DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION FOR
STEEL IMMERSED IN FRESHWATER
ABDELSALAM I S AHDASH
A project report submitted in partial fulfillment of the
requirements for the award of the degree of
Master of Engineering (Mechanical-Materials)
Faculty of Mechanical Engineering
Universiti Teknologi Malaysia
APRIL 2010
iii
DEDICATION
To my beloved parents, siblings and friends for their endless loves and supports...
iv
ACKNOWLEDGEMENT
Alhamdulillah, praise to be Allah, The Most Gracious and The Most
Merciful. First of all, I would like to express my special thanks to Professor. Dr Esah
Hamzah for her willingness to be my supervisor in this master’s project. Your
supports, encouragements, critics, guidance and friendship would never been
forgotten. The opportunity to work under your supervision was a great experience.
Special appreciations to Corrtroll company for the unconditional support,
assistance and helps.My heartfelt thanks also to my parents and my siblings for the
endless loves , supports, tolerance and understanding.
In preparing this project report, I was very lucky to have chances to learn
many new knowledge as this is a new field in corrosion protection and materials
science and technology for me. Those experiences hoped to be used and fully utilized
for my future undertaking.
My sincere appreciation also extends to all my friends for the motivations and
all the technicians in materials science laboratory and marine technology laboratory
that involved in helping me to carry out all the laboratory works.
v
ABSTRACT
Impressed current cathodic protection (ICCP) and coating give the optimum
protection against corrosion for steel immersed in freshwater. This project presents
the results of a study on the effectiveness of coating, impressed current cathodic
protection and different environment conditions in preventing corrosion of steel.
Experimental tests were carried out on coated and bare steel plates with ICCP and
without ICCP by immersing in stagnant and flowing freshwater for one month. The
results demonstrated that for coated and bare steel with ICCP and different variable
resistance, the values of the potential are sufficient to protect the bare and the coated
steel -840mV to -875mV.For coated steel without ICCP immersed in stagnant
freshwater the potential has changed from -702 mV to -630mV, but for the bare
sample the change in potential was about -10mV this may be due to oxide layer
formed on the metal surface. For coated steel without ICCP immersed in flowing
freshwater the drop in potential was about -50mV and the bare steel with the same
condition was about -100 mV. A good agreement was observed for corrosion rate
between weight loss measurement (4.29 mpy) test and electrochemical test (4.27
mpy) for bare steel in stagnant freshwater. The location of the reference electrode has
significant implications for the control the potential change of ICCP system, the
corrosion potential increases at the top of the sample (60cm below the water) and
decrease when the sample was immersed further down to 1 meter in the water level.
vi
ABSTRAK
Salutan dan perlindungan katod arus bekasan (ICCP) dapat memberikan
perlindungan yang optimum pada keluli apabila direndam di dalam air bersih. Projek
ini bertujuan untuk mengkaji kesan salutan dan perlindungan katod arus bekasan dan
keadaan persekitaran yang berbeza pada kakisan keluli. Kajian dijalankan selama
sebulan di dalam air genang dan air yang mengalir dengan menggunakan dua jenis
keluli iaitu keluli bersalut dan tanpa salutan. Ia dibahagikan kepada dua bahagian
iaitu dilengkapi sistem ICCP dan tanpa sistem ICCP. Keputusan kajian menunjukkan
nilai upaya pada keluli tanpa salutan dan keluli bersalut yang dilengkapi sistem ICCP
adalah mencukupi untuk melindungi keluli- keluli tersebut(-840mVhingga -875mV).
Manakala keputusan nilai upaya pada keluli bersalut tanpa sistem ICCP yang
direndam di dalam air genang berubah dari -702 mV kepada -630mV. Berlainan pada
keluli tanpa salutan iaitu hanya -10mV disebabkan kehadiran lapisan oksida.
Keputusan nilai upaya untuk keluli bersalut tanpa dilengkapi sistem ICCP di dalam
air mengalir adalah -50mV, manakala bagi keluli salutan adalah -100 mV. Keputusan
ujian kehilangan berat dan juga ujian elektrokimia tidak memberikan perbezaaan
yang ketara nilai kadar kakisan pada keluli tanpa salutan di dalam air genang iaitu
(4.29) mpy untuk ujian kehilangan berat dan (4.27) mpy untuk ujian elektrokimia.
Kedududukan elektrod rujukan juga memberikan kesan pada nilai upaya di dalam
sistem ICCP ini. Nilai upaya kakisan meningkat apabila kedudukan elektrod rujukan
berada di atas sampel (60sm dari paras air) dan menurun apabila diletakkan di
bahagian bawah air iaitu (1 meter dari paras air)
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
TITLE
i
DECLATATIONS
ii
DEDICATION
iii
ACKNOWLEDGMENT
iv
ABSTRACT
v
ABSTRAK
vi
LIST OF CONTENTS
vii
LIST OF TABLES
xii
LIST OF FIGURES
xiii
LIST OF APPENDICES
xvi
INTRODUCTION
1
1.1
Introduction
1
1.2
Background of the Study
1
1.3
Objectives of the Study
3
1.4
Research Questions
3
1.5
Significance of the Study
4
1.6
Scopes of the Study
4
LITERATURE REVIEW
5
2.1
General Review
5
2.2
Electrochemical Nature of Aqueous Corrosion
6
2.3
Corrosion Control
9
2.3.1
9
Design
viii
2.3.2
Materials Selection
10
2.3.3
Inhibitors
11
2.3.4
Protective Coatings
11
2.3.5
Cathodic Protection
11
2.3.5.1 The Principles of Cathodic
12
Protection
2.3.5.2 Types of Cathodic Protection
2.4
2.5
13
Current Sources
16
2.4.1
Transformer/Rectifiers
16
2.4.1.1 Circuit Breaker
20
2.4.1.2 Transformer
21
2.4.1.3 Rectifier Cells
21
2.4.2
Rectifier Efficiency
22
2.4.3
Engine Generator Sets
23
2.4.4
Batteries, Solar and Wind Generators
23
2.4.5
Thermoelectric Generators
24
2.4.6
Closed Cycle Turbo Generators
25
Anode Materials
25
2.5.1
Steel Scrap Anodes
26
2.5.2
Cast Iron Scrap Anodes
27
2.5.3
Silicon Iron Anodes
27
2.5.4
Graphite Anodes
27
2.5.5
Magnetite Andes
28
2.5.6
Lead Alloy Anodes
28
2.5.7
Platinised Titanium Anodes
29
2.5.8
Mixed Metal Oxide Based Anodes
29
2.5.9
Zinc Anodes
30
2.5.10
Aluminium Anodes
31
2.6
Distributed Anode Cables
31
2.7
Protection of Underwater Structure
32
ix
3
RESEARCH METHOLOGY
34
3.1
Introduction
34
3.2
Impressed Current Design
35
3.13.1
36
Physical Dimensions of Structure to be
Protected
3.13.2
Drawing of Structure to be Protected
36
3.13.3
Electrical Isolation
36
3.13.4
Short Circuits
37
3.13.5
Corrosion History of Structures in the
37
Area
3.3
Review pH Data
37
3.4
Variations in Temperature and Concentration
38
3.5
Current Requirement
38
3.6
Coating Resistance
40
3.7
Selection of Anode Material, Weight and
40
Dimensions
3.8
Calculate Number of Anodes Needed to Satisfy
42
Manufacturer’s Current Density Limitations
3.9
Determine Total Circuit Resistance
3.10 Calculate Rectifier Voltage to Determine Voltage
43
43
Output of the Rectifier
3.11 Power Source Selection
44
3.12 Monitoring by Measuring of the Potential
47
3.13 Electrochemical Testing
48
3.13.6 Principle of Measurement
48
3.13.7 Preparation of Working Electrode
50
3.14 Immersion Test
52
x
4
RESULTS AND DISCUSSION
53
4.1
Chemical Composition of Materials Used
53
4.2
Impressed Current Cathodic Protection
54
Calculations
4.2.1 For Coated Steel Immersed in Stagnant
54
Freshwater
4.2.2 For Bare Steel Immersed in Stagnant
56
Freshwater
4.2.3 For Coated Steel Immersed in Flowing
58
Freshwater
4.2.4 For Bare Steel Immersed in Flowing
60
Freshwater
4.3
Potential Measurement Results
62
4.3.1 Coated and Bare Steel Immersed in
62
Stagnant Freshwater with ICCP
4.3.2 Coated and Bare Steel Immersed in
64
Stagnant Freshwater without ICCP
4.3.3 Coated and Bare Steel Immersed in
66
Flowing Freshwater with ICCP
4.3.4 Coated and Bare Steel Immersed in
68
Flowing Freshwater without ICCP
4.4
The Effectiveness of the Reference Electrode
70
Location on The Protection Potrntial Result
4.5
4.6
Electrochemical Result
74
4.5.1
Visual Inspection
74
4.5.2
Polarization Result
74
Immersion Test Results
76
xi
5
CONCLUSTION AND RECOMMENDATIONS
77
FOR FUTURE WORK
5.1
Conclusions
77
5.2
Recommendations for Future work
78
REFERENCES
79
APPENDICES
81
Appendices A - C
81-92
xii
LIST OF TABLES
TABLE NO.
2.1
TITLE
Comparison between sacrificial anode system and
PAGE
15
impressed current system
2.2
Typical consumption rates of impressed current anode
26
materials
3.1
Current density and types of environment
29
3.2
Coated and bare samples immersed in different conditions
44
of freshwater
3.3
Potentiostatic polarization test parameters
48
3.4
Immersion test parameters
52
4.1
Chemical composition of low carbon steel
53
4.2
Electrochemical result
75
4.3
The result of corrosion rate of samples without ICCP
76
xiii
LIST OF FIGURES
TABLE NO.
