Marine Surveying

8/12/2019 Marine Surveying

1/138

Marine Corrosion& Coatings

Module K

Basics of Corrosion

Environmental Effects

Forms and Mechanisms of Corrosion

Survey Equipment and Methods

Choosing a Paint System

Diploma in

Marine Industry

Surveying

K

2008/09

AUTHOR

Mr. Peter Morgan


2/138

Module K Marine Corrision & Coatings

Diploma in Marine Industry Surveying

MPI Group, as a body, are not responsible for any opinions expressed in this module by contributors. All rights reserved. No

part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means,

electronic, mechanical, photocopying, recording or otherwise, without prior permission of MPI Group.

Marine Publications International Ltd and Lithgow Associates 2007


3/138

Module KMarine Corrision & Coatings


Page 1

Author

Mr. Peter Morgan


4/138

Marine Corrosion & Coatings Module K

CONTENTS

1.1 Corrosion Costs and Economics. 5

1.2 Definition of Corrosion 5

1 ELECTRO-CHEMISTRY 6

1.3 Atomic Structure and Ion Formation 6

1.4 Electrolytes, Electron Flow and Batteries 7

1.5 Electron Flow 8

1.6 Half Cells 9

1.7 Redox Reactions 91.8 Cell Potentials 10

1.9 Corrosion Potentials / Electro-chemical Series 10

1.10 Cell Voltage 11

1.11 pH and Acidity. Hydrogen Ion Concentration 12

2 CORROSION PROCESSES 13

2.1 Corrosion Sites, Anodes and Cathodes 13

2.2 Passivation 13

2.3 Process Factors affecting Corrosion Mechanisms 14

2.4 Temperature 14

2.5 Concentration Effects 14

2.6 Conductivity Effects 14

2.7 Velocity Effects 14

2.8 Pressure 14

2.9 Galvanic Effects 15

2.9a Anode size effect in Galvanic Corrosion 15

3 ENVIRONMENTAL EFFECTS 16

3.1 Marine Atmosphere and Seawater Corrosion 16

3.2 Dirt Deposits and Cargo Deposits 16

3.3 Condensation Corrosion 18

4 FORMS AND MECHANISMS OF CORROSION 19

4.1 Oxygen Concentration Cell Formation 19

4.2 Mechanism of Oxygen Corrosion 19

4.3 Crevice Corrosion 20

4.4 Pitting corrosion 21

4.5 Galvanic Corrosion 22

4.6 Carbon Dioxide Corrosion 24

4.7 Hydrogen Sulphide Corrosion 24



Page 2


5/138

4.8 Environmental Stress Cracking 24

4.9 Microbiologically Induced Corrosion (MIC) 25

4.10 Other Mechanisms and Types of Corrosion 26

5 METHODS OF CORROSION CONTROL 28

5.1 Materials selection for corrosion protection 29

5.2 Steels and Irons 29

5.3 Non-Ferrous Materials 30

5.4 Thermosets and Thermoplastics 31

6 COATINGS 32

6.1 Types of Coatings 32

6.2 Surface Preparation and Application of Coating 32

6.3 Coating Types and Application 336.4 Effect of Coatings on Cathodic Protection Design 36

6.5 Coating Evaluation and Inspection Measurements 36

7. CATHODIC PROTECTION 38

7.1 Theory 38

7.2 Impressed Current Cathodic Protection (ICCP) 39

7.3 Protective Potentials and Potential Measurements 41

7.4 Cathode Current Density 42

7.5 Importance of Coatings for CP 43

7.6 Over Protection / Under Protection 43

7.7 Types and Properties of Anodes. Anode Weight, Shape and Life 45

7.8 Calculations for Design 46

7.9 Impressed current cathodic protection 48

7.10 Power Sources 48

7.11 Types and Selection of Anodes 48

7.12 Calculations for Design 49

7.13 Interference Current 49

7.14 Transformer / rectifiers 51

7.15 Potential Surveys 51

7.16 C.P. Maintenance Factors 52

7.17 CP SAFETY 53

8.0 CORROSION PREVENTION MAINTENANCE 55

9.0 IDENTIFICATION OF DEFECTS 56

10.0 SURVEY METHODS AND EQUIPMENT 57

10.1 Non-Destructive Inspection Methods. 57

11.0 CASE HISTORIES 59

Service History 59



Page 3


6/138

12 METHODS OF CORROSION CONTROL - COATINGS 64

12.0 Minimising Corrosion Effects 64

12.1 Corrosion Prevention using Protective Coatings 64

12.2 What are Coatings? 64

13 CHOOSING A PAINT SYSTEM 71

Exposure Testing of Paint Films 71

Performance Expectation of Coatings 71

Coating Systems and their Selection 71

14 COATING SPECIFICATIONS 78

15 PRACTICAL PAINTING CONSIDERATIONS 80

16 SURFACE PREPARATION - ALTERNATIVE METHODS 82

Blast Pots/Hoses/Nozzles 85

17 PAINT APPLICATION: QUALITY CONTROL 93

18 PAINT FAULTS 95

19 FURTHER INFORMATION 101

Corrosion Societies 101

APPENDIX A 102

APPENDIX B 112

Marine Terminology and Construction 112

Naval Vessels 112



Page 4


7/138

1 BASICS OF CORROSION

1.1 Corrosion Costs and Economics.

UK Industry Cost in Million

Building and Construction 250

Food Industry 40

General Engineering 110

Marine Industry 280Government 55

Metal refining / fabrication 15

Oil, Gas and Chemical Industry 180

Power 60

Transport 350

Water 25

Total 1365 million

Table showing cost of Corrosion in the USA 1995 compared with 1975

Since 1995 it is estimated that real term corrosion costs for the oil and gas industry have been reduced

by 50%. However the above Table for corrosion costs averaged through all industries in the United States

shows a saving between 1975 and 1995 of only 14%

1.2 Definition of Corrosion

Corrosion is the deterioration of a substance, usually metal, or the deterioration of its mechanicaland metallurgical properties due to its reaction with the environment

In some cases there is no visible evidence of this deterioration that can lead to the sudden failure

(environmental cracking) of the material for no immediately apparent cause.

Iron, the main component of steel is thermodynamically unstable and tends to return to its oxide state.

Most other metals including aluminium and zinc show this tendency to return to an oxide ore state or

other thermodynamically more stable compound.

All Industries 1975 (US $ Billion) 1995 (US $ Billion) Ratio 1995 /1975

Total 82.5 296.0 3.59

Avoidable 33.0 104.0 3.15

GNP 1683.7 7033.6 4.18

Total % of GNP 4.90 4.21



Page 5


8/138

ELECTRO-CHEMISTRY

1.3 Atomic Structure and Ion Formation

Atoms are the minute building blocks of all matter and consist of a nucleus and circulating electrons. The

diameter of the path for circulating electrons can be a factor of 10,000 x larger than the diameter of the

nucleus.

The nucleus consists of uncharged particles called Neutrons and positively charged (+ve) Protons. The

orbiting electrons have an equal negative (-ve) charge to the Protons and the atom is electrically neutral.

Corrosion reactions and processes are ordinary chemical reactions in which the atoms gain or lose

electrons.

If the atom gains or loses one (or more) of its electrons it becomes an ION with an electrical charge. Loss

of n electrons equals a gain of n positive charges on the atom nucleus that becomes a CATION.

Gain of n electrons equals a loss of positive charge and the atom becomes an ANION.

(n can be 1, 2 or 3)

Chemical and corrosion processes are actually the exchange of the -ve charged electrons between the

various atoms involved.

Ion Examples: Na+ Sodium cation, Ca++ Calcium cation,

Fe++ Ferrous cation, Fe+++ Ferric cation,

Cl-

Chloride anion, SO4- - Sulphate anion

MoleculesMolecules are formed when two or more atoms combine together with a strong bond between them.

Nearly all gases occur normally in the form of molecules. Hydrogen (H), Oxygen (O), Nitrogen (N) and

Chlorine (Cl) occur as pairs of atoms (Molecules) and their formulae are written as H2, O2, N2 and Cl2.

Other gases such as Carbon Dioxide (CO2) Hydrogen Sulphide (H2S) and Water Vapour (H2 O) also occur

as molecules.

For example the Carbon Dioxide Molecule Formula is: O=C=O

The water molecule H2O is formed from a hydrogen cation H+ and an anion OH-

Water Molecule, Hydrogen & Hydroxyl Ions

Ionic CompoundsA large group of substances known as inorganic salts exist as solids in an ionic form.

Ordinary salt. Sodium Chloride (NaCl) consists of sodium cations regularly arranged next to chlorine

anions.

The +ve and -ve charges on the ions attract them together and the charges also neutralise each other so

that the solid sodium chloride is electrically neutral as shown in the following diagram..

Na+ Cl- Na+ Cl- Na+ Cl- Na+

Cl- Na+ Cl- Na+ Cl- Na+ Cl-

Na+ Cl- Na+ Cl- Na+ Cl- Na+



Page 6


9/138

Arrays of these unit cells are built up to form crystals.

