marine - application & maintenance of marine coatings

G U I D A N C E N O T E S O N

the Application and Maintenance of Marine CoatingSystemsS E C O N D E D I T I O N

ourM I S S I O N

quality & environmentalP O L I C Y

The mission of the American Bureau of Shipping is to serve the public

interest as well as the needs of our clients by promoting the security

of life, property and the natural environment primarily through the

development and verification of standards for the design, construction

and operational maintenance of marine-related facilities.

It is the policy of the American Bureau of Shipping to be responsive

to the individual and collective needs of our clients as well as those

of the public at large, to provide quality services in support of our

mission, and to provide our services consistent with international

standards developed to avoid, reduce or control pollution to the

environment.

All of our client commitments, supporting actions, and services

delivered must be recognized as expressions of Quality. We pledge

to monitor our performance as an on-going activity and to strive

for continuous improvement.

We commit to operate consistent with applicable environmental

legislation and regulations and to provide a framework for

establishing and reviewing environmental objectives and targets.

COATING SYSTEMS: A GUIDANCE MANUAL FOR FIELD SURVEYORS

IA B S

G U I D A N C E N O T E S O N

the Application and Maintenance of Marine Coating

Systems

S E C O N D E D I T I O N

First Published 1998Copyright © 1998Reprinted 2004

ABS16855 Northchase Drive USA

Houston, TX 77060


Steel structure of a vessel is prone to corrosion throughout its service life.

Due allowance must be made at the new-building stage, and by periodic maintenance

to provide effective corrosion protection to ensure continued

structural integrity of the vessel.

With the emphasis on coatings in the Enhanced Survey Scheme,

and with the unprecedented acceleration of coating technology,

the field surveyor should have some basic factual knowledge on coating systems.

It is the intent of this guide to meet this need in a straight forward and practical manner.

AA B S

INTRODUCTION


This guide draws on many sources for its contents.

ABS wishes to acknowledge the assistance and guidance provided by all

who contributed to it, in particular the following active members

of the ABS Ad Hoc Panel on Coatings:

Bernard Appleman..........................................................Steel Structures Painting Council

Michael Bentkjaer ..........................................................................................Hempel Paints

Phil Birleson ..................................................................................JRS Ship & Offshore

Ron Briggs........................................................................................Matson Navigation

Dennis Buffo.............................................................................Sabine Transportation Co.

Helena Buist ................................................................................................................NACE

Jim Denny ................................................................Courtaulds (International Paints)

Rong Huang .........................................................................................Chevron Shipping

Owen Jones ...............................................................................................Royal Chemical

Joseph Madden ...................................................................................Arco Transportation

Tom McIntyre ............................................................................Marine Transport Lines

Tom Mulligan .........................................................................................Devoe Coatings

Ramesh Raghavan........................................................................................Sigma Coatings

Bruce Sawvel............................................................................................................Esgard

Bob Stanley .........................................................................Maritime Overseas Carriers

Charles Stuckey...............................................................................Drew Ameroid/Ashland

Richard Whiteside ................................................................................BP Oil Shipping Co.

Dave Witmer.........................................BP Oil Shipping, Co, Chairman NACE TC-14B

Frank Windler.......................................................................................Scherwin Williams

Len Zagrzecki........................................................................................................Unitor

- Gus BourneufChief Surveyor

B A B S

ACKNOWLEDGEMENTS


CHAPTER ONE: What is Paint?Paint Technology........................................................................................................................................................................1Binders.........................................................................................................................................................................................2

Thermoset Coatings....................................................................................................................................................2Air Drying Resins.........................................................................................................................................2Oleoresinous Varnishes ...............................................................................................................................2Alkyd Resins.................................................................................................................................................3Epoxy Ester Resins ......................................................................................................................................3Urethane Oil/Alkyd Resins ..........................................................................................................................3Silcone Alkyd Resins ...................................................................................................................................3Styrenated and Vinyl Toluenated Alkyd Resins........................................................................................3Epoxy Resins................................................................................................................................................4Polyurethane Resins ....................................................................................................................................4Inorganic Resins ..........................................................................................................................................4

Thermoplastic Coatings..............................................................................................................................................5Chlorinated Rubber Resins .........................................................................................................................5Vinyl Resins .................................................................................................................................................5Bituminous Binders.....................................................................................................................................5

Pigments and Extenders ............................................................................................................................................................6Anticorrosive Pigments...............................................................................................................................................6

Red Lead ......................................................................................................................................................6Zinc Chromate.............................................................................................................................................6Zinc Phosphate............................................................................................................................................7Zinc ..............................................................................................................................................................7Barrier Pigments..........................................................................................................................................7Coloring Pigments ......................................................................................................................................8Extended Pigments.....................................................................................................................................8

Solvents .......................................................................................................................................................................................9Other Paint Additivites .............................................................................................................................................10

Typical Groups of Paint Additives ...........................................................................................................10

CHAPTER TWO: CorrosionIntroduction ..............................................................................................................................................................................11

Steel and the Corrosion Reaction ...........................................................................................................................12Principals of Corrosion.............................................................................................................................................13Corrosion in Humid Environment...........................................................................................................................13Corrosion in Acidic Environment ............................................................................................................................14Galvanic or Bi-metallic Corrosion ...........................................................................................................................14

Types of Corrosion ...................................................................................................................................................................16Uniform Corrosion....................................................................................................................................................16Pitting Corrosion ......................................................................................................................................................16Crevice Corrosion......................................................................................................................................................16Deposition Corrosion................................................................................................................................................16

Microbiologically Influenced Corrosion (MIC) .......................................................................................................................17Types of Bacteria that Cause Corrosion .................................................................................................................17Detecting Bacteria ....................................................................................................................................................17Coatings and MIC.....................................................................................................................................................18Protection from MIC ................................................................................................................................................18

CHAPTER THREE: Paints for PurposesAnticorrosives............................................................................................................................................................................19

Barrier Effect.............................................................................................................................................................19Inhibitor Effect .........................................................................................................................................................20Galvanic Effect..........................................................................................................................................................20

CA B S

CONTENTS


Shop Primers .............................................................................................................................................................................21Demands....................................................................................................................................................................21Properities..................................................................................................................................................................21Types of Shop Primers .............................................................................................................................................22

Antifouling Paints ....................................................................................................................................................................23Fouling ......................................................................................................................................................................23The Organisms ..........................................................................................................................................................23Micro-Organisms.......................................................................................................................................................23Macro-Organisms......................................................................................................................................................23Distribution ...............................................................................................................................................................23Antifouling Paints.....................................................................................................................................................24Classification of Antifouling Paints ........................................................................................................................24Soluble Matrix (non-polishing) ...............................................................................................................................24Insoluble Matrix (non-polishing) ............................................................................................................................25Self Polishing ............................................................................................................................................................26Roughness .................................................................................................................................................................26

CHAPTER FOUR: Surface PreparationSolvent Cleaning (SSPC-SP-1).................................................................................................................................28Hand Tool Cleaning (SSPC-SP-2) ...........................................................................................................................28Power Tool Cleaning (SSPC-SP-3) ..........................................................................................................................29

Rotary Wire Brushing................................................................................................................................29Mechanical Descaling ...............................................................................................................................29Rotary Power Discing ...............................................................................................................................29

Abrasive Blast Cleaning (SSPC-SP-5,6,7,10) ..........................................................................................................30SSPC-SP-5, NACE No. 1, Swedish Sa 3..................................................................................................30SSPC-SP-10, NACE No. 2, Swedish Sa 2 1/2.........................................................................................30SSPC-SP-6, NACE No. 3, Swedish Sa 2..................................................................................................30SSPC-SP-7, NACE No. 4, Swedish Sa 1..................................................................................................30

Water Jetting and Hydroblasting............................................................................................................................30

CHAPTER FIVE: Methods of Paint ApplicationMethods of Application ...........................................................................................................................................................31

Brush application......................................................................................................................................................31Roller application......................................................................................................................................................31Conventional spray ..................................................................................................................................................32Airless spray...............................................................................................................................................................32High-Volume, Low Pressure Spray..........................................................................................................................32

CHAPTER SIX: Alternatives to “Hard” Coatings & Cathodic ProtectionChemistry ..................................................................................................................................................................................34Method of Protection ..............................................................................................................................................................34Thickness...................................................................................................................................................................................34Opacity ......................................................................................................................................................................................34Preparation................................................................................................................................................................................34Application................................................................................................................................................................................34Cathodic Protection .................................................................................................................................................................35

CHAPTER SEVEN: Glossary of Frequently Used Coating Terms ........................................................................37

CHAPTER EIGHT: Assessment Scale for BreakdownAssessment of Existing Surface Coating Systems .................................................................................................................39

CHAPTER NINE: How to Use this Guide ..........................................................................................................................41

CHAPTER TEN: Examples...........................................................................................................................................................43

D A B S


Paint can be described as a liquid material capable of being applied or spread over a solid surface in which itsubsequently dries or hardens to form a continuous adherent barrier coat. In the past, paint technology and paint making were arts or crafts developed over many years and supported by results of practical experience.

The performance limitations of the paints produced were basically attributed to the available raw materials. These were predominantly of natural origin. For example, oxides of iron were used as pigments and various blends of vegetable oils and natural resins used as binders. These traditional coatings, however, did generally fulfill the accepted demands of time.

Significant advances in paint technology came about with the demand for higher performance and longer life coatings.These were eventually realized by the progressive introduction and development of synthetic raw materials and intermediates helping to alleviate the restrictions imposed on the paint chemist by traditional technology, and to establish a much more scientifically based industry. Reproducible products with predictable performance resulted.

