Immersed Cathodic Protection

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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


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DEDICATION

To my beloved parents, siblings and friends for their endless loves and supports...


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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 masters 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 learnmany 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.


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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 flowingfreshwater 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.


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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)


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TABLE OF CONTENTS

CHAPTER 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

1 INTRODUCTION 1

1.1 Introduction 11.2 Background of the Study 11.3 Objectives of the Study 31.4 Research Questions 31.5 Significance of the Study 41.6 Scopes of the Study 4

2 LITERATURE REVIEW 5

2.1 General Review 52.2 Electrochemical Nature of Aqueous Corrosion 62.3 Corrosion Control 9

2.3.1 Design 9


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2.3.2 Materials Selection 102.3.3 Inhibitors 112.3.4 Protective Coatings 112.3.5 Cathodic Protection 11

2.3.5.1The Principles of CathodicProtection

12

2.3.5.2Types of Cathodic Protection 132.4 Current Sources 16

2.4.1 Transformer/Rectifiers 162.4.1.1Circuit Breaker 202.4.1.2Transformer 212.4.1.3Rectifier Cells 21

2.4.2 Rectifier Efficiency 222.4.3 Engine Generator Sets 232.4.4 Batteries, Solar and Wind Generators 232.4.5 Thermoelectric Generators 242.4.6 Closed Cycle Turbo Generators 25

2.5 Anode Materials 252.5.1 Steel Scrap Anodes 262.5.2 Cast Iron Scrap Anodes 272.5.3 Silicon Iron Anodes 272.5.4 Graphite Anodes 272.5.5 Magnetite Andes 282.5.6 Lead Alloy Anodes 282.5.7 Platinised Titanium Anodes 292.5.8 Mixed Metal Oxide Based Anodes 292.5.9 Zinc Anodes 302.5.10 Aluminium Anodes 31

2.6 Distributed Anode Cables 312.7 Protection of Underwater Structure 32


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3 RESEARCH METHOLOGY 34

3.1 Introduction 343.2 Impressed Current Design 35

3.13.1 Physical Dimensions of Structure to beProtected

36

3.13.2 Drawing of Structure to be Protected 363.13.3 Electrical Isolation 363.13.4 Short Circuits 373.13.5 Corrosion History of Structures in the

Area

37

3.3 Review pH Data 373.4 Variations in Temperature and Concentration 383.5 Current Requirement 383.6 Coating Resistance 403.7 Selection of Anode Material, Weight and

Dimensions

40

3.8 Calculate Number of Anodes Needed to SatisfyManufacturers Current Density Limitations

42

3.9 Determine Total Circuit Resistance 433.10 Calculate Rectifier Voltage to Determine Voltage

Output of the Rectifier

43

3.11 Power Source Selection 443.12 Monitoring by Measuring of the Potential 473.13 Electrochemical Testing 48

3.13.6 Principle of Measurement 483.13.7 Preparation of Working Electrode 50

3.14 Immersion Test 52


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4 RESULTS AND DISCUSSION 53

4.1 Chemical Composition of Materials Used 534.2 Impressed Current Cathodic Protection

Calculations

54

4.2.1 For Coated Steel Immersed in StagnantFreshwater

54

4.2.2 For Bare Steel Immersed in StagnantFreshwater

56

4.2.3 For Coated Steel Immersed in FlowingFreshwater

58

4.2.4 For Bare Steel Immersed in FlowingFreshwater

60

4.3 Potential Measurement Results 624.3.1 Coated and Bare Steel Immersed in

Stagnant Freshwater with ICCP

62

4.3.2 Coated and Bare Steel Immersed inStagnant Freshwater without ICCP

64

4.3.3 Coated and Bare Steel Immersed inFlowing Freshwater with ICCP

66

4.3.4 Coated and Bare Steel Immersed inFlowing Freshwater without ICCP

68

4.4 The Effectiveness of the Reference ElectrodeLocation on The Protection Potrntial Result

70

4.5 Electrochemical Result 744.5.1 Visual Inspection 744.5.2 Polarization Result 74

4.6 Immersion Test Results 76


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5 CONCLUSTION AND RECOMMENDATIONS