TITLE
PAGE
2.1
Shows corrosion of pipeline
6
2.2
Electrochemical nature of corrosion processes in water
7
2.3
The principle of cathodic protection
13
2.4
(a) Sacrificial anode system
14
(b) Impressed current system
14
2.5
Operation of a single phase bridge rectifier
19
2.6
Components of a rectifier
22
2.7
Typical zinc anode
30
2.8
Marine structure anode
32
3.1
Flow chart of research methodology
35
3.2
Schematic of coated and bare samples with and without
45
ICCP in
3.3
(a) Stagnant freshwater
45
(b) Flowing freshwater
45
Actual sites in marine technology laboratory
46
(a) Stagnant freshwater side
46
(b) Flowing freshwater side
46
3.4
Wave generator towing tank
46
3.5
Silver- Silver chloride reference electrode
47
3.6
3.7
(a) Schematic
47
(b) Real
47
Copper- copper sulfate reference electrode
47
(a) Schematic
47
(b) Real
47
Cell kit setup
49
xiv
3.8
Photographs of
(a) Connection of specimen to copper wire by
50
50
brazing technique
(b) Mounting of samples
3.9
4.1
Photographs of
50
51
(a) Working electrode
51
(b) Typical surface area of a sample
51
Potential measurement of coated and bare samples in
63
stagnant freshwater with ICCP
4.2
Samples with ICCP after 1 month immersion in stagnant
63
freshwater
4.3
(a) Coated sample
63
(b) Bare sample
63
ICCP anodes after 1 month immersion in stagnant
64
freshwater for
4.4
4.5
(a) Coated sample
64
(b) Bare sample
64
The potential measurement on coated and bare samples in
stagnant freshwater without ICCP
Samples without ICCP after 1 month immersion in
65
65
stagnant freshwater
4.6
(a) Coated sample
65
(b) Coated sample
65
(c) Bare sample
65
(d) Bare sample
65
Quantitative analysis of XRD pattern of corrosion
66
products from the bare sample in stagnant freshwater
4.7
Potential measurement of coated and bare samples in
67
flowing freshwater with ICCP
4.8
Samples with ICCP after 1 month immersion in flowing
67
freshwater
(a) Coated sample
67
(b) Bare sample
67
xv
4.9
ICCP anodes after 1 month immersion in flowing
68
freshwater for
4.10
(a) Coated sample
68
(b) Bare sample
68
Potential measurement of coated and bare samples in
69
flowing freshwater without ICCP
4.11
Samples without ICCP after 1 month Immersion in
69
flowing freshwater
4.12
(a) Coated sample
69
(b) Coated sample
69
(c) Bare sample
69
(d) Bare sample
69
Effectiveness of reference electrode location on the
71
samples potential in stagnant freshwater with ICCP
4.13
Effectiveness of reference electrode location on the
71
samples potential in stagnant freshwater without ICCP
4.14
Effectiveness of reference electrode location on the
72
samples potential in flowing freshwater with ICCP
4.15
Effectiveness of reference electrode location on the
72
samples potential in flowing freshwater without ICCP
4.16
Bar chart for samples immersed in stagnant freshwater
73
4.17
Bar chart for samples immersed in flowing freshwater
71
4.18
(a) A specimen before electrochemical test
74
(b) A specimen after electrochemical test
74
Tafel extrapolation curve for bare steel in freshwater
75
4.19
xvi
LIST OF APPENDICES
APPENCIX
TITLE
PAGE
A
The potential measurement for coated and bare steel in
81
stagnant and flowing freshwater with and without ICCP
B
General properties of low carbon steel
85
C
Wave generator towing tank
86
CHAPTER 1
INTRODUCTION
1.1
Introduction
This section discuss about the introduction of the study which are background
of the study, purpose and objective of the study, significant of study and scope of
study.
1.2
Background of the Study
Corrosion can be defined as destruction or deterioration of the material
because of the reaction with the environment. Most of the materials which undergo
corrosion are metal, so some insist definition of the corrosion should be specific to
the metal. Mars G. Fontana [1] suggest that all material including ceramic, polymer
and other non-metallic material which contributes into the corrosion reaction should
be taken care.
Corrosion weakens strength and cause failure on material. Protection
materials from undergoing corrosion become crucial especially tropical country like
Malaysia which has high humility. Cost of the corrosion in United State is around
USD$ 40 billion or RM 140 million annually. Protection need to be done onto the
material so that reduce corrosion rate so that less materials and money being wasted.
2
Acidity of water varies over a wide range because variety of the compositions.
Factors affecting acidness of water is moisture, alkalinity, permeability of air,
oxygen, salts, stray currents, and biological organisms [1].
Several methods used to protect materials from being corrode, for example
coating, cathodic and anodic protection. In our research, we will only concentrate
into impressed current cathodic protection (ICCP) which is commonly used in big
structure or component protection. ICCP systems require the use of an external DC
power supply that is energized by standard AC current. There are several important
advantages for using ICCP systems, for example unlimited current output capacity,
adjustable out capacity and lower cost per ampere of cathodic protection current [2].
It’s usually cost effective to justify the adoption of an ICCP system, for
example it is much cheaper in term of long term and large structure, for build an
ICCP system than to locate and repair the underground structure leaks. Impressed
current cathodic protection (ICCP) system take advantage of natural electrochemical
reactions of the materials to minimize corrosion damage. In an ICCP system, an
external source of electrons is provided to the metal/electrolyte combination. In order
to achieve protection from the corrosion the sources of electrons must be sufficient to
raise potential of the structure to a level at which negligible corrosion occurs [3].
3
1.3
Objectives of the Study
The objectives of this study are:
1.
To design an ICCP model for steel structure immersed in freshwater.
2.
To compare between impressed current cathodic protection for a steel
structure immersed in stagnant freshwater and impressed current
cathodic protection for a steel structure immersed in flowing freshwater.
3.
To measure the potential of steel with and without impressed current
cathodic protection and determine the effectiveness of impressed
current cathodic protection design.
4.
To determine the effect of coating and ICCP protection on corrosion
behavior of carbon steel.
5.
To determine the effectiveness of the location of the reference electrode
on the protection potential.
1.4
Research Questions
The research questions are
1.
How to build an effective laboratory scale impressed current cathodic
protection setup for a structure immersed in water?
2.
How to improve current impressed cathodic protection system?
3.
How to control parameters of the ICCP – for example current, selected
anode etc.
4
1.5
Significance of the Study
The findings of this study are important to understand theory of the ICCP
system. In the current project, an effective laboratory scale ICCP system has been
designed. Comparison of results for laboratory ICCP system and real application can
be done for further understanding the effect of parameters upon ICCP system.
1.6
Scopes of the Study
The scopes of the study include the following;
1.
Literature review on corrosion principles.
2.
Design an impressed current cathodic protection for steel immersed in
freshwater by calculating the current required, selecting an anode
material, number of anodes, circuit resistance and power source
selection.
3.
Determine the effectiveness of coating and ICCP protection on
corrosion behavior of carbon steel by measuring the potential for steel
in different freshwater conditions.
4.
Determine the effectiveness of the locations of the reference electrode
on the protection potential by measuring the potential at different
positions of the samples.
CHAPTER 2
LITERATURE REVIEW
2.1
General Review
Corrosion is defined as destruction or deterioration of a material, because it is
a form of destructive attack of a metal by chemical or electrochemical reaction with
its environment. In the most common use of the word, corrosion means a loss of
electrons of metals reacting with water and oxygen. In the other way, some of the
scientists think that deterioration by physical cause is not belong to corrosion, but is
described as erosion, galling, or wear [1]. Suggest that some of the chemical attack
will accompanies physical deterioration physical deteriorations, for example
corrosion – erosion, corrosive wear, or fretting corrosion, included both destruction
and deterioration into the concept of corrosion [2].
Corrosion is an electrochemical process in which a current leaves a structure
at the anode site, passes through an electrolyte, and reenters the structure at the
cathode site as Figure 2.1 shows. For example one small section of a pipeline may be
anodic because it is in a environment with low resistivity compared to the rest of the
line. Current would leave the pipeline at that anode site, pass through the
environment, and reenter the pipeline at a cathode site. Current flows because of a
potential difference between the anode and cathode. That is, the anode potential is
more negative than the cathode potential, and this difference is the driving force for
the corrosion current. The total system—anode, cathode, electrolyte, and metallic
6
connection between anode and cathode (the pipeline in Figure 2.1) is termed a
corrosion cell [4].
Figure 2.1
Corrosion of a Pipeline Due to Localized Anode and Cathode
(Source: Technical manual, Headquarters Department of The US Army Washington,
1985)
2.2
Electrochemical Nature of Aqueous Corrosion
In our societies, water is used for a wide variety of purposes, from supporting
life as potable water to performing a multitude of industrial tasks such as heat
exchange and waste transport. The impact of water on the integrity of materials is
thus an important aspect of system management. Nearly all metallic corrosion
processes involve transfer of electronic charge in aqueous solutions. Thus, to
understand the electrochemical nature of aqueous corrosion it is necessary to start the
discussion with the electrochemical reactions. Basically all environments are
corrosive to certain degree, thus we take an example of corrosion of a metal M with
2+ as the oxidation number in HCl acid for discussion on the electrochemical
reactions as shown in Figure 2.2.
7
Figure 2.2
Simple Model Describing The Electrochemical Nature of Corrosion
Processes in HCl [5]
Metal ions go into solution at anodic areas in an amount chemically
equivalent to the reaction at cathodic areas. In the cases of iron-based alloys, the
following reaction usually takes place at anodic areas: [5]
M + 2HCl → MCl2 + H2
(2.1)
Metal reacts with acid solution forming soluble metal chloride and liberating
hydrogen bubbles on the surface. In ionic form the reaction is
M + 2H+ + 2Cl¯ → M+2 + 2Cl¯+ H2
(2.2)
8
Eliminating Cl¯ from both side of the reaction gives
M + 2H+→ M+2 + H2
(2.3)
Reaction (2.3) can be separated as follows
M → M+2 + 2e¯
2H+ + 2e- → H2
(Anodic reaction)
(Cathodic reaction)
(2.4)
(2.5)
In deaerated solution, the cathodic reaction is shown in equation (2.5). This
equation is rapid in most media, as shown by the lack of pronounced polarization
when metal is made an anode employing an external current. When metal corrodes,
the rate is usually controlled by the cathodic reaction, which in general is much
slower (cathodic control).
The most important basic principle of corrosion is during metallic corrosion,
the rate of oxidation equals to the rate of reduction’. In some corrosion reactions, the
oxidation reaction occurs uniformly on the surface while in other cases it is localized
and occurs at specific areas.
Generally, corrosion form can be represented by the equation of (2.4).
Simplest equation of reaction is in acidic deaerated solution, while aerated acidic and
alkaline solution will be represented by the equations (2.6) and (2.7)
9
O2 + 2H2O + 4e- → 4OH¯
(aerated alkaline solution)
(2.6)
O2 + 4H+ + 4e- → 2H2O
(aerated acidic solution)
(2.7)
and
In the absence of all other reduction reactions, water will be reduced by
2H2O + 2e- → H2 + 2OH¯
The equation is equivalent to reaction (2.5), assuming dissociation of water to
H+ and OH- and subtracting OH- from both sides of the reaction [5].