The way in which the atoms of a metal are bonded together is different to that in the molecule or in the

ionic crystal.

Apart from the regular arrangement of unit cells the atoms share their electrons between every other

atom in the crystal.

Electrical neutrality is still maintained because the electrons stay within the crystal, and their charges are

still neutralised by the charges on the protons in the atomic nuclei.

It is this special atomic and electron arrangement in metals which gives them their special properties of

electrical conductivity, hardness, brightness and formability.

1.4 Electrolytes, Electron Flow and Batteries

To create corrosive conditions at normal environmental temperatures corrosion cell is required. The

corrosion cell works in the same way as the well-known dry battery cell. A typical corrosion cell is shown

in Figure 1

For low temperature corrosion (< 200o

C) to take place, an electrically conductive solution that will allow

cations and anions to move freely through its bulk must be present. A solution of this type is called an

electrolyte. Water containing dissolved ionic salts is the most common and the best electrolyte, and is

always present in electrolytic corrosion. Seawater is on of the best electrolytes and is the main cause of

marine corrosion. Water corrosion is greatly increased if it contains dissolved oxygen.

The formation of ions in solution and the movement of the ions towards one or other of the electrodes

(anode or cathode) are called electrolysis.

The movement in the electrolyte occur as a result of the difference in electrical potential between the

anode and the cathode. This is the driving force. (Electro-motive Force. EMV or Volts)

If there is no difference in potential there is no driving force between the anode and the cathode andcorrosion cannot occur.



Page 7


10/138

FIGURE 1 ELECTROLYTIC CELL-BATTERY

In the battery cell, shown above, the electrolyte is a paste of zinc chloride in water. On closing the electric

circuit there is a chemical action and a flow of electrons in the circuit caused by the potential difference

of 1.5 volts between the cathode and anode.

1.5 Electron Flow

Two different metals or conductors must be connected in the battery or corrosion cell to enable currentto flow.

Negative charged electrons flow from the part of the cell which starts corroding, called the Anode, through

the metallic part of the circuit to the part of the cell which does not corrode, called the Cathode. This flow

of electrons produces a measurable electrical potential difference across the circuit.

Due to the flow of electrons the electrical neutrality of the electrodes is lost. The anode becomes

positively charged because it loses electrons through the metal portion of the circuit.

This +ve charge will attract the flow of -ve charged anions in the electrolyte.

In the same way the cathode gains electrons through the metal path and becomes negatively charged.

The cathode now attracts +ve charged cations in the electrolyte.

e-

Fe++

Fe++

H+

H+H+

H+

CORROSION

ANODE CATHODE

ELECTROLYTE: DILUTE

In any corrosion reaction the loss of electrons at theanode must equal the gain of electrons at the cathode

METALIC CONNECTION PATH

ELECTROLYTE LEVEL



Page 8


11/138

Corrosion occurs at the anode because atoms of the metal at the surface lose electrons and become

positively charged. These positively charged ions go into solution, and are transported towards the

cathode under the influence of its negative charge.

1.6 Half Cells

In Figure 1 the anode reaction is a half-cell. The cathode reaction is also a half-cell and the two together

(cathode + anode) make a complete cell.

The total reaction occurring is the sum of the reactions of the half cells.

For example.

The anode reaction is: Zn = Zn++ + 2e-

The cathode reaction is: 2H+ + 2e- = H2

The sum of the reaction is: Zn + 2H+ = Zn++ + H2

The concept of half-cells is important in the measurement of corrosion potentials and in cathodic

protection where special copper/copper sulphate half-cells are used. These uses will be discussed in

greater detail later.

1.7 Redox Reactions

Where any two half-cells are coupled together they form a complete cell.

The loss of electrons or gain in +ve charge by a metal is known as Oxidation and this type of reactionalways occurs at the anode.

The process does not necessarily form oxides. Chlorides and other salts may also be formed, butoxidation is one of the fundamental corrosion processes.

Only one type of reaction occurs at the anode. M = M++ + 2e

For an iron anode this reaction is: Fe = Fe++ + 2e-

At the anode, metal is always dissolved as cations withthe loss of electrons and oxidation.At the cathode however, any one of three reactions may take place:

1. 2H+ + 2e- = H2 (Hydrogen gas bubbles)

2. O2 + 4H+ + 4e- = 2H2 O (Water)

3. O2 + 2H2O + 4e- = 4OH- (Hydroxyl ions)

Reaction 1. is the reduction of hydrogen ions and is very common in acid solutions. (The H+ comes from

the acid)

Reaction 2. is called the reduction of oxygen and occurs in acid solutions containing air or oxidising

agents.

Reaction 3. is also reduction of oxygen but occurs in neutral or alkaline solution.

1. 2. & 3 are called Reduction reactions. The Hydrogen ions are reduced to hydrogen gas. Oxygen

molecules are reduced to water or hydroxyl ions.



Page 9


12/138

The first reaction takes place quite rapidly in acid solutions, but very slowly in alkaline or neutral media.

It can be speeded up by dissolved oxygen (O2) as shown in Reaction 2.

In a complete cell oxidation occurs at one half cell and reduction at the other cell. The overall reaction is

known as a Redox Reaction (Reduction / Oxidation)

1.8 Cell Potentials

The driving force of a corrosion cell is determined by the difference in electrical potential (volts),

measured between the anode and the cathode in the metallic path of the closed circuit, with a high

resistance voltmeter. The potential difference is measured across the cell as shown in Figure 2

FIGURE 2: MEASURING CELL POTENTIAL

Measured potential differences (E) can range from zero up to 3 volts. A zero voltage indicates that no

chemical (corrosion) action will occur, or that an equilibrium condition will exist. A few millivolts show a

low corrosion driving force and the possibility of corrosion occurring. A potential of several hundred

millivolts or more indicates a very high corrosion driving force and the possibility of very high,

unacceptable corrosion rates.

1.9 Corrosion Potentials / Electro-chemical Series

In laboratory work the corrosion potential of a metal is usually measured against a standard half-cellcalled a platinum-hydrogen electrode. This technique allows the corrosion potentials of various metals to

be compared with a standard reference.

The cell is used to define a galvanic series of metals arranged in order of their potentials as measured

against the reference standard. This series is also known as the Electrochemical Series of Metals.

A table of potentials for different metals in seawater is shown in the electrochemical series. (Table 1) In

practise cells such as the saturated calomel cell, copper sulphate cell and the silver/silver chloride cell

are used in the field.

ANODECATHODE

Electrolyte

VElectronFlow

Electrictity Conventional Flow



Page 10


13/138

Electrochemical Series (sea water) reference a saturated calomel electrode (SCE)

TABLE 1METAL POTENTIAL (v)PURE MAGNESIUM -1.65

MAGNESIUM ALLOY -1.53

ZINC -1.03

ALUMINIUM ALLOYS 7072 and 6065 -0.95 to -0.85

PURE ALUMINIUM -0.9

MILD STEEL -0.6 to -0.55

GREY CAST IRON -0.55

HIGH SILICON CAST IRON -0.4

18:8 STAINLESS STEEL, ACTIVE -0.4

LEAD -0.3 to -0.2

ADMIRALTY BRASS, -0.35

COPPER -0.35

ALUMINIUM BRONZE -0.30

SATURATED CALOMEL (REF. CELL) 0.00

18:8 STAINLESS STEEL, PASSIVE -0.1 to +0.1

DUPLEX STAINLESS STEEL PASSIVE +0.15 .

INCONEL 625 +0.2

HASTALLOY C +0.24

TITANIUM 0 to +0.15

GRAPHITE +0.3

SILVER +0.8

GOLD +1

TABLE 1

The more negative the potential of the metal the more reactive (easily corroded) the metal is likely to be.

e.g. Magnesium is much more easily corroded than copper.

Potentials between Dissimilar MetalsUsing the above Table or similar Tables that use the Hydrogen reference Electrode, the Copper / Copper

Sulphate Electrode (CSE) or other standards, the potential of a cell is easily calculated by simple

subtraction between the half-cell potentials.

Example: The Electrochemical series potential difference between the SCE reference electrode and

Aluminium is: 0.00 - (-0.9) = +0.9 volts

Example: Between Grey Cast Iron and Aluminium Bronze the potential difference is:

-0.30 - (-0.55). = +0.25 volts

1.10 Cell Voltage

The potential (volts) of a corrosion cell indicates the corrosion driving force and the amount of current

flowing (amps) gives an indication of the volume of metal corroding in unit time. Current flow is measured

with a low resistance ammeter.

The amount of metal lost in a corrosion reaction can often be found by applying the law that the mass of

a substance liberated at an electrode is proportional to current flow time. (Faradays Law). One Amp

for one second = one Coulomb and 96,500 Coulombs will liberate or dissolve the gram equivalent weight

of a metal.



