedures for monitoring the protective
layer performance when this form of failure is a possibility. In particular,
hydrogen sulfide, hydrogen fluoride, and hydrogen cyanide are very aggressive with carbon steels, and chlorides are aggressive with austenitic stainless steels (NACE Standard MR0175, “Metals for Sulfide Stress Cracking
and Stress Corrosion Cracking Resistance in Oilfield Environments,” NACE
TABLE 25-3
Corrosion Rate Classification
CR μm/yr
CR mpy
Designation
Preventive measures
CR < 25
25 ≤ CR < 50
50 ≤ CR < 400
400 ≤ CR < 825
≥ 825
CR < 1.0
1 ≤ CR < 2
2 ≤ CR < 16
16 ≤ CR < 32
CR ≥ 32
Noncorrosive
Slightly corrosive
Moderately corrosive
Corrosive
Severely corrosive
1
2
3
4
4
Preventive Measures:
1. No preventive measures except in cases where corrosion product contamination
is not allowed.
2. Corrosion allowance or protective coatings used.
3. Corrosion allowance, protective coating, cladding, lining, or corrosion-resistant
alloy used.
4. Protective cladding, lining, or corrosion-resistant alloy used.
25-13
International, Houston, Tex., 1975). Also, concentrated sodium hydroxide
will cause cracking with many alloys. When dealing with these chemicals,
special precautions are required.
DESIGN
General Process equipment is usually designed to deliver maximum performance at minimum cost, but corrosion concerns may not be
reflected in the design decisions. As a result, problems may arise in various
areas unless the design is modified to mitigate the corrosion possibility. The
following discussion covers several types of equipment with corrosion concerns that have been observed.
Heat Exchangers Heat exchangers are designed to either heat or cool a
process fluid. Corrosion issues have been observed with both types of heat
exchangers. In many systems, steam is used to heat the process stream. One
common issue that occurs in these systems is that the heat exchanger is
oversized so that only a portion of the surface area is required to heat the
fluid to the required temperature. As a result, condensate fills the bottom
of the unit, so the remaining surface area exposed to the steam heats the
process fluid. As a result, the process fluid will be overheated, and this can
cause corrosive attack in some systems. Vertical exchangers are less susceptible to this problem, but horizontal exchangers can experience severe corrosion on the upper tubes. In exchangers where the process fluid is being
vaporized on the shell side of the exchanger, it is important to maintain
the liquid level in the shell above the highest tube to prevent dry boiling
on the upper tubes. This has been the cause of corrosion and cracking in
these tubes.
The decision as to which fluids will be routed through the shell side versus the tube side of a heat exchanger is usually based on pressure drop or
fluid considerations. In most cases, these decisions do not affect the corrosion performance of the exchanger, but occasionally the decision becomes
important. For example, in a boiler using horizontal tubes with the boiler
water on the tube side being heated by flue gas, severe corrosion was
observed on the bottoms of some of the tubes. The corrosion was identified
as caustic gouging, and it was attributed to poor flow distribution of the
boiler water resulting in having the tubes almost dry at the exit. The design
in this case was inadequate for the service conditions, with the boiling
occurring on the tube side and poor flow control from the manifold feeding
the tubes.
Another example of a design issue occurred in a nitric acid plant where
the cooler-condenser hot end tube sheet suffered serious corrosion from the
condensation of nitric acid on the inlet end surface. This problem occurred
because the corrosion products that accumulated there were not washed
away by the condensing acid, and the development of a high concentration
of chrome six ions aggravated the corrosion. This was another example of
horizontal tubes in the exchanger, but with condensing rather than boiling.
Insulating the cooling water side of the tube sheet prevented the condensation and minimized the corrosion damage.
Heat exchangers used for cooling and condensing process fluids are
less susceptible to corrosion issues, but problems can occur when cooling waters are overheated. In particular, stainless steels are susceptible to
pitting and stress corrosion cracking in waters with significant chloride
contents. In both of these mechanisms it has been observed that there is
a critical temperature above which the problem becomes severe. Another
problem that has been observed is that horizontal condensers can have condensate block the lower tubes in the exchanger if the outlet is not designed
properly to allow free flow. In this case, the condensate will accumulate until
the pressure drop becomes large enough to force the accumulated liquid out
in a slug. This slug can damage downstream equipment through a fatigue
mechanism [Dean, S. W., “Chloride SSC of Stainless Steel? No – Cyclic Strain
Cracking!” Materials Performance 39(9): 78–87 (2000)].
Fired heaters also present design challenges from a corrosion perspective. In systems where steam is present in the process fluid, cool spots that
allow condensation to occur above the heated zone will drip, and the thermal shock from this dripping can cause thermal fatigue cracking of components in the furnace. In such cases the use of insulation to eliminate cool
spots solved the problem.
Distillation and Absorption Columns These units are often subject to
corrosion damage because they are involved with separating and concentrating components that may be corrosive. A cursory examination of such a process would suggest that the most corrosive areas should be at the bottom where
it is hottest, or perhaps at the top. However, this is seldom observed. In many
cases where water is present, a volatile component in the system is trapped
in the column because it ionizes at lower temperatures, thereby becoming
less volatile, and recombines at higher temperatures, increasing its volatility.
Hydrogen chloride and carbon dioxide both have been observed to exhibit this
behavior. As a result, the location where the compound concentrates causes
corrosion to occur at a high rate. Several approaches have been used to deal
25-14
MATERIALS OF CONSTRUCTION
with this problem. In the case of carbon dioxide accumulation, the addition
of sodium hydroxide to the process stream was successful in removing the
carbon dioxide from the system. In the case of hydrogen chloride, the use of a
bypass that removed some of the liquid from the point where it concentrated
was successful. Another approach is to upgrade the materials of construction
with an alloy that resists the offending contamination in the region where the
concentration occurs.
Tankage Although tankage is generally not considered a critical area for
corrosion control, there are cases where careful design is required to assure
good performance. In flat-bottom storage tanks, the tank bottoms often
suffer corrosion damage both from the inside and from the soil side. With
organic liquids that are less dense than water, a water layer can develop on
the tank bottom from condensation, pressure testing, or minor impurities in
the liquid. This layer can become very corrosive over time because in most
situations the outlet nozzle for the tank is above the bottom. Corrosion from
the soil side is a concern if the sand layer below the tank becomes filled with
water and does not drain properly. Cathodic protection systems are now
used to prevent soil-side corrosion. Sulfuric acid tanks are a special case,
and their design incorporates many important features to minimize corrosion damage (NACE Standard SP02-94, “Design, Fabrication, and Inspection
of Fresh Sulfuric Acid Tanks, 2006” and NACE Standard SP02-05, “Design,
Fabrication, and Inspection of Tanks for Storage of Petroleum Refining
Alkylation Unit Spent Sulfuric Acid at Ambient Temperature, 2015” NACE
International, Houston, Tex.).
Piping Although piping standards cover most of the issues involved
with piping designs, there are some details that are overlooked in many
cases. One important detail is the elimination of dead legs in piping systems.
Dead legs are sometimes included to allow for future expansion or changes
to the flow system. In other cases they are included to facilitate start-up
or shutdown operations where it may be necessary to introduce steam or
other material. These areas are locations where corrosive components can
accumulate and cause damage without attracting much attention. From a
design perspective, if the dead leg cannot be eliminated, an effort should be
made to prevent liquids from accumulating in it. This can be accomplished
by having the entry point at the top of the pipe rather than at the bottom.
Also, placing the valve as close to the junction point as possible can minimize the quantity of liquid that accumulates there.
Bypass lines around heat exchangers are sometimes used to control the
fluid temperature exiting the exchanger. The problem in this case is that
the mixing point where the bypassed fluid encounters the fluid from the
exchanger can be subjected to severe thermal stresses from minor flow
variations in the streams. Rather than using a tee or wye connection, it may
be better to have a concentric connection so that the mixing occurs over a
larger area and does not affect the pipe walls.
ALTERING PROCESS/ENVIRONMENT
Introduction Changing the environment or the process is sometimes
the best way to deal with corrosion issues. However, it requires knowledge
of the corrosion mechanism that occurs in the environment in question. As
a general rule, changes to the environment appear to be impossible in most
cases. The ability to find some flexibility in the conditions will often pay dividends in terms of being able to use less expensive materials. Some examples
are discussed next.
Passivation Promotors Stainless steels and other alloys with significant chromium contents owe their corrosion resistance to an oxide passive
film that forms spontaneously in the presence of oxidizing substances in
the environment. This film is insoluble in many aqueous and acid environments, but its ability to form is dependent on the concentration of oxidizing
substances in the environment. In cases where the natural formation of this
film is marginal, the corrosion resistance of the alloy can be improved by
the addition of small amounts of an oxidizing component. This approach
has been used with stainless steels in concentrated sulfuric acid. In this
case, the addition of small amounts of hydrogen peroxide or nitric acid was
successful in reducing the corrosion rate into the noncorrosive region. The
addition of air is commonly used in urea reactors to keep the stainless steel
in the passive region. In another case, the addition of a small concentration
of hydrogen peroxide was successful in reducing the corrosion of stainless
steel pipes and tubing in the condenser from a sulfuric acid concentrator.
In the case of steels, passivation is also possible, but it requires a higher
level of oxidizer and pH control. Nitrite additions have been used for this
purpose. In addition, the condensate from air compressors is usually not
corrosive to steels because of its high oxygen content.
Removal of Oxidizers In many aqueous environments, the presence
of dissolved oxygen is the driving force for the corrosion of steel. In these
systems, the removal of dissolved oxygen can reduce the corrosion rate to a
low level. This approach is commonly used in boiler feedwater preparation.
In the case of high-pressure boilers, it is particularly important to keep the
dissolved oxygen at a very low level, depending on the pressure required. In
this case, both mechanical deaeration and chemical scavengers are used.
These systems are designed to maintain the boiler tubing in good operating condition and to minimize pitting that will occur if the oxygen level
becomes too high.
Scale Enhancers In the case of cooling water, oxygen removal is not
practical when cooling towers operate by the evaporation of water. In these
cases, corrosion control is achieved by the addition of chemicals that promote the formation of a protective scale on steel and copper alloy tubes and
pipes. These chemicals usually include phosphates or molybdates, which
are effective in maintaining the system pH and minimizing corrosion of the
metals in the system. In the past, chromates were used for this purpose, but
they are no longer permitted in most applications. Maintaining the system
pH is important in order to keep the pH in a range where the solubility of
iron in the liquid is very low.
Cracking Preventives In the cases of anhydrous ammonia and several
of the lower molecular weight alcohols, the addition of about 0.1 percent
water has been found to be effective in preventing the stress corrosion cracking of carbon steels. Although in some cases this is not possible because
of product specifications, where it can be practiced it has been found to
be a practical solution to a serious problem (Dean, S. W., Materials Selector for Hazardous Chemicals, Organic Solvents, Publication MS-8, Materials
Technology Institute, St. Louis, Mo., 2011, p. 9).
Removal of Aggressive Compounds Because minor concentrations
of specific compounds can cause corrosion damage, the removal of these
compounds is another approach to controlling corrosion. The removal
of oxygen from boiler feedwater was mentioned earlier. Chloride removal
systems have been used to prepare water for use in stainless steel equipment to reduce the possibility of chloride stress corrosion cracking. Another
example is the use of a copper removal system for cooling water used in
aluminum exchangers. Aluminum alloys are susceptible to pitting damage
in cooling water, but if trace copper contamination is removed from the
water, the aluminum will give better performance. In this case, an aluminum foil contactor was placed in the cooling water inlet to the exchanger,
and it functioned by replacing copper ions with aluminum ions in the water.
Process Changes Sometimes process changes can be made to prevent
or minimize corrosion problems. For example, in distillation columns with
external reboilers, the boiling process may concentrate corrosive species
on the tube surfaces to the point where they cause rapid corrosion. This is
especially true for horizontal tube units. In this case, it might be possible to
increase the pressure in the exchanger so that boiling does not occur on the
tubes. The vaporization can occur when the fluid passes through a pressure
let-down valve entering the column bottom. Another example is the case of
a centrifugal pump where the minimum net static head is maintained at the
pump inlet to prevent cavitation that will damage the impeller.
When cooling water can be corrosive to exchangers, some possibilities
to prevent problems include using fin-fan air exchangers or using a closedloop inhibited system between the critical exchanger and the cooling tower
system.
Another example occurred in a nitration process where sulfuric acid was
used to remove the water generated by the reaction, thereby allowing the
reaction to proceed to completion. The problem in this case was that sulfuric acid is very corrosive to stainless steels in the concentration range from
80 to 91 percent. As a result, it is conventional to use a sulfuric acid concentrator that concentrates the recycled acid to 93 percent. By concentrating
the acid to 89 percent and adding a small concentration of nitric acid, it was
possible to eliminate the difficult steps required to reach 93 percent, and
also to handle the 89 percent acid with conventional stainless steel alloys.
In all of these cases the key is understanding the corrosion mechanisms
that occur and taking steps to reduce or eliminate the driving force or to
enhance the protective surface film.
BARRIERS TO PREVENT CORROSION
A common technique of corrosion prevention is to place a barrier between
the corrosive environment and the underlying alloy that would be corroded
by the environment. Barriers can be either nonmetals or an alloy that is
more corrosion resistant than the alloy it is protecting.
Paints, Coatings, and Linings Paints, coatings, and linings are systems of pigments with natural or synthetic resins that form a protective film
by drying, oxidizing, or polymerizing (Dillon, C. P., Corrosion Control in the
Chemical Process Industries, 2d ed., Materials Technology Institute, St. Louis,
Mo., 1994, p. 346). There are many different types of systems, and the more
common ones will be discussed.
The terms paint and coating are often used interchangeably. For the purpose of this section, paint will be used. Paints are applied to the exterior
of equipment to prevent corrosion by the external environment. They are
a very important means of preventing corrosion under insulation (CUI)
CORROSION PREVENTION
because insulation can often act as a water trap that can create very corrosive conditions. A lining is a system applied to the interior of process
equipment.
The correct system selection and application is critical to achieving the
desired level of corrosion prevention. All paints and linings require some
level of surface preparation. If the preparation is inadequate, proper adhesion of the system to the underlying alloy won’t be achieved, and premature failure will occur. Common methods of surface preparation are sand
or grit blasting, water blasting, and hand or tool cleaning. Different systems
require different levels of preparation. There are three common methods
of classifying surface preparation: the National Association of Corrosion
Engineers (NACE), the Steel Structures Painting Council (SSPC), and the
Swedish Pictorial Standards (SA). A commercially available paint or coating
system will state what level of surface preparation is required.
When the paint or lining is applied, the first coat is referred to as the
primer coat. Some paints/linings will require more coats. Additional coats
may be of the same type or a different type. For example, epoxies are often
topcoated with a urethane because the urethane has greater ultraviolet
light resistance. Two- and three-coat systems are commonly used. A rule of
thumb is that for a particular environment, more coats will last longer and
provide better corrosion resistance.
Common paints and linings are:
1. Inorganic zinc. This is zinc particles in a silicate binder. This is widely
used on carbon steel. The zinc particles provide galvanic protection to the
steel. It should never be used on stainless steel. In a fire situation, the zinc
could melt and cause liquid metal embrittlement of the stainless steel.
2. Epoxies. These are formulated from polyphenol and epichlorinhydrin
and require a catalyst for curing. They have good corrosion resistance to
a wide variety of conditions (Dillon, C. P., Corrosion Control in the Chemical Process Industries, 2d ed., Materials Technology Institute, St. Louis, Mo.,
1994, p. 349).
3. Epoxy mastics. These paints are very useful for maintenance painting
because they don’t require the same level of surface preparation and are
often used over old, still adherent paint systems. Be aware that if the underlying old system has lost its adherence, the new epoxy mastic will not be
properly adhered to the underlying metal.
4. Silicones. Silicones have very high temperature limits and are normally
a niche product for high-temperature applications such as furnace stacks.
5. Urethanes. Urethanes are available in a wide variety of colors and have
excellent gloss retention and weathering resistance. They are commonly
used as topcoats for aesthetic reasons.
6. Alkyds. Alkyds do not have good chemical resistance, but they are
inexpensive and easy to apply. They have limited use for protecting process
equipment but are often used for buildings, both interiors and exteriors.
The best chance of getting a good paint application is during initial fabrication. Once equipment is in service, it may be difficult to repair or replace
a paint system once it starts to fail. Restrictions on such surface preparation
techniques as grit blasting often preclude getting the desired level of surface preparation. If the equipment is insulated, removal and reinstallation
of the insulation will significantly increase repainting costs. Money spent
up front on a superior system will often pay for itself in the life-cycle cost of
the equipment.
Linings are used on the internal surfaces of process equipment. This is an
area that requires great care. Linings will almost invariably have pinholes,
referred to as holidays, that allow the process contents to contact the alloy.
Even if a lining is pinhole free on initial application, flaws in the lining can
develop once the equipment is in service. It is not good practice to rely on a
lining to protect equipment from highly corrosive process conditions. Any
flaw in the lining can lead to very rapid failure. The most common use of
linings is to either slow corrosion in mildly corrosive systems or to protect
the process contents from iron contamination that could result from even
low levels of corrosion.
Paints and linings can be applied by several methods, including spraying, rolling, and brushing. For large areas, spraying is almost always the
economic choice. Paint systems will come with instructions for application. After application, inspection is often needed to ensure quality. This
will include a thickness check to be sure the paint is neither too thin nor
too thick. For large or critical jobs, a National Association of Corrosion
Engineers–qualified coating inspector can be employed to check quality.
A coating system that can be used in lieu of painting is thermal spray
aluminum. In this process, a spray of molten aluminum droplets is applied
to a steel substrate. This method is considerably more expensive than conventional painting, but it results in a superior corrosion barrier and a much
longer life. The life-cycle costs may make this an attractive option. It can be
used on both bare and insulated steel.
Galvanizing Galvanizing is the deposition of a zinc coating on a ferritic, normally carbon steel, substrate. The zinc serves two corrosion prevention purposes. The zinc will react with the atmosphere to form a zinc oxide
25-15
corrosion product that will protect the steel by forming a barrier. Also, zinc
is active to steel in the galvanic series and will provide galvanic protection.
Galvanizing is done by two primary processes. In hot dip galvanizing, the
steel component is placed in a bath of molten zinc. Zinc will solidify on the
surface. In electrogalvanizing, zinc ions are electrodeposited on the steel.
Galvanizing is commonly used on structural steel, pipe, fasteners, and
other components that will be directly exposed to the atmosphere. Galvanized steel can be difficult to paint due to problems in getting adequate
surface preparation. If painting on galvanized steel is necessary, care
must be taken, and consulting with a painting subject matter expert is
recommended.
Metal Barriers Metal barriers are most commonly used when
equipment is constructed from a ferritic steel such as carbon steel with a
corrosion-resistant alloy (CRA) placed between it and the process environment. The use of a metal barrier has the advantage of using lower-cost ferritic steel as the underlying substrate, which provides most of the structural
strength. The higher-cost CRA is normally much thinner than the steel. This
provides for significant cost advantages over the use of solid CRA. The cost
advantage increases with increasing thickness of the equipment.
The types of metal barriers can be classified by the construction
technique.
1. Loose lining
2. Roll cladding
3. Explosive cladding
4. Weld overlay
Loose Lining Loose linings are thin sheets of a corrosion-resistant alloy
placed over the underlying substrate. There are different ways to do this, but
a common technique is to attach the sheets to the substrate by fillet welds.
As more sheets are laid down, they are overlapped and seal-welded to each
other. It is critical that all welds attaching sheets are leak free. Haynes International publication No. H-2010, Fabrication of HASTELLOY ® CorrosionResistant Alloys, shows this technique in detail (Haynes H-2010, Fabrication
of HASTELLOY ® Corrosion-Resistant Alloys, Haynes International, Kokomo,
Ind., 2003, p. 18-24).
Good welding technique is critical. A weld flaw may allow process contents to get behind the corrosion-resistant sheets and corrode the steel substrate. This type of problem can be very difficult to detect by inspection, and
the first indication of a problem can be a loss of process containment.
Roll Bonding Roll bonding is a technique where a CRA is metallurgically bonded to a ferritic steel by placing the CRA on the ferritic steel and
then pressing the two alloys together between rolls at high temperature and
high pressure. The critical element for plate quality is the bond between the
steel and the CRA. This bond can be measured by the shear strength of the
bond and the extent of any unbonded areas between the plates. ASTM A264
covers the purchase requirements for chromium-nickel steel-clad plates.
These plates can be fabricated into welded pipe.
Explosion Bonding Explosion bonding creates a metallurgical bond
between a CRA and a ferritic steel by placing the CRA on top of the steel,
covering it with an explosive charge, and then detonating the charge. The
compressive forces created by the detonation create a solid state weld bonding the CRA to the steel. For certain metals such as zirconium, explosion
bonding is the only technically viable method.
Weld Overlay Weld overlay, as its name implies, is placing a CRA on a
ferritic steel by using a welding process to lay down layers of weld metal on
the steel substrate.
Roll bonding, explosion bonding, and weld overlay are often considered
competing processes. When these techniques are used in equipment fabrication, quality control of the resulting plate is very important. Standards in
common usage to insure quality are:
ASTM A263 Standard Specification for Stainless Chromium Steel-Clad
Plate
ASTM A264 Standard Specification for Stainless Chromium-Nickel SteelClad Plate
ASTM A265 Standard Specification for Nickel and Nickel-Base Alloy-Clad
Steel Plate
ASTM B432 Standard Specification for Copper and Copper Alloy Clad
Steel Plate
ASTM B898 Standard Specification for Reactive and Refractory Metal
Clad Plate
Nonmetallic Barriers In addition to the metallic barriers discussed,
several types of nonmetallic barriers are used in the process industries.
These include:
1. Glass linings
2. Rubber linings
3. Refractory linings
Glass-Lined Equipment Glass-lined equipment is fabricated by bonding a layer of glass to a steel substrate. Glass thickness is normally 0.22 cm
(0.085 in) or less. Almost all types of equipment are available, including
25-16
MATERIALS OF CONSTRUCTION
reactors, tanks, pipe, valves, and fittings, subject to size limitations. Glass
is resistant to almost all acids except hydrofluoric. Glass-lined equipment
has wide usage where high purity is required and where even low corrosion
rates can contaminate the product.
Glass is brittle by nature, and any chipping that allows the process to
contact the steel substrate may result in a failure by corrosion. Glass-lined
equipment is repairable by the use of metal patches. Tantalum is commonly
used. However, other materials such as stainless steel can be used, as long
as the material has acceptable corrosion resistance to the process environment. The key is to detect the failure of the glass before the failure of the
steel occurs. Fault detectors are commercially available that use electrical
techniques to determine when a flaw has occurred that results in contact
between the process and the steel.