2.3
Corrosion Control
There are five popular methods to control corrosion
2.3.1
Design
As an old adage says, corrosion prevention must start at the blackboard, at the
design stage. A good design at the blackboard is no more costly than a bad design, a
bad design is always more expensive than a good design in reality. Technical design
includes the aspects of design that directly bear on the proper technical functioning
of the product attributes that describe how it works and how it is made. Design
configuration has a critical role to play in the service life of components. The
important point is that the designers must have an understanding and awareness of
10
corrosion problems. Corrosion is, however, only one of the several parameters with
which the designer is concerned and it may not be, however, important to a designer
to give consideration to corrosion unless dictated by a requirement. In many
instances, corrosion is incorporated in design of an equipment only after its
premature failure. More often, more attention is paid to the selection of corrosion
resistant materials for a specific environment, and a minimal consideration is given
to design, which leads to equipment failure. For instance, even a material, like 90-10
copper–nickel may fail prematurely as a condenser tube material, if the flow velocity
of salt water or seawater is not given a due consideration for a smooth flow in the
tube design. This has been a common observation in desalination plants in the Gulf
region. This chapter would highlight how corrosion could be prevented by adopting
good design practices [8].
2.3.2
Materials Selection.
The world of materials comprises of polymers, metals, ceramics, glasses,
natural materials and composites. Revolutionary developments have taken place in
recent years because of the highly competitive materials market and emergence of
new materials and new processing techniques. selecting a corrosion – resistant alloy
would be the answer to corrosion problems.
However, corrosion resistance is not the only property to be considered when
selecting a material. Cost dictate the selection of materials [8].
11
2.3.3
Inhibitors
A corrosion inhibitor is a substance which when added in a small quantities to
a corrosive environment reduces the corrosion rate of the metal by action at or near
the metal surface.
Whether a substance is an inhibitor or not depends on the nature of both the
metal and environment.
It is convenient to classify inhibitors according to which electrode reaction
they affect: anodic or cathodic [8].
2.3.4
Protective Coatings
The objective of a coating is to provide a barrier between the metal and the
environment.
Another advantage of protective coatings is that it is possible to
combine the protective function with aesthetic appeal. Coating can be classified into
Metallic and Non Metallic coatings [8].
2.3.5
Cathodic Protection
Cathodic protection is a method to reduce corrosion by minimizing the
difference in potential between anode and cathode. This is achieved by applying a
current to the structure to be protected (such as a pipeline) from some outside source,
or current can be passed between the cathode and the anode due to the different in
potential When enough current is applied, the whole structure will be at one
12
potential; thus, anode and cathode sites will not exist. Cathodic protection is
commonly used on many types of structures, such as pipelines, underground storage
tanks, locks, and ship hulls.
2.3.5.1 The Principles of Cathodic Protection
The principle of cathodic protection is in connecting an external anode to the
metal to be protected and the passing of an electrical dc current so that all areas of
the metal surface become cathodic and therefore do not corrode. The external anode
may be a galvanic anode, where the current is a result of the potential difference
between the two metals, or it may be an impressed current anode, where the current
is impressed from an external dc power source. In electro-chemical terms, the
electrical potential between the metal and the electrolyte solution with which it is in
contact is made more negative, by the supply of negative charged electrons, to a
value at which the corroding (anodic) reactions are stifled and only cathodic
reactions can take place. The current density and the potential are quite high and after
applying ICCP the potential decrease with decreasing the current density as shown in
Figure 2.3.
13
Figure 2.3
The Principle of Cathodic Protection
2.3.5.2 Types of Cathodic Protection
There are two main types of cathodic protection systems; there are impressed
current and sacrificial anode. Both types of cathodic protection have anodes, a
continuous electrolyte from the anode to the protected structure, and an external
metallic connection (wire). These items are essential for all cathodic protection
systems.
(a)
Sacrificial Anode Cathodic Protection
A sacrificial anode cathodic protection system in fig 2.4 (a) makes use of the
corrosive potentials for different metals. Without cathodic protection, one area of the
structure exists at a more negative potential than another, and results the occurrence
14
of corrosion on the structure. On the other hand, if a negative potential metal, such as
Mg is placed adjacent to the structure to be protected, such as a pipeline, and a
metallic connection is installed between the object and the structure, the object will
become the anode and the entire structure will become the cathode. New addition
object will be sacrificially corrodes to protect the structure. Thus, this protection
system is called a sacrificial anode cathodic protection system because the anode
corrodes sacrificially to protect the structure. Anodes materials in this system are
usually made of either Mg or zinc because of these metals higher potential compared
to steel structures [7].
(b)
Impressed Current Cathodic Protection
Impressed-current systems in Figure 2.4 (b) employ inert (zero or low
dissolution) anodes and use an external source of DC power (rectified AC) to
impress a current from an external anode onto the cathode surface [7].
Figure 2.4
(a) Sacrificial Anode System
(b) Impressed Current System
15
Table 2.1: Comparison Between Sacrificial Anode System and Impressed Current
System
Sacrificial Anode System
Impressed Current System
It requires no external source
External power is essential
It can be easily installed and maintained
More complicated system for installation
It can be used in areas where the soil Limited to use below a soil resistivity of
resistivity is low
3000 ohms-cm
It is economical
Less economical for small structure
For small structures
For big structures
In addition to the structure to be protected and the electrolyte (soil, water,
etc.), impressed current cathodic protection systems consist of the following essential
components:
1.
The current source, such as transformer/rectifiers, solar generators, etc.
2.
The impressed current anodes, buried in soil or immersed in water.
3.
The interconnecting cables [7].
An ICCP uses a rectifier (an electrical device for converting alternating
current into direct current) to provide direct current through anodes to the metal tank,
piping, or other underwater components to achieve corrosion protection.
The system may also be provided with a current control circuit to regulate the
protection level. Such regulation is particularly useful when different structures are
protected by the same current source.
Impressed current cathodic protection (ICCP) is widely employed in
conjunction with surface coatings to control the corrosion of the underwater
structures. The potential static ICCP systems normally fitted employ closed loop
control in which the current output from a DC. power supply is controlled via a
reference electrode (RE) which measures surface potential in its vicinity. This
16
potential is compared with the required protection value (set potential), typically 800
or 850 mV vs silver/silver chloride or copper/copper sulfate System current output is
then varied, via the driving voltage of the power supply, to maintain a zero error
signal and hence a constant potential at the RE. Current output is thus controlled
automatically in response to the operational conditions and the system is, therefore,
demand-responsive. The processes involved in cathodic protection are essentially
electrochemical phenomena at the interfaces between the water and the cathodic
structure (and the anodic surfaces). ICCP system current output, as determined via
the maintenance of the set potential in the vicinity of the RE(s), will be affected by a
number of factors, such as surface condition, coatings and the presence or of flow
[6].
2.4
Current Sources
2.4.1
Transformer/Rectifiers
Transformer/rectifiers are the most economical and usually most reliable
current sources for impressed current cathodic protection. They shall be of a special
design for cathodic protection service and able to operate under the prevailing
service and weather conditions.
Transformer/rectifier units can be either oil- or air-cooled. For installation
outdoors in hot climates, oil-cooled units are preferred. Units with a high current
rating are often oil-cooled although modern semiconductor technology allows
increased current capacities for air cooled units. Air-cooled units are usually smaller
and less expensive than oil cooled units with the same capabilities.
AC power for transformer/rectifier units can be either single-phase or threephase. Especially for high power units, three-phase units are preferred because they
17
normally provide a smoother DC output than single-phase units unless sophisticated
smoothing circuits are installed.
AC sources able to accelerate the corrosion of mild steel even though they are
cathodically protected in both the media [11].
The transformer/rectifier shall be provided with an isolator or Moulded Case
Circuit Breaker (MCCB) on its incoming circuit and, where applicable, on its AC
sub-circuits. Additionally, suitably sized fuses shall be installed on the
transformer/rectifier's phase AC sub-circuits and negative DC output circuits.
The rectifying elements shall be constructed with high current density silicon
diodes, so arranged as to provide full wave rectification. To prevent damage to
overload or short spikes in the supply, the current rating of the diodes shall be more
than 125 % of the maximum current rating of the rectifier and have a minimum peak
inverse voltage of 1200 V.
The unit shall be able to withstand a short circuit at the output terminals of up
to 15 s duration without damage to the circuits.
The output RMS ripple shall not exceed 5 % of the DC output current
between 5 % and 100 % of the rated current output. This is particularly important for
certain anode types such as platinised titanium.
The output voltage shall be adjustable from zero to the maximum rated output
when on load. A stepless (continuous) adjustment is preferred. If tapping switches
are used, these shall be front mounted switches with a step-size of maximum 3 % of
maximum output. Transformer tapping should not be done by relocating jumpers
unless changes in operating conditions are expected to be infrequent (e.g. when
subsequent potential or current control is used). Electronic voltage and/or current
control may be used, e.g. in combination with automatic potential control
18
For low current applications such as for well-coated structure, a ballast
resistor may be required to provide a minimum load for good operation of the
rectifier.
The transformer/rectifier shall be provided with approximately 70 mm
diameter or similarly sized square pattern meters to read the output voltage and
current. The measuring accuracy shall be better than 2 % of full scale.
The polarity of the DC terminals and AC supply terminals shall be clearly
marked. AC and DC cables shall be physically separated e.g. by an insulating panel.
A built-in timer unit may be required. The timer unit may be mechanical or
electronic and shall be capable of switching the full output current in a sequence of
50 s on and 10 s off. If more than one transformer/rectifier are protecting a single
structure, all transformer/rectifier timer units should be provided with a facility for
synchronous switching. During normal operation, the timer shall be bypassed.
If a transformer/rectifier is oil-cooled, the incoming cables shall terminate in
separate non-oil filled cable boxes and penetration into the tank shall be via bushings
above oil level. A sight glass and thermometer shall be provided [7].
The three-phase bridge is the most common circuit for rectifiers operated
from a three-phase AC power line. Each phase of a three-phase AC current is spaced
120 electrical degrees apart and therefore the voltage of each secondary winding
reaches its peak at different times.