Page 11


14/138

The most reactive metals such as Potassium (K) Magnesium (Mg) Aluminium (Al) and Iron (Fe) have

negative electrode potentials and are anodic in character when related to the Pt/H 2 Electrode. In the

presence of hydrogen ions these anodic metals will more readily corrode than the less reactive and more

noble metals such as platinum and gold that have positive electrode potentials.

If two metals in the galvanic series are coupled in a corrosion cell, the driving force or tendency for

corrosion to occur in the anodic metal is proportional to the potential difference between them.

For example, if zinc and steel are coupled the potential difference between the metals is:

-0.5 - (-1.03) = +0.53V.

The zinc corrodes at a moderate rate and the steel does not corrode. If, in the same cell, the steel is now

replaced by silver the potential difference is:

+ 0.8 -(-1.03) = +1.83V.

The zinc is at a more -ve electrode potential and its potential has greatly increased.

In this case the actual corrosion rate of the zinc also increases considerably although the silver does not

corrode.

This principle is important in design to prevent dissimilar metals contacting one another and causing

corrosion and in the application of sacrificial corrosion protection to be discussed later.

1.11 pH and Acidity. Hydrogen Ion Concentration

The symbol pH is a chemical shorthand for a method of measuring the acidity or alkalinity of a solution.

The acidity of an aqueous solution is a measurement of the negative logarithm of the hydrogen ion

concentration. This is better known as the pH value.

The symbol p is derived from the German word potenz, and means the logarithmic exponent, or power of

concentration.

The letter H is the chemical symbol for hydrogen.

A pH scale of 0 to 14 has been adopted. 0 is a very strong acid. (High H+ ion concentration) and 14 is

a strong alkali with a very low H+ ion concentration.

Pure water is considered to be neutral, neither acid nor alkaline and it has a pH of 7.0.

The pH of a liquid is extremely important as most acid solutions with low pH values (acids) are very

corrosive to metals, and if very acid they can also damage the skin and eyes of operators unless

precautions are taken.

Alkalis are often beneficial in improving the corrosion resistance of steel, but can be very damaging to

metals such as zinc and aluminium. Strong alkalis also damage the skin and eyes.



Page 12


15/138

2 CORROSION PROCESSES

Corrosion occurs due to the anode and cathode reactions in electrolytic cells as already discussed. Onany given piece of metal such as pipes or tanks the corrosion cells form at numerous microscopic sites

on the surface.

2.1 Corrosion Sites, Anodes and Cathodes

The anodic sites corrode as shown in Figure 3 with the loss of metal ions. The usual reaction at cathodes

is the production of hydrogen gas in acid solutions, reduction of oxygen to form water in oxygenated

slightly acid solutions or the production of hydroxyl ions (OH-) in neutral oxygenated solutions.

FIGURE 3 CORROSION SITES ON A METAL SURFACE

Corrosion (oxidation) of metals usually produces a soluble or easily detached corrosion product that is

lost from the surface to expose new metal, which continues to corrode.

In some cases the corrosion product forms a thin adherent film that protects the metal from further

attack, as in stainless steel. The metal is then said to be passivated.

Corrosion rates are measured in mm per year or, in USA practise, in mils (thousandths of an inch) per

year and the rates are affected by process variables.

2.2 Passivation

Metals that can be passivated corrode normally in moderate concentrations of oxidising agent but may

suddenly show a 10-3 o 10-6 decrease in corrosion rate as the oxidising agent concentration is increased

still further into the Passive region. Eventually at very concentrations of oxidant the corrosion rate will

increase again in the transpassive region.

ANODE

ANODE

CATHODE

CATHODE

CATHODE

ELECTROLYTE

Fe++

H+

H+

H+

Cl-

Cl-

CORROSION CELL ON STEEL PLATEPOSITIVE CURRENT FROM ANODE TO CATHODE

THROUGH ELECTROLYTE

CORROSION SITES ON METAL SURFACE



Page 13


16/138

Metals showing this effect include stainless steels in mild oxidising agents and carbon steels in

concentrated sulphuric or concentrated nitric acid.

The passivation effect of Stainless Steel is due to a thin layer of inert chromium oxide on the steel

surface. Other passivation effects are due to thin oxide or other inert chemical layers. Sometimes the

film is easily destroyed by moderate mechanical damage or by changes in oxidising conditions. Rapid

corrosion may then occur.

2.3 Process Factors affecting Corrosion Mechanisms

2.4 Temperature

Corrosion rates generally increase with temperature and may double for every 20o

C increase in the

reaction system temperature.

2.5 Concentration Effects

Increased chemical concentration usually increases corrosion rates up to a limiting value. Further

increases in the concentration may cause no further increases in the corrosion rate or may even cause it

to decrease.

2.6 Conductivity Effects

Increased electrical conductivity in a solution (or a soil) indicates increased ionic activity, and therefore a

probable increase in corrosion rate. Seawater with a high conductivity or low resistivity is much more

corrosive than fresh water.

Conductivity = 1 / resistivity and resisistivity is the property usually measured in corrosion with a value in

ohm cm. Seawater has a resistivity of 30 ohm cm.

The corrosion pattern occurring on a ship is rather different due to the ships movement. The ship is

continually passing through new supplies of oxygenated water so the supply at the ships hull surface is

never used up in the corrosion reactions.

Instead of occurring in the splash zone the maximum corrosion occurs at or just below the water line

where the oxygen content and is high and the movement of water is greater than occurs in the splash

zone of a static structure. The band of maximum corrosion occurs over a depth ranging from the unloaded

to fully loaded water lines.

2.7 Velocity Effects

It can be seen from the above statement that Increased velocity can cause increased corrosion due to

increases in the amount of corrosive substance passing over the surface in a given time.

At high velocities and in turbulent conditions fluids may also cause mechanical damage to materials,

accelerating corrosion effects and producing severe attack known as erosion corrosion. This occurs

particularly on ships propellers and on various pump impellers that may be used for pumping water,

controlling ballast tanks or loading cargoes.

2.8 Pressure

An increase in pressure increases the amount of gas such as oxygen or carbon dioxide that can be

dissolved in the electrolyte (usually water). This gives an increase in the concentration of corrosive gases

and causes increased corrosion rates. This is not normally a problem in marine corrosion.



Page 14


17/138

2.9 Galvanic Effects

Dissimilar metal contacts in an electrolyte can accelerate the corrosion of the more active metal. This

effect occurs frequently in marine design and maintenance such as where brass fittings are screwed intoa steel structure, steel components are fixed to an aluminium hull; bronze propellers are connected to

steel shafts. Materials such as admiralty brass or bronze fittings are commonly attached to steel

structure and can accelerate the corrosion of the steel. Pipefittings and heat exchanger tubes are other

items commonly affected by galvanic corrosion. The less noble material will be at risk of corrosion and

requires additional protection. (Refer to Table 1 )

2.9a Anode size effect in Galvanic Corrosion

If the anodic material (material at a risk of corrosion) has a large surface area in relation to the area of

the cathodic (non-corroding metal) then the amount of corrosion should not be severe because the

electrical current causing the corrosion is distributed over the large area of the anode and produces a low

current density. Since current density is directly related to corrosion rate the corrosion rate should be low.

However if the exposed anode area is small in relation to the cathode area the corrosion rate can be veryhigh.

The classic example of this effect occurred over 200 years ago when the Royal Navy tried to protect its

wooden ships from marine fouling by cladding the hulls with copper.

The copper sheet was attached to the hull by iron rivets. This produced very large cathodes (Cu) and

small anodes. Within a few months the rivets corroded away causing the copper to disbond from the

hulls.



Page 15


18/138

3 ENVIRONMENTAL EFFECTS

3.1 Marine Atmosphere and Seawater Corrosion

The commonest form of atmospheric corrosion is the uniform rusting of steel due to the combination of

iron with water and oxygen from the air.

Rusting of iron to form various oxides.

Fe + 2H2O + O2 = 2Fe(OH)2 (Unstable, oxidises)

2Fe(OH)2 + H2O + O2 = Fe(OH)3 (Ferric hydroxide)

Fe(OH)3 = H2O + FeO(OH) (usually yellow)

2FeO(OH) = H2O + Fe2O3 (orange dehydrated oxide)

Reaction rates are modified by the following:

The amount of water in the air (humidity) and rainfall, contaminants in the air, salt spay and blown sand

in marine environments or sulphuric acid in industrially polluted areas. Many harbours in industrial ports

can suffer from the double affect of marine salt and industrial air pollution.

Corrosion rates in coastal areas have been measured at 400 times the corrosion rate in desert

atmospheres and corrosion on specimens 25m from the seashore has been measured at 12 times the

corrosion rate at locations 250m from the shoreline.

High air and metal surface temperature can also increase corrosion. Chemical reaction rates, including

corrosion usually double for every 10 to 20oC increase in temperature. High surface temperatures can

cause rapid evaporation of water and then of course the corrosion stops. Remember there cannot be anyelectrolytic corrosion without an electrolyte (water). However the evaporation can cause salts toconcentrate on the surface. At some later period when the surface is again wetted the concentrated salts

will be dissolved and may form exceptionally high concentrations of salt or sulphate that can then have

an increased corrosion effect.