Rubber-Lined Equipment Rubber is a generic term that covers several
different elastomeric compounds. Several have good to excellent resistance
to certain chemicals and can be used as linings for process equipment. For
example, natural rubber is widely used to line hydrochloric acid storage
tanks. For specific applications, a subject matter expert should be consulted.
Refractory-Lined Equipment Refractory is normally used in hightemperature applications such as waste heat boilers. When used as an
internal lining, what is called the cold wall effect must be considered. The
refractory will insulate the steel, causing it to be at a considerably lower
temperature. Refractory can crack, and if hot gases reach the steel and condense, corrosion can occur, especially if the hot gases create acid conditions
on condensation.
Braze lead wire to pipe.
Coat braze to equal dielectric
and physical strength of pipe
coating.
Pipe
Sacrificial anode in
specially prepared
backfill
Sacrificial anode cathodic protection system. (“Corrosion in Petroleum
Production Operations – Types of Cathodic Protection Systems,” Metals Handbook
Volume 13C Corrosion: Environments and Industries. Reprinted with permission of ASM
International. All rights reserved. www.asminternational.org.)
FIG. 25-10
CATHODIC PROTECTION
Cathodic protection (CP) is a corrosion prevention technique that protects
equipment by using electrical circuits to force the equipment into a cathodic
state. The two basic types are sacrificial anode and impressed current. The
most common uses of cathodic protection are on equipment in contact with
the soil, including buried pipelines and the bottom sides of large storage tanks.
With sacrificial anodes, the galvanic series is used to electrically connect
steel to a less noble metal such as zinc or magnesium. Figure 25-10 is a basic
description of a sacrificial anode system.
The steel becomes the cathode and is protected by the anode, which corrodes preferentially. Anodes will be consumed as they corrode. Because of
this consumption, they need regular testing to ensure that they are providing adequate levels of protection. Once the level falls below accepted industry standards, replacement is required.
With an impressed current system, an external power supply is used with
an inert anode. The normal arrangement is to use an alternating current
supply off an available electric supply system. A rectifier converts the alternating current to direct current, with the negative lead to the equipment
to be protected and the positive lead to the inert anode, normally graphite.
This supplies electrons to the protected equipment, making it the cathode.
Figure 25-11 is an example of an impressed current system.
The choice between using sacrificial anodes and impressed current systems
is driven by installation and maintenance costs. In remote areas where a local
power supply is not available, sacrificial anodes are normally used. However,
solar cells used with some type of battery storage have been used.
Paints or other nonmetallic barriers such as a polyethylene wrap are used
on the protected equipment. The purpose is to reduce any exposed area.
This decreases the amount of protection needed in terms of electrical load.
Cathodic protection systems need to be engineered and designed for the
specific application. Only subject matter experts in this field should be used
for the design, installation, and maintenance of CP systems.
ANODIC PROTECTION
Anodic protection is the opposite of cathodic protection in that it makes the
equipment be protected by the anode. This only works with metals or alloys
Pole
Wires in conduit (ac)
Meter (kWh)
Anodes in backfill
Disconnect switch (fused)
Selenium rectifier
Bare copper ground wire
connect to rectifier cabinet
Copper ground rod
Positive direct
burial cable
2 cables in conduit (dc)
Negative dc cable
brazed to pipe
Buried pipe line
FIG. 25-11 Impressed current cathodic protection system. (“Corrosion in Petroleum
Production Operations – Types of Cathodic Protection Systems” Metals Handbook
Volume 13C Corrosion: Environments and Industries. Reprinted with permission of ASM
International. All rights reserved. www.asminternational.org.)
that exhibit active/passive behavior (see the subsection on Passivity under
Corrosion Fundamentals). The goal is to use an electrical potential to move
the equipment into the passive state. The most common usage is on sulfuric acid storage tanks to reduce iron contamination and on 316L stainless
steel coolers used in sulfuric acid production (Dillon, C. P., Corrosion Control
in the Chemical Process Industries, 2d ed., Materials Technology Institute,
St. Louis, Mo., 1994, p. 379). With these systems, design and operation are
very critical because an error can result in accelerated corrosion.
CORROSION-TESTING METHODS*
The purpose of corrosion testing is to identify the optimum choices for
process equipment in terms of materials of construction, design, and
corrosion-control measures. Optimum here means that which comprises
the best combination of cost, lifetime, safety, and reliability.
In many cases, information about the corrosion resistance of a material in
a specific environment is not available and must be derived experimentally.
There is no standard way to evaluate an alloy in an environment. Even if the
chemistry of the operating plant environment is duplicated in the laboratory, a variety of other factors affect the result. These include, for example,
velocity, hot and cold wall effects, crevices, chemical changes in the fluid
during the test, stress levels of the equipment, contamination with products of corrosion, trace impurities, and dissolved gases. Also, the rate of the
∗[Includes information from papers by Oliver W. Siebert, John G. Stoecker II, and Ann Van Orden, courtesy of NACE International; Oliver W. Siebert and John R. Scully, courtesy of
ASTM; John R. Scully and Robert G. Kelly, courtesy of ASM; and Metal Samples Company, Division of Alabama Specialty Products Company, Munford, Alabama.]
CORROSION-TESTING METHODS
corrosion reaction itself may change with time. Even with these considerations, immersion testing remains the most widely used method for selecting materials of construction.
Corrosion testing may be done either in the field—in either full-scale
operation plants or smaller pilot plants—or in laboratories using simulated
environments.
Both field and laboratory corrosion tests use specimens (coupons) or
electronic corrosion sensors that are installed in the process. The most
common electronic corrosion sensors are based on electrical resistance
measurements or electrochemical response. Laboratory corrosion tests are
generally regarded as producing less reliable results than field tests because
the laboratory environment is simulated, and so they may not accurately or
completely reproduce the many factors of an operating plant that affect corrosion behavior. Field corrosion tests are conducted in the actual process,
and they are better able to account for the variety of potential influences.
25-17
(a)
CORROSION TESTING: LABORATORY TESTS
Metals and alloys do not respond alike to the many factors that are involved
in corrosion. Consequently, it is impractical to establish any universal standard laboratory procedures for corrosion testing. However, some details of
laboratory testing need careful attention in order to produce useful results.
This section reviews important aspects of laboratory corrosion testing.
A comprehensive treatment of this subject is given in MTI’s Laboratory
Corrosion Testing of Metals and Alloys, The Materials Technology Institute,
St. Louis, Mo., 2015 (www.mti-global.org).
Coupon Immersion Test The coupon total-immersion test is a relatively simple, nonaccelerated corrosion test method that uses metal test
specimens (coupons). The total-immersion test serves quite well to eliminate materials that obviously cannot be used. Further selection among
those materials of potential use can be based on a knowledge of the properties of the materials concerned and possibly more sophisticated corrosion
tests.
The National Association of Corrosion Engineers (NACE) TMO169-95
“Standard Laboratory Corrosion Testing of Metals for the Process Industries,” and ASTM G31, “Recommended Practice for Laboratory Immersion
Corrosion Testing of Metals,” are the general guides for immersion testing.
Small pieces of the candidate metal are exposed to the medium, and the
loss of mass of the metal is measured for a given period of time. The mass
loss is usually converted mathematically to a corrosion (penetration) rate.
A disadvantage of this method is the assumed average-time weight loss. The
corrosion rate could be high initially and then decrease with time. In other
cases, the rate of corrosion might increase very gradually with time. The
description that follows is based on these standards.
Test Piece* The size and the shape of specimens will vary with the purpose of the test, nature of the material, and apparatus used. A large surfaceto-mass ratio and a small ratio of edge area to total area are desirable. These
ratios can be achieved through the use of rectangular or circular specimens
of minimum thickness. Circular specimens should be from sheet stock, not
bar stock, to minimize the exposed end grain.
Shapes of typical commercially available test specimens are shown in
Fig. 25-12. Other desired shapes are available in various sizes and can be
obtained in a variety of shapes, materials of construction, and surface finish
as needed.
All specimens should be measured carefully to permit accurate calculation of the exposed areas. An area calculation accurate to plus or minus
1 percent is usually adequate. A consistent surface finish provides more
consistent results. Specimens should be degreased by scrubbing with
bleach-free scouring powder, followed by thorough rinsing in water and in
a suitable solvent, and air-dried. For relatively soft metals such as aluminum, magnesium, and copper, scrubbing with abrasive powder is not always
needed, and it can mar the surface of the specimen. The use of towels for
drying may introduce an error through contamination of the specimens
with grease or lint. The dried specimen should be weighed on an analytic
balance. After final preparation of the specimens, they should be stored in a
desiccator until exposure if they are not used immediately.
Apparatus A common test apparatus consists of a kettle or flask of
suitable size [usually 500 to 5000 mL (16.9 to 169.1 ounces)], a reflux condenser with atmospheric seal, a sparger for controlling atmosphere or
aeration, a thermowell and temperature-regulating device, a heating device
(mantle, hot plate, or bath), and a specimen-support system. If agitation is
required, the apparatus can be modified to accept a suitable stirring mechanism, such as a magnetic stirrer. A typical resin-flask setup for this type of
test is shown in Fig. 25-13. Open-beaker tests should not be used because of
evaporation and contamination.
∗Coupons and racks/holders as well as availability information are courtesy of
Metal Samples, Munford, Alabama.
(b)
(c)
(d)
FIG. 25-12 Typical commercially available test coupons: (a) circular; (b) rectangular;
(c) welded rectangular; (d) horseshoe stressed.
In more complex tests, provisions might be needed for continuous flow
or replenishment of the corrosive liquid while simultaneously maintaining
a controlled atmosphere.
Heat flux apparatus for testing materials for heat-transfer applications
is shown in Fig. 25-14. Here the sample is at a higher temperature than the
bulk solution.
25-18
MATERIALS OF CONSTRUCTION
FIG. 25-13 Laboratory-equipment arrangement for corrosion testing. (Based on
NACE Standard TMO169-95.)
A variety of important factors influence corrosion in a laboratory test,
and applicable factors must be reported. These may include solution composition (including changes during the test), temperature, oxygen concentration or aeration/deaeration, rate of flow, pH, ratio of solution volume
to specimen area, and any other important characteristics of the test
environment.
Method of Supporting Specimens The supporting device and container should not be affected by or cause contamination of the test solution.
The method of supporting specimens will vary with the apparatus used for
conducting the test but should be designed to physically isolate the specimens from each other and any metallic container or supporting device used
with the apparatus.
The shape and form of the specimen support should assure free contact of
the specimen with the test solution, the liquid/vapor interface, and/or the
vapor phase, as desired and as shown in Fig. 25-13. Some common supports
are glass or ceramic rods, glass saddles, glass hooks, fluorocarbon plastic
strings, and various insulated or coated metallic supports.
Duration of Test The duration of any test will be determined by the
nature and purpose of the test. A procedure for evaluating the effect of time
on corrosion of the metal and also on the corrosiveness of the environment
in laboratory tests has been presented by Wachter and Treseder [Chem. Eng.
Prog. 315–326 (June 1947)]. This technique is called the planned-interval
test. Other procedures that require the removal of solid corrosion products
between exposure periods will not accurately measure the normal changes
of corrosion with time.
Materials that experience severe corrosion generally do not need lengthy
tests to obtain accurate corrosion rates. Although this assumption is valid in
many cases, there are exceptions. For example, lead exposed to sulfuric acid
corrodes at an extremely high rate at first while building a protective film;
then the rate decreases considerably, so further corrosion is negligible. The
phenomenon of forming a protective film is observed with many corrosionresistant materials, and therefore short tests on such materials may indicate
high corrosion rates that would be misleading.
Short-time tests also can give misleading results on alloys that form
passive films, such as stainless steels. With borderline conditions, a prolonged test may be needed to permit breakdown of the passive film and
subsequently more rapid attack. Consequently, tests run for long periods
are considerably more realistic than those conducted for short durations.
FIG. 25-14
Laboratory setup for the corrosion testing of heat-transfer materials.
This statement must be qualified by stating that corrosion should not proceed to the point at which the original specimen size or the exposed area is
drastically reduced or the metal is perforated.
If anticipated corrosion rates are moderate or low, Eq. (25-16) gives a suggested test duration:
78,740
corrosion rate, mm/yr
2000
=
corrosion rate, mils/yr
Duration of test, h =
(25-16)
Cleaning Specimens after Test Before specimens are cleaned,
their appearance should be observed and recorded. Locations of deposits,
variations in types of deposits, and variations in corrosion products are
extremely important in evaluating localized corrosion such as pitting and
concentration-cell attack.
Cleaning specimens after the test is a vital step in the corrosion-test procedure and, if not done properly, can give rise to misleading test results.
Generally, the cleaning procedure should remove all corrosion products
from specimens with a minimum removal of sound metal. Set rules cannot
be applied to cleaning because procedures will vary with the type of metal
being cleaned and the degree of adherence of corrosion products.
Mechanical cleaning is the most common method; it includes scrubbing, scraping, brushing, mechanical shocking, and ultrasonic procedures.
Scrubbing with a bristle brush and a mild abrasive is a widely used method.
Others are used principally as supplements to remove heavily encrusted
corrosion products before scrubbing. Other methods of cleaning that are
sometimes used include chemical cleaning and electrolytic cleaning. Care
should be used to avoid the removal of sound metal. If removal of sound
metal may occur, then that effect must be quantified and the weight loss
should be corrected accordingly.
CORROSION-TESTING METHODS
Evaluation of Results After the specimens have been cleaned and
reweighed, they should be examined carefully. Localized attack such as pits,
crevice corrosion, stress-accelerated corrosion, cracking, or intergranular
corrosion should be measured for depth and the area affected.
The depth of localized corrosion should be reported for the actual test
period and not interpolated or extrapolated to an annual rate. The rate of
initiation or propagation of pits is seldom uniform. The size, shape, and
distribution of pits should be noted. A distinction should be made between
those occurring underneath the supporting devices (concentration cells)
and those on the surfaces that were freely exposed to the test solution. The
specimen may be subjected to simple bending tests to determine whether
any embrittlement has occurred.
If localized corrosion is not present or is recorded separately in the
report, the corrosion rate (penetration rate) can be calculated as shown in
Eq. (25-17)
Weight loss × 534
= mils/yr (mpy)
(Area)(time)(metal density)
Weight loss × 13.56
= mm/yr (mmpy)
(Area)(time)(metal density)
(25-17)
Weight loss is in mg, area is in in2 of metal surface exposed, time is in
hours exposed, and density is in g/cm3. Densities for alloys can be obtained
from the producers or from various metal handbooks.
The following checklist is a recommended guide for reporting all important information and data:
• Corrosive media and concentration (changes during test)
• Volume of test solution
• Temperature (maximum, minimum, and average)
• Aeration (describe conditions or technique)
• Agitation (describe conditions or technique)
• Type of apparatus used for test
• Duration of each test (start, finish)
• Chemical composition or trade name of metals tested
• Form and metallurgical conditions of specimens
• Exact size, shape, and area of specimens
• Treatment used to prepare specimens for test
• Number of specimens of each material tested and whether specimens were
tested separately or which specimens were tested in the same container
• Method used to clean specimens after exposure and the extent of any error
expected by this treatment
• Actual weight losses for each specimen
• Evaluation of attack if other than general, such as crevice corrosion under
support rod, pit depth and distribution, and results of microscopic examination or bend tests
• Corrosion rates for each specimen expressed as millimeters (mils) per year
Laboratory tests are typically not able to simulate every plant condition
that affects corrosion. Some conditions to consider when applying laboratory test results to actual plant use include galvanic effects, concentration
cells, cold and hot wall effects, and minor constituents in the corrosive environment such as impurities and contaminants.
Electrical Resistance Corrosion Sensors The measurement of
corrosion by electrical resistance is performed by measuring the change
in resistance of a thin metallic wire or strip as its cross section decreases
from the loss of metal by corrosion. Commercial devices are available
that convert that measurement to a corrosion rate and compensate for
the effect of temperature on electrical resistance. Advantages of this
method include that these devices can measure corrosion over short
periods of time (hours or days) and that the environment does not have
to be an electrolyte or liquid, so measurements are possible in environments such as concrete and corrosive gases. Disadvantages of the
technique include that localized corrosion (pitting, crevice corrosion,
galvanic, stress corrosion cracking, fatigue, and so forth) is not reliably
measured, scatter in the readings can be caused by wide temperature
fluctuations or strain on the sensor (as from flow and turbulence), and
corrosion products or deposits that are electrically conductive will detrimentally affect the readings.
Linear Polarization This method measures a characteristic of an
electrochemical corrosion system called polarization resistance. A relationship exists between polarization resistance and the instantaneous corrosion
rate of a freely corroding alloy. The polarization resistance is determined by
measuring the amount of applied current needed to change the corrosion
potential of the freely corroding specimen by about 10 mV. The slope of the
curves thus generated is directly related to the corrosion rate by Faraday’s
law. Several commercial instruments are available that are used for linear
polarization measurements, for both laboratory and field use. The corrosion
25-19
rate measured is the instantaneous rate, and measurements can be quickly
repeated to provide a quasi–real-time view of corrosion. This method is not
sensitive to localized corrosion.
Potentiodynamic Polarization The activity of pitting, crevice corrosion, and stress-corrosion cracking is strongly dependent on the corrosion potential (i.e., the potential difference between the corroding
metal and a suitable reference electrode). Commercially available instruments (potentiostats) can control the corrosion potential at a desired
value and measure the current required to maintain it at that value. A
plot of that current over a range of potentials is called a polarization diagram. By using proper experimental techniques, it is possible to define
approximate ranges of corrosion potential in which pitting, crevice corrosion, and stress-corrosion cracking will or will not occur. With properly
designed electrode sensors, these techniques can be used in the field as
well as in the laboratory.
The potentiostat has a three-electrode system: a reference electrode
such as a saturated calomel electrode (SCE), a platinum counter (auxiliary)
electrode, and a working electrode made of the alloy of interest (Fig. 25-15).
Current passes between the counter and working electrodes to maintain
the desired potential between the reference and working electrodes. The
potentiostat controls that potential, either holding it constant, stepping it,
or scanning it anodically or cathodically at some linear rate.
For a given metal/environment system, the potentiostat provides a plot of
the current resulting from changes in potential. This is typically presented
as a plot of log current density versus potential, or an Evans diagram. A typical active/passive metal anodic polarization curve is seen in Fig. 25-16, generally showing the regions of active corrosion, passivity, and a transpassive
region.
Scan Rates Sweeping a range of potentials in the anodic (more electropositive) direction of a potentiodynamic polarization curve at a high
scan rate of about 60 V/h (high from the perspective of the corrosion
engineer, slow from the perspective of a physical chemist) can indicate
regions where intense anodic activity is likely. Second, for otherwise identical conditions, sweeping at a relatively slow rate of potential change of
about 1 V/h will indicate regions likely to be relatively inactive. The rapid
sweep of the potential range has the object of minimizing film formation, so the currents observed relate to relatively film-free or thin-film
conditions. The object of the slow sweep rate experiment is to allow time
for filming to occur. A zero scan rate provides the opportunity for maximum stability of the metal surface, but at high electropositive potentials,
the environment could be affected or changed. A rapid scan rate compromises the steady-state nature of the metal surface but better maintains the stability of the environment. Whenever possible, corrosion tests
should be conducted using multiple techniques: potentiodynamic polarization at various scan rates, crevice, stress, velocity, and so forth. An
evaluation of these several results, on a holistic basis, can greatly reduce
or temper their individual limitations.
FIG. 25-15 The potentiostat apparatus and circuitry associated with controlled
potential measurements of polarization curves.
25-20
MATERIALS OF CONSTRUCTION
Typical electrochemical polarization curve for an active/passive alloy
(with cathodic trace) showing active, passive, and transpassive regions and other
important features. (Note: Ep = primary passive potential, Ecorr = freely corroding
potential.)
FIG. 25-16
FIG. 25-17
Six possible types of behavior for an active/passive alloy in a corrosive
environment.
Slow-Scan Technique In ASTM G5, “Polarization Practice for Standard
Reference Method for Making Potentiostatic and Potentiodynamic Anodic
Polarization Measurements,” all oxygen in the test solution is purged with
hydrogen for a minimum of 0.5 hours before introducing the specimen. The
test material is then allowed to reach a steady state of equilibrium (open
circuit corrosion potential, Ecorr) with the test medium before the potential
scan is conducted. Starting the evaluation of a basically passive alloy that
is already in its “stable” condition precludes any detailed study of how the
metal reaches that protected state (the normal intersection of the theoretical anodic and cathodic curves is recorded as a zero applied current on the
ASTM diagram of potentiostatic potential versus applied current). These
intersections between the anodic and cathodic polarization curves are
where the total oxidation rate equals the total reduction rate (ASTM G3,
“Recommended Practice for Conventions Applicable to Electrochemical
Measurements in Corrosion Testing”).
Three general reaction types compare the activation-control reduction
processes. In Fig. 25-17, in Case I the single reversible corrosion potential
(anode/cathode intersection) is in the active region. A wide range of corrosion rates is possible. In Case II the cathodic curve intersects the anodic
curve at three potentials, one active and two passive. If the middle active/
passive intersection is not stable, the lower and upper intersections indicate the possibility of very high corrosion rates. In Case III, corrosion is in
the stable, passive region, and the alloys generally passivate spontaneously
and exhibit low corrosion rates. Most investigators report that the ASTM
method is effective for studying Case I systems. An alloy-medium system
exhibiting Case II and III conditions generally cannot be evaluated by this
conventional ASTM method.
The potentiodynamic polarization electrochemical technique can be
used to study and interpret corrosion phenomena. It may also furnish useful
information on film breakdown or repair.
Those wanting a more detailed review of the subject of electrochemistry
(and/or corrosion testing using electrochemistry) are directed to additional
reference material from the following: Siebert, O. W., and J. G. Stoecker,
“Materials of Construction,” in Perry’s Chemical Engineers’ Handbook, 7th ed.,
sec. 28, McGraw-Hill, New York, 1997, pp. 28-11 to 28-20; Stoecker, J. G.,
O. W. Siebert, and P. E. Morris, “Practical Applications of Potentiodynamic
Polarization Curves in Materials Selection,” Materials Performance 22(11):
13–22 (1983); Siebert, O. W., “Correlation of Laboratory Electrochemical Investigations with Field Application of Anodic Protection,” Materials
Performance 20(2): 38–43 (1981); and the assorted historical literature of
Stern, Geary, Evans, Sudbury, Riggs, Pourbaix, and Edeleanu, and other studies referred to in their publications.