Figure 2.6 shows the operation of a single phase bridge rectifier. The
direction of flow reverses 60 times per second for 60 cycles AC. In a positive halfcycle (diagram A), current originates at T2 on the secondary winding. It is blocked
by D3 (silicon diode). The current, therefore, flows through direction D1, follows the
path (3) and through diode D4 it enters the negative terminal T2. In the next half-
19
cycle (1/120th) of a second later, polarities at T1 and T2 are reversed (see diagram
B). The current is blocked by diode D4 and flows through D2, follows the path (3)
through D3 in the same direction as before. The load RL thus receives energy in the
form of pulses at 120 per second.
Although three-phase rectifiers are used as mentioned before, each single
bridge shares a pair of diodes with one of the other bridges. The three phase bridge is
like three single-phase bridges, with each bridge sharing a pair of diodes with one of
the other bridges [7]. A rectifier consists of three important components circuit
breaker, transformer and rectifying elements (stacks). Brief details are given in
Figure 2.5.
Operation of a Single Phase Bridge Rectifier. Arrows Show
Figure 2.5
Conventional (positive) Current Flow Direction
20
2.4.1.1 Circuit Breaker
These are basically switches with an internal mechanism which opens the
switch when the current exceeds a prescribed designed limit. They also serve as ‘on
and off’ switches. There are two types of switches: (1) magnetic and (2) thermal. The
circuit breaker protects equipment from over loading.
In the magnetic type, a coil is woven around a brass tube and a magnetic field
is set up by a current flowing in the coil. The magnetic slug is held at one end of a
tube by a spring. The magnetic field attracts the slug, but at or below the rated
current the slug does not move. At overload, the magnetic field pulls the slug into the
coil. When the slug is drawn to the opposite end of the tube, the circuit is completed
for the trip mechanism and the breaker switch trips. The movement of the magnetic
flux is slowed down and a time delay is provided. The breaker can trip on to 101–
125% of the rated current. Overloads
of ten times the rated currents can be sustained. The dropping is very fast
when the overload is ten times.
In thermal magnetic breakers, the thermal tripping is caused by the flowing
current through the resistor close to the bimetallic strip. When the current exceeds
the rated value, the bimetallic element trips the breaker and a long time delay is
involved before the breaker can be closed [7].
21
2.4.1.2 Transformer
This consists of two coils of wire wound around an iron core. The coils are
not connected electrically, but the core provides a magnetic link between them. AC
voltage is applied to one coil (primary), the changing magnetic field crosses to the
other coil (secondary) and induces a voltage in it. The changing field induces the AC
voltage in the secondary coil that is proportional to the turn’s ratio between the two
coils [7].
P
S
=
P
S
2.4.1.3 Rectifier Cells
The change of AC power to DC is done by rectifying elements. They act like
check valves by offering low resistance to current flow in one direction and high
resistance in the other direction. The function of the rectifying element is to allow the
current to flow readily in one direction and to block current flow in the opposite
direction fig 2.6. The Selenium cell is the most common rectifier cell. Selenium is
applied to one side of an aluminum base plate which has been nickel plated. A thin
metallic layer is applied over the selenium layer. This layer acts as counter electrode.
It collects the current and provides low resistance to the contact surface. These cells
may be arranged in stacks or parallel to produce the desired voltage and current
rating [7].
22
Figure 2.6
2.4.2
Components of a Rectifier
Rectifier Efficiency
This is the ratio between the DC power output and AC power input.
Rectifiers are used as a source of DC power. Rectifiers convert the AC current (60
cycles) to DC current through rectifier operated at maximum efficiency at the full
rated loads.
Overall rectifier efficiency =
× 100%
An efficiency filter can be used to minimize the ripples.
23
2.4.3
Engine Generator Sets
Where AC power is not available to supply rectifiers and the required power
is high, engine generator sets may be used to provide the electrical supply needed.
If a remote survey unit with alarms cannot be installed, a two-generator
system shall be used (one running, one on standby) with an automatic changeover
system.
Remote generator units are prone to failure and vandalism and require
frequent maintenance. For critical systems, alternatives such as solar power may be a
better option [7].
2.4.4
Batteries, Solar and Wind Generators
If the AC mains suffer frequent power failures, the use of batteries, charged
by mains powered battery chargers, may be used instead of transformer/rectifiers.
Batteries may also be charged by means of a wind-powered generator or by
solar cells. The batteries should be charged on a regular basis to provide a continuous
source of cathodic protection current.
Cathodic protection systems using batteries shall be provided with suitable
output voltage and/or current control equipment and a load cut off system to avoid
damage to the batteries due to a complete discharge.
24
Battery chargers and generators shall be provided with regulators to ensure
that the recommended charging rates are applied and shall be equipped with a
protection system to prevent overcharging of the batteries.
The design of wind and solar generators shall be based on extensive local
weather reports, stating average and minimum sun and/or wind periods and intensity
during all seasons, generally a one-year period, to determine the capacity of the
system. The battery capacity shall be based on the required autonomy during the
prevailing maximum time without sun or wind.
Wind and solar generators shall be rated to recharge the batteries in less than
48 hours from a partially discharged state due to an extended period of no wind/sun.
In tropical areas the generators and batteries shall be designed to operate in
high ambient temperatures. Solar generators should be designed to maintain the
design capacity at the highest ambient temperature [7].
2.4.5
Thermoelectric Generators
Thermoelectric generators are based on the “thermocouple” principle.
Heating one side of a stack of thermocouples, sized to provide the required DC
power, generates power. Heating of the unit is normally accomplished by means of
gas from the gas line that is protected by the unit.
Thermoelectric units are economical but their reliability depends largely on
the quality of the supply gas. Dust and liquids transported with the gas may block the
burner system and extinguish the flames. This can be avoided by using additional
pressure control systems or filters but this makes these units less competitive.
25
Thermoelectric units tend to operate more efficiently in cold climates
compared to hot (tropical) climates [7].
2.4.6
Closed Cycle Turbo Generators
A closed cycle turbo generator consists basically of a combustion system, a
vapour generator, a turbo alternator, an air-cooled condenser, a rectifier, alarms and
controls housed in a shelter. It can supply 200 to 3,000 Watt of filtered DC power.
The gas supply is normally provided from the structure or from a separate supply
system. The units are manufactured by specialized companies. Like thermoelectric
generators their reliability probably depends on the gas quality and cleanliness [7].
2.5
Anode Materials
Any current-conducting material could be used for the anodes or groundbeds,
but for reasons of economy and required service life, the material should have a low
consumption rate at an acceptable cost. Materials used for groundbed construction
can be carbon steel scrap, cast iron scrap, graphite cylinders, special alloy rods or
noble materials plated with “inert” materials such as platinum or mixed metal oxides.
A description of the various materials is given below and approximate current
densities and consumption rates are given in Table (2.1)
26
Table 2.2: Typical Consumption Rates of Impressed Current Anode Materials
Impressed
current anode
Material
Maximum
current density,
A/m²
Working
current
density, A/m²
Consumption
rate
Steel
-
0.5
10 kg/A.yr
Aluminium
10
4.8
2 kg/A.yr
Graphite
25
2.5 to 10
0.25 kg/A.yr
Silicon Iron
50
5 to 25
0.1 kg/A.yr
Magnetite
200
115
0.02 kg/A.yr
Lead Alloy
300
50 to 150
0.085 kg/A.yr
Platinised Titanium
2000
250 to 700
8 mg/A.yr
2000
500 to 1000
8 mg/A.yr
1000
500 to 100
1 mg/A.yr
Platinised
Tantalum or
Platinised Niobium
MMO on Titanium
2.5.1
Steel Scrap Anodes
In some cases, steel scrap is used as an impressed-current anode. This may be
for temporary protection or for economical reasons. Abandoned steel-lined oil or
water wells can be quite suitable. The sections are thin, however, and early failure is
likely. Another weakness is the anode cable connection, which should preferably not
contact the soil. For long term protection of critical installations, the use of scrap
metal is not recommended [7].
27
2.5.2
Cast Iron Scrap Anodes
Cast iron scrap generally has the advantage of being thick in section and of
such form that any one piece will be in soil of more or less uniform resistivity.
Moreover, a graphite surface is left exposed as the outer iron is consumed, so that the
remaining iron with its graphite surface acts as a graphite anode, thus reducing the
rate of iron consumption. Old engine blocks are examples. The anode cable
connection remains the weak point [7].
2.5.3
Silicon Iron Anodes
High silicon cast iron has been found to be a suitable anode material. It is
relatively inexpensive and it is used on quite a large scale for groundbeds. It is
suitable both in soil and water. In soil applications, it is normally surrounded by a
carbonaceous backfill. Current densities can be high and consumption rates are low
taking into account the high mass per anode. The anodes come in different sizes and
different cable attachments. They are quite brittle and shall be handled carefully. For
seawater applications the silicon iron is usually alloyed with about 5 % chromium to
resist pitting [7].
2.5.4
Graphite Anodes
Graphite anodes have a low rate of consumption. The choice between
graphite and silicon iron often depends on availability in a given area.
Graphite anodes are generally cylindrical in shape, though other forms are
available. The graphite is impregnated with wax or resin, which reduces flaking, or
28
disintegration of the anodes as the graphite is consumed. The anodes are supplied
with terminal connections, and with cables if required. When installed in soil,
impregnated graphite anodes are generally used with a backfill of carbonaceous
material such as coke breeze. In soil and seawater, current densities of up to 10 A/m2
may be employed, but in fresh or brackish water, the current densities should not
exceed 2.7 A/m2 in fresh water or 5.4 A/m2 in brackish water. At higher outputs, the
surface of the graphite deteriorates excessively due to the formation of gas.
Graphite anodes are brittle and require careful handling during transport,
storage, and installation. Long graphite cylinders may be broken by subsidence of
surrounding soil [7].
2.5.5
Magnetite Anodes
Magnetite (Fe3O4) anodes are made by means of a proprietary process. The
magnetite is plated onto metal (copper alloy) cylinders, which provide the electrical
connection. They are light in weight but brittle. Current output and consumption rate
are favorable. Because of single-source supply, they are used less often than other
alloys [7].
2.5.6
Lead Alloy Anodes
An alloy of lead, silver, and antimony (1 % of silver, 6 % of antimony) has
been used in salt water. At a current density of 108 A/m2, the annual consumption is
about 85 g/A. The alloy has good mechanical properties and can be cast or extruded
to any desired shape. Platinised titanium or MMO anodes have largely replaced this
type of anode [7].
29
2.5.7
Platinised Titanium Anodes
These anodes are used for salt water or fresh water where the conductivity is
very low. Titanium develops an adherent oxide layer of high electrical resistance.