3.2 Dirt Deposits and Cargo Deposits

Accumulation of dirt or mud deposits on metal surfaces can attract moisture and set up corrosion cells

that can cause severe corrosion often in crevices or corners where the corrosion can be very damaging.

Many ships carry corrosive cargoes that are held in specially designed holds. If these cargoes spill in the

wrong areas such as on deck they can cause severe deposit corrosion problems. The corrosion in these

cases occurs as pitting and the mechanism by which it occurs is usually an oxygen concentration cell as

described in the next chapter.

Cargo holds are subject to severe environments including corrosive and abrasive wear due to the loading

of bulk materials through inlet pipes, wear due to tractor movement within the holds and salt water that

may be deliberately added as ballast or accidentally leaked into the hold.

The worst conditions apply when a corrosive cargo is saturated with seawater through hatch leakages.

Corrosive cargoes include salt, gypsum, organic and inorganic fertilizers, iron ore and numerous other

materials.



Page 16


19/138

Sea Water Immersion CorrosionSea water can only dissolve some 15 parts per million of oxygen, however oxygenated sea water is

extremely corrosive to iron and carbon or low alloy steels which need to be well protected by coatings. The

pattern of seawater corrosion and the effect of oxygen are shown in Figure 4.

Where oxygen content is less than 0.1 ppm as in deep quiet water or sea mud the corrosion can be

negligible. The sections of harbour sheet piling that are buried in the mud are often uncoated and do not

corrode significantly.

FIGURE 4 SEA WATER CORROSION OF STEEL AT VARIOUS DEPTHS

Static Structure

Ships hulls are subject to heavy general corrosion at and above the waterline due to continuous expose

to oxygenated water. Well below the waterline the corrosion is more likely to be local pitting associated

with marine growths, deposits or variations in the metal plate composition, as at welds.

Seawater corrosion of ballast tanks is a major problem on ships. The movement of the water in the tanks

and by transfer pumps can cause the water to be saturated with oxygen and be extremely corrosive. Lack

of drainage in some designs allows local stagnant areas to develop where corrosion cells under deposits

can form and where bacterial activity can also increase corrosion rates. (Bacteria / microbiologicaleffects are discussed later))

High standards of painting are required to prevent corrosion with special formulations to reduce marine

growth.

Typical protective systems are heavy duty (200 to 300 microns) epoxy based coatings for hulls and tanks.

Galvanised steel is often used for submerged and splash zone steel structures.

Seawater usually causes general corrosion in carbon and low alloy but can cause pitting under deposits

and marine growth or bio fouling. Pitting corrosion is the standard form of failure in type s 304 and 316

stainless steels and in aluminium alloys. The extent of pitting can be severe and is usually greatest in the

higher strength copper containing aluminium alloys.

SEA WATER CORROSION OF STEEL AT VARIOUS DEPTHSStatic Structure

Marineatmosphere

Splash zone

High tide

Low tide

Quietseawater

Mud line

Typical corrosion rate of steel, mpy

Corrosionofsteelpilng



Page 17


20/138

3.3 Condensation Corrosion

Condensate corrosion is a particularly aggressive form of atmospheric corrosion in empty or partly filled

storage tanks. The moisture-laden air is sucked into the tank and the tank cools at night. Condensationforms on the interior walls and roof.

Frequent cycles of condensation and drying concentrates dissolved salts and can cause severe corrosion.

High quality coatings, sealed tanks, and inert atmosphere blanketing are techniques used to control this

type of problem. Tanks on land may also be designed with a floating roof that allows no air space above

the tank contents.



Page 18


21/138

4 FORMS AND MECHANISMS OF CORROSION

Atmospheric rusting and external corrosion of all metal structures are subject to accelerated corrosiondue to oxygen in contact with an electrolyte (water)

Air and water are the two items that cause the greatest amount of corrosion damage due to their

abundance in the natural environments.

Iron oxides FeO(OH), Fe2O3 and Fe3O4 are formed by steel corrosion and heavy localised corrosion pitting

occurs in soil and under deposits where oxygen concentration cells can occur.

4.1 Oxygen Concentration Cell Formation

This is one of the commonest causes of pitting corrosion and is the cause of pitting corrosion in the oil

and gas industry. It is also a major cause of internal corrosion in systems wherever oxygen can be

introduced.

4.2 Mechanism of Oxygen Corrosion

Figure 5 shows a very common corrosion problem at a point where some form of local deposit blocks of

the possibility of air contacting the surface while the surrounding area is still in contact with the air. The

deposit may be dirt, bacteria slimes, residual cargo remains. (Iron ore causes severe problems) or items

left after maintenance work.

e.g. Gloves left in a hold or tank has been a source of this type of problem.

The differential concentration of oxygen in the water or soil causes an electrolytic cell to form.

The cell reaction causes oxygen to be reduced at the cathode.

2H2O + O2 + 4e- = 4OH-

For every anode reaction there must be a corresponding anode reaction. Therefore the area deprived of

oxygen becomes the anode and corrodes.

Fe = Fe++ + 2e-



Page 19


22/138

FIGURE 5

The problem is usually overcome by ensuring the area is kept free of debris or by painting. Problems

frequently occur under residual deposits left in cargo holds or even

on ships decks. Cleaning, hosing and regular maintenance are helpful.

4.3 Crevice Corrosion

Crevice corrosion can occur on pipe flange joints and under gaskets or under the heads of nuts and bolts

as shown in Figure 6.

The crevice corrosion reaction mechanism is generally considered to be another form of the oxygen

concentration cell reaction. There is also evidence that in many cases the corrosion mechanism is

actually the same as that in pitting corrosion.

Metals, which have protective oxide layers on the surface and are prone to pitting corrosion, such as

304 / 316 type austenitic stainless steels, are also prone to crevice corrosion, particularly in seawater.

Crevice corrosion is controlled by sealing the crevices or by opening them to sufficient width for flow to

occur and reduce the differential concentration effects.

OXYGEN CONCENTRATION CELLFORMED BY DIRT DEPOSIT ELECTROLYTE

High oxygen atdeposit surfaceCathode

High oxygen atdeposit surfaceCathode

Salt, mud,waterlow oxygen at

centre Anodic area

Steel Plate

Corrosion

Steel Plate

This type of corrosion cell occurs under depositsand in crevice corrosion and in pitting



Page 20


23/138

FIGURE 6 CREVICE CORROSION

4.4 Pitting corrosion

A wide range of environments and mechanisms, each of which may form different types of pits, can cause

pitting. Oxygen concentration cells and CO2 corrosion cause wide interconnecting pits.

Chlorides in oxidising solutions cause the formation of deep small diameter pits in metals that are

covered with partially protective surface films. This type of corrosion occurs frequently and can be very

damaging to plant and pipelines.

Pitting can be caused by several sets of circumstances as follows:

localised differences in metal composition, which create a galvanic action between grains and grain

boundaries in the metal surface.

A local break down of the protective coating on a surface. This is particularly common in austenitic

stainless steels that normally have a thin protective oxide layer over the exposed surfaces. This layer

provides the steel with its corrosion protective properties and is known as the passivating surface.

If the passivation film is removed rapid corrosion can follow.

In practise local small areas of film damage occur frequently.

In oxidising conditions the damage on stainless steels may be self-healing but if shielded under deposits,cells are set up in which the unprotected area is strongly anodic to the surrounding cathodic area and

pitting commences.

As the pit deepens metal is lost as Fe++ and oxygen reduction takes place on surrounding surfaces.

As reaction proceeds an excess +ve charge builds up in the pit and attracts Cl ions from the electrolyte.

A high concentration of ferric chloride, FeCl3 develops and the product hydrolyses.

FeCl3 + 3H2O = 3HCl + Fe(OH)3

The acid HCl concentration increases in the pit and the reaction rate accelerates.

The mechanism is known as an autocatalytic reaction and explains the rapid penetration rates that occur.

Rates of +500 mils/year (12.5mm/year) are common and thick pipeline sections can be penetrated inless than one year. See Figure 7.

CREVICE CORROSION

ANODE AREA(ACTIVE)

NUT

WASHER

CATHOD AREA

(PASSIVE)

STAINLES STEEL

OXYGEN

STARVATION



Page 21


24/138

FIGURE 7 PITTING CORROSION MECHANISMS

4.5 Galvanic Corrosion

The Table of the Electrochemical Series lists metals in an order according to the ease with which they

corrode. If any two dissimilar metals are electrically connected in a corrosive solution, the one that is

most active, and has the most -ve potential, will corrode. The driving force of corrosion on the least noble

metal is directly proportional to the potential difference between the metals.

The actual rate of corrosion is proportional to the amount of current passed.

Accelerated corrosion, or in some cases decreased corrosion, of a metal caused by bringing it into

electrical contact with a dissimilar metal, is called Galvanic Corrosion.

In Figure 8 there is an open circuit between the metals in the first cell and no flow of electricity. The zinc

corrodes fairly rapidly and the iron corrodes at a slower rate.