The modern surface coating industry provides many different generic types of coatings used in many different circumstances and applied by many different methods. These range from conventional liquid paints applied by brush and drying at ambient temperatures by oxidation, to powder coatings applied by electrostatic spray and cured by heat.

The following notes describe the principle components of paints, their functions, and the properties they impart to the finished product.

PAINT TECHNOLOGY

Paints are mixtures of many raw materials, each of which in turn has been manufactured to give certain specific properties.Basically, however, paints consist of three major components and many additives which are included in minor properties.

The major components are:

• Binder (other terms used include: vehicle, medium, resin, film, polymer)• Pigment and extender• Solvent

Of these, only the first two form the final dry paint film. Solvent is necessary purely to facilitate application and initial filmformation; it leaves the film by evaporation and can therefore be considered an expensive waste product.

1A B S

CHAPTER ONE: What is Paint?


BINDERS

Binders are the film forming components of paint. They are predominant in determining the principle characteristics of thecoating, both physical and chemical. Paints are generally named after their binder component (e.g. epoxy paints, chlorinatedrubber paints, alkyd paints, etc.). The function of the binder is to give a permanent continuous film which is responsible foradhesion to the surface and which will contribute to the overall resistance of the coating to the environment.

Binders used in the manufacture of paints fall into two classes, Thermoset and Thermoplastic. This classification is solelydependent upon how they form a film, and whether that film formation is reversible. In the case of liquid paints, theychange state, i.e. from a liquid to a solid. This transformation in paint is known as drying or curing.

It will be readily appreciated that a Thermoset coating when dry will be chemically quite different from the paint in the can.Thermoset coatings are not affected by solvent wipe, once cured. With a Thermoplastic coating, the dry film and the wetpaint differ only in solvent content, but chemically these remain essentially similar. If solvent is applied to a thermoplasticcoating, it will soften and try to return to its original state.

THEROSET COATINGS

In liquid paints where solvent is involved, drying is considered a two stage process. Both stages actually occur together but at different rates.

• Stage One: Solvent is lost from the film by evaporation and the film becomes dry to touch.

• Stage Two: The film progressively becomes more chemically complex by one of the following methods:

1) Reaction with atmospheric oxygen, known as oxidation.2) Reaction with an added chemical curing agent.3) Reaction with water (moisture in the atmosphere).4) Artificial heating.5) Radiation curing (e.g. ultraviolet).

The films formed by the above methods are chemically different to the original binders and will not re-dissolve in their original solvent.

Air Drying ResinsHistorically, binders were based on various drying oils, either vegetable or animal. These materials are slow drying, cross-linking of the molecules taking place slowly in the presence of atmospheric oxygen. The addition of various metal catalystsnormally referred to as dryers can significantly accelerate the drying process. Vegetable drying oils alone are unsuitable aspaint binders. To obtain the optimum film properties, it is necessary to modify the oil with a range of natural or syntheticresins. The following notes describe the various modifications of vegetable oils to produce suitable paint binders, all of which dry by oxidation.

Oleoresinous VarnishesThese materials are developed from vegetable oils where the oil is reacted, or “cooked” with other compatible resins, e.g.rosin, ester gum, coumarone or phenolic resins. The introduction of the resin component upgrades the film properties andimproves hardness, gloss, drying time, and weathering characteristics. The description of the varnish is normally based on theamount and type of oil present. Normally only the more “oily” materials are used for exterior primers and finishes, wheregood adhesion and flexibility are important requirements. One of the more important classes of binders from this group arethe tung oil phenolics. These are used widely in both primers and finishes particularly where improved water and chemicalresistance are required from a conventional coating.

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Alkyd ResinsThese are the most widely used air drying resins. Unlike oleoresinous materials, they can be described as pure polyesters. They are formed by the reaction between a special organic acid (e.g. Phthalic acid), a special alcohol (e.g. Glycerol orPentaerythritol), and a vegetable oil or its fatty acids. The final properties of the alkyd depend on the percentage of oil (termed ‘oil length’) and also on the alcohol and organic acid used. Typical variations are shown as follows:

Long oil length alkyds (60-80% oil)Normally used in gloss finishes for brush application and also in primers for hand prepared steel.

Medium oil length alkyds (40-60% oil)Used in undercoats or in quick drying finishes where appearance and flexibility are of less importance.

Short oil length alkyds (less than 40% oil)Suitable as binders for quick drying paints. Only soluble in stronger solvents like xylol (aromatic solvent). Not normallysuitable for brushing paints.

Alkyds are not resistant to acids or alkalis and many of the modifications given below are aimed at improving this weakness,however, none provide complete resistance.

Epoxy Ester ResinsSometimes called one-pack epoxies, these resins are more closely allied in performance properties to alkyds. They aremanufactured by reacting fatty acids from vegetable oils with high molecular weight epoxy resins. The resulting materialsgive similar properties to alkyds with improved chemical resistance, but have poorer weathering properties. Epoxy ester paintsalso suffer from ‘chalking’, an effect caused by degradation of the uppermost layers of the paint by UV radiation. This results in a premature loss of gloss.

Urethane Oil/Alkyd ResinsOften referred to as a one-pack polyurethanes, but as with epoxy esters, it is better to consider them as modified alkyds. They are formed by the reaction between an alkyd and isocyanate to produce a product with improved drying, gloss, andhardness over the corresponding alkyd but which can sometimes be more difficult to overcoat after aging.

Silicone Alkyd ResinsThe modification of an alkyd with a silicon resin results in a material which probably has the best retention propertiesavailable in resistance, being stable to temperatures in excess of 200oC. Unfortunately, due to the high cost of silicone resins,silicone alkyds are three to four times more expensive than conventional alkyds. This severely limits their use.

Styrenated and Vinyl Toluenated Alkyd ResinsReaction of alkyds with either styrene or vinyl toluene monomers produces rapid drying resins with good chemical resistance,however they have a tendency to yellow on aging and their exterior durability is inferior to that of conventional alkyds.

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Epoxy ResinsThese resins are particularly important, and their development for use as binders was one of the most significant advances inpaint technology. They are examples of the modern approach to synthetic resins, and provide high polymer materials ofpredetermined structure. These have predictable properties which can be tailored to suit numerous uses.

The resins are produced by the condensation or polymerization of epichlorhydrin and diphenylol propane (bis-phenol ‘A’). The reaction conditions and relative proportions of the reactants determine the properties of the final product. For example,the molecular weight or chain length, and the number of reactive cross-linkable groups can be varied with wide limits. Thusepoxy resins can vary from low molecular weight, low viscosity liquids at room temperature, to fairly high molecular weighthigh melting point solids. Situated at the terminal ends of each resin molecule are cyclic epoxide groups which cross-link bychemically reacting with added curing agents such as amines, amine adducts, and polymides. Liquid epoxy resins give highcross-link densities whereas solid materials have fewer reactive groups resulting in a less cross-linked film. In fact, some veryhigh molecular weight epoxy resins are used as uncured one-pack coatings.

The rate of cross-linking or curing is dependent on temperature. Below 5oC the curing rate is considerably reduced, and toobtain optimal film properties, full cure is essential.

The choice of curing agent is very important as this determines the principle resistance properties of the film. There is a widechoice of both resins and curing agents which allows for formulation of products to suit most applications.

Polyurethane ResinsThese are polymers formed by reaction between hydroxy compounds and compounds containing isocyanates. The resins areavailable in both one-pack and two-pack forms. The one-pack material is based on a resin which has been partially reactedto give a prepolymer; when applied as a film, further reaction with atmospheric moisture occurs, giving full polymerization or film cure.

In two-pack systems a special polyether or polyester resin with free hydroxyl groups is reacted with a high molecular weightisocyanate curing agent. A major problem with these materials is their water sensitivity on storage and one application.

Polyurethane resins have excellent chemical and solvent resistance and are superior to standard epoxies in acid resistance.Polyurethane finish coats are very hard and have extremely good gloss, gloss retention, and can be formulated to be non-yellowing. In view of all these good film properties, polyurethane paints are considered to be the best all-around coatingsproduced. Unfortunately they are expensive and can be difficult to overcoat after aging and require very clean surfaces foroptimum adhesion. Because of the isocyanate curing agent there is also a health hazard when sprayed. This health hazard hasbeen reduced by minimizing the presence of “free” isocyantes.

Inorganic ResinsThese types comprise the silicates which are almost always used in conjunction with zinc dust. There are inorganic silicatesbased on lithium, potassium, or sodium silicate and organic silicates normally based on ethyl silicate. Inorganic types are byfar the most commonly used. Coatings based on these resins are very hard, corrosion resistant, and temperature resistant.

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THERMOPLASTIC COATINGS

These types of paint binders are simple solutions of various resins or polymers dissolved in suitable solvent(s). Drying is simply effected by the loss of the solvent by evaporation. This is termed physical drying as no chemical change takes place.The resulting film is therefore always readily soluble in the original solvent and can also be softened by heat. Since thesecoatings, by definition, require the presence of significant amounts of solvent, they are disappearing from markets wherevolatile organic content is regulated, particularly the USA. Generic types of binders in this category include:

• Chlorinated Rubber Resins• Vinyl Resins• Bituminous Binders• Cellulose Derivatives

Chlorinated Rubber ResinsThese are normally considered to be chlorinated polyisoprene (polyisoprene being synthetic rubber) but also included in thegroup are chlorinated polypropylene and chlorinated polyethylene. All of these polymers are similar in their general properties.