The oxide layer prevents corrosion by acting as a barrier. Titanium acts as an inert
support for the platinum. Platinum can withstand very high current density and it is
generally applied to a small area only. The platinum layer is normally 2.5 microns in
thickness and it has an estimated life expectancy of 10 years. Titanium sheets, 1–2
mm thick with a platinum coating of 2.5–5.0 μm, can be loaded to 10 A/dm2 or over
a period of years. Rod anodes of 10–25 mm diameter are used frequently for
protection of vessels, pipes, condensers, heat oil terminals, etc. [7].
2.5.8
Mixed Metal Oxide Based Anodes
These anodes are the latest technology in anode material and have largely
replaced other anode types, both onshore and offshore. They consist of a proprietary
mixture of (noble) metal oxides plated on a titanium or niobium substrate. This type
of anode has the same advantages (and some limitations) as platinised anodes but is
generally cheaper. They can be made in various shapes such as ribbons, rods, wires,
mesh etc. Ribbon shapes are often used as distributed anodes for localised protection
of structure or under structure bottoms. Applicable current densities are high and
consumption rate is low [7].
30
2.5.9
Zinc Anodes
Zinc anodes are frequently used for protection of submarine pipelines. They
are commercially available in weights from 5 to 60 lb. They have a driving potential
of –1.10V compared to a Cu–CuSO4 reference electrode. The details of zinc anodes
are shown in Figure 2.7.
Figure 2.7
Shows Typical Zinc Anode
Corrosion products insulate the anodes and the anodes are, therefore, installed
below the water table in soils with no free carbonate or phosphate so that passivity
does not occur [8].
31
2.5.10 Aluminium Anodes
These are mostly employed for seawater applications. The base metal
contains 98–99% of aluminum. Aluminum anode has some characteristics which are:
1.
The cost is low and they are light in weight.
2.
The corrosion products do not contaminate the water.
3.
The rate of consumption varies between 7 and 9 lb/A-year. The
efficiency varies between 87 and 95%.
4.
The anodes are easily passivated and must be rinsed with NaCl to
reactivate. Backfill must be used with aluminum anodes [8].
2.6
Distributed Anode Cables
Distributed anode cables consist of a copper core sheathed by a conductive
polymer that allows passage of cathodic protection current to the water. The current
density of the anode is usually low, and such cables are mainly used for localised
protection of structure. They have also been used successfully for the protection of
coated buried tanks and vessels and for the protection of coated external tank
bottoms. These anodes require a specialized design and should not be operated above
their rated current density. Consumption rates or anode life can be obtained from the
Supplier [7].
The cathodic protection current decreases with the time of the immersion, and
attains stable value after approximately 15 days, probably due to the solidification of
32
the coating and/or the accumulation of the corrosion products in the coating pores
[10].
Cathodic protection current density increases with increasing distance
between cathode and anode [9].
2.7
Protection of Underwater Structure
Structures in seawater are protected by so-called bracelets (annular anodes) as
shown in Fig. (2.9). In marine structures, corrosion is at maximum at a small distance
below the water line and decreases with depth. Corrosion is less severe in mud.. In
the impressed current system non-consumable graphite anodes are required, whereas
in the galvanic system a magnesium anode is the best material. Zinc anode is also
used as galvanic anodes, but the cost is high [9].
Figure 2.8
Marine Structure Anode
33
The potential necessary to protect buried steel is −0.85 V, however, in the
presence of sulfates, reducing bacteria a minimum potential of –0.95V with respect
to copper sulfate electrode would be necessary. Approximately 10 mA/m² current is
needed for protection of bare steel in sluggish water. In rapidly moving water, 30
mA/m² for bare steel in a flowing water would be necessary. Current requirements in
various environments can be found abundantly in the literature as well as cathodic
protection specifications [7].
In ICCP design it’s difficult to know the expected potential distribution over
the underwater structure that leads to reliance to current density measurement as a
mean of assessment. The corrosion influenced by the environment factors such as
velocity and pH. Accordingly, when ICCP system is designed, various protection
factors need to reflect in accord one with the underwater environment. The current
density increases with increasing velocity, but it decreases with increasing pH [6].
For coated steel containing defect under appropriate CP potentials, cathodic
reaction is dominated by reduction of oxygen. Mass-transfer of oxygen through
solution layer and the defect with a narrow [12].
CHAPTER 3
RESEARCH METHODOLOGY
3.1
Introduction
This chapter introduces the experimental procedures for the design of
impressed current systems that shall be carried out in the laboratory approved by the
principals to make impressed current design. Figure 3.1 is the general flow chart of
experimental procedures.
35
Literature Review
Review pH data
Variations in Temperature and Concentration
Current Requirement
Coating Efficiency
Select Anode Material, Weight and Dimensions
Calculate number of anodes
Determine Total Circuit Resistance
Select Area Placement of Anode
Calculate Power Source Voltage
Select Rectifier
Monitoring
Corrosion Rate Measurment (Electrochemical & Immersion Tests)
Figure 3.1
A Flow Chart Showing a Summary of Research Methodology
36
3.2
Impressed Current Design
Before starting the design of impressed current, cathodic protection system,
there are certain preliminary data must be gathered.
3.2.1 Physical Dimensions of Structure to be Protected
One important constituent in designing an impressed current cathodic
protection system is the structure's physical dimensions (for example, length, width,
height and diameter). These data are used to calculate the surface area to be protected
[13].
3.2.2 Drawing of Structure to be Protected
The installation drawings must include sizes, shapes, material type, and
locations of parts of the structure to be protected [13].
3.2.3 Electrical Isolation
If a structure is to be protected by the impressed current cathodic system, it
must be electrically connected to the anode,. Sometimes parts of a structure or
system are electrically isolated from each other by insulators. For example, in a gas
pipeline distribution system, the inlet pipe to each building might contain an electric
insulator to isolate in house piping from the pipeline. Also, an electrical insulator
might be used at a valve along the pipeline to electrically isolate one section of the
37
system from another. Since each electrically isolated part of a structure would need
its own cathodic protection, the locations of these insulators must be determined[13].
3.2.4 Short Circuits
All short circuits must be eliminated from existing and new cathodic
protection systems. A short circuit can occur when one structures contact with each
other, causing interference with the cathodic protection system. When updating
existing systems, eliminating short circuits would be a necessary first step [13].
3.2.5 Corrosion History of Structures in the Area
Studying the corrosion history in the area can prove very helpful when
designing an impressed current cathodic protection system. The study should
reinforce predictions for corrosivity of a given structure and its environment, in
addition, it may reveal abnormal conditions not otherwise suspected. Facilities
personnel can be a good source of information for corrosion history [13].
3.3
Review pH Data
Corrosion is also proportional to electrolyte pH. In general, steel's corrosion
rate increases as pH decreases [9].
38
3.4
Variations in Temperature and Concentration
Differences in temperature and concentration can in principle lead to
corrosion cell formation, but have little effect below the water line.
Cathodic protection current density and limiting current density increase with
increasing temperatures [9].
3.5
Current Requirement
A critical part of design calculations for impressed current cathodic
protection systems on existing structures is the amount of current required per square
meter (called current density) to change the structure’s potential to -0.85 volt
(NACE). The current density required to shift the potential indicates the structure's
surface condition. A well coated structure (for example, a structure well coated with
coal-tar epoxy) will require a very low current density (about 10 milliampere per
square meter for stagnant freshwater and 30 milliampere per square meter for
flowing freshwater based on PETRONAS technical standard); an uncoated structure
would require high current density (about 10 milliamperes per square meter).The
amount of current required for complete impressed current cathodic protection can be
determined two ways:
1.
An actual test on existing structures using a temporary impressed
current cathodic protection setup.
2.
A theoretical calculation based on coating efficiency.
The second methods above can be used on existing and new structures.
39
Current requirements can be calculated based on coating efficiency and
current density desired. The efficiency of the coating as supplied will have a direct
effect on the total current requirement, as equation (3.1) shows:
Is = S x js x 10-3 ( 1- CE)
(3.1)
where:
Is:
is total protective current.
S:
is total structure surface area in square meter.
Js:
is required current density.
CE: is coating efficiency.
Equation 3-1 may be used when a current requirement test is not possible, as
on new structures, or as a check of the current requirement test on existing structures.
Coating efficiency is directly affected by the type of coating used and by quality
control during coating application. The importance of coating efficiency is evident in
the fact that a bare structure may require 100,000 times as much current as would the
same structure if it were well coated [13] Current density depends on the type of the
environment as in the table 3.1.
Table 3.1
Current Density and Types of Environment
Environment
Current density (mA/m²)
Soil, 50 to 500 Ω.cm
20 to 40
Soil, 500 to 1500 Ω.cm
10 to 20
Soil, 1500 to 5000 Ω.cm
5 to 10
Soil, over 5000 Ω.cm
5
Fresh water
10 to 30
Moving fresh water
30 to 65
Brackish water
50 to 100
Sea-mud zone
20 to 30
40
3.6
Coating Resistance
A coating's resistance decreases greatly with age and directly affects
structure-to-electrolyte resistance for design calculations. The coating manufacturers
supply coating resistance values.
Platform productions are coated only in exceptional cases or for the purposes
of investigation because the life of the structure is greater than the life of the coating.
Therefore in the design of the cathodic protection, only the protection potential of the
steel need be considered [13].
3.7
Selection of Anode Material, Weight and Dimensions
The choice of anode is arbitrary at this time economy will determine which
anode is the best.
Cylindrical anodes are suitable for use in water to protect steel-water
constructions and offshore installations, and for the inner protection of tanks. In
addition to graphite magnetite and high-silicon iron, anodes of lead-silver alloys are
used as well as titanium, niobium or tantalum coated with platinum or lithium ferrite.
These anodes are not usually solid, but are produced in tube form. In the case of lead
silver anodes, the reason is their heavy weight and relatively low anode current
density; with coated valve metals, only the coating suffers any loss. Then, the tubular
shape gives larger surfaces and therefore higher anode currents. The same types of
connection apply to lead-silver anodes. The cable can be directly soft soldered onto
the anode if a reduction in the tensile load is required. This is not possible with
titanium. Such anodes are therefore provided with a screw connection welded on
where appropriate, which is also of titanium. The complete connection is finally
41
coated with cast resin or the whole tube is filled with a suitable sealing compound.
Because of the poor electrical conductivity of titanium, with long and highly loaded
anodes it is advisable to provide current connections at both ends.