If the pieces of metal are connected through an external circuit the corrosion pattern changes. Zinc

corrosion increases and iron corrosion is stopped. The galvanic effect with the zinc protecting the iron is

the principle of cathodic protection.

The lower part of the Figure shows typical practical problems that occur when dissimilar metals are in

contact in a corrosive environment

In engineering structures, many different metals come into contact with each other. This is particularlytrue of process vessels and heat exchangers.

Conditions for galvanic corrosion to occur can be summarised as follows:

Presence of dissimilar metals in electrical contact

Presence of a corrosive electrolyte

Small anode and large cathode

Small cathodes and large anodes, as in the copper rivet example, create cells with lower anode current

densities and, consequently, lower corrosion rates than occur with large cathodes and small anodes as

in the aluminium example.

PITTING CORROSION MECHANISMS

O2

O2 O2CL-

OH-OH-

OH-OH-

CL-CL-

CL-

CL-

CL-

Fe++

Fe++

Na+Na+

Fe++

Fe++

ALKALIALKALI

CATHODE

PRECIPITATEPRECIPITATE

CATHODE

ACID

ACID

ANODE



Page 22


25/138

The smaller the difference between dissimilar metals on the electrochemical and galvanic series table,

the lower is the corrosion driving force and the lower is the potential for corrosion to occur.

Prevention of Galvanic CorrosionDissimilar metals should be separated with suitable electrical insulating material. When insulation is not

possible, make the more anodic metal the easier to replace when corrosion becomes evident. Otherwise

ensure that the anodic metal is thicker and of larger surface area than the cathodic metal to reduce

corrosion rates.

FIGURE 8 GALVANIC CORROSION

V

CLOSED CIRCUITOPEN CIRCUIT

GALVANIC EFFECT OF ZINC COUPLED TO IRONIN A CORROSIVE ELECTROLYTE

Zinc corrodes more rapidly than iron

(Sacrificial Protection)

GALVANIC CORROSION OF STEEL UNDER COPPER RIVET

GALVANIC CORROSION OF ALUMINIUM RIVETIN CONTACT WITH STEEL PLATE

Zinc corrodes rapidly, iron is protected

COPPER RIVET

STEEL

ALUMINIUM RIVET

STEEL



Page 23


26/138

4.6 Carbon Dioxide Corrosion

Carbon dioxide (CO2) gas forms only 1% of the atmosphere.

Carbon dioxide (CO2) readily dissolves in water (H2O) to form carbonic acid (H2CO3), a weak acid that in

saturated solution at 15o

C and 1 bar (14.5 psi or 000kPa) pressure, has a pH of 3.6. due to dissociation

to produce hydrogen ions.

H2CO3 = H+ + HCO3-

The solubility of CO2 is increased by pressure and decreased by temperature.

Solubility of dissolved minerals, particularly calcium carbonate, is increased by the presence of carbon

dioxide in water. This increases the conductivity of the electrolyte and therefore increases the corrosion

tendency.

CO2 corrosion is not a significant gas in marine environments.

The gas may be present in natural gas and liquefied gas carried by special transporters but does notnormally present a corrosion risk.

4.7 Hydrogen Sulphide Corrosion

Hydrogen Sulphide (H2S) is a colourless, inflammable and highly toxic gas. It is heavier than air and has

a very strong smell of rotten eggs. Quite low concentrations are sufficient to cause rapid death.

H2S can occur in fresh or salt waters containing large amounts of rotting vegetation.

The gas is often present in sewers, sewer outfalls and in rotting organic matter in shallow waters. However

the concentration is rarely enough to cause safety problems or the accelerated corrosion of steels.

4.8 Environmental Stress CrackingStress Corrosion Cracking (SCC)This type of corrosion is one of the most important and dangerous causes of failure in metal structures

and vessels. It is a combination of corrosion and mechanical stress and can only occur when the

structure, in the area of the corrosive substance, is under a tensile stress.

The stresses may be due to many sources:

mechanical loads

internal pressure

weld restraint

thermal stresses

residual surface stresses

Cracking may proceed slowly at first, but when the load bearing capability of the metal has been reduced

sudden complete failure occurs due to mechanical overload. This rapid failure, and the fact that SCC is

often difficult to detect, makes this type of corrosion very dangerous.

Brasses undergo stress corrosion in ammonia solution.

Carbon steel is susceptible to stress corrosion in contact with carbonates, in strong, hot caustic soda and

in nitrates (NO3-) solutions.

Ships cargoes that may contain nitrates or carbonates can sometimes cause a risk of stress corrosion

cracking if they become wet and are in contact with steel surfaces. Inspectors and surveyors should be

aware of the risk.



Page 24


27/138

Austenitic Stainless Steel Stress Corrosion CrackingStainless steels of the 18% chromium and 8% nickel type are readily stress corroded in brines or sea

water at temperatures above 60o

C. The Chloride Ion (Cl-) in association with oxygen or oxidising agents

usually causes this type of attack on stainless steel.

Chloride Stress Corrosion Cracking. (CSCC)CSCC in stainless steel often has a characteristic branched appearance, like an aerial view of a river

estuary system, when studied under a microscope. The cracks can be intergranular or transgranular in

form. A stress of 60% of the ultimate tensile strength and a temperature of 60o

C is normally required to

induce this type of cracking.

One of the principle areas of stainless steel CSCC has been on heated and insulated pipes and vessels

where chloride from the environment or from the insulation, concentrates on the steel surface and

causes cracks to form.

On board ship there are may be a number of stainless steel systems that could be affected by this type

of failure. Heat exchangers and hot water pipework may be affected. Some specialist ships have

extensive cargo areas and pipes in stainless steel for carrying foodstuffs or special corrosive chemicals.

A pipe that is not heated can still be at risk. For example piping on a deck may heat up to well over 600

C

in hot sunlight. Small amounts of salt water left in the pipe could also heat up and then cause cracking

at weld joints or bends.

Hydrogen EmbrittlementThe presence of atomic hydrogen in some metals, particularly in high strength steels and in titanium,

reduces the metal ductility rendering it brittle. This phenomenon is known as hydrogen embrittlement.

Until a steel containing hydrogen actually cracks, there is no permanent damage and in many cases the

original properties of the steel can be restored by suitable heat treatment to bake out the hydrogen.

Stress raisers increase the effect of hydrogen embrittlement.

Hydrogen from corrosion reactions and electro-plating processes is a cause of hydrogen embrittlement.

Cathodic over protection is also thought to pose a risk to high strength quenched and tempered steels.

Chromium plated and cadmium plated high strength steels can fail due to this effect.

4.9 Microbiologically Induced Corrosion (MIC)

Many types of bacteria can live in tanks, vessels, cargo holds, and pipelines in the slimes that often coat

submerged pier supports and sheet piling. Bio-fouling, the build up of small shell fish, crustaceans and

barnacles on submerged structure and ships hulls can also act as centres for bacteria growth.

In optimum environments the organisms reproduce rapidly and large colonies can grow in a few days.During growth the bacteria can convert nutrients into highly corrosive chemicals including hydrogen

sulphide and sulphuric acid.

Colonies of bacteria can also act as deposits and create concentration cells. All of these activities can

lead to increased corrosion in a system. Bacteria slimes can also grow to an extent where they can

completely block pumps and process pipes.

Two principle forms of bacteria are encountered.

Anaerobic Bacteria. These bacteria live and grow where there is no free oxygen. The commonest typesare known as Sulphate Reducing Bacteria (SRBs). These bacteria absorb sulphate from water or other

nutrients such as sewage, various oils and even bitumen coatings. They then reduce the sulphate to

sulphite and finally to hydrogen sulphide that is then free to cause corrosion.



Page 25


28/138


29/138

FIGURE 9 EROSION CORROSION OF PIPE ELBOW

CavitationCavitation is a special form of erosion corrosion. This is caused when vacuum bubbles are created inturbulent flow of high velocity; these bubbles collapse creating small areas of high stress and severe

pitting can occur as pieces of metal are torn out of the surface by the mechanical forces involved in the

implosion of the bubbles.

Cavitation occurs at places where there are large and rapid changes of pressure.

Typical examples are on the trailing edges of impellers and ships propellers or on downstream areas of

high pressure reducing or proportioning valves.

Corrosion FatigueFatigue is the failure of a metal at a stress considerably below its normal yield strength when it is

subjected to continued cyclic stress. If this cyclic stress is imposed in a corrosive environment the fatigue

life of the metal will be substantially reduced. Failure often occurs by intergranular cracking.

The time to failure by fatigue is related to the amplitude of the cyclic stress and the number of stress

cycles that occur. This is shown in Figure 10.