The resins are dissolved in aromatic hydrocarbon solvents like xylene, and form solutions which are film-forming in their ownright. However, they produce brittle films. Thus, in most formulations, high levels of plasticiser are required, normally between30% and 50% of the total binder. The plasticisers normally used are chlorinated paraffin.

Chlorinate rubber resins have very good chemical and water resistance. These properties are somewhat adversely affected bythe addition of plasticiser and the chemical make-up of these binders provides the reason for their poor resistance to someoils and greases. Another inherent property of the binder is its thermoplasticity which makes these coatings unsuitable for useat temperatures above 80oC. This temperature sensitivity can lead to various film defects when used in very hot climates. Inaddition, white and pale colors have a pronounced tendency to yellow when exposed to bright sunlight. Due to the viscosityof chlorinated rubber resin solutions, it is only possible to make paints with maximum volume solids of about 45%. Forspraying versions, much lower volume solids are necessary. Chlorinated rubber paints will dry at low temperatures and giveexcellent intercoat adhesion in both freshly applied and aged systems, making them particularly suitable and popular formaintenance purposes.

Vinyl ResinsThese are based on film forming polymers consisting of varying ratios of polyvinyl chloride, polyvinyl acetate, and polyvinylalcohol. Composition of the final polymer depends on the ratios of the components used in the polymerization. Whereaschlorinated rubbers are soluble in aromatic solvents, vinyl resins require ketone or ester solvents. Like chlorinated rubbers,vinyls form brittle films and therefore require plasticising. The plasticising level necessary is about 20-25% of the total resincomponent. Plasticiser types used are tricresyl phosphate or dioctyl phthalate. The volume of solids in vinyl resin paints arerelatively low because of their high solution viscosity. No more than about 25-30% volume solids can be achieved withoutthe paint becoming too thick for application. This obviously creates a problem in formulating high build coatings. Highervolume solid materials can be produced by blending the vinyl resin with other materials such as acrylic resins. Thesemodifications detract from some of the more positive properties of the vinyl resins, such as chemical and water resistance.Generally the film properties and weathering characteristics also show good low temperature drying and excellent intercoatadhesion characteristics.

Bituminous BindersAsphaltum and similar materials have been used for thousands of years as water proofing systems. The majority of materialsused today in paint form are based on either petroleum bitumen or coal tar pitch. The latter material has the better waterresistance and is more chemically complex requiring stronger solvents for solution. Pigmentation of both types produces easy to use cheap products. Because of their inherent composition, only dark colored paints can be made.

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PIGMENTS AND EXTENDERS

Pigments and extenders are used in paints in the form of fine powders. These are dispersed into the binder to particle sizesof about 5-10 microns for finishing paints and approximately 50 microns for primers.

These materials can be divided into the following types:

Type Purpose

Anticorrosive pigments To prevent corrosion of metals by chemical and electrochemical means.

Barrier pigments To increase impermeability of the paint film

Coloring pigments To give permanent color

Extending pigments To help give film properties required

ANTICORROSIVE PIGMENTS

Red LeadThis is probably the best known anticorrosive pigment, however, because of environmental and health considerations, lead is not used on coatings in most developed countries. When used in conjunction with linseed oil it gives excellent corrosionprotection. The afforded is due to the lead complexes (‘soaps’) which suppress the corrosion reaction of steel, while alsoreinforcing the film and giving good penetration and adhesion even to weathered corroded steel. The lead soaps are notformed with “non-oil” binders, consequently in quick drying binders based on chlorinated rubber, vinyl or epoxy resins redlead is much less effective. The slow drying time of the linseed oil, together with the toxicity of the red lead, drasticallyrestricts the use of this type of coating.

Other anticorrosive pigments based on lead are metallic lead. It depends largely on its own inertness, but the actualmechanism of corrosion protection is uncertain, calcium plumbate is often used for use on galvanized surfaces. All of thesetraditional pigments are decreasing in importance for the same reasons as red lead.

Zinc ChromateThere are two types, (1) zinc potassium chromate, (2) zinc tetroxy chromate. Both function as anticorrosive pigments byreleasing water soluble chromate ions which help to passivate the steel surface. They are used in a wide variety of bindersand are not restricted to oil based types. Once again, health considerations out weigh the benefits “heavy metals “ bring to coatings, therefore, the use of chromates is becoming rare.

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Zinc PhosphateThis is also a widely used anticorrosive pigment although the actual mechanism of protection is uncertain. However, it isthought that under normal exposure condition protection is afforded by a barrier effect. This is reflected in the necessity ofhaving to have high pigmentation levels to give adequate protection. In acidic atmospheres, zinc phosphate primers performsurprisingly well, and it is assumed that this is because the phosphate ion becomes more active in these conditions andelectrochemically protects steel in a way similar to that of chromate ions.

Zinc phosphate can be incorporated into almost any binder, and because of its low opacity or transparent nature, paints ofany color can be produced.

ZincMetallic zinc is widely used in primers giving excellent resistance to corrosion of steel. Initial protection is by galvanic action.However, as the coating is exposed to the atmosphere, a progressive build up of zinc corrosion products occurs, producing animpermeable barrier with little or no galvanic protection. To give good galvanic and barrier protection, high levels of zinc arerequired, about 80% of zinc in dry film. Because corrosion products of zinc with rain water are alkaline, reaction would occurwith saponifiable binders such as alkyds, and consequently these types should not be used as binders for zinc primers.

The most suitable resins are epoxy, silicate, or chlorinated rubber. Obviously, for the zinc to function correctly, it has to be inintimate contact with the steel substrate. High degrees of surface cleanliness are therefore necessary.

Other available anticorrosive pigments which have not found general acceptance because of their lack of cost effectiveness are:

• Barium Metaborate• Calcium Molybate• Zinc Molybate• Lead Silicochromate

Barrier PigmentsThe most common types of these pigments are aluminum (leafing aluminum) and micaceous iron oxide (M.I.O.). Both have particle shapes which are termed lamellar (plate-like). These materials are often used in combination, the aluminumlightening the almost black shade of M.I.O. M.I.O. pigmented films have durability, but to achieve this, high levels of M.I.O.are necessary, of the order of 80% if the total pigment. Aluminum has been used for many years as the principle pigment in paints for use underwater, the lamellar shape helping to make the film more water impermeable. The diagram belowillustrates how the impermeability of the film is increased and how the binder is protected, both benefits being promoted by the lamellar structure of the incorporate pigment. Glass flake is also often used as a barrier pigment.

Figure 1-1

7A B S

UV W ATER

DIRECTPATH

NON-LAMELLAR PIGMENTS

UV W ATER

LAMELLAR PIGMENTS


Coloring PigmentsThese pigments provide both color and opacity and can be divided into either inorganic or organic types. Inorganic pigmentscan be either naturally occurring or synthetically produced whereas all the organic pigments are nowadays syntheticallymanufactured.

Figure 1-2

The most common coloring pigment is titanium dioxide, which is white. Commonly used inorganic pigments include oxides of iron, which can be black, red-brown or yellow-brown in color and lead chromates which can vary from yellow to scarlet.Yellows, blues, greens, and reds are more commonly obtained as organic pigments. They are generally bright in color andtheir low toxicity makes them more generally acceptable for decorative coatings. In paint, all pigments are normally dispersedto a very fine particle size in order to give maximum color and opacity (hiding power).

Extended PigmentsMore commonly known as extenders, and as the name suggests, they basically adjust or “extend” the pigmentation of thepaint until the required pigment volume concentration (PVC) is achieved. They are all inorganic powders with various particleshapes and sizes. Although making little or no contribution to the color opacity of the paint, they can have significantinfluence on physical properties. These include flow, degree of gloss, anti-settling properties, sprayablility, water and chemicalresistance, mechanical strength and hardness, firm build (volume solids, hold up thixotrophy). Mixtures of extenders are oftenused to obtain the desired properties. They are relatively inexpensive when compared to resins, anticorrosive pigments andcoloring pigments.

Common Extenders

Extender Uses

Barytes (Barium Sulphate) A medium hard powder which helps to reinforce the film.

China Clay Sometimes used to vary the level of gloss. Has a relatively high oil absorption.

MICA The lamellar properties of this material enable it to act to some extent as a barrier pigment to reduce permeability. It can also improve film durability by preventing cracking.

TALC Can have a similar effect to MICA but to a lesser degree.

8 A B S

INORGANIC

NATURAL

ORGANIC

SYNTHETIC


SOLVENTS

Solvents are used in paints principally to facilitate application. Their function is to dissolve the binder and consequentlyreduce the viscosity of the paint to a level which is suitable for the various methods of application, i.e. brush, roller,conventional spray, airless spray, dipping, etc. After application, the solvent evaporates and plays no further part in the finalpaint film, the solvent therefore becomes a high cost waste material. Liquids used as solvents in paints can be described inone of three ways:

True SolventsA liquid which will dissolve the binder and is completely compatible with it.

Latent SolventA liquid which is not a true solvent. However, when mixed with a true solvent, the mix has stronger dissolving properties than the true solvent alone.

Diluent SolventA liquid which is not a true solvent. Normally used as a blend with true solvent/latent solvent mixes to reduce the cost.Binders will only tolerate a limited quantity of diluent.

There are numerous solvents used in the paint industry. This is partly due to the number of different properties which have to be considered when selecting a solvent or solvent mixture. In addition to commercial factors such as price and availability,these include toxicity, volatility, flammability, odor, compatibility, and suitability.1

The table below outlines typical solvents and their uses.

Solvent Type Typical Solvent Name Typical Paint Types

Aliphatic White Spirit Most conventional paints based on vegetable oils, e.g. alkyds.