Disc and ingot-shaped anodes are also used in water besides the cylindrical or
conical shapes. Several parallel-connected rod anodes as well as hurdle-shaped racks
are sometimes used for the protection of larger objects such as sheet steel lining and
loading bridges if sufficient space is available and there is no likelihood of the
anodes being damaged, e.g., by anchors. These are situated on the ground and
contain several anodes, mostly rod anodes, next to one another in insulated fixtures.
Floating anodes are used for offshore installations in which the current outflow
surface is attached to the outside of a cylindrical or spherical float which is attached
to the seabed by the anchor rope, so that the anode body floats at a predetermined
depth in the water. The advantage of this is the ability to carry out repairs without
interrupting the operation of the offshore installation. Furthermore, a desired uniform
current distribution can be achieved by distancing the anode from the protected
object [7].
Aluminum anodes with the same protection effect and life as zinc anodes
have much less weight. This is a very important advantage for the uncoated surface
that is to be protected. Several thousands of tons aluminum anodes are used on
platforms at greater depths, which must be taken into account of construction and
transport to the installation site. The anode mountings are welded to lap joints in the
yard, and the anodes are installed at a minimum distance of 30 cm from the structure
to achieve the most uniform current distribution. Non uniform potential distribution
occurs even with this distance. important factor. The number of anodes has to be
small so the anodes need to be relatively large, which will result in too negative a
potential if the distance is not sufficiently great. A minimum distance of 1.5m is
prescribed, but this involves considerable construction effort due to the effects of
heavy seas. Besides the so-called restriction on impressed current installations, there
is the requirement that the corrosion protection be switched off when diving work is
being carried out. This regulation is not justifiable. Work on the underwater region of
production platforms takes place continuously, as far as the weather allows if the
42
protection must be switched off each time, the impressed current protection becomes
very limited [7].
Other anodes used most often are made of mexid metal oxide MMO, zinc or
magnesium. When impressed current-type cathodic protection systems are used to
mitigate corrosion on an underwater steel structure [7].
3.8
Calculate Number of Anodes Needed to Satisfy Manufactuere's Current
Density Limitations
Impressed current anodes are supplied with a recommended maximum
current density. Higher current densities will reduce anode life. To determine the
number of anodes needed to meet the current density limitations.
� =
��
�
Where:
MT = LF x C x Is
C:
Consumption rate of anode .
LF:
Life time (How many years).
� :
Mass of anode.
(3.2)
43
3.9
Determine Total Circuit Resistance
The total circuit resistance (cables of anode and cathode) will be used to
calculate the rectifier size needed.
RT = + Rca + Rcc
RT:
Total circuit resistance.
Rca:
Anode cable resistance.
Rcc:
Cathode cable resistance.
Rca=
Rcc=
(3.3)
L ca
(3.4)
K×S ca
L cc
(3.5)
K×S cc
K:
Cable conductivity.
Type of cable conductor usually is copper specific conductivity K = 56
sm/mm2
3.10
Lca:
Length of anode cable (mm).
Lcc:
Length of cathode cable (mm).
Sca:
Size of anode cable (mm2).
Scc:
Size of cathode cable (mm2)
Calculate Rectifier Voltage to Determine Voltage Output of the Rectifier
U = RT x IS
U:
Output Voltage
(3.6)
44
3.11
Power Source Selection
Many power sources are available commercially; one that satisfies the
minimum requirements of (I) and (Vrec) should be chosen. Besides the more
common rectifiers being marketed, a solar cathodic protection power supply (for
D.C. power) may be considered for remote sites with no electrical power.
P = U x IS
P:
(3.7)
Output power
After all the calculations above to calculate the output current, voltage and
output power start immersing eight samples (40cm x 7.5cm x 4mm) in different
freshwater conditions as shown in table 3.2. Schematic of the design immersed in
stagnant and flowing freshwater are shown in figure 3.2
Table 3.2: Coated and bare samples immersed in different conditions of freshwater
Sample
Condition
Coated sample with ICCP
Coated sample without ICCP
Stagnant Freshwater
Bare sample with ICCP
Bare sample without ICCP
Coated sample with ICCP
Coated sample without ICCP
Bare sample with ICCP
Bare sample without ICCP
Flowing Freshwater
45
a
b
Figure 3.2
Schematic of Coated and Bare Samples With and Without ICCP in
(a) Stagnant Freshwater
(b) Flowing Freshwater.
This work has done in marine technology laboratory, actual sites for stagnant
and flowing freshwater are shown in Figure 3.3.
46
a
b
Figure 3.3
Actual Sites in Marine Technology Laboratory
(a) Stagnant Freshwater Side
(b) Flowing Freshwater Side
For the flowing sude the wave has been generated by using wave generator
tank Figure 3.4. More details for wave generator towing tank refer to appendix C.
Figure 3.4
Wave Generator Towing Tank
47
3.12
Monitoring by Measuring of the Potential
The potential can be measured by using copper - copper sulfide or silversilver chloride electrode shown in Figures 3.3 and 3.4 respectively.
a
b
Figure 3.5
Copper- Copper Sulfate Reference Electrode
(a) Schematic
b
a
Fig 3.6
(b) Real
Silver- Silver Chloride Reference Electrode
(a) Schematic
(b) Real
48
3.13
Electrochemical Testing
An electrochemical corrosion test was carried out by the potentio-dynamic
anodic polarization using Potentiostat Galvanostat instrument according to the
ASTM Standard G-5. Two replicate tests of each measurement were performed. The
test was carried out in freshwater solutions. The temperature of solution was at
24+2°C. All the parameters are tabulated in Table 3.2.
Table 3.3: Potentiostatic Polarization Test Parameters
Parameters
Unit
Exposure time
10 to 20 minutes
Corrosive solution
Temperature
Freshwater
Room temperature (25°C)
3.13.1 Principle of Measurement
The electrochemical test was conducted according to the ASTM G5.
The potentiostatic measuring equipment consists of three electrodes
procedure. They are Working Electrode, WE, Reference Electrode, RE and Auxiliary
Electrode, AE. Working electrode represents the specimen to be tested, reference
electrode to provide datum against which the potential of the working electrode is
measured and the auxiliary electrode which carries the current created in the circuit.
49
A filtered direct current (DC) power supply, PS, supplies current (I) to the working
electrode is measured with respect to a reference electrode, with a series-connected
potentiometer, P.
The experimental arrangement placed the reference electrode which is
Saturated Calomel electrode separately from the electrochemical cell where the
junction test tube was filled with saturated KCl solution figure 3.7. The reference
electrode was then placed into the test tube. The Luggin probe is usually included to
minimize ohmic resistance interferences in the electrolyte. The luggin probe was
placed as near as possible to the surface of the metal being studied, as it allows
potential to be detected close to the metal surface. The working electrode becomes
the anode while the auxiliary electrode becomes the cathode [14].
Auxiliary electrode
Reference
electrode
Test solution
Working electrode
Figure 3.7
Cell kit Set-up
50
3.13.2 Preparation of Working Electrode
The low carbon steel specimens were cut using precision cutter into small
pieces approximately 30mm x 20mm. Brazing technique was applied to connect the
specimen to the copper rod for ease of connection to the electrochemical cell
(Figures 3.5 (a) and (b)). Then the specimen was mounted by embedding in epoxy
resin for 24 hours as shown in Figures 3.6 (a) and (b). The surface of each sample
was smoothened and cleaned to remove any unwanted particles or grease [14].
b
a
Figure 3.8
Photographs of
(a) Connection of Specimen to Copper Wire by Brazing Technique;
(b) Mounting of Samples
51
a
b
Figure 3.9
Photographs of
(a) Working Electrode (WE)
(b) Typical Surface Area of a Sample
52
3.14
Immersion Test
Immersion test was conducted to determine corrosion rate using weight loss
method in which a specimen known initial weight is exposed to the corrosive
environment for a specified period of time. By the end of the test, the specimen is
cleaned and weighed to determine the weight loss and the pits behaviour. The
immersion test is in accordance to ASTM G31-72 [15]. The parameters for the
immersion test are given by table 3.4.
Table 3.4: Immersion Test Parameters
Parameters
Unit
Exposure time
30 Days
Corrosive solution
Freshwater
Temperature
Room temperature (25°C)
Calculation of corrosion rate in mm/yr for immersion test result is as follow:
Corrosion penetration rate, r (mpy),
K = Constant (3.45x106)
W = mass loss, g
A = Exposed surface area, cm²
T = Time of exposure, hour
D = Density of specimen, g/cm³
r=
��
���
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Chemical Composition of Materials Used
Chemical composition of the material used is obtained by using GDS (Glow
Discharge Spectrometer). There is only one material that is used in the test. It is
low carbon steel. The following is the result obtained from GDS. Table 4.1 show
the chemical compositions for low carbon steel.