Crevice Corrosion Prediction The most common type of localized
corrosion is the occluded mode crevice corrosion. Pitting can, in effect,
be considered a self-formed crevice. A crevice must be wide enough to
permit liquid entry, but sufficiently narrow to maintain a stagnant zone. It
is nearly impossible to build equipment without mechanical crevices. On
a micro level, scratches can be sufficient crevices to initiate or propagate
corrosion in some metal/environment systems. The conditions in a crevice can, with time, become a different and much more aggressive environment than those on a nearby clean, open surface. Crevices may also be
created by factors foreign to the original system design, such as deposits,
corrosion products, and so forth. In many studies, it is important to know
or to be able to evaluate the crevice corrosion sensitivity of a metal to a
specific environment and to be able to monitor a system for predictive
maintenance.
A common method to test for crevice corrosion is by the immersion
test technique, creating a crevice by clamping two metal test specimens
together, or clamping a metal specimen in contact with an inert plastic or
ceramic material.
Velocity* For corrosion to occur, an environment must be brought
into contact with the metal surface, and the metal atoms or ions must be
allowed to be transported away. Therefore, the rate of transport of the environment with respect to a metal surface is a major factor in the corrosion
system. Changes in velocity may increase or decrease attack, depending on
its effect involved. A varying quantity of dissolved gas may be brought in
contact with the metal, or velocity changes may alter diffusion or transfer
of ions by changing the thickness of the boundary layer at the surface. The
boundary layer, which is not stagnant, moves except where it touches
the surface. Many metals depend on the development of a protective surface
for their corrosion resistance. This may consist of an oxide film, a corrosion
product, an adsorbed film of gas, or other surface phenomena. The removal
∗See review paper by David C. Silverman, courtesy of NACE International.
CORROSION-TESTING METHODS
Gas
dispersion
tube
Rotating
electrode
Thermometer
Luggin-Haber
capillary
Teflon®
cover
Pyrex®
container
Platinum screen
counter electrode
Reference
electrode
compartment
FIG. 25-18 Rotating cylinder electrode apparatus.
of these surfaces by the effect of the fluid velocity exposes fresh metal, and
as a result, the corrosion reaction may proceed at an increasing rate. In
these systems, corrosion might be minimal until a so-called critical velocity is attained where the protective surface is damaged or removed and the
velocity is too high for a stable film to re-form. Above this critical velocity,
the corrosion may increase rapidly.
The NACE Landrum Wheel velocity test, originally TM0270-72, is
typical of several mechanical-action immersion test methods to evaluate the effects of corrosion. Unfortunately, these laboratory simulation
techniques did not consider the fluid mechanics of the environment or
metal interface, and service experience very seldom supports the test
predictions. A rotating cylinder within a cylinder electrode test system
has been developed that operates under a defined hydrodynamics relationship (Figs. 25-18 and 25-19). The assumption is that if the rotating
electrode operates at a shear stress comparable to that in plant geometry,
the mechanism in the plant geometry may be modeled in the laboratory.
Once the mechanism is defined, the appropriate relationship between
fluid flow rate and corrosion rate in the plant equipment as defined by
the mechanism can be used to predict the expected corrosion rate. If
fluid velocity does affect the corrosion rate, the degree of mass-transfer
control, if that is the controlling mechanism (as opposed to activation
control), can be estimated. Conventional potentiodynamic polarization
scans are conducted as described previously. In other cases, the corrosion potential can be monitored at a constant velocity until steady state
is attained. While the value of the final corrosion potential is virtually
independent of velocity, the time to reach steady state may be dependent on velocity. The mass-transfer control of the corrosion potential
can be proportional to the velocity raised to its appropriate exponent.
The rate of breakdown of a passive film is velocity-sensitive. To review a
very detailed and much needed refinement of the information and application of the rotating electrode technique as used for evaluation of the
effect of velocity on corrosion, the reader is directed to a study by David
C. Silverman, “The Rotating Cylinder Electrode for Examining VelocitySensitive Corrosion—A Review,” Corrosion 60(11): 1003–1023 (2004).
Contact for
rotator brush
Top of
Teflon®
liner
End-cap
to hold
assembly
Sample
Extension
of sample
under
Rulon®
spacer
Rulon®
spacer
FIG. 25-19 Inner rotating cylinder used in laboratory apparatus of Fig. 25-18.
25-21
Environmental Cracking The problem of environmental cracking of
metals and their alloys is very important. Stress corrosion cracking (SCC)
is cracking caused by the combined effects of stress and corrosion. Stress
corrosion cracking is an incompletely understood corrosion phenomenon.
Much research activity (aimed mostly at mechanisms) plus practical experience have led to the development of empirical guidelines, but they contain a large element of uncertainty. No single test method has proven to be
reproducible enough for known crack-causing environment/alloy systems
to justify a high confidence level.
Most SCC testing is done using immersion tests with stressed metal
specimens. There are a variety of methods of stressing the specimens,
including bending, welding, compressed C-rings, and constant extension of
tensile bars (slow strain rate). Prestressed samples, such as those shown in
Fig. 25-20, have been used for laboratory and field SCC testing. The variable
observed is “time to failure or visible cracking.” Such tests do not provide
acceleration of failure.
SCC often shows a fairly long induction period (months to years), so such
tests must be conducted for long periods before reliable conclusions can be
drawn. Several months’ duration is not unusual.
In the constant-strain method, the specimen is stretched or bent to a fixed
position at the start of the test. The most common shape of the specimens
used for constant-strain testing is the U-bend, hairpin, or horseshoe type.
A bolt is placed through holes in the legs of the specimen, and it is loaded
by tightening a nut on the bolt. In some cases, the stress may be reduced
during the test as a result of creep. In the constant-load test, the specimen
is supported horizontally at each end and is loaded vertically downward at
one or two points and has maximum stress over a substantial length or area
of the specimen. The load applied is a predetermined, fixed dead weight.
Specimens used in either of these tests may be precracked to assign a stress
level or a desired location for fracture to occur or both, as is used in fracture
mechanics studies. These tensile-stressed specimens are then exposed to
the environment of study.
Slow Strain-Rate Test In the slow strain-rate test (SSRT), a tension
specimen is slowly loaded in a test frame to failure under prescribed test
conditions. The normal test extension rates are from 2.54 × 10−7 to 2.54 ×
10−10 m/s (10−5 to 10−8 in/s). Failure times are usually 1 to 10 days. The failure mode will be either SCC or tensile overload, sometimes accelerated by
corrosion. An advantage of the SSRT, compared to constant-strain tests, is
that the protective surface film is ruptured mechanically during the test,
thus giving SCC an opportunity to progress. It is common for the potential
to be monitored during the SSRT. The strain rates that best generate SCC
in various alloys are reported in the literature. The SSRT is generally considered a severe test for SCC. A disadvantage of this test is that indications
of failure are not generally observed until the tension specimen is plastically stressed, sometimes significantly, above the yield strength of the metal.
Such high-stressed conditions can be an order of magnitude higher than the
intended operating stress conditions. Another disadvantage is that crack
initiation must occur fairly rapidly in order to have crack growth sufficient
to be detected using the SSRT. The occurrence of SCC in alloys requiring
long initiation times may go undetected.
Conjunctive Use of Slow- and Rapid-Scan Polarization Potentiodynamic polarization curves can be used to predict SCC-sensitive potential ranges. The technique involves conducting both slow- and rapid-scan
sweeps in the anodic direction of a range of potentials. Comparison of the
two curves indicates potential ranges within which high anodic activity
in the film-free condition reduces to insignificant activity when the time
requirements for film formation are met. This indicates heightened susceptibility to SCC. Some SCC theories predict these domains of SCC behavior
to be between the primary passive potential and the onset of passivity. This
technique helps identify those SCC potential ranges.
Electrochemical Impedance Spectroscopy (EIS) and AC
Impedance* Many direct-current test techniques assess the overall corrosion process occurring at a metal surface but treat the metal/solution
interface as if it were a pure resistor. Problems of accuracy and reproducibility often encountered in the application of direct-current methods have
led to the use of electrochemical impedance spectroscopy (EIS).
Electrode surfaces in electrolytes generally have a surface charge that is
balanced by an ion accumulation in the adjacent solution, thus making the
system electrically neutral. The first component is a double layer created by
a charge difference between the electrode surface and the adjacent molecular layer in the fluid. Electrode surfaces may behave at any given frequency
as a network of resistive and capacitive elements from which electrical
impedance may be measured and analyzed.
∗
Excerpted from papers by Oliver W. Siebert, courtesy of NACE International and
ASTM.
25-22
MATERIALS OF CONSTRUCTION
(a)
(b)
(c)
(g)
(i)
(h)
(j)
(d)
(e)
(f)
FIG. 25-20 Specimens for stress-corrosion tests. (a) Bent beam. (b) C ring. (c) U bend. (d) Tensile. (e) Tensile.
( f ) Tensile. ( g) Notched C ring. (h) Notched tensile. (i) Precracked, wedge. Open-loading type. ( j ) Precracked,
cantilever beam. [Chem. Eng. 78: 159 (Sept. 20, 1971).]
The application of an impressed alternating current on a metal specimen
can generate information on the state of the surface of the specimen. The
corrosion behavior of the surface of an electrode is related to how that surface responds to this electrochemical circuit. The AC impedance technique
involves the application of a small sinusoidal voltage across this circuit. The
frequency of that alternating signal is varied, and the voltage and current
responses of the system are measured.
A method of analyzing electrical impedance of a corrosion system uses
so-called white-noise analysis by the fast Fourier transform technique
(FFT). The entire spectrum is derived from one signal. The impedance
components thus generated are plotted on either a Nyquist (real versus
imaginary) or Bode (log real versus log frequency plus log phase angle
versus log frequency) plot. These data are analyzed by computer to determine the polarization resistance, and thus the corrosion rate if Tafel
slopes are known. EIS measurements are also used with coated specimens to better understand how the coating’s dielectric properties change
with time.
Other Electrochemical Test Techniques A summary of electrochemical test techniques can be found in Chapter 7, “Electrochemical Tests,” 2005
ASTM Manual 20, by John R. Scully. He presents theories associated with
the mechanisms of corrosion, and he describes common test methods used
to predict corrosion. These include methods based on concentration polarization effects, frequency modulation methods, electronic noise resistance,
and the scratch-repassivation method for local corrosion.
(no heat-treatment scale remaining unless this is specifically part of the
test), and should be identified by stamping. See Fig. 25-21 for a typical
plant test assembly.
The choice of materials for the specimen holder is important. Materials
must be durable enough to ensure satisfactory completion of the test. It
is good practice to select very resistant materials for the test assembly.
Common insulating materials used are resistant plastics such as nylon
and PTFE.
The method of supporting the specimen holder during the test is important. The holder must be located so as to cover the conditions of exposure
to be studied. It may have to be submerged, or exposed only to the vapors,
or located at liquid level, or holders may be called for at all three locations.
Various means are used to support the holders in liquids or in vapors. The
simplest is to suspend the holder by means of a heavy wire or light metal
chain. Holders have been strung between heating coils, clamped to agitator
shafts, welded to evaporator tube sheets, and so on.
For tests in pipelines of 75-mm (3-in) diameter or larger, a spool holder
such as that shown in Fig. 25-22 has been used. This frame is designed so
that it may be placed in a pipeline in any position without permitting the
disk specimens to touch the wall of the pipe. As with the strip-type holder,
this assembly does not materially interfere with the fluid through the pipe,
and it permits the study of corrosion effects prevailing in the pipeline.
CORROSION TESTING: PLANT TESTS
It is not always practical or convenient to investigate corrosion problems in the laboratory. It may be difficult to discover the conditions of
service and reproduce them exactly. This is especially true with process
streams that change with time, such as those that may occur in batch
processes, evaporation, distillation, polymerization, sulfonation, synthesis, and processes using recycled mother liquors. Laboratory tests may
also experience contamination of the test solution by corrosion products,
or depletion of a corrosive component, which significantly affects test
results.
In such cases, it is usually preferable to carry out the corrosion-testing
program by exposing specimens in operating equipment under actual conditions of service. Such in-plant testing has the additional advantages that
it is possible to test a large number of specimens at the same time and that
little technical supervision is required.
Test Specimens In carrying out plant tests, it is necessary to install
the test specimens so that they will not come into contact with other metals and alloys. This avoids having their normal behavior disturbed by galvanic effects. It is also desirable to protect the specimens from mechanical
damage.
There is no standard size or shape for corrosion-test specimens. They
usually weigh from 10 to 50 g (0.35 to 1.76 ounces) and preferably have a
large surface-to-mass ratio. Disks 40 mm (1½ in) in diameter by 3.2 mm
(⅛ in) thick and similarly dimensioned square and rectangular specimens are the most common. Surface preparation varies with the aim of
the test, but machine grinding of surfaces or polishing with a 120 grit
is common. Samples should not have sheared edges, should be clean
FIG. 25-21 Assembly of a corrosion-test spool and specimens. (Mantell, ed.,
Engineering Materials Handbook, McGraw-Hill, New York, 1958.)
CORROSION-TESTING METHODS
Spool-type specimen holder for use in a 3-in-diameter or larger pipe.
(Mantell, ed., Engineering Materials Handbook, McGraw-Hill, New York, 1958.)
FIG. 25-22
Another way to study corrosion in pipelines is to install in the line short sections of pipe of the materials to be tested. If possible, these test sections should
be electrically isolated from each other and from the rest of the piping system.
It is occasionally desirable to expose corrosion-test specimens in operating equipment without the use of specimen holders of the type described.
This can be done by attaching specimens directly to some part of the operating equipment and by providing the necessary isolation against galvanic
effects as shown in Fig. 25-23.
Test Results The methods of cleaning specimens and evaluating
results after plant corrosion tests are identical to those described earlier for
laboratory tests.
On-Line Corrosion Monitoring* On-line corrosion monitoring
is used to provide in-situ, real-time information about corrosion in an
operating plant. The most common devices are based on either electrical
resistance or linear polarization resistance (LPR) electrochemical measurements. These devices are particularly useful to detect changes in process
conditions (upsets) that render equipment susceptible to corrosion.
Electrical resistance sensors measure changes in the electrical resistance of a metal element as it gets thinner from corrosion. As the metal gets
thinner, its resistance increases. The most common metal elements used
in plant monitoring are thin cylinders with welded ends. These sensors
are only quasi-real-time, and they may require days or weeks to accumulate enough corrosion to indicate a rate. The higher the corrosion rate, the
FIG. 25-23 Methods for attaching specimens to test racks and to parts of moving
equipment. (Mantell, ed., Engineering Materials Handbook, McGraw-Hill, New York,
1958.)
∗Excerpted from papers by Oliver W. Siebert, courtesy of NACE International and
ASTM.
25-23
shorter the time required. These sensors are affected by temperature variations and strain effects (e.g., from flow or vibration), both of which affect
electrical resistance.
Linear polarization resistance (LPR) sensors provide instantaneous
corrosion rate measurements, and they work on the principle outlined in
the ASTM guide on making polarization resistance measurements (G59,
“Standard Practice for Conducting Potentiodynamic Polarization Resistance Measurements”). LPR sensors measure the current density required
to create a small shift (usually 5 to 10 mV) in the corrosion potential of the
test electrode, which provides an indication of the corrosion rate.
Unique Uses of On-Line Corrosion Monitoring The major use of
corrosion monitoring is to measure the corrosion rate in the plant or the
field. As noted previously, on-line corrosion monitoring can also be used to
detect process upsets that change the corrosivity of the process. Monitoring
can also be used to optimize the chemistry and level of corrosion inhibitors used. Too little inhibitor allows unacceptable levels of corrosion, and
too much inhibitor increases costs without adequate benefit. Optimizing
inhibitor use in terms of concentration, location in the process, and method
of addition can be helped by the use of carefully placed corrosion sensors.
Corrosion sensors may also be used to monitor scale formation, and some
sensors claim to provide information about localized corrosion.
Other Types of Sensors Other types of sensors are used to measure
corrosion, or to measure environmental characteristics known to influence
corrosion.
The electrochemical techniques of EIS and polarization testing we have
discussed as laboratory methods may also be adapted for in-plant use. Electrochemical noise monitoring is another method that may be used in the
plant. This method uses naturally occurring fluctuations in current and
potential of a metal to provide information on the reaction kinetics at the
surface and the corrosion rate.
Ultrasonic thickness monitoring may be permanently mounted on the
outside of process equipment to provide real-time indications of metal loss.
Such devices provide measurements for small areas, or multiple transducers may be ganged to provide measurements for larger areas.
Some sensors do not measure corrosion directly, but instead yield measurements that are useful to indicate a change in corrosive conditions.
Examples include:
Pressure probes. Pressure monitors or transducers may be of use in corrosion monitoring in environments where a buildup of gases such as hydrogen
or H2S may contribute to corrosion.
Gas probes. The hydrogen patch probe is mounted on the outside of a
pipe or vessel, and it measures the amount of corrosion-generated hydrogen
coming through a metal wall.
pH probes. Monitoring pH may aid in the early detection of corrosion. The
acidity or alkalinity of the environment is often a controlling parameter for
corrosion, and it may be controllable.
Ion probes. Measuring metal ions in solution with specific ion electrodes
may provide a direct indication of corrosion, and measuring other types
of ions (chloride for example) may allow one to infer a change in corrosive
conditions.
Considerations for Measuring Corrosion Rate with Coupon
Specimens Corrosion rates may vary during testing, as described in the
discussion about laboratory testing. Because the rate obtained from coupon
testing is averaged over time, one must be aware that actual rates may have
varied during the time of testing.
The frequency of sampling is important. Generally, measurements made
over longer times are more valid. This is especially true for low corrosion
rates, under 1 mil/yr, mpy (0.001 in/yr). When corrosion rates are this low,
longer times should be used.
A variety of factors may throw off these rates, as outlined in ASTM G31,
“Standard Practice for Laboratory Immersion Corrosion Testing of Metals.”
Coupon-type tests cannot be correlated with changing plant conditions
that may dramatically affect process equipment lifetimes. Other methods
must be used if more frequent measurements are desired or correlation with
plant conditions is necessary.
A plot of mass loss versus time can provide information about changes
in the conditions under which the test has been run. One example of such
a plot comes from the ASTM Standard G96, “Standard Guide for Online
Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods.)” As mentioned previously, weight loss measurements are not
appropriate for the measurement of localized corrosion, such as pitting and
crevice corrosion. Accurate corrosion penetration rates for localized corrosion require direct measurement on the coupon. Instruments commonly
used for this include pit-depth gages, calibrated microscopes, and profile
meters based on laser or optical principles.
Heat Flux Tests Removable tube test heat exchangers can be useful in the field for monitoring heat flux (corrosion) conditions, per NACE
TMO286-94 (similar to laboratory test, Fig. 25-14).
25-24
MATERIALS OF CONSTRUCTION
ALLOY DESIGNATIONS
The Unified Numbering System (UNS) has become the most widely
accepted system for identifying metals and alloys. The UNS is built around
a series of 18 primary alphanumeric designations, as shown in Table 25-4.
ASTM E527 is the standard practice for numbering metals and alloys in
the UNS system.
Each metal and alloy to receive a UNS designation is placed in one of the
primary systems and then given a unique five-digit identifier. In most cases
a UNS specification characterizes the chemical composition of the metal or
alloy. It does not include other criteria, such as mechanical properties, heat
treatment, or form.
Prior to the adoption of UNS, there was no universally recognized
system for numbering alloys. Different organizations such as the
Aluminum Association (AA), Copper Development Association (CDA),
and American Iron and Steel Institute (AISI) had their own systems. In
many cases, UNS has incorporated these commonly known numbers
from those systems. For example, AISI 304 stainless steel has the UNS
number S30400.
In addition to the UNS system, other organizations generate materials
standards of importance to the chemical process industries. Some of these
are:
1. American National Standards Institute (ANSI), formerly American
Standards Association (ASA). ANSI promulgates the piping codes used in
the chemical-process industries.
2. American Society of Mechanical Engineers (ASME). This society generates the Boiler and Pressure Vessel Codes.
TABLE 25-4
Axxxxx
Cxxxxx
Dxxxxx
Exxxxx
Fxxxxx
Gxxxxx
Hxxxxx
Jxxxxx
Kxxxxx
UNS Numbering System
Aluminum and aluminum
alloys
Copper and copper alloys
Lxxxxx
Specified mechanical
propertied steels
Rare earth and rare
earth-like alloys
Cast irons
Nxxxxx
Low-melting-point metals and
alloys
Miscellaneous nonferrous
metals and alloys
Nickel and nickel alloys
Pxxxxx
Precious metals and alloys
Rxxxxx
Reactive and refractory metals
and alloys
Stainless steels, valve steels,
superalloys
Tool steels
Welding filler metals
Zinc and zinc alloys
AISI and SAE carbon and
alloy steels
AISI H-steels
Cast steels
Miscellaneous steels and
ferrous alloys
Mxxxxx
Sxxxxx
Txxxxx
Wxxxxx
Zxxxxx
3. American Society for Testing and Materials (ASTM). This society
generates specifications for most of the materials used in the ANSI Piping
Codes and the ASME Boiler and Pressure Vessel Codes.
4. International Organization for Standardization (ISO). This organization is engaged in generating standards for worldwide use.
ASTM and ASME have incorporated UNS designations into their
specifications.
FERRITIC STEELS
CARBON STEEL
Carbon steel is the most common material used in industry due to its
relative low cost, high strength, favorable properties, and general application in many chemical processing services. Carbon steel has predictable
behavior, can be used at elevated temperatures up to 540°C (1000°F), and
seldom fails in an unexpected fashion. Carbon steel’s excellent ductility
permits mechanical manipulation into various shapes; its formability coupled with ease of welding allows different component parts to be joined
into one continuous piece of equipment, such as a pressure vessel or a
long run of piping.
Conventional carbon steel contains mostly iron with small amounts of
carbon (a few tenths of a percent) that strengthen the material and create
its excellent mechanical properties. The grades most commonly used in the
chemical process industries have tensile strengths in the 345- to 485-MPa
(50,000 to 70,000 lbf/in2) range. Manipulating the alloy elements, different
heat treatments, and cold working creates higher-strength steel with other
property enhancements or weaknesses.
Carbon steel has routine applications in processing organic chemicals
found in the refining and chemical industries. Refining process plants are
often constructed almost entirely of conventional carbon steel, with limited
applications of other alloys where high temperatures, corrosion, or metallurgical damage preclude steel’s use.
Carbon steel has limited applications above 425°C (800°F) due to a metallurgical transformation called graphitization, a breakdown of iron carbides
after long periods of high-temperature exposure. Oxidation also increases
to unacceptable levels above 510°C (950°F) when carbon steel is exposed
to air (typical furnace conditions). With temperatures above 425°C (800°F),
low-alloy steels (discussed next) become the first alternative material
considered.
