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 A
TITLE The potential measurement for coated and bare steel in
PAGE 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.
Figure 2.5 Operation of a Single Phase Bridge Rectifier. Arrows Show 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 =
DC
× 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.
𝑀
𝓃𝑏 = 𝑚 𝑇
𝑎
Where: MT = LF x C x Is
C:
Consumption rate of anode .
LF:
Life time (How many years).
𝑚𝑎 :
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.
(3.3)
L
Rca=K×Sca
(3.4)
ca
L
Rcc=K×Scc
(3.5)
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
Temperature
Freshwater
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
𝐾𝑊
r = 𝐷𝐴𝑇
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
MT ma
nb
0.00033332 0.000666 anode 1anode 0.5
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 Anode cable resistance:
K = 56 sm/mm2
Rca
Lca k S ca
Rca
5 0.14 56 0.64
Rcc
Lcc k S cc
Rcc
5 0.14 56 0.64
Cathode cable resistance:
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
MT ma
0.0006666 0.5 nb 0.00133 anode 1anode nb
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 S ca
Rca
5 0.14 56 0.64
Lcc k S cc
Rcc
5 0.14 56 0.64
Cathode cable resistance: Rcc
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
MT ma
0.00033332 0.5 nb 0.000666 anode 1anode nb
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 S ca
Rca
5 0.14 56 0.64
Rcc
Lcc k S cc
Rcc
5 0.14 56 0.64
Cathode cable resistance:
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
MT ma
0.00133328 0.5 nb 0.00266 anode 1anode nb
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 S ca
Rca
5 0.14 56 0.64
Rcc
Lcc k S cc
Rcc
5 0.14 56 0.64
Cathode cable resistance:
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