Table 4.1: Chemical Composition of Low Carbon Steel
Element
Fe
C
Mn
S
Si
V
Mo
Ti
Al
Sb
Sn
Pb
Compositions (%)
98.4
0.0555
0.524
0.0163
0.145
0.00431
0.0252
0.0150
0.00124
0.0115
0.0519
0.00635
54
4.2
Impressed Current Cathodic Protection Calculations
4.2.1 For Coated Steel Immersed in Stagnant Freshwater
Sample dimensions:
Length
40 cm
Width
7.5 cm
Thickness
4 mm
Surface area
S= (40x7.5x2) + (40x0.4x2) + (7.5x0.4x2)
S = 638 cm² = 0.0638 m²
Current density for steel in stagnant freshwater based on P.T.S
JS = 10 mA/m2
IS = S x Js
IS = 0.0638 x 10 x 10-3(1- CE)
CE:
Coating Efficiency = 80%
IS = 0.1276 mA
Current + 40% spare = 0.2 mA
Current layout
IS = 1 mA
Type of anode (Aluminium)
Mass of anode:
Ma = 0.5 kg
Life time of anode:
LF = 2 Months = 0.166666 year
Consumption rate of anode: C = 2 kg / A year
55
Calculation of required mass of anodes
MT = LF x C x Is = 0.16666 x 2 x1x10‫־‬³ = 0.33332 g
Number of anodes
nb 
nb 
Total circuit resistance
MT
ma
0.00033332
 0.000666 anode  1anode
0.5
RT = Rca + Rcc
Length of anode cable (Lca = 5 m )
Size of anode cable (Sca = 0.64 mm2 )
Length of cathode cable (Lcc = 5m) Size of cathode cable (Scc= 0.64 mm2)
Type of cable conductor is copper specific conductivity
Anode cable resistance:
Rca 
Lca
k  Sca
K = 56 sm/mm2
Rca 
5
 0.14 
56  0.64
Rcc 
5
 0.14 
56  0.64
Cathode cable resistance:
Rcc 
Lcc
k  Scc
Total circuit resistance
RT = 0.14 + 0.14 = 0.28 
Current layout
Is = 1 mA
Output voltage
U = RT x IS
U = 0.28 Ω x 1 mA = 0.28 mV
U= 1 mV
Output power
p = U x IS
P = 1mV x 1mA = 1 mW
Layout of power source
1 mA / 1 mV/ 1 mW
56
4.2.2 For Bare Steel Immersed in Stagnant Freshwater
Surface area
S= (40x7.5x2) + (40x0.4x2) + (7.5x0.4x2)
S = 638 cm² = 0.0638 m²
Current density for steel in stagnant freshwater based on P.T.S
JS = 10 mA/m2
IS = S x Js
IS = 0.0638 x 10 x 10-3
IS = 0.638 mA
Current + 40% spare = 1 mA
Current layout
IS = 2 mA
Type of anode (Aluminium)
Mass of anode:
Ma = 0.5 kg
Life time of anode:
LF = 2 Months = 0.166666 year
Consumption rate of anode:
C = 2 kg / A year
Calculation of required mass of anodes
MT = LF x C x Is
MT = 0.16666 x 2 x2x10‫־‬³ = 0.666664 g
Number of anodes
57
nb 
nb 
MT
ma
0.0006666
0.5
nb  0.00133 anode  1anode
Total circuit resistance
RT = Rca + Rcc
Length of anode cable (Lca = 5 m ) Size of anode cable (Sca = 0.64 mm2 )
Length of cathode cable (Lcc = 5m) Size of cathode cable (Scc= 0.64 mm2)
Type of cable conductor is copper specific conductivity
K = 56 sm/mm2
Anode cable resistance:
Rca 
Lca
k  Sca
Rca 
5
 0.14 
56  0.64
Rcc 
5
 0.14 
56  0.64
Cathode cable resistance:
Rcc 
Lcc
k  Scc
Total circuit resistance
RT = 0.14 + 0.14 = 0.28 
Current layout
Is = 2 mA
Output voltage
U = RT x IS = 0.28 Ω x 2 mA = 0.56 mV
U = 0.56 mV
U= 1 mV
Output power
p = U x IS = 1mV x 2mA = 2 mW
Layout of power source
2 mA / 1 mV/ 2 mW
58
4.2.3 For Coated Steel Immersed in Flowing Freshwater
Surface area
S= (40x7.5x2) + (40x0.4x2) + (7.5x0.4x2)
S = 638 cm² = 0.0638 m²
Current density for steel in stagnant freshwater based on P.T.S
JS = 30 mA/m2
IS = S x Js
IS = 0.0638 x 30 x 10-3(1- CE)
CE:
Coating Efficiency = 80%
IS = 0.3828 mA
Current + 40% spare = 0.6 mA
Current layout:
IS = 1 mA
Type of anode (Aluminium)
Mass of anode
Ma = 0.5 kg
Life time of anode:
LF = 2 Months = 0.166666 year
Consumption rate of anode
C = 2 kg / A year
Calculation of required mass of anodes .
MT = LF x C x Is
MT = 0.16666 x 2 x1x10‫־‬³ = 0.33332 g
Number of anodes
59
nb 
nb 
MT
ma
0.00033332
0.5
nb  0.000666 anode  1anode
Total circuit resistance
RT = Rca + Rcc
Length of anode cable (Lca = 5 m )
Size of anode cable (Sca = 0.64 mm2 )
Length of cathode cable (Lcc = 5m) Size of cathode cable (Scc= 0.64 mm2)
Type of cable conductor is copper specific conductivity
K = 56 sm/mm2
Anode cable resistance:
Rca 
Lca
k  Sca
Rca 
5
 0.14 
56  0.64
Rcc 
5
 0.14 
56  0.64
Cathode cable resistance:
Rcc 
Lcc
k  Scc
Total circuit resistance
RT = 0.14 + 0.14 = 0.28 
Current layout
Is = 1 mA
Output voltage
U = RT x IS
U = 0.28 Ω x 1 mA = 0.28 mV
U= 1 mV
Output power
p = U x IS
P = 1mV x 1mA = 1 mW
Layout of power source
1 mA / 1 mV/ 1 mW
60
4.2.4 For Bare Steel Immersed in Flowing Freshwater
Surface area
S= (40x7.5x2) + (40x0.4x2) + (7.5x0.4x2)
S = 638 cm² = 0.0638 m²
Current density for steel in flowing freshwater based on P.T.S
JS = 30 mA/m2
IS = S x Js
IS = 0.0638 x 30 x 10-3
IS = 1.914 mA
Current + 40% spare = 2.6796 mA
Current layout
IS = 4 mA
Type of anode (Aluminium)
Mass of anode
Ma = 0.5 kg
Life time of anode
LF = 2 Months = 0.166666 year
Consumption rate of anode
C = 2 kg / A year
Calculation of required mass of anodes .
MT = LF x C x Is
MT = 0.16666 x 2 x4x10‫־‬³ = 1.33328 g
Number of anodes
61
nb 
nb 
MT
ma
0.00133328
0.5
nb  0.00266 anode  1anode
Total circuit resistance
RT = Rca + Rcc
Length of anode cable (Lca = 5 m )
Size of anode cable (Sca = 0.64 mm2 )
Length of cathode cable (Lcc = 5m) Size of cathode cable (Scc= 0.64 mm70)2
Type of cable conductor is copper specific conductivity
K = 56 sm/mm2
Anode cable resistance:
Rca 
Lca
k  Sca
Rca 
5
 0.14 
56  0.64
Rcc 
5
 0.14 
56  0.64
Cathode cable resistance:
Rcc 
Lcc
k  Scc
Total circuit resistance
RT = 0.14 + 0.14 = 0.28 
Current layout
Is = 4 mA
Output voltage
U = RT x IS
U = 0.28 Ω x 4 mA = 1.12 mV
U= 3 mV
Output power
p = U x IS
P = 3mV x 4mA = 12 mW
Layout of power source 4 mA / 3 mV/ 12 mW
62
4.3
Potential Measurement Results
Corrosion of steel in freshwater with pH=7.04 was monitored by measuring
the potential of steel by using Cu/CuSO4 reference electrode and all these
measurements were taken at the center of the samples.
4.3.1 Coated and Bare Steel Immersed in Stagnant Freshwater with ICCP
From the results obtained shown in figure 4.1 the potential of coated steel
was initially -713mV Cu/CuSO4 and after applying ICCP system the potential has
shifted into the negative direction until it reached the protection level between (867mV to -875mV) Cu/CuSO4 and these values were maintained until the end of
the test. Coated sample with ICCP after 1 month immersing in stagnant freshwater is
shown in figure 4.2 (a)
For the bare steel, the initial potential was -654mV and after applying ICCP
system and adjusting the variable resistance the potential has become more –ve until
reached the protection level between (-830mV to -854mV) Cu/CuSO4 and these
values were almost the same until the end of the test and in this case the anode has
corroded more than the coated steel as shown in Figure 4.3. Bare sample with ICCP
after 1 month immersing in stagnant freshwater is shown in Figure 4.2 (b). Detail of
the potentials measurement is given in appendix A.
63
Figure 4.1
The potentials measurment for coated and bare steel in stagnant
freshwater with ICCP
b
a
Figure 4.2
Samples with ICCP after 1 month immersion in stagnant freshwater
(a) Coated Sample
(b) Bare Sample
64
b
a
Figure 4.3
ICCP anodes after 1 month immersion in stagnant freshwater for
(a) Coated Sample
(b) Bare Sample
4.3.2 Coated and Bare Steel Immersed in Stagnant Freshwater without ICCP
From the results obtained shown in figure 4.4 that for the coated steel the
initial potential was -702mV Cu/CuSO4 and it started shifting immediately after few
hours into the positive direction. With increase in time the potential shifted to less
negative values until it reached -663 mV Cu/CuSO4 after 30 days which means out
of protection region.
For the bare sample the potential was initially -689 mV Cu/CuSO4 and then
has shifted slightly into the positive direction and only very little change has
happened until the potential reached -682 mV Cu/CuSO4.Among the two samples
the bare one has almost stable values of potentials.The steel showed some red rust
products as shown in figures 4.5 (c) and (d). Based on XRD analysis of the red rust
showed that the little change was due to oxide layer formed on the metal surface as
shown in figure 4.6. Detail of the potentials measurement is given in appendix A.
65
Figure 4.4
The Potential measurment on coated and bare samples in Stagnant
freshwater without ICCP
a
b
c
d
Figure 4.5
Samples without ICCP after 1 month immersion in stagnant freshwater
(a) Coated Sample
(b) Coated Sample
(c) Bare Sample
(d) Bare Sample
66
Figure 4.6
Quantitative analysis of XRD pattern of corrosion products from the
bare sample in the stagnant freshwater
4.3.3 Coated and Bare Steel Immersed in Flowing Freshwater with ICCP
In all samples immersed in flowing freshwater the measurement were taken
when the freshwater was stagnant ( out of operation hours) because sometimes the
height of the waves reach to 0.44 meter and the length up to 6 meter. From the results
obtained, the potential of coated steel was initially -708mV Cu/CuSO4 and instantly
after applying ICCP system it has been found that the value of the potential has
become more negative until it reached the optimum protection level between (845mV to -866mV) Cu/CuSO4 and these values were remained until the end of the
test as shown in Figure 4.7. Coated sample with ICCP after 1 month immersing in
flowing freshwater is shown in figure 4.8 (a).
For the bare steel, the initial potential was -648 mV Cu/CuSO4 and after
applying ICCP system and adjusting the variable resistance the potential has shifted
67
into the protection level between (-841mV to -853mV) Cu/CuSO4 and these values
were almost the same until the end of the test and in this case the anode has corroded
more than the coated steel as shown in figure 4.9. Bare sample with ICCP after 1
month immersing in flowing freshwater is shown in figure 4.8 (b). Detail of the
potentials measurement is given in appendix A.