FOR FUTURE WORK

77

5.1 Conclusions 775.2 Recommendations for Future work 78

REFERENCES 79

APPENDICES 81

Appendices A - C 81-92


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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Comparison between sacrificial anode system and

impressed current system

15

2.2 Typical consumption rates of impressed current anode

materials

26

3.1 Current density and types of environment 29

3.2 Coated and bare samples immersed in different conditions

of freshwater

44

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 754.3 The result of corrosion rate of samples without ICCP 76


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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

ICCP in

45

(a) Stagnant freshwater 45

(b) Flowing freshwater 45

3.3 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

(a) Schematic 47

(b) Real 47

3.6 Copper- copper sulfate reference electrode 47

(a) Schematic 47

(b) Real 47

3.7 Cell kit setup 49


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3.8 Photographs of 50

(a) Connection of specimen to copper wire by

brazing technique

50

(b) Mounting of samples 50

3.9 Photographs of 51

(a) Working electrode 51

(b) Typical surface area of a sample 51

4.1 Potential measurement of coated and bare samples in

stagnant freshwater with ICCP

63

4.2 Samples with ICCP after 1 month immersion in stagnant

freshwater

63

(a) Coated sample 63

(b) Bare sample 63

4.3 ICCP anodes after 1 month immersion in stagnant

freshwater for

64


(b) Bare sample 64

4.4 The potential measurement on coated and bare samples in

stagnant freshwater without ICCP

65

4.5 Samples without ICCP after 1 month immersion in

stagnant freshwater

65


(b) Coated sample 65

(c) Bare sample 65

(d) Bare sample 65

4.6 Quantitative analysis of XRD pattern of corrosion

products from the bare sample in stagnant freshwater

66


flowing freshwater with ICCP

67

4.8 Samples with ICCP after 1 month immersion in flowing

freshwater

67


(b) Bare sample 67


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4.9 ICCP anodes after 1 month immersion in flowing

freshwater for

68


(b) Bare sample 68


flowing freshwater without ICCP

69

4.11 Samples without ICCP after 1 month Immersion in

flowing freshwater

69


(b) Coated sample 69

(c) Bare sample 69

(d) Bare sample 69

4.12 Effectiveness of reference electrode location on the

samples potential in stagnant freshwater with ICCP

71


samples potential in stagnant freshwater without ICCP

71


samples potential in flowing freshwater with ICCP

72


samples potential in flowing freshwater without ICCP

72

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

4.19 Tafel extrapolation curve for bare steel in freshwater 75


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LIST OF APPENDICES

APPENCIX TITLE PAGE

A The potential measurement for coated and bare steel in

stagnant and flowing freshwater with and without ICCP

81

B General properties of low carbon steel 85

C Wave generator towing tank 86


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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 thematerial so that reduce corrosion rate so that less materials and money being wasted.


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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].

Its 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].


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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 currentcathodic protection and determine the effectiveness of impressed

current cathodic protection design.

4. To determine the effect of coating and ICCP protection on corrosionbehavior of carbon steel.

5. To determine the effectiveness of the location of the reference electrodeon the protection potential.

1.4 Research Questions

The research questions are

1. How to build an effective laboratory scale impressed current cathodicprotection 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.


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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 oncorrosion behavior of carbon steel by measuring the potential for steel

in different freshwater conditions.

4. Determine the effectiveness of the locations of the reference electrodeon the protection potential by measuring the potential at different

positions of the samples.


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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 isdescribed 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 systemanode, cathode, electrolyte, and metallic


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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.


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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 + 2HClMCl2 + 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+ + 2ClM+2 + 2Cl+ H2 (2.2)


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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 (Anodic reaction) (2.4)

2H+ + 2e- H2 (Cathodic reaction) (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 localizedand 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)


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O2 + 2H2O + 4e- 4OH (aerated alkaline solution) (2.6)

and

O2 + 4H+ + 4e- 2H2O (aerated acidic solution) (2.7)

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


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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

coppernickel 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].


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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


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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.

2.3.5.1The 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.


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Figure 2.3 The Principle of Cathodic Protection

2.3.5.2Types 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


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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


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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

resistivity is low

Limited to use below a soil resistivity of

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 areference electrode (RE) which measures surface potential in its vicinity. This


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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 three-

phase. Especially for high power units, three-phase units are preferred because they


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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 currentcontrol may be used, e.g. in combination with automatic potential control


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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 half-

cycle (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 thepath (3) and through diode D4 it enters the negative terminal T2. In the next half-


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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


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2.4.1.1Circuit 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].