Aromatic Tobuol Quick drying primers for automatic plants.

Xylol Modified alkyds. Chlorinated rubbers, some stoving paints. A diluent solvent for epoxies, vinyl, polyurethanes.

Ketones Acetone Quick drying primers for automatic plants.

Methylethy Ketone (MEK) True solvent for vinyls. Quick drying primers.

Methyl Isobutyl Ketone (MIBK) True solvent for vinyls used sometimes for epoxies.

Cyckihexanone A slow evaporating true solvent to give good flow in vinyls and epoxies.

Alcohol Isopropanol Latent solvents for vinyls, wash and etch primers.

Butanol Some stoving paints used for epoxies in conjunction with aromatics.

Esters Butyl Acetate, Cellosolve Acetate Polyurethane and vinyls.

Water Water Emulsion paints-some special epoxies.

1 In certain countries certain types of solvents are not allowed. This is especially true in the USA, where the Hazardous Air PollutantSubstances Act, (HAPS) dictates a time line for removing many solvents and extenders from coatings. Application properties, drytimes, and overcoat windows will most likely be affected as this AACT is implemented.

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OTHER PAINT ADDITIVES

Modern paints along with the principle ingredients, i.e. pigment/binder/solvent, require various other additives which aidmanufacture, shelf life, application, film formation, film curing and film properties of the paint. The term additive covers a very wide range of materials which are essential to good formulation, but which are used in minor proportions of the whole paint.

Typical Groups of Paint Additives

Aids to Manufacture• Dispersion agents• De-foamers

Aids to Shelf Life• Stabilizers• Antisetting agents• Antiskinning agents (antioxidants)• Preservatives• Thickening agents• Moisture absorbers

Aids to Application• Flow promoting agents• Solvent retarders• Conductivity controllers• Antistatic agents

Aids to Smoothness• Antifloating agents• Pattern additives• Matting agents• Thixotropes• Antigas Checking agents

Aid to Film Curing• Driers• Curing agents/catalysts• Adhesion promoters

Aids to Film Formation• Heat stabilizers• Fire retarders• Optical brighteners• Slip and anti-slip agents• Anti-fouling agents• Fungicides• Bactericides• Insecticides• Absorbers• Antiscuff agents• Corrosion inhibitors

Others• Deodorants• Flash point controllers

10 A B S


Corrosion of metals may be defined as an electromechanical process in which the metal reacts with its environment to form an oxide, or other compound, similar to the ore from which it was originally won. The strength of any metalis determined by the arrangement of atoms within the metal, but it is the atomic structure at its very surface that

determines its tendency to corrode.

Figure 2-1: The Reversion Reaction

The majority of metals are found in nature in the mineral state, that is, in their stable oxidized condition as oxides, chlorides,carbonates, sulfates, sulfides, etc. The extraction of a metal from the appropriate mineral involves a reduction process inwhich a great deal of energy is absorbed. As a consequence of this large energy input the metal is in a high energy conditionand will endeavor to return to its former stable oxidized low energy state as quickly as environmental conditions will allow. Itis this energy difference between the pure metal and its oxidized forms which is the driving force for corrosion of the metal.Many corrosion products show a chemical similarity to the corresponding minerals. Iron, for example, is extracted from itsores, mainly oxide and carbonate, by reduction with carbon in a blast furnace. In the presence of moisture the iron metal soobtained is oxidized rust, which if analyzed is found to have a composition similar to the mineral ore.

11A B S

CHAPTER TWO:Corrosion

IRON ORE

Iron Oxide

REVERSIONREA CTION

HEA T IRON METAL

OXYGEN W ATER

RUSTIRON OXIDE

Chemically similar to origin


Steel and the Corrosion Reaction

All surfaces strive to achieve a certain balance, equilibrium, with their surroundings. On the atomic scale there is no distinctborderline between a metal and its environment. The metal will surrender atoms to its environment and in return absorbsome atoms from the environment.

Metal atoms leave the metal and they go into solution as positively charged metal ions, leaving behind negatively chargedelectrons. Consequently, the metal becomes more negatively charged. This makes it increasingly difficult or impossible for the remaining metal atoms to escape as positively charged ions as they are being held by the negative charge of the metal.

For the process of releasing atoms as positively charged ions to continue, or for the corrosion process to continue, the excessof electrons in the metal must be done away with. The manner and speed with which these excess electrons can be removeddetermines the rate of corrosion.

The important step in the corrosion of steel is the transformation of a metal atom to metal ion by the loss of two electrons.This is the anodic reaction:

(1) Fe = Fe++ + 2e

IRON ATOM = IRON ATOM ELECTRONS

(FERROUS)

This reaction can only occur if there is a suitable electron acceptor to combine with the electrons released by the ion atom.Freshwater or seawater contain dissolved atmospheric oxygen which readily serves this purpose. The oxygen is electrochemicallyreduced to hydroxyl ions in the cathodic reaction.

(2) O2 + 2H2O + 4e = 4OH

OXYGEN WATER ELECTRONS = HYDROXYL IONS

The heterogeneous character of the metal surface allows for some areas or sites to favor reaction (1) anodes and othersreaction (2) cathodes. The whole surface of the metal is therefore divided up into large numbers of anodes and cathodes.

Figure 2-2 shows the progress of corrosion where metal is lost from the anode, causing it to progressively become thinner,accompanied by a flow of electrons from the anode to the cathode which in turn react with both oxygen and water to formhydroxyl ions.

Figure 2-2

12 A B S

Mild Steel W ater

AnodeFe

e

e

OH Ð

O2

Fe++

OH ÐO2

Cathode

Cathode


(3) Fe+ + 2OH = Fe (OH)2

FERROUS IONS HYDROXYL IONS = FERROUS HYDROXIDE

Ferrous hydroxide in the presence of an abundant supply of oxygen is oxidized to the familiar reddish brown rust.

(4) 2Fe (OH)2 + O2 = Fe2O32H2O

FERROUS HYDROXIDE OXYGEN = RUST

The above reactions, (1) (2) (3) (4), are the basic reactions which occur when iron or steel transforms to rust or, more specifically, to iron oxide. In practical situations the process is not so simple. For instance, corrosion of steel in seawater or in pollutedatmospheres results in more rapid and complicated reactions producing corrosion products in association with iron oxide. Some of these iron salts are water soluble and can cause major problems to paint coatings if not removed before application.

Principles of Corrosion

Corrosion in Humid Environment

Steel immersed in seawater or exposed to high humidity will corrode as shown in Figure 2-3.

Figure 2-3

Areas from where iron ions go into solution (the anodes) and areas where electrons are consumed in a reaction with theenvironment (the cathodes), may continuously change resulting in a comparatively even corrosion.

In an electrolyte of seawater, there is nothing to retard the dissolution of iron, and the rate or speed of corrosion is almostexclusively determined by the rate at which oxygen reaches the cathode areas.

Hydroxyl ions and iron ions with the addition of oxygen react further to form iron hydroxide, a major component of rust.

In given environments certain metals will form protective corrosion products, oxides, that prevent or retard further corrosion.Aluminum and stainless steel are examples of this.

13A B S

eÐ

eÐ

eÐ

eÐ

eÐ

eÐ

eÐeÐ

eÐ

eÐ

eÐ

eÐ

O2

2H2O

4OH Ð

Fe++

Fe++

Fe++

Fe++

Fe++

Fe++


Corrosion in an Acidic Environment

In a very acidic environment, such as diluted sulfuric acid, steel will corrode evenly while creating hydrogen gas. See figure 2-4.

Figure 2-4

The rate of corrosion of steel in an acid solution is much higher than in a neutral environment, partly because the hydrogengas easily and quickly escapes the steel surface and partly because the corrosion products are soluble in acid. This preventsthe formation of a rust layer that could retard the corrosion process.

Galvanic or Bi-metallic Corrosion

Since steel is less noble, i.e. releases electrons more easily than copper, metallic contact between the two would cause anyexcess electrons to travel from the steel to the copper. See figure 2-5.

Figure 2-5

In accordance with the traditional concept, an electrical current travels from the copper to the steel (from the positivecathode to the negative anode).

The cathode will strive to rid itself of the excess electrons it receives. The anode, on the other hand, will strive to regain it’snegative potential by releasing more metal ions, corroding. An expression for the rate of corrosion is the flow of electricalcurrent that can be maintained through the electron consuming process on the cathode, and the electron generating processon the anode.

14 A B S

H2eÐ

eÐ

Fe++

Fe++

Fe++

Fe++

Fe++

eÐ

eÐeÐ

eÐ

eÐ

eÐ

eÐ

eÐ

eÐ eÐ

eÐ eÐ

2H+

STEEL COPPERÐ +

H2O 2O2Fe++

eÐ eÐ

eÐeÐeÐ

eÐeÐ

eÐ

eÐeÐ

eÐ

eÐeÐeÐeÐ

Fe++

Fe++Fe++

Fe++Fe++

Fe++

Fe++

Fe++

Fe++

4OH Ð

eÐ


Corrosion of the steel is accelerated through its contact with copper. The steel suffers galvanic corrosion while the copper isprotected cathodically. The most positive (noble) material will be protected against corrosion at the cost of the most negative(ignoble) material.

See figure 2-6 table of materials in the Galvanic Series.

Figure 2-6 Galvanic Series

15A B S

+0.2 0 Ð0.2 Ð0.4 Ð0.6 Ð0.8 Ð1.0 Ð1.2 Ð1.4 Ð1.6

Alloys are listed in the order of the potential they exhibit in flowing seawater. Certain alloysindicated by the symbol: in low-velocity or poorly aerated water, and at shielded areas,may become active and exhibit a potential near Ð0.5 volts.