Carbon steel can tolerate neutral and basic conditions very well. Industry available data can provide predicted corrosion rates with exposure to
different chemicals or conditions. Less predictable corrosion occurs in
applications where organic chemical streams with trace amounts of water
are being processed; water will drop out in low spots and may be acidic
or permit scaling that accelerates corrosion in these areas. Carbon steel
is generally the most economic alternative in chemical exposure applications with corrosion rates less than 0.12 mm/yr (5 mils/yr). Typically, steel
equipment has a designed corrosion allowance that permits some corrosion to occur while still giving the equipment an acceptable design life
(nominally 20 years).
For highly corrosive environments, the wetted interior of a carbon steel
vessel can be internally alloy clad or weld overlaid to protect the underlying carbon steel. Roll/explosion-bond cladding or weld overlay uses highly
corrosion-resistant alloys such as stainless steels, a high nickel alloy, titanium, or zirconium. In many cases, a base carbon steel vessel using cladding
or weld overlay provides more favorable equipment properties or fabrication costs than a solid corrosion-resistant alloy.
Carbon steel often experiences corrosion under installed thermal insulation, which is used to retain heat or protect personnel. Several techniques
minimize corrosion under insulation, including organic coatings (paint) or
spraying the surface with TSA (thermal sprayed aluminum).
CAST IRONS
Cast irons contain mostly iron with a few percent of carbon, and they cost
less than carbon steel. Cast irons have very low ductility (they are brittle),
and they can fail in a less predictable fashion than carbon steel. There are
several types of cast iron, including gray iron, ductile iron, white iron, and
malleable iron. Ductile iron is the most common grade used in the chemical industry because of its superior mechanical properties. Cast irons have
limited use in the chemical, petrochemical, or refining industries. Some
applications include pump parts, such as impellers and pump casings. Cast
iron has low corrosion rates in low-temperature water, which enables it to
be used in low-pressure water services.
LOW-ALLOY STEELS
Low-alloy steels contain one or more alloying agents to provide mechanical and corrosion-resistant properties superior to those of carbon steel.
These alloying elements allow for the production of a large variety of standard and proprietary grades. Nickel increases toughness and improves
low-temperature properties and corrosion resistance. Chromium and
silicon improve hardness, abrasion resistance, corrosion resistance, and
resistance to oxidation. Molybdenum provides strength at elevated temperatures. The addition of small amounts of alloying materials greatly
improves corrosion resistance to atmospheric environments but has little
effect against liquid corrosives. The alloying elements produce a tight,
dense, adherent rust film; however, in acid or alkaline conditions, these
minor alloying elements only create corrosion resistance equivalent to
that of carbon steel.
STAINLESS STEELS
The most common low-alloy steels used in chemical processing and
refining contain up to 9 percent chromium and up to 1 percent molybdenum, referred to as low-chrome or chrome-moly (Cr-Mo steels). The
primary reasons to use a low-alloy steel instead of carbon steel include
the following:
• Improved high-temperature strength, useful up to at least 649°C (1200°F).
• Improved resistance to sulfidation corrosion in the absence of highpressure hydrogen. Carbon steel is limited to about 260°C to 290°C (500°F
to 550°F). Typically, at least 5 percent Cr is needed to improve sulfidation
resistance. API RP 939-C gives information on avoiding sulfidation
corrosion.
• Improved resistance to high-temperature hydrogen attack; carbon steel
is limited to as little as 232°C (450°F), depending on the hydrogen partial
pressure. API RP 941 has information to set practical limits on the use of
carbon and low-alloy steels in hydrogen service at elevated temperatures
and pressures.
25-25
Most welded low-alloy steels have hard welds and weld heat affected
zones (HAZs), which can crack following fabrication or when put into service. Certain process environments promote cracking. Post-welding heat
treatment (PWHT), generally prescribed by fabrication codes (ASME/
ANSI), will reduce hardness, improve fracture toughness, and provide more
resistance to process environment cracking. Low-alloy steels carry a significant cost premium compared to carbon steel due to the additional alloying
elements and the required PWHT during fabrication.
The higher strength of low-alloy steels permits process equipment
designs with a reduced or lower minimum thickness. Thinner wall thickness reduces the amount of welding required, reduces equipment weight,
and translates into cost savings. Newer low-alloy steels with small additions of vanadium allow an even greater reduction in wall thickness. However, vanadium-enhanced low-alloy steels have more stringent fabrication
requirements, which add complication but reduce overall costs relative to
nonenhanced low-alloy steels.
STAINLESS STEELS
Stainless steel is typically defined as a ferrous alloy containing at least
10 percent (weight percent) chromium. Since their discovery in the early
part of the 20th century, stainless steels have become primary construction
materials for process vessels, heat exchangers, storage tanks, and process
piping. This growth has been driven by the need for corrosion-resistant
materials in the chemical, petrochemical, power generation, pulp and
paper, pharmaceutical, and food processing industries.
Today there are literally hundreds of different stainless steel alloys or
“grades,” and the choice of a grade for any specific application is based on
the required level of corrosion resistance, strength, and toughness as well as
other factors such as fabrication requirements, availability, and cost. Stainless steels are divided into groups or families based on their crystal structure
and strengthening mechanism. The five primary stainless steel families are
austenitic, ferritic, duplex, martensitic, and precipitation-hardened stainless steels. For a more complete listing of commonly specified grades used
for wrought plate, sheet, and strip, see the following ASTM standards:
• A240, Standard Specification for Chromium and Chromium-Nickel
Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and General
Applications
• A666, Standard Specification for Annealed or Cold-Worked Austenitic
Stainless Steel Sheet, Strip, Plate, and Bar
• A693, Standard Specification for Precipitation-Hardening Stainless Steel
and Heat Resistant Plate Sheet, and Strip
structure. The compositions of the austenitic stainless steels are adjusted to
produce the desired microstructure or properties. This is done by maintaining a balance between the austenite stabilizing elements (Ni, C, Mn, and N)
and the ferrite stabilizing elements (Cr, Mo, Si, and Nb).
As a family, the austenitic stainless steels tend to have a relatively low yield
strength, high work hardening rate, high tensile strength, good ductility, and
excellent low-temperature toughness. They also tend to be more susceptible
to chloride stress corrosion cracking than are the ferritic and duplex stainless
steels. Austenitic stainless steels have good weldabilty and are readily fabricated into complex shapes. This family of stainless steels cannot be hardened
or strengthened by heat treatment, but it can be strengthened by cold forming
or work hardening (see ASTM A666). The austenitic family of stainless steels
can be further divided into subgroups, depending on their alloying content.
300 Series Austenitic Stainless Steel The most widely used of all
stainless steels are the AISI 300 series austenitic grades, which include
standard grades such as 304 (S30400), 304L (S30403), 316 (31600), and 316L
(S31603). Some of the more commonly specified 300 series grades and their
chemical compositions are summarized in Table 25-5. The microstructure
of these standard grades consists primarily of austenite phase with a small
amount of ferrite phase, typically in the range of 3 to 8 vol%. These grades are
used widely across all industry sectors and are available in a wide variety of
product forms, including plate, sheet, strip, tubing, piping, forgings, and bar.
The 304 and 304L grades are often termed 18-8 stainless steels, which
refers to the nominal 18 percent (wt%) chromium and 8 percent nickel content. The 316 and 316L grades have just over 16 percent Cr, a minimum Ni
content of 10 percent, and a 2 to 3 percent Mo addition, which improves
the resistance to localized corrosion, especially by chlorides and other
halides. The L refers to low carbon content (≤0.030%) which is desirable for
welded applications due to the improved resistance to chromium carbide
precipitation. In fact, because the composition of an L grade is a subset of
the regular grade, many 300 series stainless steels are promoted as being
“dual-certified,” that is, meeting the requirements for both 304 and 304L, as
AUSTENITIC STAINLESS STEELS
Austenitic stainless steels are by far the largest and most widely used of all
the stainless steels. This group of stainless steels is characterized by having
a microstructure that is either entirely or predominately austenite phase,
which is a nonmagnetic phase that has a face-center cubic crystal structure. Some of the austenitic stainless steels also contain a small amount of
ferrite phase, which is a ferromagnetic phase with a body-centered crystal
TABLE 25-5
List of Commonly Used 300 Series Austenitic Stainless Steels*
Common name
UNS number
C
Mn
Si
Cr
Ni
Mo
N
301
301L
302
304
304L
305
309S
310S
321
347
316
316L
317
317L
317LM
317LMN
S30100
S30103
S30200
S30400
S30403
S30500
S30908
S31008
S32100
S34700
S31600
S31603
S31700
S31703
S31725
S31726
0.15
0.03
0.15
0.07
0.03
0.12
0.08
0.08
0.08
0.08
0.08
0.03
0.08
0.03
0.03
0.03
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1
0.75
0.75
0.75
0.75
0.75
1.5
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
16–18
16–18
17–19
17.5–19.5
17.5–19.5
17.0–19.0
22.0–24.0
24.0–26.0
17–19
17–19
16–18
16–18
18–20
18–20
18–20
17–20
6.0–8.0
6.0–8.0
8.0–10.0
8.0–10.5
8.0–12.0
10.5–13.0
12.0–15.0
19.0–22.0
9.0–12.0
9.0–13.0
10.0–14.0
10.0–14.0
11.0–15.0
11.0–15.0
13.5–17.5
13.5–17.5
…
…
…
…
…
…
…
…
…
…
2.0–3.0
2.0–3.0
3.0–4.0
3.0–4.0
4.0–5.0
4.0–5.0
0.1
0.2
0.1
0.1
0.1
…
…
…
0.1
…
0.1
0.1
0.1
0.1
0.2
0.10–0.20
∗Single values are maximum values unless otherwise noted.
Other
Ti 5 × (C + N) min, 0.70 max
Nb 10 × (C) min, 1.0 max
25-26
MATERIALS OF CONSTRUCTION
TABLE 25-6
List of Commonly Used 200 Series Austenitic Stainless Steels*
Common name
UNS number
C
Mn
Si
Cr
Ni
Mo
N
0201
201L
201LN
202
Nitronic® 30
XM-19 Nitronic® 50
XM-17
XM-18
Nitronic® 60
XM-11 Nitronic® 40
XM-29 Nitronic® 33
S20100
S20103
S20153
S20200
S20400
S20910
S21600
S21603
(S21800)
(S21904)
(S24000)
0.15
0.03
0.03
0.15
0.03
0.06
0.08
0.03
0.1
0.04
0.08
5.5–7.5
5.5–7.5
6.4–7.5
7.5–10.0
7.0–9.0
4.0–6.0
7.5–9.0
7.5–9.0
7.0–9.0
8.0–10.0
11.5–14.5
1
0.75
0.75
1
1
0.75
0.75
0.75
3.5–4.5
0.75
0.75
16–18
16–18
16–17.5
17–19
15–17
20.5–23.5
17.5–22.0
17.5–22.0
16–18
19–21.5
17–19
3.5–5.5
3.5–5.5
4.0–5.0
4.0–6.0
1.5–3.0
11.5–13.5
5.0–7.0
5.0–7.0
8.0–9.0
5.5–7.5
2.3–3.7
…
…
…
…
1.5–3.0
1.5–3.0
2.0–3.0
2.0–3.0
…
…
…
0.25
0.25
0.10–0.25
0.25
0.15–0.30
0.20–0.40
0.25–0.50
0.25–0.50
0.08–0.18
0.15–0.40
0.20–0.40
Other
Cu 1.0 max
∗Single values are maximum values unless otherwise noted.
an example. Chromium carbide precipitation can also be minimized by the
addition of the stabilizing elements titanium, niobium, or tantalum, which
are added to the stabilized grades, type 321 and 347. The 317L (S31703),
317LM (S31725), and 317LMN (S31726) grades contain progressively higher
levels of Cr, Ni, and Mo, which provide increased corrosion resistance and
slightly higher strength levels than the 316L grade.
200 Series Stainless Steel The AISI 200 series austenitic stainless steels
contain a lower level of Ni and higher levels of Mn and N than the 300 series
stainless steels (see Table 25-6). This subgroup of stainless steels tends to have
higher strength levels and larger strain hardening coefficients than the 300
series grades. Because of the lower nickel contents, the 200 series grades are
often considered a lower-cost alternative to the 300 series stainless steels.
The most widely used grades in this category are the 201 (S20100) and
201L (S20103) grades, which have a corrosion resistance that approaches
that of the 304 and 304L grades. The 201/201L grades are used in a wide
range of applications, including structural members, hose clamps, piston
rings, roofing, and transit cars. The 201LNTM (S20153) is a modified 201
grade that has a very stable austenite phase at low temperatures and is ideally suited for cryogenic tanks and vessels down to −196°C (−320°F).
Other commonly specified 200 series stainless steels include the Nitronic
30® (S20400), Nitronic 60® (S21800), XM-11 (S21904), XM-17 (S21600), and
XM-18 (S21603). The Nitronic 30® (S20400) grade is a nitrogen strengthened
grade that has good resistance to abrasion and metal-to-metal wear. The
Nitronic 60® (S21800) grade is known for having outstanding resistance to
galling and is often specified for fasteners, pins, and bushings. The XM-11
(S21904) grade has a corrosion resistance slightly better than types 304 and
304L and is twice as strong based on yield strength. The XM-17 (S21600) and
the low-carbon equivalent XM-18 (S21603) stainless steels have a corrosion
resistance that approaches the 316/316L grades, and the XM-19 (S20910)
grade’s resistance is similar to type 317L.
High-Performance Austenitic Stainless Steel The more highly
alloyed austenitic grades are often termed high-performance or super austenitic grades. These steels have increased levels of Cr, Ni, Mo, and occasionally N and are designed to provide increased resistance to aggressive
environments, such as stronger acids and bases and higher chloride-bearing
environments such as seawater, brackish water, and brines (see Table 25-7).
A Cu addition used in the 904L (N08904) and alloy 20 (N08020) stainless
steels provides improved resistance to reducing acids. As a group, the highperformance grades tend to have higher strength levels and greater resistance to stress corrosion cracking than the 300 and 200 series stainless steels.
TABLE 25-7
The 6 percent Mo super austenitic stainless steels, which include the
grades AL-6XN® (N08367), 254 SMO® (S31254), and 25-6Mo (N08926),
contain approximately 20 percent Cr, 6 percent Mo, and 0.20 percent N.
These grades show very good resistance to various chemical environments
and can resist long-term exposures to seawater. In many environments,
6 percent Mo grades have a corrosion resistance that is just below that of
the Ni/Cr/Mo alloys such as N06625.
The most corrosion-resistant austenitic stainless steels are the 7 percent
Mo grades 654 SMO® (S32654) and 27-7MO (S31277), which have a resistance in many environments that approaches that of the C276 (N10276)
Ni/Cr/Mo alloy.
FERRITIC STAINLESS STEELS
The ferritic family of stainless steels is the AISI 400 series and is the second
most widely used group of stainless steels. This family is characterized by having a microstructure that consists of ferrite phase and possibly small amounts
of carbides and nitrides. Because the ferrite phase is ferromagnetic, this
group of stainless steels has a magnetic attraction similar to carbon steels.
Ferritic stainless steels tend to have higher strength and much better resistance to chloride stress corrosion cracking than the austenitic grades; however, they do have reduced formability and weldability. The ferritic grades
cannot be hardened or strengthened by heat treatment. They have limited
toughness compared to other types of stainless steels. The toughness can be
further reduced by a large grain size and thicker cross section. Because of
the toughness limitations, the ferritic grades are only produced as thinner
sheet and strip products. It is rare to find products thicker than 0.150 inches,
which restricts their use to tubing and other thin-gage applications.
The ferritic family includes stainless steels with a wide range of corrosion
resistance (see Table 25-8). The most widely used ferritic grade is type 430,
which has a corrosion resistance just below that of the 304 austenitic stainless steel. On the low end of corrosion resistance is type 409, which is widely
used in automotive exhaust systems, and on the upper end are the super ferritic stainless steels such as SEA-CURE® (S44660) and AL 29-4C® (S44735),
which have a corrosion resistance in many environments that is equal to
or slightly better than the 6 percent Mo super austenitic grades. Because of
their very low Ni content, ferritic stainless steels are less costly on a weight
basis than the austenitic grades, with a similar corrosion resistance.
Ferritic grades that contain more than about 12 percent Cr are susceptible to a loss of ductility due to alpha prime precipitation that occurs when
List of Commonly Used High-Performance Austenitic Stainless Steels*
Common name
UNS number
C
Mn
Si
Cr
Ni
Mo
N
Cu
Other
Alloy 20 20Cb-3
Alloy 28
3127 hMo Alloy 31
AL-6XN®
904L
25-6MO, 1925 hMo
254 SMO®
UR 66
27-7MO
254N
654 SMO®
4565
N08020
N08028
N08031
N08367
N08904
N08926
S31254
S31266
S31277
S32053
S32654
S34565
0.07
0.03
0.015
0.03
0.02
0.02
0.02
0.03
0.02
0.03
0.02
0.03
2
2.5
2
2
2
2
1
2.0–4.0
3
1
2.0–4.0
5.0–7.0
1
1
0.3
1
1
0.5
0.8
1
0.5
1
0.5
1
19.0–21.0
26.0–28.0
26.0–28.0
20.0–22.0
19.0–23.0
19.0–21.0
19.5–20.5
23.0–25.0
20.5–23.0
22.0–24.0
24.0–25.0
23.0–25.0
32–38
29.5–32.5
30–32
23.5–25.5
23.0–28.0
24.0–26.0
17.5–18.5
21.0–24.0
26.0–28.0
24.0–26.0
21.0–23.0
16.0–18.0
2.0–3.0
3.0–4.0
6.0–7.0
6.0–7.0
4.0–5.0
6.0–7.0
6.0–6.5
5.2–6.2
6.5–8.0
5.0–6.0
7.0–8.0
4.0–5.0
…
…
0.15–0.25
0.18–0.25
0.1
0.15–0.25
0.18–0.25
0.35–0.60
0.30–0.40
0.17–0.22
0.45–0.55
0.40–0.60
3.0–4.0
0.6–1.4
1.0–1.4
0.75
1.0–2.
0.5–1.5
0.5–1.0
1.0–2.5
0.5–1.5
…
0.3–0.6
…
Nb 8 × C min., 1.00 max.
∗Single values are maximum values unless otherwise noted.
W 1.5–2.5
Nb 0.10
STAINLESS STEELS
TABLE 25-8
25-27
List of Commonly Used Ferritic Stainless Steels*
Common name
UNS number
C
Mn
Si
Cr
Ni
Mo
N
403
405
409
430
439
434
436
444
XM-27 Sea-Cure®
AL29-4C®
S40300
S40500
S40910
S43000
S43035
S43400
S43600
S44400
S44660
S44735
0.15
0.08
0.03
0.12
0.03
0.12
0.12
0.025
0.03
0.03
1
1
1
1
1
1
1
1
1
1
0.5
1
1
1
1
1
1
1
1
1
11.5–13.0
11.5–14.5
10.5–11.7
16.0–18.0
17.0–19.0
16.0–18.0
16.0–18.0
17.5–19.5
25.0–28.0
28.0–30.0
0.6
0.6
0.5
0.75
0.5
…
…
1
1.0–1.35
1
…
…
…
…
…
0.75–1.25
0.75–1.25
1.75–2.50
3.0–4.0
3.6–4.2
…
…
0.03
…
0.03
…
…
0.035
0.04
0.045
Other
Al 0.10–0.30
Ti 6 × (C = N) min., 0.50 max., Nb 0.17
Ti [0.2 + 4(C + N)] min., 1.10 max., Al 0.15
Nb 5 × C min., 0.80 max.
(Ti + Nb) [0.20 + 4(C = N)]min., 0.80 max.
(Ti + Nb) 0.20–1.0, Ti + Nb 6 × (C = N) min.
(Ti + Nb) 0.20–1.00, (Ti + Nb) 6 × (C + N) min.
∗Single values are maximum values unless otherwise noted.
austenite/ferrite phase balance. As a group, the lean grades have high
strength, particularly yield strength, and exhibit a pitting and crevice corrosion resistance that is similar to or just below that of the 316L austenitic
grade. Because of their relatively high yield strength, the lean DSSs are well
suited for tank construction and structural applications.
Standard Duplex Stainless Steels The standard DSSs typically contain 21 to 25 percent Cr, 2 to 3 percent Mo, and 0.15 percent N. Grades within
this category are used widely across all industry sectors, and type 2205
(S32205) has become the most widely used of all of the duplex grades. The
2205 grade has a yield strength that is more than twice that of type 316 and
a pitting and crevice corrosion resistance that is similar to that of type 904L
austenitic stainless steel. Applications for standard DSSs include tanks, piping, process vessels, and structural applications.
Super Duplex Stainless Steels The super DSSs typically contain
25 percent Cr, 3.5 to 4.0 percent Mo, and 0.25 to 0.27 percent N. The pitting
and crevice corrosion resistance of the super DSS are essentially equivalent to
the 6 percent Mo super austenitic stainless steels. In North America, the most
widely used super DSS grades are the 2507 (S32750) and Zeron 100 (S32760)
grades. Common applications include piping, heat exchangers, tanks, and
process vessels used for chemical processing and marine applications.
Hyper Duplex Stainless Steels The hyper DSSs, SAF 3207® (S33207)
and SAF 2707® (S32707), are the most highly alloyed of the duplex family. They contain 26 to 30 percent Cr, 3.5 to 5.0 percent Mo, and 0.30 to
0.50 percent N. The hyper DSSs are designed for more aggressive acid and
chloride-containing environments. The hyper grades are produced in limited quantities and are currently only available as seamless tubing.
heated to temperatures of 315°C to 525°C (600°F to 950°F). Because of this,
the higher alloyed ferritic grades typically have a maximum service temperature of 315°C (600°F).
DUPLEX STAINLESS STEELS
Duplex stainless steel (DSS) is called “duplex” because it has a two-phase
microstructure consisting of ferrite and austenite. Wrought DSS products
typically have an austenite/ferrite phase balance of 50 to 55 vol% austenite (45 to 50 percent ferrite). The duplex microstructure gives this family of
stainless steels a desirable combination of properties, including relatively
high strength, good toughness, and improved chloride stress corrosion
cracking resistance compared to the 300 series austenitic stainless steels.
Because the levels of Ni and Mo are reduced compared to an austenitic
grade with similar corrosion resistance, the duplex grades are often less
expensive than their austenitic counterparts. Since the mid-1990s, the use of
DSS in North America has grown substantially, and many new duplex grades
have been developed. Although the development of new duplex grades provides users with more material options, the lack of availability of all product
forms for many of the newer grades can be an issue.