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2.4.1.2Transformer

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 turns ratio between the two

coils [7].

P

S =

P

S

2.4.1.3Rectifier 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].


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Figure 2.6 Components of a Rectifier

2.4.2 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.


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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.


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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.


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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)


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Table 2.2: Typical Consumption Rates of Impressed Current Anode Materials

Impressed

current anode

Material

Maximum

current density,

A/m

Working

current

density, A/m

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.yrPlatinised

Tantalum or

Platinised Niobium2000 500 to 1000 8 mg/A.yr

MMO on Titanium 1000 500 to 100 1 mg/A.yr

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 scrapmetal is not recommended [7].


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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 acarbonaceous 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 areavailable. The graphite is impregnated with wax or resin, which reduces flaking, or


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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].


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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, 12

mm thick with a platinum coating of 2.55.0 m, can be loaded to 10 A/dm2

or over

a period of years. Rod anodes of 1025 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].


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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 CuCuSO4 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].


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2.5.10 Aluminium Anodes

These are mostly employed for seawater applications. The base metal

contains 9899% 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 toreactivate. 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


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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


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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/m current is

needed for protection of bare steel in sluggish water. In rapidly moving water, 30

mA/m 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 its 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 throughsolution layer and the defect with a narrow [12].


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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.


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Figure 3.1 A Flow Chart Showing a Summary of Research Methodology

Corrosion Rate Measurment (Electrochemical & Immersion Tests)

Monitoring

Select Rectifier

Calculate Power Source Voltage

Select Area Placement of Anode

Determine Total Circuit Resistance

Calculate number of anodes

Select Anode Material, Weight and Dimensions

Coating Efficiency

Current Requirement

Variations in Temperature and Concentration

Review pH data

Literature Review


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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


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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].


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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 structures 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 impressedcurrent cathodic protection setup.

2. A theoretical calculation based on coating efficiency.

The second methods above can be used on existing and new structures.


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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/m)

Soil, 50 to 500 .cm 20 to 40

Soil, 500 to 1500 .cm 10 to 20

Soil, 1500 to 5000 .cm 5 to 10

Soil, over 5000 .cm 5

Fresh water 10 to 30

Moving fresh water 30 to 65

Brackish water 50 to 100

Sea-mud zone 20 to 30


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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


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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 outflowsurface 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 distributionoccurs 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


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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 CurrentDensity 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.

=

(3.2)

Where:

MT = LF x C x Is

C: Consumption rate of anode .

LF: Life time (How many years).

: Mass of anode.


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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 (3.3)

RT: Total circuit resistance.

Rca: Anode cable resistance.

Rcc: Cathode cable resistance.

Rca=Lca

KSca (3.4)

Rcc=Lcc

KScc (3.5)

K: Cable conductivity.

Type of cable conductor usually is copper specific conductivity K = 56

sm/mm2

Lca: Length of anode cable (mm).

Lcc: Length of cathode cable (mm).

Sca: Size of anode cable (mm2).

Scc: Size of cathode cable (mm2)

3.10 Calculate Rectifier Voltage to Determine Voltage Output of the Rectifier

U = RT x IS (3.6)

U: Output Voltage


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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 (3.7)

P: 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

Stagnant FreshwaterCoated sample without ICCP

Bare sample with ICCP

Bare sample without ICCP

Coated sample with ICCP

Flowing FreshwaterCoated sample without ICCP

Bare sample with ICCP

Bare sample without ICCP


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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.

a

b


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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

a b


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3.12 Monitoring by Measuring of the Potential

The potential can be measured by using copper - copper sulfide or silver-

silver chloride electrode shown in Figures 3.3 and 3.4 respectively.

Figure 3.5 Copper- Copper Sulfate Reference Electrode

(a) Schematic (b) Real

Fig 3.6 Silver- Silver Chloride Reference Electrode

(a) Schematic (b) Real

a b

a b


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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+2C. 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 Freshwater

Temperature Room temperature (25C)

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 ismeasured and the auxiliary electrode which carries the current created in the circuit.


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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 becomesthe anode while the auxiliary electrode becomes the cathode [14].