Magnesium

Zinc

Berylium

Aluminum Alloys

Cadmium

Mild Steel, Cast Iron

Low Alloy steel

Austanitic Nickel Cast Iron

Aluminum Bronze

Naval Brass, Yellow Brass, Red Brass

Tin

Copper

Pb-Sn Solder (50/50)

Admiralty Brass, Aluminum Brass

Manganese Bronze

Silicone Bronze

Tin Bronzes (G & M)

Stainless Steel Ñ T ypes 410, 416

Nickel Silver

90-10 Copper-Nickel

80-20 Copper-Nickel

Stainless Steel Ñ T ype 430

Lead

70-30 Copper-Nickel

Nickel Aluminum Bronze

Nickel-Chromium Alloy 600

Silver Bronze Alloys

Nickel 200

Silver

Stainless Steel Ñ T ypes 302, 304, 321, 347

Nickel-Copper Alloys 400, K-500

Stainless Steel Ñ T ypes 316, 317

Alloy Ò20Ò Stainless Steels, Cast and Wrought

Nickel-Iron-Chromium Alloy 825

Ni-Cr-Mo-Cu-Si Alloy B

Titanium

Ni-Cr-Mo Alloy C

Platinum

Graphite


Types of Corrosion

Uniform CorrosionThe most common type of corrosion encountered is the general attack of a more or less uniform nature, although the loss ofmetal is concentrated at the anode sites and there is a continual change in the surface with time. With progressive metal loss,areas which were initially anodic cease to be active and new anodic sites take over. There is thus a continuous interchangebetween the anodic and cathodic areas, such that over a period of time the loss of metal over the entire surface is fairlyuniform. This is the easiest form of corrosion to combat or allow for because structural life time can be predicted, a featurewhich is not possible with the following corrosion forms.

Pitting CorrosionThe characteristic of this type of attack is that it is extremely localized and the penetration is deep in relation to the areaattacked. Pitting is one of the most dangerous forms of corrosion and often occurs in places where it cannot be readily seen.Pitting corrosion can be extremely intense on mill scaled steel left outside, as shown in figure 2-7 Pitting Corrosion.

Figure 2-7

Crevice CorrosionIntense localized corrosion, ranging from small pits to extensive attack over the whole surface, can occur within narrowcrevices formed by the geometry of a structure, for example: riveted plates or threaded joints. Crevice corrosion ischaracterized by a geometrical configuration in which the cathode reactant, oxygen, can readily gain access to the metalsurface outside the crevice and less access to the crevice. The metal within the crevice is therefore anodic to the surroundingsteel and suffers preferential corrosion.

Deposition CorrosionThis is a similar type of attack to that occurring in crevices. Wherever loose debris collect, there will be a depletion in acrevice. Consequently, the attack is localized beneath the loose debris or sediment. See figure 2-8 Deposition Corrosion.

Figure 2-8

16 A B S

ACCUMULA TEDDEBRIS

HIGHO2

Anode

Cathode LOW O 2 STEEL

RUST

W et Surface

Mil Scale Cathode

O2Fe2O2

STEEL

Anode

e e

OH Ð OH Ð


Microbiologically Influenced Corrosion (MIC)All metals, even stainless steel, may incur corrosion from microbiologically influenced corrosion, MIC. This type of corrosionhas been in existence for a long time and is either completely overlooked or goes unrecognized. However, it is gainingattention in the marine environment as a leading cause of corrosion in cargo, ballast and void spaces.

Corrosive bacteria thrive in low oxygen or oxygen free environments and require some type of food source. In an idealenvironment microbes may double their mass every 20 minutes, but such conditions rarely exist on board vessels. On boardships microbes can thrive in the water layer at the bottom of oil cargo tanks and in the sediment in ballast tanks. Oncebacteria become established, they become difficult to control and may corrode steel up to 1/16 to 1/8 inch per year.

The reaction of microbes and steel is not very clear, but this much is known in what they can achieve:1. Microbes produce acids2. Destroy coatings3. Create corrosive cells4. Produce hydrogen sulfide

Types of Bacteria that Cause CorrosionThe two most common types of bacteria that cause corrosion are Acid Producing Bacteria (APB) and Sulfate ReducingBacteria (SRB). Both live in colonies attached to the surface of the steel where they assist each other in their growth.

Acid Producing Bacteria ( APB’s )These are generally the first to colonize the surface of the steel inside ballast and cargo tanks. APB’s feed on organiccompounds such as oil and oil products to form organic acids. APB’s thrive in conditions where there is very little oxygen.Stagnant waters in ballast tanks create this condition. In utilizing the residual oxygen about them they create a very acidicenvironment beneath the colony. By consuming the small amount of oxygen about them, an oxygen free environment isdeveloped making conditions ideal for Sulfate Reducing Bacteria to develop.

Sulfate Reducing Bacteria ( SRB’s )Sulfate Reducing Bacteria thrive only in an oxygen free environment. The oxygen they require is extracted by removingoxygen from the sulfate ion in seawater. SRB’s also utilize hydrocarbons as a food source, but prefer the organic acids givenoff by the APB’s. Once the oxygen is removed from the sulfate ion, they discard the sulfide in the form of hydrogen sulfide.The combination of the hydrogen sulfide and organic acids under the colonies create a condition that is highly aggressive to steel and the characteristic craters or pits are formed.

Detecting BacteriaInitial detection is first achieved visually by noting a black slime deposit on the surface of the steel. Also, the detection of Hydrogen Sulfide, rotten egg smell, could possibly be noted.

Corrosion attributed to MIC is almost always highly localized pitting. The pits are generally filled with a black ferrousproduct. The walls of the pit are generally terraced. The metal surface below the corrosion products is often bright and active.

Another method for detecting the presence of MIC can be accomplished by taking samples and sending them to a recognizedlaboratory where cultures are grown to determine the type of bacteria involved. Enzyme tests may also be utilized.

See figure 2-9.

17A B S

AnodeM → M2 + 2eÐ

O2 + 2H2O + 4eÐ → 4OHÐ

Oxygen-ContainingW ater

O2ClÐ

Oxide Layer

Aerobic Bacteria

M etal-DepositingBacteria

ClÐ


Coatings and MIC

Soft coatingsRapid attack by bacteria on soft coatings is the result of the base product, paraffin and oil, being a nutrient for bacteriagrowth.

AlkydsAlkyd coatings use an organic binder dissolved in solvents to form a protective barrier after the solvent evaporates. Becausealkyds have a high base organic and hydrocarbon compounds, they would probably be rapidly attacked by bacteria.

Vinyl’sVinyl’s are also rapidly attacked by some types of bacteria, especially molds and mildew, and would not perform well whenexposed to bacteria.

EpoxiesEpoxies are chemically cured and their resistance to bacteria would be dependent on the type of epoxy and the solvent usedas a curing agent. If the epoxy became porous or mechanical damage should occur, it would come under attack by bacteria.

Amine cured epoxies are the most susceptible to bacteria acid attack, with the imide cured epoxies being the most resilient.Epoxy Novolac is an example of a good, albeit expensive coating.

Coal Tar Epoxy has good resistance against acid attack due to the properties of the coal tar pitch. It resists bacterial attackdue to its phenol content.

UrethanesUrethanes are also chemically cured coatings and have shown good resistance to bacterial attack. Bacteria will grow onurethane, but will not degrade the coating.

Cathodic ProtectionSupplementing a coating protection system with anodes causes the pH within the tank or space to increase. The high pHcreates a calcareous deposit to occur on the surface of the steel. Bacteria do not usually thrive in areas where the pH level inthe space is in the range of 10 to 11. If the pH level goes below this level the bacteria growth level will return to normallevels in less than a half a day.

Protection from MIC

Utilizing a protective coating system or a combination of coatings and cathodic protection in cargo and ballast tanks canprotect a vessel from the onslaught of MIC. Locations where MIC can become a problem are in the ballast tank, where watercan become stagnant, and in the sediment in the bottom of the tank. MIC is a problem in cargo tanks in the water layer thatcollects in the bottom of cargo tanks where generally there is a good supply of hydrocarbon food source in this area.

Coatings act as a barrier between the surface of the steel and the environment it is being subjected to. Anodes protect thesteel at the imperfections in the coating. Bacteria are able to utilize some of the compounds within the various coatingsystems as nutrients, causing deterioration within coating systems.

Utilizing biocides for treating bacterial affected areas have been used, but these have short time effects. Biocides such aschlorine, hydrogen peroxide, iodine and quaternary amine have been used.

Also, at the new construction phase, design features should be utilized in eliminating areas for the accumulation of mud andsediment. Also, proper drainage of all liquids to a common stripping area is desired to eliminate the possibility of stagnantwater accumulating. Good design is safe, environmentally acceptable and permanent.

References

1. John S. Smart, Ph.D. — Microbiologically Influenced Corrosion of Ships2. Gregory Kobrin — Corrosion by Microbiological Organisms in Natural Waters3. E. C. Hill and G. C. Hill — Microbiological Pitting Corrosion — Old Problems in New Places4. Bruce Sawvel — Microbiologically Influenced Corrosion

18 A B S


Most of the important properties of a paint are determined by its binder and the manner in which the film is formed.These general properties cannot be notably changed, but in the formulation of paints it is possible to enhance,modify, or even add to these basic characteristics. The result is a much wider range of paint “types” than would off

hand appear from the generic classification. Some of these types are more often referred to by their function or purpose,than by their generic type. In this chapter the following are described:

• Anticorrosives• Shop Primers/Holding Primers• Antifoulants

ANTICORROSIVES

In corrosion prevention with paints, three main principles are employed:

• Create a barrier that keeps out charged ions and retards the penetration of water and oxygen.• Ensure that water on its passage through the paint coating takes on special properties or compounds inhibiting it’s

corrosive action.• Ensure metallic contact between the steel and a less noble metal, such as zinc, affords cathodic protection of the steel by

utilizing the galvanic effect.