FIGURE 10 FATIGUE AND CORROSION FATIGUE

CORROSION FATIGUE

100

%st

ress

Cycles to Failure

Steel test piece cycles to failure in

a non-corrosive environment

Steel test piece cycles to failure in

a corrosive environment

80

60

40

10 102 3 4 5 6

10 10 1010

EROSION CORROSION OF PIPE ELBOW

Laminar FlowTurbulence

InternalErosion Corrosion

Velocity + 25m/second, GasVelocity +5m/second, Liquid with sand, debris



Page 27


30/138

FIGURE 11 CYCLIC STRESS

A distinguishing feature of corrosion fatigue is the presence of numerous cracks in addition to the one

that caused failure.

Fatigue failures can occur in all types of structures and rotating equipment.

Catastrophic failure of several bulk tankers in recent years has been attributed to corrosion fatigue by a

mechanism similar to that shown in Figure 11.

If considerable structural corrosion has also occurred in the critical stress areas fatigue cracks can

develop and spread until eventually there are so many cracks that the load bearing capability of the

structure is exceeded and a rapid complete failure ensues that can cause the ship to break in half. The

corrosion may be a result of a cargo getting wet or leaking in poorly protected holds.

CYCLIC STRESS

Constant amplitude reverse cyclic stress pattern

Exagerated diagram of wave actions on hull stress and fatigu

(S)Stress

MPa

n = No. of cycles

+

_

Wave Wave

Bending Moment

Wave

Bending Moment in opposite direction



Page 28


31/138

5 METHODS OF CORROSION CONTROL

5.1 Materials selection for corrosion protection

A number of internationally recognised specifications exist for selecting materials. The American Society

produces these for Testing Materials, ATM. The American Iron and Steel Institute AISI. British Standards

Institute BSI and European Standards Institute Euronorm.

5.2 Steels and Irons

Over 80% of ships, marine structures, pipelines and vessels in marine associated industries are made of

carbon or low alloy steel, usually with application of some form of protective coating.

British Standard, Euro Standard and ASTM Specifications are the primary documents for the purchase of

carbon and low alloy steels.

Carbon increases strength but decreases ductility in steels.

Alloys such as manganese confer strength; chromium and molybdenum confer corrosion resistance.

Other alloy elements assist in grain refining and improving machinability.

Some high strength load bearing steels e.g. AISI4340, high strength steel plates and duplex stainless

steel forgings require heat treatment to achieve optimum properties.

ANNEALING; Slow cool in air from a high temperature.NORMALISING: Natural cool in air.QUENCHING AND TEMPERING: Produces tough, high strength structures.Safety critical and very high pressure pump casings and valve bodies are generally made from high

integrity low alloy steel forgings.

For less critical items cast irons can be used. A range of high (15%) nickel cast irons known as Ni-Resists

are excellent for moderately corrosive conditions.

Steels containing more than 12% chromium are classed as stainless steels. 12% to 14% chromium steel

(AISI 405) without any nickel is a typical ferritic stainless steel.

Steels with higher alloy content and many non-ferrous materials are considerably more expensive and

their use has to be fully justified.



Page 29


32/138

Typical Stainless Steel Compositions:

Austenitic stainless steels are prone to pitting and crevice corrosion in chloride waters, especially in the

absence of oxygen which maintains the passive film surface.

5.3 Non-Ferrous Materials

Nickel based AlloysChromium free nickel alloy such as Alloy 400 63% min. Ni, 28-34% Cu.

(Monel 400) and the higher strength K-Monel 63% min Ni, 27-33% Cu and 2.3 -3.3% Al are suitable for

service in neutral and reducing conditions.

They are excellent for use in seawater, fire water systems and many heat exchangers as shafts, impellers

and tubes. They are susceptible to failure in moderate to strong oxidising conditions. Nickel copper alloys

are also susceptible to corrosion by sulphur compounds.

Chromium containing nickel alloys can be used in oxidising conditions and very severe environments,

pump shafts, valve trim and other critical areas.

Suitable materials for these conditions include the following:-

Inconel 600 (UNS N006600) 75%Ni 16%Cr 8%Fe

Inconel 625 (UNS N006625) 61%Ni 22%Cr 9%Mo 5%F

Incoloy 825 42%Ni 21.5%Cr 30%Fe 3%Mo 2%Cu

Hastalloy C276 (UNS 102761) +50%Ni 16%Cr 5%Fe 16%Mo 4%WHigh strength Alloy X750 is used extensively for springs in corrosive service.

Copper AlloysCartridge Brass 70% Cu, 30% Zn ,

This is very ductile and used for low strength, low corrosion resistant fittings and tubing. Finished items

must be stress relieved at 280o

C otherwise they are susceptible to stress corrosion.

Admiralty Brass 70%Cu. 29%Zn, 1%Sn has improved corrosion resistance and is used extensively in heat

exchanger tubes.

Aluminium BronzeCopper aluminium alloys Cu + 2-12% Al have good resistance to corrosion and erosion corrosion and are

very useful in seawater as castings and forgings.

Type of Steel A ISI or UNS NoGrade Composition

Cr C Ni Mn Mo Si N

Austenitic 304L 18-20 0.03 8-12 2 - 1 -

Austenitic 316L 16-18 0.03 10-14 2 2-3 -

Duplex S310803 21-23 0.03 4.5-6.5 2 2.5-3.5 0.08-2

Duplex

(Feralium 255)S32550 24-27 0.04 4.5-6.5 1.5 2.9-3.9 0.1-0.25

Super Duplex

(Zeron 100)S32760 24-26 0.03 6-8 1 3-4 0.2-0.3

Super Austenitic N08028 26-28 0.03 39.5-42.5 2.53-4

(Sanicro 28)



Page 30


33/138

Copper NickelCopper Nickel 90/10 CuNi and 70/30 CuNi alloys are widely used for seawater piping, condenser tubing

and firewater pipework.

Aluminium AlloysLightweight alloys with a wide range of strengths are available. They have moderate corrosion resistance

to seawater. All are susceptible to acid and alkali corrosion with susceptibility increasing with increasing

strength.

lightweight makes aluminium attractive for ships deck housings and structures and some fittings.

Aluminium is also extensively used for the complete hulls of small boats.

Its comparatively low strength and low modulus of rigidity makes it unsuitable for the hulls or highly

stressed sections of larger ships of (say) plus 100 tons dead weight.

Zinc, Cadmium and Magnesium.Zinc is used extensively in the form of hot dipped galvanizing for the protection of steel in seawater and

in marine atmosphere environments. It corrodes at a linear rate directly related to coating thickness and

also provides sacrificial protection to the steel. Galvanizing is more commonly applied on static structures

than on ships. Bolts and fasteners that are galvanized can seize up in seawater due to zinc corrosion

products filling the threads.

Magnesium alloy is sometimes used for fittings on high cost racing boats because of its lightweight and

relatively high strength. High cost and high susceptibility to corrosion make it impractical for most ship

applications.

Zinc and magnesium bars and other shapes are also used as sacrificial anodes on ships hulls, propellers

heat exchangers, subsea piping and marine piling.

CadmiunCadmium is an excellent marine atmosphere protective electro-plating for steel. High toxicity during the

plating process and production of toxic fumes if vaporised by welding has caused it to be replaced by zinc

tin alloys and other materials, less toxic but with generally poorer performance.

5.4 Thermosets and Thermoplastics

Thermoset plastics such as glass, aramid or carbon fibre reinforced epoxies are extensively used in the

manufacture of hulls and deck structures on a wide variety of small to medium sized boats and yachts.

Royal Navy mine sweepers have been made of these materials to avoid magnetic fluxes triggering mines.

The Worlds biggest Composite ship, the yacht Mirabella V with a 75m long fibreglass hall and 90m tall

fibreglass mast was completed in 2003.

The materials have very high resistance to sea water corrosion but cost, strength and fabrication

problems make them unsuitable for very large commercial ships, except as parts of deck structures,

cable trays, walking grids, storage tanks, rigid piping and instrument housings.

ThermoplasticsThermoplastics such as PVC and polyethylene are used for flexible piping and cable ducting..



Page 31


34/138

6 COATINGS

6.1 Types of Coatings

Corrosion management by coatings has been used extensively since the mid 18th century when natural

bitumens were the common protective material.

Major advances in the 20th century were the development of sophisticated epoxies, improved pigments,

faster drying and curing to meet demands for improved performance and faster application and re-coat

times.

Since the 1970s there has been a big demand for improved high performance marine coatings. However

these demands have coincided with new requirements to reduce the toxicity of pigments and solvents

and also to reduce the amount of volatile organic compounds (VOCs) given off to pollute the atmosphere

during coaing application and drying.

These needs conflict with one another, and consequently a large amount of coating and process has had

to be undertaken to develop the wide range of modern coatings.

The main recent development milestone areas are:

Improved resins for chemical and water resistance

Moisture tolerant coatings for applying to wet surfaces

High solids coatings for applying >250m DFT in one coat

100% solids low viscosity resin coatings for high build and low volatile organic compound

(VOC) emission

Chromate and Lead free coatings of low toxicity

Tin free low toxicity environmentally friendly coatings for the protection of ships hulls. These modern

coatings are also designed to keep the hulls free from bio-fouling and achieve low friction in the water,

thereby saving fuel and allowing increased speeds.