Figure 3.7 Cell kit Set-up

Working electrode

Reference

electrode

Auxiliary electrode

Test solution


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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].

Figure 3.8 Photographs of

(a)Connection of Specimen to Copper Wire by Brazing Technique;

(b) Mounting of Samples

a b


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Figure 3.9 Photographs of

(a) Working Electrode (WE)

(b) Typical Surface Area of a Sample

a

b


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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 (25C)

Calculation of corrosion rate in mm/yr for immersion test result is as follow:

Corrosion penetration rate, r (mpy), r =

K = Constant (3.45x106)

W = mass loss, g

A = Exposed surface area, cm

T = Time of exposure, hour

D = Density of specimen, g/cm


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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 Compositions (%)

Fe 98.4

C 0.0555

Mn 0.524

S 0.0163Si 0.145

V 0.00431

Mo 0.0252

Ti 0.0150

Al 0.00124

Sb 0.0115

Sn 0.0519

Pb 0.00635


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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 cm = 0.0638 m

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


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Calculation of required mass of anodes

MT = LF x C x Is = 0.16666 x 2 x1x10 = 0.33332 g

Number of anodes

anodeanoden

m

Mn

b

a

T

b

1000666.05.0

00033332.0

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:

14.064.056

5ca

ca

ca

caR

Sk

LR

14.064.056

5

cc

cc

cc

cc RSk

L

R

Total circuit resistance RT = 0.14 + 0.14 = 0.28

Current layout Is = 1 mA

Output voltage

U = RT x IS

U = 0.28 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

Cathode cable resistance:


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4.2.2 For Bare Steel Immersed in Stagnant Freshwater


S = 638 cm = 0.0638 m


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



Mass of anode: Ma = 0.5 kg


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 = 0.666664 g

Number of anodes


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anodeanoden

n

m

Mn

b

b

a

T

b

100133.0

5.0

0006666.0

Total circuit resistance

RT = Rca + Rcc

Length of anode cable (Lca = 5 m ) Size of anode cable (Sca = 0.64 mm2

)


Type of cable conductor is copper specific conductivity

K = 56 sm/mm2


14.064.056

5ca

ca

ca

caR

Sk

LR

14.064.056

5

cc

cc

cc

ccR

Sk

LR


RT = 0.14 + 0.14 = 0.28


Output voltage

U = RT x IS= 0.28 x 2 mA = 0.56 mV

U = 0.56 mV

U= 1 mV

Output power p = U x IS = 1mV x 2mA = 2 mW




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4.2.3 For Coated Steel Immersed in Flowing Freshwater


S = 638 cm = 0.0638 m


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 layout: IS = 1 mA


Mass of anode Ma = 0.5 kg


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 = 0.33332 g

Number of anodes


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anodeanoden

n

m

Mn

b

b

a

T

b

1000666.0

5.0

00033332.0

Total circuit resistance RT = Rca + Rcc

Length of anode cable (Lca = 5 m ) Size of anode cable (Sca = 0.64 mm2 )



K = 56 sm/mm2


14.064.056

5

ca

ca

ca

caR

Sk

LR

14.064.056

5

cc

cc

cc

ccR

Sk

LR


RT = 0.14 + 0.14 = 0.28


Output voltage

U = RT x IS

U = 0.28 x 1 mA = 0.28 mV

U= 1 mV


P = 1mV x 1mA = 1 mW




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4.2.4 For Bare Steel Immersed in Flowing Freshwater


S = 638 cm = 0.0638 m

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




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 = 1.33328 g

Number of anodes


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anodeanoden

n

m

Mn

b

b

a

T

b

100266.0

5.0

00133328.0


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


K = 56 sm/mm2


14.064.056

5ca

ca

ca

caR

Sk

LR

14.064.056

5

cc

cc

cc

ccR

Sk

LR

Total circuit resistance RT = 0.14 + 0.14 = 0.28


Output voltage

U = RT x IS

U = 0.28 x 4 mA = 1.12 mV

U= 3 mV


P = 3mV x 4mA = 12 mW




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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.


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Figure 4.1 The potentials measurment for coated and bare steel in stagnant

freshwater with ICCP

Figure 4.2 Samples with ICCP after 1 month immersion in stagnant freshwater

(a) Coated Sample (b) Bare Sample

a b


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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

Immersed Cathodic Protection

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