Because of the high yield strength of the duplex grades, the required section thickness for tanks and pressure vessels can often be reduced with DSS
construction, resulting in substantial cost savings for heavy-wall or highpressure constructions. In order to avoid precipitation of the undesirable
alpha prime in the ferrite phase, most DSSs have a maximum service temperature of 315°C (600°F). As with the other stainless steel families, DSSs
have a range of corrosion resistance and can be divided into subgroups
depending on alloying content (see Table 25-9).
Lean Duplex Stainless Steels The lean duplex grades such as LDX
2101® (S32101), UR 2202 (S32202), ATI 2102® (S82011), and 2304 (S32304)
are characterized by having relatively low levels of Ni and Mo. To compensate for the reduced Ni content, which is a strong austenite former, these
grades have increased levels of N and Mn, which provide an acceptable
TABLE 25-9
MARTENSITIC STAINLESS STEELS
The martensitic stainless steels also belong to the AISI 400 series but differ
from the ferritic grades by having the ability to be strengthened by heat treatment. This family of stainless steels is characterized by having a microstructure that consists predominately of the martensite phase and possibly lesser
List of Commonly Used Duplex Stainless Steels*
Common name
UNS number
C
Mn
Si
Cr
Ni
Mo
N
Cu
1.35–1.70
1.00–2.80
3.0–5.5
1.0–2.0
0.1–0.8
0.45
0.05–0.6
0.1–1.0
0.20–0.25
0.18–0.26
0.1–0.8
…
0.15–0.27
0.50–0.60
2.5–3.5
3.0–3.5
2.9–3.9
0.08–0.20
0.14–0.20
0.10–0.25
…
…
1.5–2.5
3.0–5.0
3.0–4.0
0.24–0.32
0.20–0.30
0.5
0.50–1.00
4.0–5.0
3.0–5.0
0.30–0.50
0.40–0.60
1
1
Other
Lean duplex stainless steels
LDX 2101® 2101
URANUS® 2202 2202
2304
ATI 2102® 2102
S32101
S32202
S32304
S82011
0.04
0.03
0.03
0.03
4.0–6.0
2
2.5
2.0–3.0
2205
2205
255
S31803
S32205
S32550
0.03
0.03
0.04
2
2
1.5
1
1
1
2507
ZERON 100 Z100
S32750
S32760
0.03
0.03
1.2
1
0.8
1
1
1
1
1
21.0–22.0
21.5–24.0
21.5–24.5
20.5–23.5
Standard duplex stainless steels
21.0–23.0
22.0–23.0
24.0–27.0
4.5–6.5
4.5–6.5
4.5–6.5
Super duplex stainless steels
24.0–26.0
24.0–26.0
6.0–8.0
6.0–8.0
W 0.5–1.0
Hyper duplex stainless steels
SAF 2707 HD 2707
S32707
S33207
0.03
0.03
1.5
1.5
∗Single values are maximum values unless otherwise noted.
0.5
0.8
26.0–29.0
29.0–33.0
5.5–9.5
6.0–9. 0
Co 0.5–2.0
25-28
MATERIALS OF CONSTRUCTION
amounts of secondary phases such as ferrite, austenite, and carbides. At
elevated temperatures of approximately 1070°C (1900°F), this family of stainless steels has an austenitic structure that can be transformed into a highly
strained body-centered tetragonal phase, martensite, when cooled to room
temperature. In order to achieve a suitable combination of high strength (high
hardness) and toughness, steels hardened by the martensite transformation
must be tempered at lower temperatures, 93°C to 704°C (200°F to 1300°F). The
martensitic grades are typically alloyed with 10.5 to 18 percent Cr and lesser
amounts of elements such as Ni and Mo. Common grades include 410 (S41000),
416 (S41600), 420 (S42000), 440A (S44002), and 440C (S44004). This family of
stainless steels is ferromagnetic, similar to carbon steel (see Table 25-10).
Martensitic stainless steels have high strength, good wear resistance, low
toughness, and a relatively high ductile-to-brittle transition temperature.
They are very difficult to weld, and they typically require a post-weld heat
treatment. Because of this, the martensitic grades are often restricted to
nonwelded applications.
The martensitic grades do not have very high Cr levels, and some of the
Cr that is present is tied up as carbides. This results in a relatively low corrosion resistance, and the martensitic stainless steels are less resistant than
the standard 304/304L austenitic grades. Because of their limited toughness
and corrosion resistance, the martensitic grades are only specified when
high strength is required. Common applications include turbine blades, cutlery, furnace parts, fasteners, and shafts.
resistance. Common applications for the PH grades include fasteners, aircraft fittings, shafts, gears, bellows, and jet engine parts. Some of the more
commonly specified PH grades and their chemical compositions are summarized in Table 25-11.
Martensitic PH Stainless Steels This group of stainless steels
includes the 17-4PH® (S17400), 15-5 PH® (S15500), 13-8 PH® (S13800),
Custom 450® (S45000), and Custom 455® (S45500) grades. With the martensitic PH grades, an untempered martensitic microstructure is produced
during cooling from the solution anneal temperature. A subsequent agehardening heat treatment between 482°C and 621°C (900°F and 1150°F)
increases the strength due to the precipitation and tempers the martensite
structure for improved toughness and ductility. The martensitic PH grades
are well suited for bar, wire, and forging applications.
Semi-Austenitic PH Stainless Steels The semi-austenitic grades
have a balanced composition that results in a solution-annealed microstructure that is predominately austenite phase. This more ductile structure
can then be formed using standard techniques. After fabrication, a thermal
treatment is used to transform the austenite to martensite, followed by an
aging heat treatment, which increases the strength due to a precipitation
mechanism. Common semi-austenitic PH grades include 17-7PH® (S17700),
15-7 PH® (S15700), AM-350® (S35000), and AM 355® (S35500) grades. The
semi-austenitic PH grades are used primarily for flat-rolled applications and
applications that require extensive forming.
Austenitic PH Stainless Steels The A286 (S66286) grade is the only
PH grade that remains primarily austenitic after the aging heat treatment.
The A286 grade is hardened after solution annealing by a one-step aging heat
treatment to precipitate gamma prime phase. The A286 grade is used in applications that require high strength and good corrosion resistance at temperatures up to 704°C (1300°F). Common uses include jet and automotive engine
components, gas turbine blades, high-temperature fasteners, and springs.
PRECIPITATION HARDENING STAINLESS STEEL
The precipitation hardening (PH) stainless steels belong to the AISI 600
series, and like the martensitic stainless steels, they can be strengthened by
heat treatment. The defining characteristic of this family of stainless steels
is that they all rely on a precipitation mechanism for some or all of their
strengthening. An age-hardening heat treatment is used to produce fine
intermetallic precipitates that provide increased strength. The PH stainless steels can be subdivided into martensitic, semi-austenitic, and fully
austenitic grades. Because of their higher Cr levels, the PH grades have
better corrosion resistance than the 400 series martensitic grades, and
they are used for high-strength applications that require more corrosion
TABLE 25-10
HIGH-TEMPERATURE STAINLESS STEELS
Alloys that are designed primarily for higher-temperature applications are
often termed high-temperature or heat-resistant alloys. Although not formally
defined, a high-temperature application for stainless steels is an environment
List of Commonly Used Martensitic Stainless Steels*
Common name
UNS number
C
Mn
Si
Cr
Ni
Mo
403
410
XM-30
414
415
416
420
431
444A
444B
444C
S40300
S41000
S41040
S41400
S41500
S41600
S42000
S43100
S44402
S44403
S44404
0.15
0.08–0.15
0.18
0.15
0.05
0.15
0.15 min
0.2
0.60–0.75
0.75–0.95
0.95–1.20
1
1
1
1
0.50–1.00
1.25
1
1
1
1
1
0.5
1
1
1
0.6
1
1
1
1
1
1
11.5–13.0
11.5–13.5
11.0–13.0
11.5–13.5
11.5–14.0
12.0–14.0
12.0–14.0
15.0–17.0
16.0–18.0
16.0–18.0
16.0–18.0
0.6
0.75
…
0.75
3.5–5.5
…
…
1.25–2.50
…
…
…
…
…
…
1.25–2.50
0.50–1.00
0.6
…
…
0.75
0.75
0.75
Other
Nb 0.05–0.30
S 0.15 min.
∗Single values are maximum values unless otherwise noted.
TABLE 25-11
Common name
List of Commonly Used Precipitation Hardening Stainless Steels*
UNS number
C
Mn
Si
PH 13-8 Mo
15-5 PH
17-4 PH
Custom 450
Custom 455
S13800
S15500
S17400
S45000
S45500
0.05
0.07
0.07
0.05
0.05
0.2
1
1
1
0.5
PH 15-7 Mo
17-7 PH
AM-350
AM-355
S15700
S17700
S35000
S35500
0.09
0.09
0.07–0.11
0.10–0.15
1
1
0.50–1.25
0.50–1.25
A-286
S66286
0.08
2
Cr
Ni
Mo
Other
7.5–8.5
3.5–5.5
3.0–5.0
5.0–7.0
7.5–9.5
2.00–2.50
…
…
0.50–1.00
0.5
Al 0.90–1.35, N 0.10
Cu 2.5–4.5, (Nb + Ta) 0.15–0.45
Cu 3.0–5.0, (Nb + Ta) 0.15–0.45
Cu 1.24–1.75, Nb 8 × C min.
Ti 0.9–1.4, Cu 1.5–2.5, (Nb + Ta) 0.1–0.5
6.50–7.75
6.50–7.75
4.0–5.0
4.0–5.0
2.00–3.00
…
2.50–3.25
2.50–3.25
Al 0.75–1.50
Al 0.75–1.50
N 0.07–0.13
N 0.07–0.13
24.0–27.0
1.0–1.5
Ti 1.90–2.35, Al 0.35 max, V 0.1–0.5,
B 0.003–0.010
Martensitic PH stainless steels
0.1
1
1
1
0.5
12.25–13.25
14.0–15.5
15.0–17.5
14.0–16.0
11.0–12.5
Semi-austenitic PH stainless steels
1
1
0.5
0.5
14.0–16.0
16.0–18.0
16.0–17.0
15.0–16.0
Austenitic PH stainless steels
∗Single values are maximum values unless otherwise noted.
1
13.5–16.0
NICKEL ALLOYS
TABLE 25-12
Common name
25-29
List of Commonly Used High-Temperature Stainless Steels*
UNS number
C
Mn
Si
Cr
Ni
Mo
N
Other
Wrought high-temperature stainless steels
304H
153 MA®
316H
321H
347H
348H
S30409
S30415
S31609
S32109
S34709
S34809
0.04–0.1
0.04–0.06
0.04–0.1
0.04–0.1
0.04–0.1
0.04–0.1
2
0.8
2
2
2
2
0.75
1.0–2.0
0.75
0.75
0.75
0.75
18–20
18–19
16–18
17–19
17–19
17–19
8–10.5
10–Sep
14–Oct
12–Sep
13–Sep
13–Sep
…
…
2.0–3.0
…
…
…
…
0.12–0.18
…
…
…
…
253 MA®
309H
310H
N08810
S30815
S30909
S31009
800H
0.05–0.1
0.04–0.1
0.04–0.1
0.05–0.10
0.8
2
2
1.5
1.4–2.0
0.75
0.75
1
20–22
22–24
24–26
19–23
12–Oct
15–Dec
19–22
30–35
…
…
…
…
…
…
…
…
405
439
446
S40500
S43035
S44600
0.08
0.03
0.2
1
1
1.5
1
1
1
11.5–14.5
17–19
23–27
0.6
0.5
0.75
…
…
…
…
0.03
0.2
HC
HD
HE
HF
HH
HI
HK
HL
HN
HP
HT
HU
J92605
J93005
J93403
J92603
J93503
J94003
J94224
N08604
J94213
N08705
N08605
N08004
0.5
0.5
0.2–0.5
0.2–0.4
0.2–0.5
0.2–0.5
0.2–0.6
0.2–0.6
0.2–0.5
0.35–0.75
0.35–0.75
0.35–0.75
1
1.5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2.5
2.5
2.5
Ce 0.03–0.08
Ti 4 × (C + N) min, 0.70 max
Cb 8 × (C) min, 1.0 max
(Cb + Ta) 8 × (C) min, 1.0 max, Ta 0.10,
Co 0.20
N 0.14–0.20, Ce 0.03–0.08
Cu 0.75 min, Fe 39.5 min., Al 0.15–0.60,
Ti 0.15–0.60
Al 0.1–0.3
Ti [0.2++4(C + N)] min, 1.10 max; Al 0.15
Cast high-temperature stainless steels
26–30
26–30
26–30
18–23
24–28
26–30
24–28
28–32
19–23
24–28
15–19
17–21
4
7–Apr
11–Aug
12–Aug
14–Nov
14–18
18–22
18–22
23–27
33–37
33–37
37–41
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
∗Single values are maximum values unless otherwise noted.
that exposes construction materials to temperatures of 500°C (950°F) or
higher. Heat-resistant stainless steels are often considered as a separate
group, and a list of some of the more common heat-resistant stainless steels is
given in Table 25-12. Because many of the commonly specified heat-resistant
grades are only available as castings, Table 25-12 also includes cast grades.
High-temperature environments place special demands on construction materials, including the need for good metallurgical stability and
sufficiently high strength and creep resistance. In addition, many hightemperature environments can be very corrosive to construction materials.
Common modes of high-temperature attack include:
• Oxidation
• Carburization and metal dusting
• Nitridation
• Halogen corrosion
• Sulfidation
• Molten salt corrosion
• Molten metal attack
• Corrosion by ash/salt deposits
Common applications for high-temperature stainless steels are industrial
furnaces, turbochargers, gas turbines, power-plants, and equipment used
to produce petrochemical products, chemicals, cement, and glass. When
choosing a heat-resistant stainless steel, the required high-temperature
mechanical properties and the resistance to high-temperature corrosion
must be considered.
NICKEL ALLOYS
Nickel alloys are vital to the process industries. This is because their environmental resistance and attainable high-temperature strengths are generally much greater than those of the austenitic stainless steels. Furthermore,
they are relatively easy to form and weld into complex components.
Advantages of nickel as an alloy base include its reasonable price, its
inherent nobility (resistance to liquid and gaseous environments), and the
fact that beneficial elements, such as chromium, are highly soluble in nickel.
It also has an ideal atomic structure ( face-centered cubic, or gamma phase)
throughout its solid range; this imparts excellent ductility (ability to deform
under stress prior to fracturing). The nickel alloys can be divided into two
major groups, as follows:
Those designed primarily for use at temperatures below 500°C (932°F) in
corrosive liquids.
Those designed for use at temperatures above 500°C (932°F) in corrosive
gases, or just very hot air, which causes oxidation of metallic materials.
These are more commonly known as the corrosion-resistant nickel alloys
and the high-temperature nickel alloys. For the former, the main concern
is their resistance to the environment. For the latter, there are additional
concerns, such as:
1. The stability of the structure at high operating temperatures
2. Material strength at high operating temperatures
3. Creep (slow, progressive, plastic deformation of the material, even at
low stresses)
CORROSION-RESISTANT NICKEL ALLOYS
Introduction Nickel alloys are extremely important to the process
industries, being significantly more resistant to aqueous corrosion than the
stainless steels. Commercially pure nickel, for example, is the material of
choice for sodium hydroxide, while the nickel-copper (Ni-Cu) alloys are outstanding in hydrofluoric acid. The nickel-molybdenum (Ni-Mo) alloys are
exceptional in perhaps the two most important industrial chemicals: hydrochloric and sulfuric acids. The nickel-chromium-molybdenum (Ni-Cr-Mo)
alloys offer great versatility, being resistant to a wide range of chemicals,
including chlorides, which are notorious for causing pitting, crevice attack,
and stress corrosion cracking of lesser materials.
The role of chromium in the corrosion-resistant nickel alloys is to enable
passivation (i.e., the formation of protective oxide or hydroxide films).
Copper, which is closer to the noble end of the electromotive series than
nickel, enhances its resistance to corrosion in the absence of passive films.
Molybdenum, which is a potent solid solution strengthening agent in nickel,
greatly enhances the corrosion resistance of nickel in so-called reducing
25-30
MATERIALS OF CONSTRUCTION
TABLE 25-13
Nickel Alloy Families
Type
Commercially pure nickel
Nickel-copper (Ni-Cu)
Nickel-molybdenum (Ni-Mo)
Nickel-chromium (Ni-Cr)
Nickel-chromium-molybdenum
(Ni-Cr-Mo)
Nickel-chromium-iron (Ni-Cr-Fe)
Nickel-iron-chromium (Ni-Fe-Cr)
Primary attributes
Resistant to caustic alkalis and foodstuffs,
useful electrical and magnetic properties
Resistant to seawater and hydrofluoric acid
Resistant to pure, reducing acids
Resistant to oxidizing acids, resistant to
chloride-induced stress corrosion cracking
Resistant to both oxidizing and reducing acids,
extremely resistant to chloride-induced
pitting, crevice corrosion, and stress corrosion
cracking
Lower-cost alternate to Ni-Cr
Even lower-cost alternate to Ni-Cr
acids (i.e., those that induce hydrogen evolution at cathodic sites) and, when
combined with chromium, contributes to the integrity of passive films,
especially their ability to withstand chlorides.
There are several nickel alloy families, grouped according to their major
(10 wt% or greater) alloying elements, as shown in Table 25-13.
Commercially Pure Nickels Numerous materials are sold under the
guise of commercially pure nickel. Most contain in excess of 99 percent
nickel, and most contain small, elemental additions to control specific
properties. Several are used in the electronics industry, where the electrical
and magnetic properties of pure nickel are used to advantage.
From a corrosion standpoint, the commercially pure nickels are important for two reasons. First, they have outstanding resistance to the caustic
alkalis (caustic soda and caustic potash) over wide ranges of concentration
and temperature. Second, they are very easy to form into complex shapes
and have inherent resistance to mild corrosives; thus they are suitable for
food processing equipment.
The commercially pure nickel most widely used in the chemical
process industries is nickel 200. For applications above approximately
300°C (572°F), nickel 201 is generally preferred. This has a lower carbon
content and is thus resistant to graphitization; it also has higher creep
resistance.
Two of the commercially pure nickels (alloys 300 and 301), are agehardenable. The former was designed for electronic applications. The latter
was designed as an alternate to nickel 200, for applications requiring high
strength. The applications of alloy 301 include plastic extrusion press parts,
glass molds, and corrosion-resistant springs.
Ni-Cu Alloys Nickel and copper, neighbors in the Periodic Table, share
the same atomic structure ( fcc). Moreover, this structure is retained in all
mixtures of the two elements, at all temperatures in the solid range. This
has given rise to several commercially important nickel-copper and coppernickel alloys. The nickel-copper alloys contain approximately 30 to 45 wt%
copper. The primary attributes of the Ni-Cu alloys are their resistance to
seawater, brackish water, and hydrofluoric acid. They also have moderate
resistance to other nonoxidizing acids. They withstand cavitation erosion
and are therefore ideally suited to applications in flowing water, such as propellers and pumps.
Ni-Mo Alloys The Ni-Mo, or B-type, alloys’ chief benefit is their high
resistance to pure hydrochloric and sulfuric acids over large ranges of concentration and temperature. They also resist pure hydrobromic acid, some
concentrations of hydrofluoric acid, food-grade phosphoric acid, acid
chlorides, and other nonoxidizing halide salt solutions. The primary limitation of the Ni-Mo alloys is that they cannot tolerate either oxidizing acids,
such as nitric, or acids that contain oxidizing species. Such species include
oxygen, hydrogen peroxide, chlorine, bromine, ferric ions, and cupric ions.
The most widely used wrought material in this family is B-3 alloy, which
was developed in the early 1990s. B-3 alloy has deliberate additions of chromium and iron, which provide the material with more structural stability
than its predecessors. The most widely used cast Ni-Mo alloy is N-7M.
Ni-Cr Alloys Early experiments involving the addition of chromium to
nickel not only resulted in materials resistant to oxidizing acids, but also
paved the way for the development of a wide range of oxidation-resistant
high-temperature alloys. Indeed, some of the first Ni-Cr materials were, and
still are, used for heating elements in domestic appliances. It is not surprising, therefore, that the most common corrosion-resistant Ni-Cr materials,
namely alloys 600 and 625, were designed primarily for use at high temperatures. The primary corrosion-related attributes of alloy 600 include excellent resistance to sodium hydroxide and good resistance to stress corrosion
cracking (SCC) relative to many stainless steels.
Alloy 625 was developed in the 1950s for use as steam-line piping in supercritical steam power plants. To impart solid solution strength, significant
additions of molybdenum and niobium were used. During the development,
it was discovered that niobium induces a very effective hardening precipitate, known as gamma double-prime, and for some time the development of
alloy 625 was delayed to allow the development of a gamma double-prime
strengthened superalloy.
Ni-Cr-Mo Alloys The chief attribute of the Ni-Cr-Mo (C-type) alloys is
their versatility. This stems from the fact that they contain high levels of
both chromium and molybdenum. Chromium induces passivation in oxidizing acids, as it does in the stainless steels. Molybdenum provides resistance to reducing acids, in particular hydrochloric acid.
Hydrochloric acid and the chloride salts are the chemicals most
responsible for the commercial success of the Ni-Cr-Mo alloys. These
compounds are encountered throughout the chemical process industries, and they can be very damaging to the stainless steels. The Ni-Cr-Mo
alloys are particularly resistant to pitting, crevice attack, and stress corrosion cracking, insidious and unpredictable forms of corrosion caused by
chlorides. Just as the Ni-Cr-Mo alloys resist hydrochloric acid and associated salts, they also resist the corresponding compounds of bromine and
fluorine. Indeed, the Ni-Cr-Mo alloys are among the few metallic materials that withstand warm hydrofluoric acid. Among their other attributes,
the Ni-Cr-Mo alloys resist sulfuric and phosphoric acids, various hightemperature organic acids, and certain concentrations of caustic soda
and caustic potash.
The first Ni-Cr-Mo material, alloy C, was introduced in the early 1930s
in cast form. Wrought products of this alloy became available in the 1940s,
notably with a lower carbon limit. Post-weld annealing was necessary with
alloy C due to the fact that, prior to the 1960s, there was no way to minimize
the carbon and silicon contents during melting. With the advent of argonoxygen decarburization (AOD) during melting, however, low-carbon, lowsilicon versions became reality, the first being C-276 alloy. For applications
requiring even greater resistance to weld heat-affected zone sensitization,
C-4 alloy was introduced in the 1970s. To increase its thermal stability, additions of iron and tungsten were omitted.
The chromium content of C-276 and C-4 alloys is approximately 16 wt%.