Epoxy and polyester powder coatings for high build and zero VOC emission

Glass flake filled epoxies and polyesters for exceptional chemical and abrasion resistance.

Water based high performance coatings for low VOC emissions

Improved quality control and quality assurance on materials and application procedures

There is now a wide choice of both general purpose and highly specialised coatings available to the

Specifier.

6.2 Surface Preparation and Application of Coating

Ship coatings, offshore structures and all steel systems requiring a high standard of corrosion protection

requires abrasive blasting using iron shot, copper slag or grit before being painted.

Abrasive blasting cleans and roughens the metal surface to provide a key for the adhesion of the paint.

If the surface is too smooth the paint will not adhere. If the surface is too rough, then high points may

stick up through the surface of the coating and reduce its efficiency.



Page 32


35/138

Surface ProfileSelecting the correct grade of blasting grit produces the surface profile or peak to valley height. The

profile is required to obtain good paint adhesion even when the surface is clean.

The surface profile is measured as peak to valley height or as a centre line average between the peaks

and valleys as shown in Figure 11.

The profile should be varied to suit the type of coating. Heavy-duty coatings of

+ 250microns dry film thickness need a profile of 50 to 75 microns.

See Section 8 for methods of measuring the profile.

FIGURE 11a

Surface CleanlinessSurfaces must be properly cleaned, by using blasting, grinding, wire brushing, mechanical sanding and

chipping, or solvent washing techniques.

Cleanliness of the surface profile is necessary to get a good bonding of the primer coat. Bare metal

surfaces easily corrode in any humid or moist atmospheric conditions. This type of corrosion results in

the formation of an oxide film, which is not bonded to the metal and interferes with the paint bonding.

Blasting Standards Steels Structures Painting Council of America S.S.P.C.

NACE Standards for surface preparation by Abrasive Blast Cleaning

ISO STD.8501 Standard for Painting Steel Surfaces

British Standard Specification for Surface Finish of Blast Cleaned Steel for Painting. B.S.

7079

Dry Film Thickness (DFT)Once a coating has been applied and has dried, it is necessary to monitor the thickness to ensure that

the specified amount has been applied to the surface.

6.3 Coating Types and Application

Heavy duty two pack coating and lining systems, based on organic resins, are one of the most frequently

used forms of controlling corrosion.

Virtually all external steel work is painted. Underwater and splash zone areas are coated with high build

paint systems. Interiors of many tanks and vessels are lined or coated with specialised resin systems.

SURFACE PROFILE

Peak

Trough

Rogue Peak

Amplitude



Page 33


36/138

Additional coating systems used are the metallic coatings, such as, hot dip galvanising, metal spraying

and electro or electroless plating.

Systems based on organic (carbon containing) resins can be paint coatings or linings.

Paint Coating SystemsPaints are made up from a mixture of solvent resin and pigments. Sometimes special additives called

catalysts and hardeners also have to be added to the paint just before it is used. This depends on the

systems chosen. All paint must be thoroughly mixed to make the paint flow correctly.

Many paint systems consist of three different layers as follows:

a. Primer Coat,

b. Undercoat,

c. Finishing Coat.

Thickness is usually specified in microns (m) or mils (thousandths of one inch) for the dry film thickness

(DFT).

1 mil = 0.001inch = 25 m. 40mils = 1mm

Typical heavy duty coating thickness may vary from 200 to 500 m (5 to 20 mils) in thickness.

Special bituminous coatings, glass flake coatings and 3 layer FBE / PE coatings may be up too 3mm

thick.

Specifying PaintsA European Standard ISO 12944 provides a classification of environments and the paint types and

thickness needed to give various life times in the given environment.

The Standard is not clear for requirements on immersion service and internal pipelines. Some typical

coatings that might be used are as follows:

Ships hull Coating2 coats of Epoxy to a DFT of + 500microns + 1 coat of antifouling paint.

Note: Modern antifouling paints are highly specialised. They contain compounds that are toxic to bio

fouling organisms but have much lower toxicity than the Tri Butyl Tin compounds that were used before

2000. Some coatings rely on self-polishing to retain freedom from bio-fouling. This is the gradual

deliberately engineered loss of paint during service. The paint loss also carries away the fouling

organisms and maintains a clean surface.

It has been estimated that severe fouling of a ships bottom can add 30% to the fuel bill.

Sea water Piling and structures Immersion and Splash Zone1. 3 coat epoxy to a Total Dry film thickness (DFT) of 400 microns

2. 1 coat epoxy primer + 1 coat high build epoxy to a DFT of + 400 microns

3. Coal tar epoxy to 400 microns

4. 2 coats epoxy glass flake to 500 microns (+15 year life)

Marine Atmosphere Coatings1. 2 coats epoxy mastic + 1 coat polyurethane to 350 microns DFT

2. 2 coats epoxy + 1 coat polyurethane to 350 microns DFT



Page 34


37/138

Marine Immersion1. 2 coats of epoxy to a DFT of +500 microns

Buried or Immersed Pipeline External Coating1. Fusion Bonded Epoxy 650 microns

2. Coal Tar Enamel 3mm

3. 3 layer FBE + adhesive + polyethylene (2.5 to 3mm)

Tank Interiors1 coat Epoxy High Build 350 microns DFT

Note. Potable water tanks must have coating certified for use by relevant authority

Pipe Interiors1. Epoxy

2. FBE

3. Cement

4. Polyethylene Liner

The above are only general examples and should not be used for specification purposes.

Some paints and their uses are:Zinc Silicate Primer: Used in damp corrosive conditions where a lot of mechanical damage may occur.

Also heat resisting. Not for permanent immersion.

Epoxy Coatings: Most commonly used high performance systems for external coatings and

many tank linings. Epoxies are sometimes loaded with granite or silica fillers

and applied as non slip abrasion resistant surfaces for decks and floors.

Polyurethane: Used as topcoat on hulls, tank exteriors and platforms to give durable good

appearance. Phenolic / Epoxy Phenolic: Used on the interior of tankscontaining hot solutions.

Alkyds: General purpose paints.

Glass Flake Polyester: Modern Chemical (acid) resistant coatings for vessels and tanks. (Expensive

but long life)

Chlor -rubber: Fairly cheap but soft chemical resistant coating. (now being phased out due

to environmental unacceptability)

Fusion Bonded CoatingsThese coatings consist of dry powdered resins. Epoxy is the most commonly used type of resin in a

system called fusion-bonded epoxy (FBE).

In order to apply the coating, the components have to be pre-heated by an electrical induction heater and

the powdered resin is sprayed onto the surface. The resin melts and spreads over the pipeline as a

viscous liquid before it hardens off due to chemical reactions.

Coating thickness is usually about 70 m (3mils) dry film thickness for indoor metal furniture and up to

800m for severe immersion exposure. (32 mils).

The equipment required to apply the coating is complex and costly and almost all fusion-bonded coatings

are applied to pipe lengths, at a pipe coating mill, before despatch to the site.



Page 35


38/138

Metallic Coatings - GalvanisingDipping steel articles in a bath of molten zinc forms galvanised coatings. The zinc layer formed on the

surface is sacrificial to steel and protects it by galvanic action, even if there is a scratch in the coating.

Although zinc corrodes in air or seawater it does so at a fairly slow rate. Galvanised coatings are used in

industrial and marine atmospheres for steel protection. Heavy duty galvanized coatings are also suitable

for full immersion service. They are not satisfactory in acid or alkaline conditions.

6.4 Effect of Coatings on Cathodic Protection Design

Coatings are usually the principle corrosion protection on submerged or buried structures. However the

coating always contains defects or damage areas and deteriorates further with time. Cathodic protection

(CP) provides the protection required for the damaged coating areas.

The coating also reduces the current demand on the hull or other component to be protected by up to

98% with a very high-grade coating.

6.5 Coating Evaluation and Inspection Measurements

It is often necessary to monitor the condition of a surface as it is being prepared for a coating, as the

coating is being applied, and after the coating has dried and weathered.

The different monitoring conditions need the use of different inspection instruments.

The coating thickness criteria being measured include the following:

a. surface profile

b. surface cleanness

c. wet film thickness

d. climatic conditions

e. destructive thickness

f. dry film thickness

g. porosity or holiday detection

h. adhesion quality

Surface Profile MonitoringThe blasting surface profile can easily be measured in the field by visual comparison with a special

standard set of profiles or by a profile gauge. The latter method uses a plastic film that is pressed on the

surface and then peeled off to provide a replica of the surface.

The replica is then measured for a change in thickness from its original condition, by a specialmicrometer. This thickness change corresponds to the peak to valley height of the actual surface.

Surface CleanlinessSurfaces must be properly cleaned, by using blasting, grinding, wire brushing, mechanical sanding and

chipping, or solvent washing techniques.

Dry Film ThicknessOnce a coating has been applied and has dried, it is necessary to monitor the thickness to ensure that

the specified amount has been applied to the surface.