Barrier EffectBarrier effect is obtained by applying thick coatings, 10 to 20mils., of paint with very low water permeability.

Typical representatives are:• Bitumen• Coal tar epoxy• Epoxy

By adding flake pigments, such as leafing aluminum, a barrier effect can be achieved at lower film thickness. The flakepigments are oriented parallel to the steel surface, and water trying to pass through has to select the more complicated andlonger passage around the pigments. See figure 3-1

Figure 3-1

For permanently immersed steel, the first and often the only choice of protection in coating protection is to utilize thebarrier effect. If a barrier coating is damaged, the damaged area is open for corrosion to begin. Corrosion can then proceedinto the steel substrate and outwards under the intact coating, known as under rusting. Thus, where there is a risk ofmechanical damage, additional protection such as cathodic protection is sometimes provided.

19A B S

CHAPTER THREE: Paints for Purposes


Inhibitor EffectA corrosion inhibitive effect is achieved by using primers containing inhibitors. These are soluble or basic pigments designedto suppress the corrosion process. Example of types of inhibitors utilized are:

• Zinc Chromate• Zinc Phosphate• Zinc Metaborate• Red lead• Calcium Plumbate(The first three inhibitors do not contain metallic zinc particles)

Inhibitors are and must be somewhat water soluble. To prevent them from being washed out of the primer coats, top coatswithout inhibitors are applied to provide the barrier necessary for the inhibitive primer to last.

Due to water solubility of the pigments used, inhibitive primers are not suited for prolonged immersion. This exposure wouldresult in blistering and early breakdown of the coating system.

When damaged, a reasonable protection against rust creeping or under rusting is afforded, but the damaged area is exposedto corrosion.

Galvanic EffectProtection of steel through the galvanic effect, cathodic protection, can be achieved with paints containing large amounts of metallic zinc. A condition for effective protection is that the paint is formulated to give metallic contact between theindividual zinc particles and between zinc particles and the steel. Typical binder for zinc dust paints are:

• Epoxy• Ethyl Silicate• Alkali Silicate

The very nature of these paints requires an absolutely clean steel surface and, especially for the zinc silicates, a rather welldefined surface profile for a lasting coating system.

When applied, zinc silicates are initially rather porous. After a while the pourousness is filled with corrosion products from thezinc, and a barrier is formed.

When damaged, the galvanic effect is re-established at the damaged area and protected effectively against rust creeping.

Since the corrosion products of zinc (zinc salts) are slightly water soluble, zinc dust paints are not normally suited forpermanent water immersion service, but zinc silicate, because of its superior solvent resistance, is the favored protection in ballast tanks.

20 A B S


SHOP PRIMERS

Shop primers, also referred to as pre-construction primers, are anticorrosives designed for application in automated plants toplates or profiles prior to assembly or construction.

DemandsThe special demands of these primers are:• Provide protection against corrosion during the construction period.• Spray applicable in a variety of automatic installations.• Time between application and dry to handle is very brief.• Should not influence the speed of welding or cutting.• Must not produce noxious or toxic fumes during the welding or cutting process.• Must not influence the strength of the welds.• Should be able to withstand comparatively rough handling.• Should form a suitable base for the widest possible range of coating systems.

PropertiesConsequent to the requirements listed above, shop primers possess properties not normally found in paints designed for otherpurposes. They are applied in low film thickness, 3 to 5 mils., and do not interfere with the speed of cutting or welding.Reasonable protection at such low film thickness can only be achieved if the coating follows the contours, (surface profile),of the blasted steel.

Figure 3-2

Inherent in the formulation of shop primers are fast drying and retarded flow properties. A side effect of this is low cohesivestrength. Shop primers applied with excessive dry film thickness, DFT, have a pronounced tendency to crack.

To achieve the desired protection and avoid immediate or subsequent cracking, the dry film thickness of the primer must beclosely monitored and the manufacturer’s specification followed closely. This exactness is usually found in automated paintfacilities.

Where temporary protection to blast cleaned steel is applied by hand spray, as in a maintenance situation, one would choosea suitable anticorrosive, holding primer with a reasonably long re-coating interval and apply it in a lower than usual filmthickness. Anticorrosive primers suitable, and frequently used, for this purpose are referred to as holding primers. They areideally based on the same generic type as the top coat to follow.

21A B S


Types of Shop Primers

The most widely accepted shop primers are:• Poly Vinyl Butyral (PVB)• Epoxy Iron Oxide• Zinc Epoxy• Zinc Silicate

A general summary of the more important properties of these shop primers are in Table 3-1

Type of Shop Primers

Property Zinc Expoxy Zinc Silicate Epoxy Iron PVBOxide

Delivery two pack two pack or two pack two pack or one pack one pack

Solvents and esters, water or esters, alcohols, Thinners ketones, alcohols ketones ketones,

aromatics aromatics aromatics

Dry Film 12-20 um 12-20 um 20-20 um 15-25 umThickness

Anticorrosive very good excellent good fairProperties

Mechanical good excellent good goodStrength

Recoatability sometimes critical sometimes goodcritical critical

Resistance to 4 Not usually Not usually good limitedICCP used with used with

ICCP ICCP

Table 3-1

22 A B S


ANTIFOULINGS

Ships’ underwater hulls are painted to protect the building material, usually steel, and prevent undue roughness. The effectof roughness on the hull area is an increase in resistance to movement, resulting in reduced speed and/or increased fuelconsumption. The penalty is a higher operating cost.

FoulingThe most severe hull roughness is that caused by fouling, the growth of various marine plants, animals, and organisms. Some 20% of the fuel bill for the Royal Navy, it is estimated, is caused by fouling.

The OrganismsMany of the species populating the oceans swim or are carried by the ocean currents. Others are compelled to attachthemselves to a firm surface to fulfill their life cycles. Every available space is contested and covered by a variety of marinegrowth. The number of species involved in fouling is estimated to be as high 4 to 5,000.

Micro-OrganismsThe micro-organisms are the first to settle. They form the primary bio-film or slime layer. The most significant ones are:• Bacteria• Diatoms (unicellular algae)

Macro-OrganismsMacro-organisms are big enough to be seen without the aid of a microscope. Example of macro-organisms are:• Algae (sea weed or grass)• Animals (Hard and Soft Shelled)

Among the hard shelled animals, barnacles, goosenecks, tubeworms, bryozoans, and mussels are the most common. Softbodied animals found most commonly are hydroids and tunicates.

DistributionFouling is generally regarded as being most dominant in ports, harbors, and coastal areas where firm surfaces on which to settle abound.

The distribution of macro-organisms on a ship is seldom uniform. This is due partly to differences in flow conditions on thehull and partly to differences in the settling pattern of the organisms. A relatively well defined vertical zone division cannormally be observed.

Since algae depends on light for its existence, this type of fouling is predominant to a depth of approximately two meters.This is the algae zone and is fouled first and most heavily.

On the vertical bottom, below the algae zone, scattered fouling of barnacles, encrusting bryozoans, tubeworms andgoosenecks are common.

The flat bottom is dominated by hydroids, barnacles, mussels, tunicates, bryozoans, and sometimes goosenecks.

Severity and distribution of macro-organisms on a ship’s underwater hull will be influenced by the ship’s trading andoperational pattern. Algae settles in a matter of hours, whereas animals normally require a couple of days. Very few organismscan attach themselves at speeds in excess of four knots. The settlement of micro-organisms, bacteria and diatoms, are notconfined to specific zones or areas. It is comparatively uniform and independent of the vessel’s service pattern.

23A B S


Antifoulant PaintsThe antifouling paints used today are based on physically drying binders. They prevent fouling by releasing bioactivematerials that interfere with the biological processes of the fouling organisms. Bioactive materials used today are mainlycuprous oxide or organo-tin compounds. These compounds are used either individually or as a combination of each.

Classification of Antifouling PaintsAntifouling paints differ basically in their load or concentration of bioactive material and in the mechanism controlling therelease of that material.

According to the manner in which bioactive material is leached from the coating, antifoulants may be grouped as thefollowing:

• Soluble Matrix Types (non polishing)• Insoluble Matrix Types (non polishing)• Self Polishing Types

Particularly in the latter two groups a great number of grades (type /concentration of bioactive material) will be found.

Soluble Matrix (non polishing)Such antifoulants have, as a main constituent of their binder, a sea water soluble resin. As the binder dissolves, the bioactive material is released. The release rate is uneven and since the binder is comparatively weak the load of bioactivematerial is low.

Figure 3-3 Soluble Matrix (non polishing)

General Properties• Effective protection rather limited (up to

approximately 12 months).• Only antifoulant type that can be safely applied over

soft, bitumen, primers.• The binder oxidizes and is sensitive to sunlight.

Therefore the vessel must be launched or floated soon after curing.

• Sensitive to oil pollution (mineral and fish oils)• Due to the low strength of the binder, subject to

cold flow.

24 A B S

0 1⁄2 LT LT

TV

0 LT

Release rate ofBIOACTIVE MATERIAL

Non-Polishing-soluble matrix

TIME

Bioactive materials

Binder

Anticorrosive primer

LT Lifetime of antifouling

TV Threshold Value, i.e., minimum release ratof bioactive material required for protectiagainst fouling.