By the mid-1980s, it was realized that many industrial environments contain oxidizing impurities, and that higher chromium contents might be
generally beneficial to the versatility of the nickel-chromium-molybdenum
alloys. Thus, C-22 alloy, 686 alloy, alloy 59, and C-2000 alloy, introduced in
the 1980s and 1990s, contain 21 to 23 percent chromium. C-2000 alloy is
unique among the Ni-Cr-Mo materials in having a deliberate copper addition, to enhance its resistance to sulfuric acid. The original cast alloy C is
still made today, under the guise of its AFS designation, CW-12MW, although
newer cast materials (CW-2M and CX-2MW) with enhanced properties are
also widely used.
Due to the needs of the process industries for such versatile materials,
the Ni-Cr-Mo system is still an active field of development. Recent advances
include an age-hardenable version of C-22 alloy (C-22HS alloy) and a highmolybdenum version (HYBRID-BC1 alloy).
Ni-Cr-Fe Alloys The nickel-chromium-iron alloys were originally
designed to fill the performance gap between the high-molybdenum stainless steels and the nickel-chromium-molybdenum alloys. Thus, they possess
good resistance to chloride-induced phenomena, such as pitting, crevice
corrosion, and stress corrosion cracking, and they exhibit moderate resistance to the halogen acids, in particular hydrochloric.
The first low-carbon, wrought material in this family, G-3 alloy, which was
introduced in the 1970s, quickly became established in two major applications. First, cold-reduced tubes of G-3 alloy became standard for moderately
sour oil and gas wells. Second, G-3 alloy found use in evaporators used to
concentrate “wet process” phosphoric acid in the agrichemical industry.
The success of G-3 alloy in these applications led, in the 1980s, to the
introduction of G-30 and G-50 alloys. G-30 alloy was a high-chromium
variant with enhanced resistance to phosphoric acid, and G-50 alloy was
a high-molybdenum variant with enhanced resistance to stress corrosion
cracking in elevated-temperature environments that contained hydrogen
sulfide, as found in oil and gas wells. Copper was added to G-3 and G-30
alloys to enhance their resistance to sulfuric acid. It should be mentioned
that a material developed in the 2000s, G-35 alloy, has superseded G-30 alloy
for “wet process” phosphoric acid applications. However, it falls within the
Ni-Cr category and contains 33 wt% chromium.
Ni-Fe-Cr Alloys Alloy 825 and its age-hardenable cousin alloy 925 contain significantly more iron than chromium and thus constitute a separate
grouping. With iron contents of 30 and 28 wt%, respectively, they are compositionally close to the high-nickel, austenitic stainless steels. The negative
aspects of such high iron contents are reduced solubilities of key elements,
such as molybdenum, and reduced resistance to environmental cracking.
On the other hand, high iron contents reduce the cost of making the alloys.
One of the key additions to alloy 825 is copper. As in other materials, it provides enhanced resistance to sulfuric acid.
Table 25-14 shows the nominal composition of several nickel-based alloys.
NICKEL ALLOYS
TABLE 25-14
25-31
Common Nickel-Based Alloys
Type
Alloy
UNS
Ni
Fe
Cr
Mo
W
Cu
Mn
Ni
Ni-Cu
Ni-Mo
Ni-Cr
Ni-Cr
200
400
B-3
600
625
N02200
N04400
N10675
N06600
N06625
Bal.
Bal.
Bal.
Bal.
Bal.
0.2
1.2
1.5
8
2.5
—
—
1.5
15.5
21.5
—
—
28.5
—
9
—
—
3*
—
—
0.1
31.5
0.2∗
0.2
—
0.2
1
3∗
0.5
0.2
Ni-Cr-Mo
Ni-Cr-Mo
Ni-Cr-Mo
Ni-Cr-Mo
Ni-Cr-Mo
Ni-Cr-Fe
Ni-Fe-Cr
Ni-Cr-Mo
Ni-Cr-Mo
Ni-Cr-Mo
C-22
C-276
C-2000
59
686
G-30
825
CW-12MW
CW-2M
CX-2MW
(UNS N26022)
N06022
N10276
N06200
N06059
N06686
N06030
N08825
N30002
N26455
N26022
Bal.
Bal.
Bal.
Bal.
Bal.
Bal.
Bal.
Bal.
Bal.
Bal.
3
5
3∗
1.5∗
2∗
22
16
23
23
20.5
30
21.5
16.5
16
21
13
16
16
16
16
5.5
3
17
16
13
3
4
—
—
3.9
2.5
—
4.5
1
3
0.5∗
0.5∗
1.6
—
—
2
2.2
—
—
—
0.5∗
1∗
0.5∗
—
—
1.5∗
15
30
6
2
4
Other
Si 0.2, C 0.08
Si 0.2, C 0.2
Si 0.1∗, C 0.01∗, Al 0.5∗
Si 0.2, C 0.08
Si 0.2, C 0.05, Al 0.2, Ti 0.2,
Nb + Ta 3.6
Si 0.08∗, C 0.01∗, V 0.35∗
Si 0.08∗, C 0.01∗, V 0.35∗
Si 0.08∗, C 0.01∗, Al 0.5∗
Si 0.8∗, C 0.03∗, Co 5∗, Nb 0.8
Si 0.2, C 0.03, Al 0.1, Ti 0.9
C 0.12*
C 0.02*
C 0.02*
0.5
1∗
1∗
1∗
∗Maximum composition.
can be categorized as high-temperature corrosion-resistant alloys, precipitation-strengthened alloys, and cast alloys.
Ni-Fe-Cr Alloys The examples that can be grouped into this category are
alloys 800/800H/800HT®, RA-330®, HR-120®. In general, these alloys are considered an upgrade over austenitic stainless steels, while they are also used as
low-cost alternatives to high nickel–containing alloys. These alloys offer optimum high-temperature strength and resistance to oxidation, carburization,
and other types of high-temperature degradation processes. Alloy 800H is a
refinement of alloy 800 by the addition of higher carbon, while 800HT is an
improvement over 800H, offering better creep strength by virtue of a higher
Al + Ti content. In contrast, RA-330 is a Ni-Fe-Cr alloy modified with silicon
addition to offer better corrosion resistance at high temperatures.
Ni-Cr-Fe Alloys The alloys grouped into this category are alloys 600,
601, and X, all of which have relatively low Fe contents compared to the previous class. These exhibit good high-temperature strength and corrosion
resistance. They are also readily available and have excellent fabricability.
Alloy 601 offers higher strength and better corrosion resistance than alloy 600
due to the addition of Al and a higher Cr content (Table 25-15) (INCONEL®
600 and 601 Tech Brochures, www.specialmetals.com; HASTELLOY® X Tech
Brochure, www.haynesintl.com).
Ni-Cr-Mo/W Alloys Alloys such as 625, 617, and 230® can be grouped
into this class since they exhibit superior properties compared to those of
the other groups. They are used for outstanding high-temperature strength,
corrosion resistance, thermal fatigue resistance, and fabricability. However,
none of the alloys in this class requires a separate precipitation hardening
treatment to impart additional strength. Among the three alloys, alloys
230 and 617 exhibit higher creep and stress rupture strength than alloy 625
above 760°C (HAYNES® 230® Tech Brochure, www.haynesintl.com), while
alloy 617 offers better stress rupture strength and alloy 230 better creep
strength. In addition, an issue with alloy 625 is that it is susceptible to a
loss in room-temperature ductility after long-term thermal exposures in
the temperature range 760°C to 871°C (1400°F to 1598°F) (Radavich, J. F.,
and A. Fort, Effects of Long-Term Exposures in Alloy 625 at 1200°F, 1400°F,
HIGH-TEMPERATURE NICKEL ALLOYS
High-temperature nickel-base alloys have found widespread use in the
chemical and petrochemical process industries, primarily for their hightemperature strength and environmental resistance in hot gaseous atmospheres. Among the mechanical properties, tensile strength is important,
but in most cases creep/stress rupture strength dictates the stress allowable
for the alloy at higher temperatures. Most of the high-temperature components are subjected to thermal cyclic exposures and loading; thus they are
also required to be resistant to thermal fatigue and low cycle fatigue (LCF).
In addition, various other properties, such as thermal stability, susceptibility
to stress relaxation cracking, fabricability, and repairability are quite relevant from an alloy selection standpoint. It is noteworthy that the alloy properties and their behaviors vary significantly across the temperature ranges.
Therefore, the alloy is often selected based on its unique characteristics
and properties in the temperature range of interest along with its availability and cost. The properties of the alloy may still vary due to other factors,
including grain size changes, heat-to-heat compositional variations, “rich”
or “lean” compositions, property variations from differences in the product
forms (plate, bar, sheet, foil, pipe, and tube) and most importantly, homogeneity and the presence of impurities within the alloy.
The high-temperature strength in nickel-base alloys is imparted by solid
solution strengthening from alloying elements such as Mo, Ta, and W, or
precipitation hardening. Most of the high-temperature alloys rely on the
formation of a chromia (Cr2O3) scale for high-temperature corrosion protection. However, in certain unique high-temperature corrosive environments,
alumina (Al2O3) or silica (SiO2) scale-forming alloys are also used.
The nominal compositions of various wrought and cast nickel-base
alloys used in the chemical process industries for high-temperature
applications are given in Table 25-15. The solid solution strengthened
chromia-forming alloys can be broadly grouped according to their primary
alloying elements as follows: Ni-Fe-Cr, Ni-Cr-Fe, and Ni-Cr-Mo/W, while
additional nickel-base alloys that do not fall within the preceding classes
TABLE 25-15
Common High-Temperature Nickel-Base Alloys
Alloy
UNS
Type
Ni
Cr
Co
Fe
Mo
W
Ti
Al
800
N08800
Bal.
21
—
39.5
—
—
0.3
0.3
—
Alloy X
N06002
Bal.
22
1.5
18
9
0.6
—
—
—
C 0.10 max, Al + Ti
0.3-1.20
C 0.10
602CA®
N06025
Bal.
25
—
9
—
—
0.15
2.1
—
C 0.20, Y 0.05-0.12
718
N07718
Bal.
19
—
18.5
3
—
0.9
0.5
230
N06230
Bal.
22
5∗
3∗
2
14
—
0.3
—
HR-160
N122160
Bal.
28
29
2∗
1∗
1∗
0.5
0.4∗
—
HK-40
HP Mod.
J94204
N28701
High-temperature
corrosion alloy
High-temperature
corrosion alloy
High-temperature
corrosion alloy
Precipitationstrengthened superalloy
Solid solution
strengthened superalloy
High-temperature
corrosion alloy
Casting
Casting
20
35
25
25
—
—
—
—
0.5
—
0.5
0.5
—
—
—
—
—
1
∗Maximum composition.
Nb
5.1 (Nb + Ta)
Other
C 0.04, Si 0.2, Mn 0.2
C 0.1, La 0.02, Si 0.4,
Mn 0.5
C 0.05, Si 2.75,
Mn 0.5
C 0.35-0.45
25-32
MATERIALS OF CONSTRUCTION
and 1600°F, Superalloys 718, 625, 706 and Various Derivatives, ed. E. A. Loria,
The Minerals, Metals & Materials Society, 1994). In terms of oxidation resistance, alloy 230 offers superior oxidation resistance among the chromiaformers (Deodeshmukh, V., and S. Srivastava, “Static Oxidation Data for
High Temperature Alloys,” Haynes International, unpublished results, 2012).
Other High-Temperature Alloys The nickel-base alloys resistant to
certain types of high-temperature corrosion often contain elements such as
Al and Si along with Cr. The examples include alumina-forming alloys 214®
and 602CA® for oxidation and carburization resistance, and alloy HR-160®,
which forms a subscale of silica scale, for sulfidation resistance.
High-Temperature Cast Alloys High-temperature nickel castings are
important, particularly for furnace tubes. These tubes are centrifugally cast.
For many years the HK-40, 25Cr/20Ni alloys were the most common. These
represented an improvement over wrought alloys because of superior tensile strengths. In recent years, the HP modified 25Cr/35Ni/Nb grades have
been introduced to the commercial market. These have superior stress rupture properties, allowing for the use of thinner tubes with better heat transfer. The HP-modified grades are often marketed under trade names and can
have significantly different compositions and properties. ASTM 608 is the
specification for high-temperature nickel alloy centrifugally cast tubes.
REACTIVE METALS
CORROSION RESISTANCE OF REACTIVE AND REFRACTORY
METALS
Reactive and refractory metals have very unique properties compared to
some of the more common materials like carbon and stainless steels. For
corrosion resistance, these materials are similar in some aspects, but they
behave much differently in various environments. Titanium and niobium
prefer oxidizing media but will have difficulty in alkaline environments.
Zirconium can be used in both strong acids and strong caustics. Tantalum
can be used in strong acids, but caution should be exercised when placing tantalum in strong, hot caustics. Ti, Zr, Nb, and Ta are generally very
resistant to organic compounds. In organic media, some water is needed to
maintain passivity, but the amount will depend on the alloy used. Generally,
titanium and niobium are used up to the boiling point in lower-concentration acids, whereas zirconium and tantalum can be used in many environments above the boiling point. All of these materials are very susceptible to
attack in hydrofluoric acid and fluoride-containing solutions. All of these
materials have excellent resistance to chloride media. Of the four materials,
titanium is one of the materials most used in chloride solutions. All of these
alloys have good resistance to crevice corrosion, with the exception of some
titanium commercially pure (CP) grades, which will suffer crevice corrosion
under certain conditions. This susceptibility can be alleviated by the addition of Pd or Ru, or by the addition of Ni and Mo (e.g., titanium grade 12).
PHYSICAL AND MECHANICAL PROPERTIES
Both titanium and zirconium are hexagonal, close-packed materials.
Niobium and tantalum are body-centered cubic materials. All four of these
materials have high melting points, and their densities vary from the lowdensity titanium metal at 0.163 g/cm3 (4.51 lb/in3) to the very high-density
tantalum metal of 0.600 g/cm3 (16.6 lb/in3).
All of the reactive and refractory metals are protected by stable, adherent
oxide films. Because these materials are very reactive, they will re-form the
oxide film instantaneously (in the presence of water or oxygen) if the oxide
layer is damaged. The oxide film that forms will depend on the environment
to which the metal is exposed. Table 25-16 shows the oxide compositions
TABLE 25-16 Comparing Oxide Forms on Titanium, Zirconium,
Niobium, and Tantalum
Metal
Possible
oxides
Most stable and
corrosion
resistant oxide
Conditions favoring formation of most
corrosion resistant oxide
Ti
TiO
Ti2O3
TiO2
(Ti3O5 rare)
TiO2
Ti’s film is often a mixture of three oxides.
The relative amounts of each form
depends on the oxidizing power of
the media. The amount of protective
TiO2 present decreases with increasing
reducing power of the media. Ti’s oxide
film is, therefore, conditional.
Zr
ZrO2
ZrO2
Zr always forms ZrO2 on its surface, even
under very reducing conditions. Zr’s
protective oxide film is, therefore, very
reliable.
Nb
NbO
NbO2
Nb2O5
Nb2O5
Like Ti, Nb will form lower oxides under
reducing conditions. Nb’s oxide film is,
therefore, conditional.
Ta
Ta2O5
Ta2O5
Ta’s oxide film is likely to be Ta2O5 even in
reducing media. Ta’s oxide film is,
therefore, reliable.
formed on these metals. Some of these materials form multiple oxide films
(Ti and Nb) in various types of media (either reducing or oxidizing). Their
corrosion resistance will be best when the most stable oxide is formed on
the surface.
HYDROGEN EMBRITTLEMENT
As the term suggests, hydrogen embrittlement is a degradation process
related to the absorption of hydrogen and a resulting loss of ductility.
Although these materials are very reactive, they have differences in their
susceptibility for hydrogen pickup. All of these materials (Ti, Zr, Nb, Ta)
will have a tendency to absorb hydrogen under certain conditions with Zr
having the lowest solubility for hydrogen and lower tendency for hydrogen
absorption. Because these materials are very reactive, caution should be
exercised when they are galvanically connected to other dissimilar metals
in corrosive media. For tantalum, it has been shown that very small deposits
of platinum (over-voltage element) were effective in preventing hydrogen
pickup. These deposits or “spots” of platinum can be applied either by platinum spot welding or by the electroplating processes.
TITANIUM
Introduction Titanium is a lightweight, corrosion-resistant material. There are a number of titanium alloys available for use in chemical
applications. The chemical processing alloy grades range from the very
low-strength, high-ductility Ti Grade 1 alloy to the very high-strength Ti
Grade 38 alloy. Many of these alloys are approved for use in ASME Boiler and
Pressure Vessel Code construction. The common CP grades are variations
of Grades 1 and 2. The CP grades have good corrosion properties in many
applications, but these properties will improve in the more severe corrosive
media with the addition of Pd, Ru, or Ni and Mo, as in the case of Ti Grade 12.
While these alloying elements will help increase titanium’s resistance to the
higher concentrations of stronger acids, it will also enable the alloy to resist
crevice corrosion. Table 25-17 shows those alloys and alloy compositions
approved for use in ASME Boiler and Pressure Vessel Code construction.
TABLE 25-17 Titanium Alloys Approved for Use in ASME Boiler
and Pressure Code Construction
Alloy grade
UNS
Common name
1
2
2H
3
7
7H
9
11
12
16
16H
17
26
26H
27
28
38
R50250
R50400
R50400
R50550
R52400
R52400
R56320
R52250
R53400
R52402
R52402
R52252
R52404
R52404
R52254
R56323
R54250
CP Ti
CP Ti
CP Ti
CP Ti
Ti-Pd
Ti-Pd
Ti325
Ti-Pd
Ti-Code 12
Ti-Pd Lean
Ti-Pd Lean
Ti-Pd Lean
Ti-Ru
Ti-Ru
Ti-Ru
Ti-3-2.5 Ru
Ti425
Composition
Unalloyed Ti
Unalloyed Ti
Unalloyed Ti
Unalloyed Ti
Grade 2 + 0.15 Pd
Grade 2 + 0.15 Pd
3Al-2.5V
Grade 1 + 0.15 Pd
Grade 2 + 0.3Mo-0.8Ni
Grade 2 + 0.05 Pd
Grade 2 + 0.05 Pd
Grade 2 + 0.05 Pd
Grade 2 + 0.1Ru
Grade 2 + 0.1Ru
Grade 2 + 0.1Ru
Grade 2 + 0.1Ru
Ti 4Al-2.5V-1.5Fe
REACTIVE METALS
Water and Seawater One of the largest chemical applications for titanium is in seawater and chloride applications. Titanium has excellent corrosion resistance in water and seawater, even in excess of 315°C (600°F).
When the temperature exceeds 75°C (170°F) in a high chloride (seawater)
environment, the possibility of crevice corrosion can exist. (See preceding
discussion of Crevice Corrosion.)
Oxidizing Media (Peroxides, Chlorine) Titanium has good corrosion resistance in oxidizing media and oxidizing acids, such as chromic
acid, wet chlorine, chlorites, hypochlorites, and perchlorates. The corrosion
resistance of titanium in these media can, however, depend on the oxidizing
ions (acid purity) in solution. Titanium should never be used in dry chlorine
since this can cause a rapid attack and possible ignition.
Oxidizing Acids (Nitric, Chromic, Perchloric) Titanium also has
good resistance to the oxidizing acids. For example, titanium has good
resistance to nitric acid when the temperature is below boiling and there
are certain metal species (Ti, Fe, Cr, Si) or metal ions (Pt or Pd) present in
the solution. Titanium should not, however, be used in red fuming nitric
acid since a pyrophoric reaction may occur with a potentially explosive
effect.
Reducing Media (Hydrochloric, Sulfuric, Phosphoric) Titanium is
used in some reducing media in the lower concentrations below the boiling
point as shown in Figs. 25-24 and 25-25. Titanium is not very resistant to
strong reducing acids because these media will cause a breakdown of the
oxide film. In these media, the titanium alloys with the Pd or Ru additions
or with Ni/Mo alloying elements will have better corrosion resistance than
the unalloyed CP grades.
Alkaline Media (Sodium Hydroxide, Potassium Hydroxide,
Ammonium Hydroxide) Titanium alloys are generally resistant to alkaline media at the lower concentrations but will begin to absorb hydrogen at
the higher concentrations and temperatures. In these conditions the titanium may suffer hydrogen embrittlement when the temperature exceeds
80°C (175°F) and where the pH is greater than 12.
Organics and Organic Acids (Acetic, Citric, Formic, Lactic)
Titanium generally has good resistance to organic media and organic acids.
225
Grade 7
Grade 12
Grade 2
0.127 mm/yr
isocorrosion lines
93
79
200
175
66
150
52
125
38
100
24
0
40
50
10
20
30
Concentration of H2SO4, %
Temperature, °F
MAJOR CHEMICAL PROCESSING APPLICATIONS
FOR TITANIUM
107
Temperature, °C
Crevice Corrosion Titanium can experience crevice corrosion
when exposed to hot chlorides, bromides, iodides, fluorides, or sulfatecontaining solutions. Crevice corrosion in titanium will generally occur
in chloride solutions where the temperature is greater than 70°C (180°F)
or the pH is less than 10. This type of localized corrosion may occur in
tight crevices, such as under gaskets, deposits, or scales, or in certain
metal joints. One way to effectively prevent this type of corrosion in titanium is to use grades with Pd or Ru additions and alloys with Ni and Mo
(Ti Grade 12).
25-33
75
60
FIG. 25-25 Isocorrosion curve of titanium alloys in sulfuric acid; 0.13 mm/yr (5 mpy)
lines. (“Corrosion of Titanium and Titanium Alloys,” Metals Handbook Volume 13B
Corrosion: Environments and Industries. Reprinted with permission of ASM International. All rights reserved. www.asminternational.org.)
The presence of oxidizing species or certain metal ions and aeration will
improve the corrosion resistance of titanium in these media.
Fabrication of Titanium Titanium can be fabricated into solid
and clad equipment. It can be machined and formed using conventional
equipment. Titanium can be welded using inert gas welding processes
such as gas tungsten arc (GTAW), gas metal arc (GMAW) and plasma arc
welding (PAW).
ZIRCONIUM
Zirconium is a highly corrosion-resistant material used in many chemical environments. Zirconium is unique in the sense that it exhibits
excellent resistance to both strong acids and strong alkaline media,
including the mineral acids such as nitric, sulfuric, hydrochloric, formic,
and acetic. Zirconium for chemical processing applications is generally available in three grades: Zr700 for explosion cladding use, Zr702
for all types of chemical equipment, and Zr705 for pumps, valves, and
fasteners.
MAJOR CHEMICAL PROCESSING APPLICATIONS
FOR ZIRCONIUM
135
275
Grade 7
Grade 12
Grade 2
0.127 mm/yr isocorrosion lines
121
225
Boiling point curve
93
200
79
175
66
150
52
125
38
100
24
0
5
10 15 20 25 30
Concentration of HCI, %
Temperature, °F
Temperature, °C
107
250
75
35
FIG. 25-24 Isocorrosion curve of titanium alloys in hydrochloric acid; 0.13 mm/yr
(5 mpy) lines. (“Corrosion of Titanium and Titanium Alloys,” Metals Handbook Volume
13B Corrosion: Environments and Industries. Reprinted with permission of ASM International. All rights reserved. www.asminternational.org.)