Magnetic dry film gauges are able to measure the thickness of non magnetic coatings such as paint,

epoxy resin, glass, zinc and plating substances, and of non-conductive coatings such as glass fibre,



Page 36


39/138

rubber, plastic, and polyurethane sheeting on magnetic surfaces (carbon steel, but not stainless steel).

These gauges work on the principle that magnetic forces are reduced as the magnet is moved further

away from the steel.

Another type of thickness measuring devise is the Eddy Current Gauge that can be used on magnetic or

non-magnetic substrates.

Porosity or Holiday DetectionIdeally, the finishing coat should provide a nonporous protective shield of durable thickness and quality

that will resist penetration by moisture to any undercoats.

The coating may contain pores due to solvent bubbles trapped inside it or due to areas of contaminated

metal surfaces that prevent the coating from bonding.

Holiday detectors are non-destructive test instruments which show the position of pores or very thin

coatings. The technique is used mainly on high performance coatings for water immersion or buried

service.

The painted item to be examined has to be electrically earthed in order to carry out the test. After

earthing the coated area is traversed by a metal brush or metal loop (various designs exist) that is

supplied with a very high voltage input. The voltage can be varied depending on the thickness of the

coating.

When a holiday or thin area is located the electrical insulation of the coating breaks down and a spark

passes from the holiday detector to the suspect area that is marked out for rectification.

Adhesion TestingAdhesion Testing is a form of destructive testing that determines the adhesive or bonding quality of a

coating system. The technique used is based upon the principle of pulling off the coat from the protected

surface material.

Two such methods are; the loaded spring tool that exerts a specific pull on a test dolly that has itself been

glued to the coating surface, and the crosshatch cutting tool.



Page 37


40/138

7. CATHODIC PROTECTION

Cathodic protection (CP.) is one of the most important methods of corrosion control on Ships, jetties,sheet piling, tanks and pipelines.

7.1 Theory

Some of the theory of CP has already been covered in the Sections on General Corrosion and Electro-

chemistry. The Battery Cell as shown in Figure 8 is a simple example of an anode and cathode

For cathodic protection to work there must be an electric circuit for the transport of electrons. This circuit

is usually the metal to be protected, a suitable metal anode, a connecting wire and the soil or water

(electrolyte) in which the system is buried.

Seawater has a very low electrical resistance of 30 ohm cm. Soils have a relatively high electrical

resistance value. Typical values may be 300 - 1,000 ohm cms and 2,000 - 21,000 ohm cms for low and

high resistivity soils respectively.

The composition of the metal surface, such as the presence of mill scale and variations in chemistry

determine where anodes and cathodes will be present in the corrosion risk areas of the structure.

When corrosion is taking place electrons flow through the metal circuits that exist in the different

compositions of metal grains in the plate metal.

The electrons flow through the metal from the anodes to cathodic areas of the pipe surface, the anode

areas corrode as iron ions are released into the water or soil whereas the cathodic areas are protected.

Pitting occurs at the anode areas and eventually the pitting undercuts some cathodic grains that then fall

out of the body of the metal even though they are not corroded. Corrosion then is a continuous process

of actual dissolution of anode areas and undermining and breakdown of cathodic areas.

The electrical potential established between a steel surface and adjacent water or soil is generally in the

range of -0.4V to 650mV when measured against a standard copper/copper sulphate reference cell.

This is the natural corrosion potential of the steel. (Reference cells are discussed later)If a new metal could be introduced into the corrosion circuit and controlled at a potential that causes the

current flow to be reversed on all of the most negative area found on the metal structure that requires

protection, it follows that the new metal would become anodic to the whole of the pipeline.

In this case there would be a flow of positive current from the new metal, through the water or soil onto

the metal that requires protection. The new metal would become the anode and the whole of the system

would be cathodic to it, with the result that the corrosion of the structure would cease. This condition canbe obtained if the structure metal to water or soil potential can be changed to be equal to, or more -ve

than 850mV (-0.85V) with reference to the standard Cu/CuSO4 electrode.

Highly electro-negative metal sacrificial anodes are made from zinc, aluminium or magnesium.

Aluminium is the most popular seawater anode, If zinc is used the weight of nodes required is much

higher than for aluminium. Magnesium produces a higher potential against steel ( - 1.4 to -1.7V) and is

generally only used in soil with a high resistivity.

Positive current flows from the protected structure through insulated wiring to the anode. From the anode

the current flows back through the conductive electrolyte (sea water or soil) onto the surface that is

protected.



Page 38


41/138

The current can be masked by coatings or by corners in the structure. In the following illustration no

current would reach the back of the structure and a separate anode, or set of anodes would be needed

to protect the back.

Also the current only protects the face of the metal that sees the current. The inside face is not

protected all.

The sacrificial anodes are consumed by corrosion instead of the protected structure and the require

replacement at calculated time intervals.

7.2 Impressed Current Cathodic Protection (ICCP)

This is the system used for large boats or large structures.

In this case the anode may be graphite, cast iron, coated titanium or metal oxide.

Applying a DC current to the system, which pushes the electrons around the circuit, creates the flow of

electrons.

The anode material is not consumed by corrosion and ca have a long life. Also it is possible to use much

smaller and lighter anodes than are required for sacrificial protection.

Typical CP installation schematics are shown as follows:

FIGURE 12 SCHEMATIC DIAGRAM OF SACRIFICIAL CP

SCHEMATIC DIAGRAM OF SACRIFICIAL CP

+ve current flow

Zn or AlAnode

Sea Water

Boat orStructure

Note: Only the outer surface is protectedas shown. Also the protective current will

not flow around the back faces of the

boat or structure.



Page 39


42/138

A number of rectangular aluminium anodes are distributed around the hull, below the water line and

attached to the hull by bolting or through a welded doubler plate. Several anodes concentrated around

the stern of the boat can be used to protect the rudder and propeller. Special connections have to used

to ensure a complete electrical circuit. The

propeller requires conductive slip rings on the shaft to ensure a good connection.

FIGURE 13 IMPRESSED CURRENT CP Plan view

A range of reactive metals such as zinc, aluminium and magnesium can be used to provide sacrificial CP

systems while silicon iron, platinised titanium or mixed metal oxides are typical low corrosion rate

impressed current anodes.

Impressed current CP is applied to the hulls of most large ships. Two to six anodes are bolted on the

outside of the hull at carefully selected points. The connecting cables are fed through watertight

insulating glands in the hull to an adjustable direct current source (Transformer /Rectifier or TR) inside

the ship.

The anodes are usually bars, plates or discs of platinised titanium or mixed metal oxide construction.

Relatively small anodes can carry the amp current loading necessary to protect the hull and the anodes

breakdown very slowly by self corrosion. Mechanical damage is one of the main risks and designs must

take this into consideration.

Anode shields, as shown in Figure 13 are important in the design. The dielectric insulators are 3 to 5

times the anode length and their purpose is to prevent the majority of current flow taking the shortest

route to the metal and causing and depleting the flow to points further along the hull.

From shipsPower Supply

-ve return path

+ve +ve

-ve return path

Anode bolts insulated from hull

DC from TR

Anode shield - Fibreglass or paint

ships hull steel plates

Anode

Anode shield - Fibreglass or paint

Conductive path to

return current to TR

Transforma Recliner

+ve current flow onto hull +ve current flow onto hull

+ve current flow onto hull

Anode Anode

IMPRESSED CURRENT CP - Plan view

Impressed current CP for ships hull

Anode installation detail



Page 40


43/138

FIGURE 14

For details of anode assemblies see BS7631 Cathodic Protection. Part 1. 1991.

7.3 Protective Potentials and Potential Measurements

To achieve corrosion protection on steel the specified potential between the ship or structure to be

protected and the surrounding water is generally accepted as being

between -800mV and -1250mV as measured with a silver / silver chloride reference electrode, also

known as a half-cell. The design objective is to try and get all areas of the protected structure to meet

these requirements.

If the structure is in soil a copper / copper sulphate electrode is used instead of the silver chloride type.

This electrode is more stable in soil use.

The potential required against the copper / copper sulphate electrode is between -850 and -1300 mV.

The reference electrodes are used to measure the structure to soil potential.

The structure / water or earth junction forms one half cell and the reference electrode/ water or earth

junction is the other half of the complete cell.

To achieve accurate results a high impedance 10 or 20 meg ohm digital Voltmeter must be used and the

connecting cables must have a low resistance. The Structure is always connected to the +ve connection

of the voltmeter.

reduced currentdensity

reduced currentdensity

high currentdensity

high currentdensity

LARGE CURRENT FLOW VARIATION WITHOUT ANODE SHIELDIMPRESSED CURRENT CP



Page 41


44/138

FIGURE 15 Measuring CP Potentials in Sea Water

Achieving the PotentialA suitable anode has to be selected to achieve the potential. The type depends on mainly on the water or

soil resistivity and the area or length o

Date post:	03-Jun-2018
Category:	Documents
Upload:	mohanakrishnan-rajasekaran
View:	237 times
Download:	1 times

Marine Surveying

Documents