Insoluble Matrix (non polishing)These antifoulants may be based on a vinyl binder. These have high mechanical strength and are not affected by seawater, therefore, the load of bioactive material must be high enough for the particles of this material to be in contact with each other.

Figure 3-4 Insoluble Matrix (non polishing)

General Properties• Medium extended effective protection (up to 24 to 30

months, depending on trading pattern).• No risk to cold flow.• Re-activation (removal of empty matrix) possible

through scrubbing (“Scamp” cleaning).• Empty matrix may become blocked by pollutants.• An empty matrix, because of its sponge like properties,

should be scaled off prior to a fresh or new coat of antifoulant is applied.

25A B S

0 1⁄2 LT LT

TV

0 LT

Non-Polishing-insoluble matrix

Self-Polishing

Bioactive materials

Binder

Anticorrosive primer

LT Lifetime of antifouling

TV Threshold Value, i.e., minimum release rateof bioactive material required for protectionagainst fouling.


Self PolishingThese are based on acrylic binders incorporating organo-tin in the very polymers of the binder. In contact with sea water itdissolves at an even and predictable rate. As the antifoulant wears off/polishes the bioactive material, organo-tin is releasedat an even rate.

Figure 3-5 Self Polishing

General Properties• Medium to very high effective protection against fouling (up to 48 months).• Excellent mechanical strength and stability.• Effective life is directly proportional to applied film thickness.• No sealer coat required at subsequent dry dockings .• Remaining antifoulant becomes an effective part of the new system (can overcoat).

RoughnessSelf polishing antifoulants cannot turn a rough hull into a smooth one. They can maintain and preserve an existingsmoothness and, since the polishing rate is higher on roughness peaks than valleys, even improve smoothness.

The use of self polishing antifoulants prevents the accumulation over the years of old and spent antifoulant systems, and thus greatly contribute to maintaining a smooth underwater hull.

Roughness development versus time, as a function of the type and grade of antifoulant, can be qualified by computersimulation and the economical impact predicted. This can serve as the basis for selecting the most economical coating system for each individual vessel.

26 A B S

0 1⁄2 LT LT

TV

0 LT


Good surface preparation is, perhaps, the most important part of the entire coating job, in that the greatest percentageof coating failures can be traced directly to poor surface preparation. No paint system will give optimum performanceover a poorly prepared surface. All paint systems will fail prematurely unless the surface has been properly prepared to

receive the coating material. If contaminants such as oil, grease, dirt, salts, chemicals, etc. are not removed from the surfaceto be coated, adhesion will be compromised, and/or osmotic blistering will occur. Loose rust left on the surface will cause aloosening of the coating and eventual loss of adhesion. Also, good surface preparation roughens the surface to assist inobtaining the proper surface profile, thereby promoting better coating performance in the areas of adhesion, abrasionresistance, chemical and water resistance, as well as the long term cosmetic appearance of the paint system.

The following table provides a Summary of Surface Preparation Standards and a cross-reference of those Standards by various world-wide agencies. There are differences which can be important in some instances; care is advised when acrossover is required. While these standards are limited to steel substrates by use of common sense, many of the techniques, with their inherent advantages and disadvantages, hold true for substrates.

PREPARATION STANDARDS

Nonabrasive Blast Cleaning NACE1 SSPC2

Solvent Cleaning SSPC-SP-1

Hand Tool Cleaning SSPC-SP-2

Power Tool Cleaning SSPC-SP-3

Power Tool Cleaning to Bare Metal SSPC-SP-11

Abrasive Blast Cleaning

White Metal NACE-1 SSPC-SP-5

Near-White Metal NACE-2 SSPC-SP-10

Commercial NACE-3 SSPC-SP-6

Brush-Off NACE-4 SSPC-SP-7

Water Blasting NACE-5 SPS-SP-12

Figure 4-1

NACE = National Association of Corrosion EngineersSSPC = Steel Structures Painting Council

27A B S

CHAPTER FOUR:Surface Preparation


Solvent Cleaning (SSPC-SP-1)

A process of using solvents, expanded more recently to include other cleaning compounds, to remove oil, grease, and othercontaminants. This process is best utilized as a preliminary step in the total surface preparation procedure.

Solvents are no longer the recommended cleaner, as they may become an impediment rather than a help, if not properlyremoved. If solvent cleaning is chosen, then safety is very important. Adequate ventilation and minimizing the potential firehazard are paramount. Clean-up rags should be changed often to prevent smearing. A proprietary water soluble oil andgrease remover and fresh water washing is the perfect method of achieving this standard. (Some will also etch the surface,further promoting adhesion.)

Hand Tool Cleaning (SSPC-SP-2)

This method is the slowest and usually the least satisfactory method of surface preparation. Wire brushing, in fact, can makethe surface worse by polishing rather than cleaning the rusted surface. Scrapers, chipping hammers, or chisels can be used toremove loose, non adherent pain, rust or scale, but is usually considered incomplete. For this reason, the area to be preparedshould be sufficiently small to allow for the time required.

28 A B S


Power Tool Cleaning (SSPC-SP-3)

The advantage of power (electrical or air) tool methods over hand tools, is that they are generally less laborious, but, as withmanual, easier to feather loose coatings back to tight impact paint. The effectiveness of cleaning will depend on the effortand endurance of the operator, and becomes especially tiring when working above shoulder height. Some of the morepopular methods are as follows:

Rotary Wire BrushingThis method does have some value, depending upon the condition of the surface. Loose “powdery” rust can be removed buthard scale will resist the abrasion of the wire bristles. When rust scale is intact and adherent to the substrate, rotary wirebrushing tends to merely burnish the surface of the rust scale, but does not remove it. Care should be exercised, in that theburnished surface may give the appearance of a well cleaned surface, which is often misleading.

Mechanical DescalingNeedle Guns, Roto-Peen, and other pounding type instruments are effective to some degree in removing thick rust and scale.The action of these types of devices is dependent upon cutting blade or point pounding the surface and breaking away thescale. Cleaning is only effective at the actual points of contact. The intermediate areas are only partially cleaned, because thebrittle scale disintegrates, but the lowermost layer of rust and scale remains attached to the substrate. This may be sufficientfor surface tolerant epoxies.

Rotary Power DiscingOf the power tool methods, this one is the most effective in producing a surface suitable for the application of most types ofcoating systems, especially for most on-board maintenance. While effective, discing should generally be limited to localizedareas of fairly severe corrosion or more widespread light corrosion, because, once again, this method can be quite slow andlabor intensive. Normally silicon carbide discs are used and the grade selected to suit the conditions of the surface to beabraded. It is important to change the discs at regular intervals in order to maintain efficiency. Care should be exercised inthe selection of the grit size and type of disc to be utilized, so that the surface is not excessively smoothed, thereby reducingthe ability of the paint to adhere. Irregular and pitted surfaces may require a combination of the various power tool cleaningmethods to maximize effectiveness.

29A B S


Abrasive Blast Cleaning (SSPC-SP-5,6,7,10)

This is by far the most efficient and effective method of removing paint, rust, mill scale, etc. from substrates, also, it isgenerally considered to provide the proper surface profile to promote coating adherence. However, compared to the methodsdiscussed above, it is also the most expensive method. For this reason, it is chosen to reduce the time for surface preparation,or to achieve standards of cleanliness that are only attainable by some type of abrasive blasting. It is not feasible to provide acomplete treatise on the types of blasting, types, and sizes of grit available, surface profiles, and mechanics of abrasive gritblasting, however, each abrasive blast standard will be defined as follows:

SSPC-SP-5, NACE No. 1, Swedish Sa 3White Metal Blast Cleaned Surface Finish. This blast standard is defined as a surface with a gray-white, uniform metalliccolor, slightly roughened to form a suitable profile for coatings. This surface shall be free of all oil, grease, dirt, visible millscale, rust, corrosion products, oxides, paint, or any other foreign mater. This surface shall have a color characteristic of theabrasive media used.

SSPC-SP-10, NACE No. 2, Swedish Sa 2 1/2 Near White Blast Cleaned Surface Finish. This finish surface is defined as one from which all oil, grease, dirt, mill scale, rust,corrosion products, oxides, paint or other foreign matter have been removed except for very light shadows, very light streaks,or slight discolorations. At least 95% of a surface shall have the appearance of a surface blast cleaned to a white metalsurface finish, and the remainder shall be limited to the light discoloration mentioned above.

SSPC-SP-6, NACE No. 3, Swedish Sa 2Commercial Blast Cleaned Surface Finish. This finish is defined as one from which all oil, grease, dirt, rust scale, and foreignmatter have been completely removed from the surface and all (Sa 2 provides for almost all) rust, mill scale, and old painthave been completely removed except for slight shadows, streaks, or discolorations. At least 67% of the surface area shall befree of all visible residues and the remainder shall be limited to light discoloration, slight staining, or light residues mentionedabove.

SSPC-SP-7, NACE No. 4, Swedish Sa 1Brush Off Blast Cleaned Surface. This finish is defined as one from which oil, grease, dirt, rust scale, loose mill scale, looserust and loose paint or coatings are removed completely, but light mill scale and tightly adhered rust, paint, and coatings arepermitted to remain, provided they have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks ofthe underlying metal fairly uniformly distributed over the entire surface.

Water Jetting and Hydroblasting

As discussed previously dry abrasive blasting is the most commonly used method of surface preparation. Accompanied withthis style of preparation are some known problems. In general with abrasive blasting the resultant flying abrasive particles anddrifting dust may damage equipment, clog filters and create possible environmental problems.