Water and Seawater Zirconium has good resistance in all types of
water, seawater, brine, and chloride environments, even at elevated temperatures to approximately 300°C (572°F).
Oxidizing Media (e.g., Nitric) Zirconium will have excellent corrosion resistance in oxidizing media such as nitric acid. Zirconium is used
to the full range of concentrations, even at the higher temperatures as
shown in Fig. 25-26. At concentrations above 70 percent, zirconium will
have the tendency to stress corrosion crack. This susceptibility can be
eliminated by the use of a stress-relief anneal heat treatment on the final
equipment.
Reducing Media (Hydrochloric, Sulfuric, Phosphoric) Zirconium
has excellent resistance to sulfuric acid at the low concentrations to well
over the boiling point and up to about 70 percent above the boiling temperature, as shown in Fig. 25-27. Zirconium is one of the few materials capable
of handling sulfuric acid above the boiling temperature. In sulfuric acid
concentrations and temperatures exceeding the “weld limit line,” the welds
must be heat treated, or else preferential attack on the welds will occur. In
hydrochloric acid, zirconium also has good corrosion resistance through
the full range of 0 to 36 percent acid at temperatures exceeding the boiling
point, as shown in Fig. 25-28. The presence of oxidizing impurities in the HCl
will, however, cause zirconium to suffer from localized pitting and stress
corrosion cracking. Laboratory studies as well as commercial applications
have shown that the use of acid pickling followed by a stress-relief heat treat
will reduce or eliminate this tendency for localized corrosion. Zirconium
also has good resistance to phosphoric acid, but the presence of the fluoride
ion (caused by the ores used in producing the phosphoric acid) will increase
the corrosion rate of the zirconium significantly.
25-34
MATERIALS OF CONSTRUCTION
260
260
5
0.5
450
450
>5 mm/yr
0.13
220
220
0–0.13 mm/yr
140
250
100
Temperature, °C
350
Temperature, °F
Temperature, °C
180
350
180
0–0.13 mm/yr
140
0.13–0.5 mm/yr
100
250
Boiling point curve
Temperature, °F
0.5–5 mm/yr
Not tested
Boiling point curve
150
60
150
60
0–0.13 mm/yr
20
20
0
20
40
60
80
Concentration of HNO3, %
100
Organic Solutions (Acetic, Formic) Zirconium has excellent corrosion resistance to all organic media. One of the largest chemical applications for zirconium is its use in acetic acid environments where zirconium
will exhibit very low corrosion rates up to 260°C (500°F). Zirconium will
even handle the purer acetic acid environments if the water content is
maintained to at least 600 ppm. Zirconium also has excellent resistance to
formic acid environments.
Alkaline Media (Sodium Hydroxide, Potassium Hydroxide,
Aluminum Hydroxide, and Ammonium Hydroxide) Zirconium has
excellent resistance to most caustics even above the boiling temperature.
Urea (Ammonium Carbamate Media) Zirconium has excellent
resistance to the urea synthesis reaction at temperatures exceeding 220°C
(428°F) and in the presence of ammonium carbamate, which is very corrosive to many other materials. This application was one of the earliest chemical processing applications for zirconium.
Fabrication of Zirconium Zirconium can be fabricated and welded
into heat exchangers, columns, piping systems, pressure vessels, valves, and
pumps for the chemical process industry. Because zirconium is a reactive
metal, it is sensitive to contamination during welding and thereby limited
to inert gas welding processes such as gas tungsten arc (GTAW) and plasma
arc welding (PAW).
500
0.13–0.5 mm/yr
(5–20 mpy)
<5 mpy
(>0.13 mm/yr)
450
400
350
300
250
120
Boiling point curve
80
Temperature (°F)
Temperature (°C)
160
>0.5 mm/yr
(>20 mpy)
200
150
40
0
60
80
20
40
Sulfuric acid concentration (weight %)
20
30
40
Isocorrosion curve of zirconium in hydrochloric acid. (“Corrosion
of Zirconium and Zirconium Alloys,” Metals Handbook Volume 13B Corrosion:
Materials. Reprinted with permission of ASM International. All rights reserved.
www.asminternational.org.)
FIG. 25-28
Isocorrosion curve of zirconium in nitric acid. (“Corrosion of Zirconium and Zirconium Alloys,” Metals Handbook Volume 13B Corrosion: Environments
and Industries. Reprinted with permission of ASM International. All rights reserved.
www.asminternational.org.)
Weld
200 limit
line
10
Concentration of HCI, %
FIG. 25-26
240
0
100
100
FIG. 25-27 Isocorrosion curve of zirconium in sulfuric acid. (Reprinted with permission from ASTM STP728 Industrial Applications of Titanium and Zirconium. Copyright
ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.)
TANTALUM
Tantalum alloys for chemical processing applications include pure unalloyed tantalum (UNS R05200) and Ta2.5W (UNS R05252) and, on a limited
basis, Ta40Nb (UNS R05240). Tantalum is virtually immune to corrosion,
including resistance to both strong oxidizing and highly reducing media,
but Ta and its alloys are susceptible to hydrogen pickup and embrittlement
in strong, hot alkaline media.
MAJOR CHEMICAL PROCESSING APPLICATIONS
FOR TANTALUM
Oxidizing Media (Nitric, Chromic, Perchloric, Chlorine, Conc.
Sulfuric) Tantalum has excellent resistance to even severely oxidizing
environments such as concentrated nitric acid well above boiling, chromic
acid, and chlorine environments.
Reducing Media (Hydrochloric, Sulfuric, Hydrobromic,
Phosphoric, Formic, Oxalic) Tantalum resists most reducing media
even well above the boiling temperatures.
Tantalum is also very resistant to hydrochloric acid to temperatures
approaching 200°C (392°F). As the temperature exceeds 200°C (392°F),
care should be taken due to tantalum’s hydrogen absorption susceptibility at the higher temperatures (Fansteel published Corrosion Data Survey
on Tantalum, “Hydrogen Embrittlement of Tantalum,” Fansteel, Inc., North
Chicago, Ill., May 8, 1972, pp. 123–133).
Tantalum is resistant to sulfuric acid through the full range of concentrations and temperatures below 150°C (302°F). Tantalum can even be
used in concentrated 98 percent sulfuric acid as high as 200°C (392°F).
Although there are a number of materials that could be used below sulfuric acid’s boiling point, tantalum is one of the only choices (along with
zirconium) above the boiling point, as shown in Fig. 25-29 (Sutherlin, R. C,
“Zirconium and Zirconium Alloys for Use in Sulfuric Acid Applications,”
presented at the 2003 ACHEMA Conference, Frankfurt am Main, Germany,
May 19–24, 2003).
Tantalum can also be used in strong bromine and hydrobromic acid
environments.
Fabrication of Tantalum Tantalum, like niobium, is used as thin tubing or as a liner (cladding) in chemical equipment. Although tantalum is
not approved as an ASME Boiler and Pressure Vessel Code material, it can
be used in ASME BPV code equipment as a corrosion-resistant liner within
the pressure envelope of an approved ASME BPV material. Tantalum is
extremely ductile and can be easily formed. Welding of tantalum is limited
to the use of gas tungsten arc (GTAW), plasma arc (PAW), and electron beam
(EBW) welding. This metal is extremely sensitive to contamination during
welding and should only be welded by fabricators experienced in refractory
metals. It is used in lined piping systems as a corrosion barrier, in thermowells to protect thermocouples, and as rupture disks in pressure-relief
devices.
OTHER METALS AND ALLOYS
25-35
200
375
175
325
Ta
Ta
Zr
150
275
Zr
125
Boiling point curve
Ta
Zr
Pb
Ta
225
Si-Fe
Pb
Ta Si-Fe
Ta Zr Si-Fe
Pb Ni-Mo-Cr
175
Ta Zr Pb Ni-Mo-Cr
Si-Fe
Ni-Mo-Cr Ni-Mo
Zr
Pb
Ta
Ta Zr Pb
Si-Fe Ni-Mo
Ta
Pb
Si-Fe
Ta
Ta
Zr
Ta
Si-Fe
75
Pb
125
Alloy 20
Ni-Mo
Ni-Mo-Cr
Ta Si-Fe
Ta Ni-Mo-Cr
Zr Pb Alloy 20
All except Zr
and alloy 20
All
except
Zr
75
0
Ni-Mo
Ni-Mo
Zr
Pb Si-Fe
100
Si-Fe
Temperature (°C)
Temperature (°F)
Ta
40
60
80
20
Sulfuric acid concentration (weight %)
50
Ni-Mo
25
100
Isocorrosion curve of various materials for use in sulfuric acid, Note: Materials shown
in this figure will exhibit a corrosion rate of less than 0.5 mm/yr (20 mpy) except for Zr and Ta, which
have a corrosion rate of less than 0.125 mm/yr (5 mpy). (Reprinted with permission of ATI Specialty
Alloys and Components.)
FIG. 25-29
NIOBIUM
Niobium resists a wide variety of chemical media, including mineral acids,
organic media, and most salt solutions. Niobium is resistant to most oxidizing acids such as nitric acid, but it may be subject to hydrogen embrittlement in reducing acids. Like titanium and tantalum, niobium may be
subject to embrittlement in certain alkaline media.
The niobium alloys used in chemical processing applications include
pure niobium (Grades 1 and 2) and Nb1Zr.
MAJOR CHEMICAL PROCESSING APPLICATIONS FOR NIOBIUM
Niobium has good resistance to most organic and mineral acids
at temperatures to 100°C (212°F). Like titanium, niobium exhibits
better corrosion resistance if oxidizing impurities are present in the
corrosive environment. Niobium will, however, suffer an increased
corrosion attack in acids if the fluoride ion is present. Niobium
exhibits especially good resistance in chromic acid and bromine
environments.
Niobium will be embrittled in alkaline solutions at even the lower concentrations and higher temperatures or the higher concentrations and ambient
temperatures.
Fabrication of Niobium Niobium can be used in chemical equipment
such as heat exchangers and lined vessels. Niobium’s fabrication methods
and welding processes include welding techniques similar to those used for
tantalum equipment.
OTHER METALS AND ALLOYS
The metals and alloys listed previously are by no means the only construction materials used in the chemical process industries. Some additional
metals and alloys are described next.
ALUMINUM ALLOYS
Aluminum alloys have some specific applications. Although not normally considered a corrosion-resistant alloy, certain grades of aluminum
are used to store and handle concentrated (>93%) nitric acid at ambient temperature. The most commonly used grades are A91100, A93003,
A95052, and A95454. It is important to use low-silicon welding rods during fabrication or repair (Dillon, C. P., Corrosion Control in the Chemical Process Industries, 2d ed., Materials Technology Institute, St. Louis,
Mo., 1994, p. 228). Aluminum is also very important in the fabrication
of heat exchangers in gas liquefaction and evaporation units. It has very
good low-temperature impact properties, making it useful for cryogenic
services.
Aluminum is an amphoteric material, meaning it can be corroded by
both acids and bases. It is rapidly attacked by caustic and most acids except
as noted with nitric acid.
There are a large number of aluminum alloys categorized by composition, heat treatment, and mechanical working. Some of the more
common aluminum alloys used in the process industries are shown in
Table 25-18.
COPPER ALLOYS
The most important copper alloys used in the process industries are
brasses (Cu-Zn alloys) and copper-nickel alloys. Some of the common
copper-based alloys are shown in Table 25-19. The main use has been for
heat exchanger tubes. Cu-Ni alloys are widely used in seawater exchangers.
25-36
MATERIALS OF CONSTRUCTION
TABLE 25-18
Some Commonly Used Aluminum Alloys
Composition (nominal)
Common name
UNS
Al
1100
3003
5052
5454
A91100
A93003
A95052
A95454
99.0 min
Rem.
Rem.
TABLE 25-19
Si
Cu
0.6 max
0.25 max
0.25 max
0.5–0.20
0.5–0.20
0.10 max
0.10 max
Fe
Zn
Mn
0.7 max
0.4 max
0.4 max
0.10 max
0.10 max
0.10 max
0.25 max
0.50 max
1.0–1.5
0.10 max
0.50–1.0
Other
Si + Fe 1.0 max
Mg 2.2–2.8, Cr 0.15–0.25
Mg 2.4–3.0, Cr 0.05–0.20, Ti 0.20
Some Commonly Used Copper Alloys
Composition (nominal)
Common name
UNS
Cu
Zn
Red brass
Admiralty brass
90-10 Copper-nickel
85-15 Copper-nickel
70-30 Copper-nickel
C23000
C44300
C70600
C72200
C71500
85
72
86
83
68
15
27
1.0 max
TABLE 25-20
Ni
Sn
0.9–1.2
10
15
30
1.0 max
Fe (max)
Other
0.05 max
0.06 max
1.0–1.8
0.5–1.0
0.4–0.7
Pb 0.06 max.
Pb 0.07 max.
Pb 0.05 max., Mn 1.0 max.
Cr 0.3–0.7, Mn 0.4–0.9
Pb 0.05 max., Mn 1.0 max.
Some Commonly Used High-Cobalt Alloys
Composition (nominal)
Common name
UNS
Ni
Co
Fe
Cr
Mo
W
Mn
Si
C
La
HAYNES 556
HAYNES 188
HAYNES 25
HAYNES HR-160
ULTIMET
R30556
R30188
R30605
N12160
R31233
20
22
10
37
9
18
39
51
29
54
31
3∗
3∗
2∗
22
22
20
28
26
3
—
—
1∗
2.5
14
15
1∗
1
1.25∗
1.5∗
5
2
0.4
0.35
0.4
2.75
0.3
0.1
0.1
0.1
0.05
0.06
0.02
0.03
—
—
—
3
0.5
0.8
Other
Ta 0.6, Al 0.l, N 0.20, Zr 0.02
B 0.015∗
—
Nb 1∗
N 0.08
∗Maximum.
Brasses are susceptible to dezincification attack. All copper alloys can be
attacked by ammonia and amines, depending on concentration and the
presence of oxygen.
COBALT ALLOYS
Alloys with a high cobalt content are important in high-temperature applications, particularly when sulfidation attack is a concern. In addition, high
cobalt-containing alloys are also used for corrosion and wear resistance
applications. Alloys 556® (UNS R30556) and HR-160® (UNS N12160) are
widely used because of their resistance to sulfidation, while ULTIMET®
alloy offers excellent aqueous corrosion and wear resistance, along with
resistance to weld-related cracking. Other high-cobalt alloys HAYNES® 188
(UNS R30188) and 25 (UNS R30605) combine excellent high-temperature
strength with good resistance to oxidizing and sulfidizing environments
up to 980°C (1796°F) for prolonged exposures. Alloy 25 also offers excellent resistance to metal galling. Some common cobalt-containing alloys
are shown in Table 25-20.
LEAD
Lead was once commonly used because of its good resistance to sulfuric
acid. Its use has dramatically decreased because of its toxicity.
LOW-TEMPERATURE AND CRYOGENIC MATERIALS
Selecting metals and alloys for low-temperature and cryogenic applications is a complex subject that involves many considerations, including special material testing and fabrication issues. It should only be
done by a subject matter expert familiar with the applicable codes and
standards. The information that follows is a short introduction to the
subject.
As temperatures fall, carbon and low-alloy steels show a sharp reduction in ductility and resistance to fracture. This reduction can occur over
a narrow temperature range. The temperature at which this reduction
occurs varies by grade and heat. In general, this becomes a consideration whenever the temperature falls below 24°C (75°F). The generally
accepted design and fabrication codes take this reduction of properties
into account.
There are special grades of carbon steel for lower temperatures. These
include ASTM A516 normalized for plate, ASTM A333 for pipe (includes
both carbon and low-alloy steels), and ASTM A334 for tube (includes both
carbon and low-alloy steels).
The addition of nickel to carbon steels as an alloying element can lower
the allowable design temperature at which adequate physical properties are
retained. These grades include ASTM A203, ASTM A353, and ASTM A645
for plate, ASTM A333 for pipe, ASTM A334 for tubes, ASTM A320 for bolting,
ASTM A350, ASTM 420, and ASTM A522 for flanges and fittings. The specified nickel additions are 2¼, 3½, 5, 8, and 9 percent. As the nickel content is
increased, the allowable temperature decreases.
Austenitic stainless steels do not undergo the sudden decrease in ductility
and resistance to fracture that carbon and other ferritic steels experience.
For this reason they are often used for very low temperatures, including
cryogenic applications. Type 304 stainless steel is probably the most common grade used for cryogenic applications.
Although not normally considered a structural material, aluminum and
some aluminum alloys have important applications in cryogenic processes
as heat transfer materials. Aluminum is used in heat exchangers to transfer
heat between different liquid streams in processes such as liquefied natural
gas processing and gas separation units.
NONMETALLIC MATERIALS FOR CORROSION CONTROL
25-37
NONMETALLIC MATERIALS FOR CORROSION CONTROL
INTRODUCTION
Nonmetallic materials can be classified as inorganic nonmetallic, such as
refractories and glass, and organic nonmetallic, such as plastics, also known
as polymers, polymerics, or polymer-based materials.
INORGANIC NONMETALLICS
Glass and Glassed Steel Glass is an inorganic product of fusion that
is cooled to a rigid condition without crystallizing. With unique properties
compared with metals, these materials require special considerations in
their design and use.
Glass has excellent resistance to all acids except hydrofluoric and hot,
concentrated phosphoric acid (H3PO4). It is also subject to attack by hot
alkaline solutions. Glass is particularly suitable for piping when transparency is desirable.
The chief drawback of glass is its fragility and brittleness, and it is also
subject to damage by thermal shock. However, glass armored with epoxypolyester fiberglass offers some protection against breakage. Similarly,
glassed steel combines the corrosion resistance of glass with the working
strength of steel on the outside. Glass linings are resistant to all concentrations of hydrochloric acid to 120°C (250°F), to dilute concentrations of sulfuric to the boiling point, to concentrated sulfuric to 230°C (450°F), and to
all concentrations of nitric acid to the boiling point.
Porcelain and Stoneware Porcelain and stoneware materials are
about as resistant to acids and chemicals as glass but with greater strength,
a property offset by a greater potential for thermal shock. Porcelain enamels
are used to coat steel, but the enamel has slightly inferior chemical resistance. Some refractory coatings, capable of taking very high temperatures,
are also available.
Brick Construction Brick-lined construction can be used for many
severely corrosive conditions under which high alloys would fail. Brick
linings can be installed over metal, concrete, and fiberglass structures.
Acid-resistant bricks are made from carbon, red shale, or acid-resistant
refractory materials. Red-shale brick is not used above 175°C (350°F)
because of spalling. Acid-resistant refractories can be used up to 870°C
(1600°F).
A number of cement materials are used with brick. Standard are polymer
resin, silicate, and sulfur-based materials. The most widely used resins are
furane, vinyl ester, phenolic, polyester, and epoxies. Carbon-filled furanes
and phenolics are good against nonoxidizing acids, salts, and solvents. Silicates and silica-filled resins should not be used in hydrofluoric or fluorosilicic acid applications. Sulfur-based cements are limited to 93°C (200°F),
while resins can be used to about 180°C (350°F). Silicate-based cements are
available for service temperatures up to 1000°C (1830°F).
Brick porosity, which can be as high as 20 percent, requires an intermediate lining of lead, asphalt, rubber, or plastic. This membrane functions as
the primary barrier to protect the substrate from corrosion damage. The
brick lining provides thermal and mechanical protection for the membrane.
The membrane system also allows for the differential thermal expansion
between the brick lining and supporting substrate. The design of brick linings exposed to higher operating pressure should take into account chemical expansion in addition to thermal expansion.
Cement and Concrete Concrete is an aggregate of inert reinforcing
particles in an amorphous matrix of hardened cement paste. Concrete
made of Portland cement has limited resistance to acids and bases and
will fail mechanically following absorption of crystal-forming solutions
such as brines and various organics. Concretes made of corrosion-resistant
cements (such as calcium aluminate) or polymer resins can be selected for
specific chemical exposures.
Soil Clay is the primary construction material for settling basins and
waste treatment evaporation ponds. Since there is no single type of clay even
within a given geographic area, the shrinkage, porosity, absorption characteristics, and chemical resistance must be checked for each application.
Geotextiles can be incorporated into basin and pond clay construction to
improve the performance of the structure.
ORGANIC NONNMETALLIC MATERIALS
These materials are based on long-chain organic molecules shaped into
forms that can be used in chemical handling applications. A full description
of these materials is done in reference to:
1. Generic type, that is, the organic chemical nature. Examples are polymer families such as styrenics, vinyls, and fluoropolymers.
2. Mechanical properties: rigid [modulus >690 MPa (>100,000 psi)], semirigid [modulus between 69 MPa and 690 MPa (10,000 to 100,000 psi)], and
nonrigid [modulus <69 MPa (10,000 psi)].
3. Thermal processing: thermoplastic (materials that can remelted and
reprocessed) or thermosetting (materials that cannot be remelted and
reprocessed due to post-molding cross linking or curing).
The word elastomers refers to mechanical property basis; such materials are nonrigid and have the unique feature of high elongation and high
recovery. Composites are a special class of materials that are made by using
the polymer as a continuous phase and fibers or particulates as a discrete
phase. Various combinations are possible, depending on the final mechanical properties desired. Examples are wood and fiber-reinforced plastic.
Polymeric materials have an inherent limitation in that their maximum service temperature for chemical handling is about 204°C (400°F). They also
have lower mechanical properties, such as tensile, elongation, stiffness, and
impact. They often are heterogeneous and anisotropic, that is, they have
different properties in different directions. Hence their use is limited to linings of metallic housings or self-supporting structures at low temperatures.
They are also workmanship sensitive since the final material of construction
is often formed during fabrication. Most importantly, they are viscoelastic,
that is, they creep readily, making their performance more time and temperature sensitive than that of metals. There are fewer nondestructive testing
techniques for in-service condition assessment. Yet there are many applications where only polymeric materials can be used due to their outstanding
chemical resistance and favorable strength-to-weight ratio. Various manufacturing techniques such as injection molding, extrusion, blow molding,
transfer molding, contact molding, and compression molding are used to
manufacture finished parts.
COMMONLY USED THERMOPLASTIC MATERIALS
General use thermoplastics (in roughly ascending order of performance and
cost) include the following:
Polyethylene (PE) (-CH2-CH2-)n Various grades such as linear low
density, high density, and ultrahigh molecular weight are available, although
high-density grade is the most popular, particularly the cross-linked type.