Figure 4.7
The Potential measurment on coated and bare samples in Flowing
freshwater with ICCP
a
b
Figure 4.8
Samples with ICCP after 1 month immersion in flowing freshwater
(a) Coated Sample
(b) Bare Sample
68
a
b
Figure 4.9
ICCP anodes after 1 month immersion in flowing freshwater for
(a) Coated Sample
(b) Bare Sample
4.3.4 Coated and Bare Steel Immersed in Flowing Freshwater without ICCP
From the results it was shown in figure 4.10, for the coated steel the initial
potential was -702mV Cu/CuSO4 and it started shifting immediately after few hours
into the positive direction and this shifting increase with the time until reached -649
mV Cu/CuSO4 after 30 days and these values are farther down the scope of
protection and small cracks in the coating developed and caused coating elimination
shown in figures 4.11(a) and 4.11 (b) .
For the bare sample the potential was initially -643mV Cu/CuSO4 and then
has abruptly shifted into the positive direction with time and very high change in
potential has happened till the potential reached -549 mV Cu/CuSO4. Due to high
velocities of waves the steel could not form oxide layer similar with bare steel in
stagnant freshwater as shown in figure 4.11(c) and 4.11(d). Detail of the potentials
measurement is given in appendix A.
69
Figure 4.10 The Potential measurment on coated and bare samples in Flowing
freshwater without ICCP
a
b
c
d
Figure 4.11
freshwater
Samples without ICCP after 1 month immersion in flowing
(a) Coated Sample
(b) Coated Sample
(c) Bare Sample
(d) Bare Sample
70
4.4
The Effectiveness of the Reference Electrode Location on the Protection
Potrntial Result
Based on figures 4.12, 4.13, 4.14, and 4.15 the relation between the average
corrosion potential (mV) Cu/CuSO4 and the depth of the reference electrode (cm) is
negative linear. The corrosion potential has the maximum average negative potential
at the top of the sample (60 cm below the water) and this potential systematically
decreases in the negative direction when the depth of the reference electrode increase
until the end of the sample (1 meter below the water).
71
Figure 4.12
The Effectiveness of Reference Electrode Location on The Samples
Potential in Stagnant Freshwater With ICCP
Figure 4.13
The Effectiveness of Reference Electrode Location on The Samples
Potential in Stagnant Freshwater Without ICCP
72
Figure 4.14
The Effectiveness of Reference Electrode Location on The Samples
Potential in Flowing Freshwater With ICCP
Figure 4.15
The Effectiveness of Reference Electrode Location on The Samples
Potential in Flowing Freshwater Without ICCP
73
Figure 4.16
Bar chart for samples immersed in stagnant freshwater.
Figure 4.17
Bar chart for samples immersed in flowing freshwater.
74
4.5
Electrochemical Result
4.5.1 Visual Inspection
Figure 4.18 shows the surface area of specimens before and after the
electrochemical test. Visual inspection is important in order to observe any change
on the surface appearance. It was found that after the test, most of the metal surface
has changed due to the reaction of ions in the electrolyte and metal surface.
a
Figure 4.18
b
Specimens (a) before and (b) after electrochemical test.
4.5.2 Polarization Result
Table 4.2 shows the potentiodynamic anodic polarization data obtained when
the test was carried out in freshwater at room temperature. The value of icorr is
shown graphically in figure 4.19
75
Table 4.2: Electrochemical Result
Parameter
E(I=0) (mV)
icorr (µA)
Ca. Beta (mV)
An. Beta (mV)
Corrosion Rate (mpy)
Chi-Square
Fit Range (mV)
Density (g/cm³)
Surface Area (cm²)
Equivalent Weight (g)
Figure 4.19
Value
-413.158
5.219e+001
409.777
453.579
4.267e+000
4.70
(-436), (-389)
7.87
3
15
Tafel extrapolation curve for bare steel in freshwater
76
4.6
Immersion Test Results
Corrosion rate determination at different condition of freshwater was carried
out by conventional immersion test for the exposure period of 1 month, the initial
weights for the bare samples in stagnant and flowing freshwater were 1318 g and
1313 g respectively and the initial weights for the coated sample in stagnant and
flowing freshwater were 1324 g ang 1319 g respectively the weight loss for each
sample and the corrosion rate results are in table 4.3.
Table 4.3: Corrosion Rate of Samples Without ICCP expressed in (mpy)
Stagnant freshwater
Coated sample
Flowing freshwater
Bare sample
Coated sample
Bare sample
Weight Corrosion Weight Corrosion Weight Corrosion Weight Corrosion
loss (g) rate(mpy) loss (g) rate(mpy) loss (g) rate(mpy) loss (g) rate(mpy)
3.45
3.292
4.5
4.29
8
7.634
17.6
16.8
A good agreement was observed for corrosion rate between weight loss
measurement (4.29 mpy) test and electrochemical test (4.27 mpy) for bare steel in
stagnant freshwater.
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE
WORK
5.1
Conclusions
The result obtained from the experimental work had successfully fulfilled the
objectives of the project. The conclusions derived from this project are listed as
follows:
1.
Impressed current cathodic protection and coating give the optimum
protection for steel immersed in freshwater.
2.
Steel immersed in flowing freshwater required current density higher
then steel immersed in stagnant freshwater.
3.
The potential of steel shifted from less negative values to more negative
until it reached the optimum protection level when ICCP system
applied.
4.
Results from immersion test indicated that, bare steel immersed in
flowing freshwater exhibits the highest corrosion rate (16.8 mpy) and
coated steel immersed in stagnant freshwater exhibits the lowest
corrosion rate (3.29 mpy).
78
5.2
Recommendations for Future Work
From the study that has been successfully carried out, the following are some
recommendations may be considered for the future work:
1.
Applying impressed current cathodic protection on various shapes of
steel in different environments and at different temperatures with other
types of anodes.
2.
Further studies should be made to evaluate the effect of the life time of
the anodes on the economy of the whole process.
REFERENCES
1.
R. Winston River and Herbert H Uhlig, Corrosion and Corrosion Control; an
Introduction to Corrosion Science and Engineering, 4th edition (2008).
2.
Mars G Fontana, Corrosion Engineering, 3rd edition (1987).
3.
www.wikipedia .com ~corrosion.
4.
U.S. Army Corps Of Engineers, Naval Facilities Engineering Command, Air
Force Civil Engineer Support Agency, UFC3-570-02A (2005).
5.
Pierre R. Roberge, Handbook of Corrosion Engineering (2000).
6.
Jin-Seok Oh* & Jong-Do Kim. KSME International Journal Vol, 18 No, 4. A new
protection strategy of impressed current cathodic protection in ship (2004).
7.
PETRONAS Technical Standard, Design & Engineering Practice (CORE),
Manual Cathodic Protection, PTS 30.10.73.10.
8.
Zaki Ahmad. Principles of Corrosion Engineering and Corrosion Control, 1st
edition (2006).
9.
Dr. Sami Abulnoun Ajeel, Ghalib A. Ali. Variable Conditions Effect On
Polarization Parameters Of Impressed Current Cathodic Protection Of Low
Carbon Steel Pipes (2007).
10.
Sanja Martinez, Lidija Valek Z ulj, Frankica Kapor. Corrosion Science 51.
Disbonding of underwater-cured epoxy coating caused by cathodic protection
current (2009).
11.
Dae-Kyeong Kim, Srinivasan Muralidharan, Tae-Hyun Haa, Jeong-Hyo
Baea,Yoon-Cheol Haa, Hyun-Goo Lee , J.D. Scantlebury. Electrochimica Acta
51. Electrochemical studies on the alternating current corrosion of mild steel
under cathodic protection condition in marine environments (2006).
80
12.
C.F. Dong, A.Q. Fua, X.G. Li , Y.F. Cheng. Electrochimica Acta 54. Localized
EIS characterization of corrosion of steel at coating defect under cathodic
protection (2008).
13.
API Recommended practice 575, "Cathodic Protection of underwater structure,
American Petroleum Institute, Second Edition (1997).
14.
ASTM G5 – 94 Standard Reference Test Method for Making Potentiostatic and
Potentiodynamic Anodic Polarization Measurements (2004).
15.
ASTM Standard: G31-72, Standard practice for laboratory Immersion Corrosion
Test of metal (2004).
APPENDICES
APPENDIX A
The potential measurement for coated and bare steel in stagnant freshwater
with ICCP.
Time (Day)
Bare Sample Potential
(mV)
Coated Sample Potential
(mV)
Before applying ICCP
-654
-713
2
-838
-874
4
-852
-875
6
-850
-876
8
-848
-875
10
-847
-874
12
-848
-875
14
-849
-875
16
-851
-873
18
-850
-872
20
-849
-873
22
-848
-870
24
-846
-871
26
-842
-870
28
-840
-870
30
-840
-866
82
The potential measurement for coated and bare steel in stagnant freshwater
without ICCP.
Time
Bare Sample Potential
Coated Sample Potential
(Day)
(mV)
(mV)
0
-689
-702
2
-688
-699
4
-687
-697
6
-686
-697
8
-685
-695
10
-685
-691
12
-685
-690
14
-684
-689
16
-684
-687
18
-684
-685
20
-685
-678
22
-684
-677
24
-683
-674
26
-683
-672
28
-682
-669
30
-682
-663
83
The potential measurement for coated and bare steel in flowing freshwater
with ICCP.
Time
Bare Sample Potential
Coated Sample Potential
(Day)
(mV)
(mV)
Before applying
ICCP
-648
-708
2
-842
-845
4
-853
866
6
-853
-865
8
-852
-864
10
-851
-864
12
-849
-862
14
-849
-862
16
-848
-861
18
-848
-861
20
-847
-861
22
-847
-860
24
-845
-860
26
-844
-858
28
-842
-857
30
-841
-857
0
84
The potential measurement for coated and bare steel in flowing freshwater
without ICCP.
Time
Bare Sample Potential
Coated Sample Potential
(Day)
(mV)
(mV)
0
643
702
2
640
697
4
636
695
6
632
694
8
622
692
10
612
689
12
603
689
14
589
685
16
577
682
18
574
676
20
568
671
22
568
668
24
566
663
26
564
657
28
560
655
30
549
649
85
APPENDIX B
General properties of low carbon steel
Physical
Properties
Mechanical
Properties
Electrical
Properties
Density
Modulus of
Elasticity
Bulk Modulus
Poissons Ratio
Shear Modulus
Electrical
Resistivity
Metric
7.872 g/cc
English
0.2844 lb/in³
200 GPa
29000 ksi
140 GPa
20300 ksi
0.29
0.29
80.0 GPa
11600 ksi
0.0000174 ohm-cm 0.0000174 ohm-cm
86
APPENDIX C
Wave Generator Towing Tank
87
88
89
90
91
92