Commonly used forms are piping and rotomolded storage tanks. PE has
excellent resistance to mineral acids up to 43°C (110°F). PEs have limited
resistance to organic solvents. Among the polymer materials, urethanes,
nylons, and polyethylene have the best abrasion resistance, but only PE is
commonly used for chemical handling.
Polyvinyl Chloride (PVC) (-CH 2-CHCl-)n- A polymer of vinyl chloride monomer is used in either plasticized or unplasticized form. A chlorinated version (CPVC) is also used for higher temperatures and chemical
resistance. Common forms are piping and sheet linings for vessels and
columns. It has good resistance to sulfuric acid and limited resistance to
organic solvents.
Polypropylene (PP) (-CH2-CHCH3-)n In terms of chemical and
temperature resistance, PP is superior to PE and PVC. It is used as selfsupporting pipe or as lining for metallic piping. It is also used as vessel linings in sheet form.
Mid-Performance Partially Fluorinated Polymers The presence
of fluorine significantly increases the chemical and temperature resistance
of polymers. They are used as linings for piping, valves, and vessels. These
include the following materials:
Polyvinyledene difluoride (PVDF) (-CH2-CF0-)n Trademark: Kynar by
Arkema. Maximum use temperature 121°C (250°F).
Ethylene Chloro-tri-fluoroethylene (ECTFE) (-CH2-CH2-CF2-CFCl-)n
Trademark Halar of Ausimont. Maximum use temperature 135°C (275°F).
Ethylene tetrafluroethylene (ETFE) (-CH2-CH2-CF2-CF2-)n Trademark
Tefzel of Chemours. Maximum use temperature 149°C (300°F).
High-Performance Fully Fluorinated Polymers Chemical and temperature resistance is higher than that of the partially fluorinated polymers.
Among all polymer-based materials, these have the highest chemical and
temperature resistance. Because their mechanical properties are lower
than the partially fluorinated, they are used principally as linings for vessels, piping, valves, hoses, and expansion joints. These materials include the
following:
Polytetrafluoroethylene (PTFE) (-CF2-CF2-)n PTFE was the first grade
of fluoropolymer discovered. This polymer is processed by the powder metallurgy technique of compaction and sintering due to its very high melt viscosity. This is also the most commonly used material for lining pipes, hoses,
expansion joints, and gaskets. Its maximum service temperature is 260°C
(500°F). One trade name is Teflon PTFE.
25-38
MATERIALS OF CONSTRUCTION
Fluorinated Ethylene Propylene (FEP) (-CF2-CF2-CF2-CFCF3-)n
This grade is melt processible and therefore can be easily welded. It is
extruded and injection- and transfer-molded but with a lower service temperature, 204°C (400°F), than that of PTFE. A common example is Teflon
FEP by Dupont.
PerfluoroAlkoxy (PFA) The highest-performing material of this class.
Melt processible, and the maximum service temperature is 260°C (500°F).
An example is Teflon FEP by Dupont.
COMMONLY USED THERMOSETTING MATERIALS
Introduction Used with glass, carbon, or polyester reinforcement,
thermosetting resins are used for fiber-reinforced plastic (FRP) vessels
and piping. FRP is excellent for storing and transporting acids such hydrochloric, sulfuric, and phosphoric. It is also used for hydrofluoric acid up to
5 percent concentrations, but it has limited resistance to organic solvents.
The addition of SiC4 in the corrosion barrier can improve the abrasion
resistance. Typical FRP laminates consist of a corrosion barrier of 2.54 to
5.08 mm (0.1 to 0.2 in) made up of a resin-rich layer or layers of veil (with
90 percent resin) followed by two to four layers of chopped strand mat. The
veils can be of glass, carbon, or polyester. Together the corrosion barrier has
70 percent resin and 30 percent glass. The corrosion barrier is followed by
structural layers of either the hand layup construction, which consists of
alternate layers of mats and woven roving, or the filament-wound construction, which uses continuous strand roving. The structural thickness typically has 30 percent resin. These resins are also used with fibers, fabric, or
particulate fillers as coatings. All resins are used as copolymers with styrene
as a reactive diluent.
BisA Fumerate Polyester Resins for FRP The earliest resins used for
FRP vessels and piping with good chemical resistance in general service. An
example is Atlac 382 resin by Reichold.
Chlorendic Anhydride–Based Polyester Resins Chlorinecontaining polyester resins used for handling chlorine-containing chemicals. Example is Hetron 192 by Ashland Company. Used as copolymers
with styrene.
Epoxy Vinyl Ester and Novolac Epoxy Vinyl Ester Resins
Currently these classes of resin are the most commonly used materials
for FRP applications. They are always used as copolymers with styrene.
Brominated versions of these resins are also available for fire retardancy.
Novolac epoxies have better temperature and solvent resistance than
the straight vinyl esters. Examples are the Derakane family of resins by
Ashland Company.
Furan A polymer of furfural alcohol, Furan has excellent organic solvent resistance in addition to resistance to acids. Due to changing environmental regulations, the corrosive nature of the resin catalyst, and greater
difficulty with handling during fabrication, Furan is used less often than the
vinyl esters.
Epoxy Resins These are more brittle than the vinyl esters but have
excellent chemical resistance in certain applications. They tend to be stronger and hence are used in high-pressure piping, particularly for downhole
applications.
COMMONLY USED ELASTOMERS (THERMOSETTING
RUBBERS)
Introduction Elastomers or rubbers are used as linings for vessels,
valves, hoses, and expansion joints. They are used for seals and gaskets in
very large quantities. Elastomers are broadly classified as natural and synthetic rubbers. They are generally more economical than other options, easy
to manufacture, and more forgiving in terms of installation and repair. Natural rubbers (Cis-1,4-polyisoprene) are made of the rubber latex produced by
rubber trees, Hevea brasiliensis. Synthetic rubbers are a product of the petrochemical industry. In addition to the natural rubbers in their soft, semihard,
and hard forms, the following are used for chemical handling:
1. Butyl or chlorobutyl rubbers—poly(isobutylene-co-isoprene)
2. BUNA N or nitrile rubber—poly(butadiene-co-acrylonitrile)
3. EPDM (ethylene propylene diene monomer), also known as EPM
4. Chloroprene—chlorinated polyisoprene or polychloroprene, popularly
known as neoprene
5. Chlorosulfonated polyethylene (Hypalon®)
Fluorine-containing elastomers such as FKM (e.g., Viton®) or FFKM (e.g.,
Kalrez®) are used for high-temperature and highly corrosive applications,
principally as seals and gaskets.
LIFE CYCLE OF EQUIPMENT
Life cycle deals with materials selection, design, fabrication, shipping, operation, maintenance, and replacement issues.
STORAGE TANKS, REACTORS, TRANSPORTATION
EQUIPMENT: FRP
Material Selection Material selection involves choosing the appropriate resin and reinforcement for both the corrosion barrier and the structural
layers and a cure system per ASTM C581 or the new MTI (Materials Technology Institute) guidelines, which involve coupons exposed to a medium at the
application temperature. The cross sections of these coupons are studied
for the type and extent of damage. Using this information, the life of the
laminate can be estimated.
Design, Construction, Inspection, and Transportation FRP is a
multilayered construction of corrosion barrier and structural thickness
built by contact-molded (also known as hand layup) or filament-wound
techniques. Using a thermosetting resin and curing with either methyl ethyl
ketone peroxide (MEKP), benzoyl peroxide (BPO), or cumene hydroperoxide (CHP), the structure is completed. For vessels, ASME RTP-1 is a widely
accepted industry code for design, fabrication, inspection, and transport in
North America, and EN13121 is a comparable code in Europe. The rest of the
world follows one of them. For pressures higher than 0.10 MPa (14.7 psig),
ASME Section X is used. Large-diameter field-erected or field-fabricated
tanks are less common, but new guidelines are available from the Materials
Technology Institute (Begsjo, P., and S. Rohmhild, Accelerated Testing of FRP,
Materials Technology Institute, St. Louis, Mo., 2014).
Maintenance (In-Service Inspection, Condition Assessment, Fitness
for Service, NDT, and Destructive Techniques) of FRP In-service
inspection consists primarily of visual examination. Damage observed is
blisters, cracking (gross and fine), delamination, fiber prominence, chemical attack, and abrasion. Acoustic emission testing is often used to determine overall structural integrity. The frequency of in-service inspection is
determined by the nature of service and the consequences of failure. Repair
of damaged FRP is possible, but test patches are recommended to ensure
success. Destructive testing in the form of a cutout is possible, but it should
be considered a last resort. With proper interpretation of this information,
remaining life estimates can be made for the corrosion barrier, which is
effectively the remaining life of the unit itself. If the cross-sectional examination indicates damage to the structural layer, retained mechanical properties should be determined in case the unit needs to be taken out of service
immediately.
After all the testing is completed, the path forward is one of operate as
is, re-rate, repair/resurface, or replace. Re-rating is usually for storage vessels; it is an option if it is acceptable to operate the unit at a reduced capacity. Resurfacing is usually costly, so replacing the unit is often economically
justified, and it eliminates uncertainty.
VESSELS WITH LININGS
Vessel linings are classified in three ways.
1. Chemical nature
2. Thickness
3. Application method—sheet, spray, or trowel
The most commonly used classification is the thickness. Thick linings are
defined as those greater than 0.635 mm (0.025 in). Usually they are 1.01 mm
(0.040 in) and higher. Thick linings are used where the corrosion rate of carbon steel is greater than 10 mils per year. Thin linings have thicknesses less
than 0.635 mm (0.025 in) and are used for situations where the corrosion
rate of carbon steel is less than 0.254 mm (0.010 in) per year, or where the
corrosion is localized, such as pitting or crevice corrosion. Thin linings are
also used for nonstick applications or for product purity.
Selection of Lining Testing for chemical compatibility is done in two
steps, screening and application specific. Screening is done by exposing simple coupons in liquid and vapor phases and tracking the changes in mechanical properties over time. Criteria such as those shown in Table 25-21 are
used. High-weighted-value candidates are selected for further evaluation.
EVALUATION OF SCREENING TESTING BY COUPON
IMMERSION
After screening, the candidate materials are subjected to a one-sided test
(Atlas cell test), ASTM C868. Postexposure evaluation is done by visual
examination and by a peel-pull test for loss of adhesion to and condition of
the substrate. Since linings are permeable, particularly the fluoropolymer
ones, permeation testing is carried out. Permeation is a complicated topic,
and engineering decisions are usually made using the rules of thumb shown
in Table 25-22. Other factors such as shop versus field, repairability, and
NDT also play a role in lining selection.
Design and Fabrication of Vessels to Be Lined Vessels need to be
designed and fabricated in such a way that coatings and linings can be effectively applied or installed. This usually means no sharp corners or crevices.
NONMETALLIC MATERIALS FOR CORROSION CONTROL
TABLE 25–21
Evaluation of Screening Testing by Coupon Immersion
Weighted
value
Weight∗
change
Volume∗
change
Hardness
change (units)
Mechanical†
property retained
10
9
8
7
0–0.25
> 0.25–0.5
> 0.5–0.75
> 0.75–1.0
0–2.5
> 2.5–5.0
> 5.0–10.0
> 10.0–20.0
0–2
> 2–4
> 4–6
> 6–9
> = 97
94 < 97
90 < 94
85 < 90
6
5
> 1.0–1.5
> 1.5–2.0
> 20.0–30.0
> 30.0–40.0
> 8–12
> 12–15
80 < 85
75 < 80
4
> 2.0–3.0
> 40.0–50.0
> 15–18
70 < 75
3
> 3.0–4.0
> 50.0–70.0
> 18–21
60 < 70
2
> 4.0–6.0
> 60.9–90.0
> 21–25
50 < 80
1
> 6.0
> 90.0
> 25
> 0 < 50
0
Visual‡ observed change
Permeation rate
(μg/cm2/min)
BTT (min)
≤1=0
> 1–2 = 2 > 1 ≤ 2
> 2–5 = 3 > 2 ≤ 5
> 5–10 = 4 > 5 ≤ 10
No change
Slightly discolored; slightly
bleached
Discolored; yellow, agent; flexible
Possible; stress crack agent;
flexible; possible oxidizing agent;
slightly crazed
Distored; wraped; softened; slight
swelling; blistered; known stress
crack agent
Cracking; crazing; plasticize,
oxidizer; softened; swelling;
surface hardened
Severe distortion; oxidizer; and
plastixizer, deterioratedd
Decomposed
Solvent dissolved, disintegrated
25-39
≤ 0.9
> 0.9–9
> 10–30 = 5 > 10 ≤ 30
> 30–120 = 6 > 30 ≤ 120
> 9–90
> 120–240 = 7 > 120 ≤ 240
> 90–900
> 240–480 = 9 > 240 ≤ 480
> 480–960 = 9 > 480 ≤ 960
> 900–9000
> 960 = 10 > 960
> 9000
∗All values are given as percent change from original.
†
Percent mechanical properties retained include tensile strength, elongation, modulus, flexural strength, and impact. If the percent retention is greater than 100 percent, a value
of 200 minus the percent property retained is used in the calculations.
‡
Due to the variety of information of this type reported, this information can be used only as a guideline.
Welded joints also need to be smooth and in some cases ground flush.
Nozzle sizes cannot be less than 50.8 mm (2 in) with projections of no more
than 152.4 mm (6 in). NACE standard SP0178 should be followed. Surface
preparation for most nonmetallic linings is white metal blasting according
to Steel Structure Paint Council standard SSPC SP 5.
Spray-Applied Thin Coatings Thermosetting coatings of epoxy, phenolic, and phenolic-modified epoxy are the most commonly used. They are
multicoat systems averaging about 0.31 to 0.41 mm (0.012 to 0.016 in) of dry
film thickness.
Spray, Brush, or Trowel-Applied Thick Linings Vinyl ester and
furan resins are used with chopped fibers, fabric, or particulate fillers for a
combination of corrosion control and strength. Broadly they are classified
as flake glass linings or mat linings. A high-voltage spark tester is used to
ensure continuity.
Elastomeric (Rubber) Sheet Lining These are typically 6.35 mm
(0.25 in) thick and are used for handling and transporting hazardous mineral acids such as hydrochloric, hydrofluoric, and phosphoric acids and
abrasive slurries. Most commonly used elastomeric linings are the natural
rubbers—soft, semihard, or hard, depending on the temperature and chemical resistance required. Selection of rubbers is done by screening test by
coupon immersion and application-specific testing. The interpretation of
coupon data requires a great deal of skill and experience. Table 25-23 shows
basic guidelines.
Rubber linings are applied as uncured sheets and are vulcanized (cured)
in the autoclave (in the shop) or with steam in the field. The level of cure
is indicated by Shore A or D scales of Durometer hardness. Integrity of the
final lining is tested by a high-voltage spark test. Degradation of rubber in
service occurs by blistering, chemical attack (hardening or softening), separation of laps, cracking (micro or gross), and mechanical damage such as
abrasion or tears. Damaged rubber can be repaired relatively easily with
patching done by chemical cure rather than steam vulcanizing.
Thermoplastic Linings PVC, CPVC, polypropylene, and fluoropolymers are used as linings in a variety of ways. For PVC, CPVC, and polypropylene, the most common form is sheet linings directly “wall-papered” on
the inside of a metallic vessel. The sheets need to be welded to each other, so
welding technology is an important part of this type of lining. Fluoropolymer
linings such as PFA, PTFE, and FEP are installed in one of five ways, as shown
in Table 25-24. The choice depends on such factors as vessel geometry, complexity, size, shop or field, and cost. In loose linings, bonded linings, and duallaminate structures, welding of the fluoropolymers is involved. The types of
welding procedures used are mostly flow fusion and hot gas hand welding.
The quality of welding is determined by a short-term tensile test as described
by ASTM C 1147 as well as German Welding Institute (DVS) standards in the
qualification stage. High voltage (15 kV to 25 kV) is used to detect pinholes
or breaks in the weld seam. In-service condition assessment is done mostly
by visual inspection and by destructive peel test if required. Spark testing of
in-service lining should be avoided as far as possible, but if necessary the
voltage should be reduced to 50 percent of the original setting. Typical degradation modes of fluoropolymer liners are discoloration, blistering, cracking,
separation of welds, and occasionally environmental stress cracking. Direct
TABLE 25-23
Coupons
TABLE 25–22
Interpretation of Immersion Testing of Elastomers
Root cause of property damage
Permeation Rules
A
Permeation variables
Factor
Change
Effect on permeation
Permeant concentration
Temperature
Pressure
Permeant/polymer chemical similarity
Voids in polymer
Permeant size/shape
Polymer thickness
Polymer crystallinity
Polymer chain stiffness
Polymer interchain forces
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
−
−
−
−
−
Key to Table 25-22: As the concentration of the permeant goes up (+), the
permeation rate goes up (+).
B
C
Deleterious Extraction of Medium
attack on
compound
process
ingredients the filler
medium
system
absorption
Hardness Decrease
Increase
Usually
decreases
Mass and Increase
Decrease
Increase
volume
Decrease
Increase (or Decrease
Tensile
decrease)
strength
at break
Modulus Decrease
Increase (or Decrease
decrease)
(Usually)
Increase
Elongation Increase
at break
Decease
Property
change
D
E
Attack and degradation of
cross-links and/or the
polymer backbone
Increase
Decrease
(hard/brittle) (soft/gummy)
(Often) Increase Increase
Decrease
Decrease
Increase
Decrease
Decrease
Increase or
decrease
(often increase)
25-40
MATERIALS OF CONSTRUCTION
TABLE 25-24
Fluoropolymer Lining Options
Lining technology
Materials available
Adhesively bonded fabric
backed sheets to carbon steel
All fluoropolymers possible. Most
commonly used: PVDF, ETFE, FEP, and
PFA. Thickness range from 60 mils to
120 mils
All fluoropolymers possible. Most
commonly used: ETFE, modified PTFE,
ETFE, FEP, and PFA. Thickness range
from 60 mils to 120 mils
Same as adhesively bonded
Loose lining of sheets
Dual laminate ( fluoropolymers
inside FRP)
Rotolining
Spray and baked coatings
-Electrostatic powder coating
-Liquid dispersin coating
ETFE and PFA (two-coat system).
Thickness range from 90 mils to
250 mils
ETFE, FEP, PFA (with or without wire
mesh reinforcement), PVDF (with or
without fabric reinforcement), ECTFE
Fabrication
Welding, vacuum
bagging, use of
adhesive
Welding, thermoforming
Liner fabricated first by
welding and FRP then
laminated on top
Liner rotationally
molded in a CS vessel
Multicoat application of
dry powder, or liquid
dispersion and baked
chemical attack is rare, but permeation is a bane of fluoropolymers. It cannot be eliminated, but it can be mitigated by following some rules shown in
Table 25-24. Repairs to fluoropolymer linings can be difficult because polymer morphology could change due to absorption and permeation. “Cold”
welding—that is, welding very close to the melting temperature—slow-speed
welding, and using nitrogen can help in the repair procedures.
Piping Most commonly used plastic piping is custom-made FRP and
PTFE-lined steel. PP and PVDF-lined piping are also used for less corrosive
or lower-hazard services. Commodity or machine-made FRP piping is usually not recommended for hazardous service. Rubber-lined piping and solid
thermoplastic piping such as PE, PP, and PVC/CPVC are rarely used in North
America but are still rather common in other parts of the world. ASME
B31.3 is the governing code for materials, design, fabrication, inspection,
and joining of pressure piping. ASTM F1545 is used for all lined piping for
material, construction, and more importantly for qualifying the producer.
For in-service inspection of FRP piping, internal boroscopic examination
is the only possible nondestructive examination technique, and it requires
the pipe circuit to be clean and out of operation. Often, removing sections
of piping and destructively testing them on a statistical sampling basis
provides valuable information. A very large percentage of FRP failures are
at the wrapped joints, particularly those made in the field. For the PTFE
or PVDF-lined piping, the failures are often at the flanged joints. Collapse
of the liners due to temperature and pressure cycling is a common failure
mode. Like FRP pipe, lined pipe systems must be shut down and clean to
allow for proper inspection.
Valves Valves with thermoplastic liners are very common. PTFE, modified PTFE, PFA, FEP, ETFE, and PVDF are the most commonly used materials. Ball, plug, butterfly, diaphragm, and clamp valves use these materials for
body liners, ball or plug covers, and diaphragms.
Design
Pressure determined by ASME code, shop
and field, full vacuum up to 120 F, max temp
determined by adhesive and permeation
consideration
Very limited vacuum handling ability.
Pressure determined by ASME code. Max
temperature determined by permeation
consideration
Very limited vacuum handling ability.
Pressure determined by ASME code. Max
temperature determined by permeation
consideration
Pressure determined by ASME code. Vacuum
limit not determined but expected to be
somewhat higher
Pressure determined by ASME code. Vacuum
limit not determined but expected to be
somewhat higher
Size limitation
None
Determined by body
flange. Usually 3 ft
diameter by 6 ft
14 ft diameter max
6 ft diameter by 22 ft
20 in diameter by 40 ft
Expansion Joints Solid PTFE expansion joints are common, although
some companies choose not to use them for safety reasons. Unlike vessels,
piping, and valves, PTFE expansion joints have no metallic housings. PTFElined elastomeric expansion joints are a better option.
Hoses Many polymeric material components are in chemical hoses.
Inner liners are typically of PE, PTFE, ETFE, PVDF, or elastomers such as
natural rubber, synthetic butyl, or chlorobutyl rubber, EPDM, and chloroprene. Carbon-filled PTFE is also used for static charge dissipation where
required. The overbraiding can be metallic, such as stainless steel or highnickel alloys, or polymeric, such as PVDF. Sometimes an elastomeric cover
is also used for additional handling protection. End connections can be
flanged or couplings. Transfer hoses are typically 25.4 to 50.8 mm (1 or 2 in)
with lengths of 6.1 m (20 ft). Transfer hoses should not be used as permanently installed flexible piping. Most operations replace the hoses at sixmonth to one-year frequencies.
Seals and Gaskets All elastomers are used as gaskets. Usually
elastomeric gaskets are full-faced. PTFE ( filled, nonfilled, or expanded)
is the most commonly used nonelastomeric material. PTFE is also used in
the semimetallic forms, such as envelope or spiral-wound configurations.
The selection of gaskets involves two considerations, chemical compatibility (determined by coupon testing) and compression set (at the application
temperature). Proper joint assembly techniques, proper gasket preload, and
achieving recommended torque are vital. Gaskets are changed every time
the joint is disassembled. O-rings are commonly used for severe service
applications such as valves and mechanical seals. Other cross sections such
as T- and U-rings are also sometimes used.
Column Internals ETFE and PP are the most commonly used column
packing materials. They can be produced in all the common forms, such as
saddles or rings. The most common form of failure is embrittlement.