Also, it is possible to trap contaminants on the surface of the substrate being cleaned.

Government regulations are continuously investigating and developing more environmentally sensitive and user friendlymethods of surface preparation. The use of hydroblasting is becoming an increasing viable means to accomplish this.Standards from both NACE and SSPC are being developed to satisfy this need.

It should be noted that hydroblasting does not produce a profile on the steel surface as does abrasive blasting. It does,however, expose the original abrasive blast surface profile. To be an effective agent the water being used should be pureenough that it does not contaminate the surface being cleaned.

Pressures used in hydroblasting can exceed 35,000 psi. This pressure removes most contaminants, such as salts, dirt, greaseand rust scale. Utilized sometimes in this type of preparation are inhibitors which are added to the water to help preventflash rusting prior to coating being applied.

Advantages to hydroblasting are:• Water as a cleaning material is generally available in inexpensive large quantities.• Lack of contamination of surrounding areas because there are no abrasive particles.• Lack of dust.

30 A B S


The objective in applying paint coatings are to provide films which will give protection and, normally to a lesser extent, decoration to the structure being painted. The variables which govern the success of any application are:

• Surface preparation• Film build and total thickness of system• Methods of application• Atmospheric conditions during application

Methods of Application

The normal methods of application of paint coatings are by:• Brush• Roller• Conventional Spray• Airless Spray• High-Volume, Low Pressure spray (HVLP)

Other methods may also be encountered, such as dipping and pouring, and more sophisticated adaptations of spraying suchas electrostatic, powder coatings application, and automatic plants, but in this paper we will concentrate on the four basicmethods detailed.

Brush ApplicationThe “historical” method of paint application is not as fast as spraying or rolling and is generally used for the coating of smallcomplicated or complex areas or where the need for ‘clean’ working with no overspray precludes the use of spray application.

When painting it is important to dip the brush in paint frequently and not to ‘over-brush’ the surface, as this will result inlarge variations in film thickness, the inherent problem with brush application. Choice of brush, both size, length and type ofbristle, and shape, are important, and the type of paint being applied will modify the selection. Thus, large flat brushes arenormally used for the majority of purposes, but round brushes are better for painting bolt-heads and ‘difficult’ areas. Specialbrushes are available with offset heads and long handles to facilitate painting the ‘backs’ of structures and inaccessible areas.

Brush application is most suited to the slower drying, normal build type of coatings, and will not always be suitable for themore sophisticated ‘fast-drying’ or ‘hi-build’ materials. It is often not possible to achieve the required film thickness in thesame number of coats as with spray application, and multi-coat applications are necessary to give the specified film build.

Roller ApplicationRoller application is faster than brush on large, flat surfaces, such as tank sides and tops and walkways and deck areas, but itis not so good for ‘difficult’ areas. It is hard to control film thickness, however, and care must always be taken that thecoating is not ‘over-rolled’ in the same manner that it can be ‘over-brushed’. Choice of roller pile (short or long hair, spongeor lambs wool) is dependent on type of coating and roughness and irregularity of surface being coated.

31A B S

CHAPTER FIVE:Methods of Paint Application


32 A B S

Conventional SprayThis is a widely accepted, rapid method of applying paint to large surfaces. The equipment is relatively simple and is usually confined to fairly low-viscosity paints, although newer techniques using ‘pressure-pot’ or ‘hot spray’ apparatus allowapplication of some of the ‘higher build’ type coatings. Whatever type of equipment is used, the mechanisms is the same.Paint and air are fed separately to the spray gun and mixed at the nozzle, where the paint is atomized and air is mixed withthese droplets forming a fine mist of paint which is carried by the air pressure to the work surface. It is important to haveonly sufficient air to provide good atomization, as excess air gives rise to overspray and ‘rebound’ from the work surface. The gun should be held at right angles to the work surface with the nozzle some 6-8” (15-20 cms) away. Normal air pressureis from 40-80 psi (2.8-5.6 kg/cm2). The pattern of the ‘fan’ so produced is controlled by adjusting the air and fluid pressures.A change in paint type can be accommodated by different sizes of nozzles.

Airless SprayBy far the most important and efficient method for the application of heavy duty marine coatings.

As the name implies, it is a technique of spray application which does not rely on the mixing of the paints with air to provideatomization, which is achieved by forcing the paint through a specially designed nozzle or ‘tip’ at very high pressures, ascompared with air pressure associated with conventional spray, 2500-3500 psi, (176-246 kg/cm2).

High-Volume, Low Pressure SprayHigh-Volume, Low Pressure spray systems use a high volume of air delivered at 10 psi or less to atomize a coating into a softlow velocity pattern. This reduction in the air stream compared to conventional spray systems (40 - 70 psi) results in: • A more controlled spray pattern• Reduced bounce back• Reduced overspray• Reduced VOC emissions• Savings in materials• Less hazardous waste

Reduced overspray improves visibility, which reduces operator error, and therefore improves finish quality. For example, twothirds or more of every gallon of coating sprayed by conventional methods can be lost to overspray, compared with one pintper gallon or less when using high-volume, low pressure.

HVLP is made more graphic by the current trend in regulations regarding the amount of material being applied versus theamount lost to over spray. California enacted legislation requiring all spray equipment deliver at least 65% transfer ofmaterial to the surface being coated. Similar legislation is pending in other states.


There are several different alternate coatings to choose from other than a hard coating. The alternative coatings aredivided into two categories dependent on their properties: • Soft Coatings• Semihard Coatings

Soft CoatingsSoft Coatings are coatings that remain soft, so that they wear off at a low mechanical impact or evenly only when touchedby hand. These coatings will give temporary protection of rusted steel surfaces and must be maintained or re-coated every year or every second year. These coatings always remain soft and can be damaged or removed by walking or touching.

Semihard CoatingsSemihard Coatings are coatings that dry in such a way that they stay soft and flexible, but are hard enough to touch and walk upon and will not wear off or erode by ballast water movement. This coating also gives temporary protection of rusted steel.

Of the two aforementioned coatings, the semi-hard is the preferred coating for protection of steel surfaces when a good hard coating is not applied.

It is important to note that alternative coating products are very diverse and can vary by:• Chemistry• Method of Protection• Thickness• Opacity• Preparation• Application

33A B S

CHAPTER SIX:Alternatives To Hard Coatings & Cathodic Protection


34 A B S

Chemistry

Products can be one or a combination of the following:• Lanolin/wool grease based• Petroleum based• Vegetable oil based• Organic or inorganic

Each type has its own unique characteristics and corrosion protection capabilities.

Method of Protection

Products can be classed by one or a combination of the following:• Corrosion inhibitor (interacts with oxides to prevent further oxidation)• Corrosion barrier (prevents oxygen from reaching metal surface)

It should be noted that a pure corrosion barrier product will still allow a corrosion cell to be active underneath the product,while a corrosion inhibitor stops this activity.

Thickness

Product film thickness can vary from a thin film of 3 mils. up to a thick film of 80 mils. This is an important feature toconsider when inspecting a tank, since a thicker product may be a safety hazard and require spot removal in order to viewthe steel surface or structure underneath.

Opacity

The products are either:• Opaque (dark or black)• Gray• Transparent

This feature will have an impact on the inspection of the tank. The opaque products will require spot removal to allow forinspection of the steel surface, whereas the transparent product could allow the inspector to view most of the steel surface,without removing the coating.

Preparation

All silt, mill scale, sheet scale, cement, oil, grease or chemical contaminants must be removed. The bare metal substrate mustbe visible. This is to a SSPC Standard of SP-2. De-scaling until all loose material is removed is required. Old grease, if stillremaining, should be scraped off, followed by degreasing using hot water jetting. Hydroblasting, when used, should be at arecommended pressure of 6000 psi.

Prior to application the surface should be dry. This to be accomplished either by ventilation or dehumidification.

Application

Products are to be applied by either brush, roller, or airless spray. Most soft coating are applied via a float method. Thismethod will require additional product, but no staging. Care must be taken to adhere to pollution prevention procedureswhen applying coatings in this method.


35A B S

Cathodic Protection

Metallic Corrosion is an electrochemical phenomenon, metal degradation being accompanied by the passage of electrons.Consequently, as a metal corrodes it takes on its own electrical potential, known as the corrosion potential, with respect to a fixed reference. Table 6-1 gives the relative tendencies of commercially available metals to corrode in seawater.

Metal Electrode Potential(VOLTS Versus: Standard Calomel Electrode)

Magnesium..........................................................................-1.50 BASE (IGNOBLE)

Zinc ........................................................................................-1.03 CORRODED END

Aluminum ............................................................................-0.79

Cast Iron ...............................................................................-0.61

Mils Steel..............................................................................-0.61

Lead........................................................................................-0.50

Tin...........................................................................................-0.42

Brass.......................................................................................-0.30

Copper...................................................................................-0.28

Cupro Nickel........................................................................-0.25

Bronze ...................................................................................-0.23

Nickel.....................................................................................-0.14

Silver......................................................................................-0.13

Titanium................................................................................-0.10

CORROSION RESISTANT END

Table 6-1: Galvanic Series

When two dissimilar metals become electrically connected and are exposed to the same solution (seawater) their individualcorrosion behavior can become significantly altered, particularly if the difference in their corrosion potentials is large. Whenmild steel and copper are in contact, for example, the mild steel, being the more base metal, suffers greater corrosion than it would if the two metals were isolated. The corrosion of the more noble copper is reduced.

Figure 6-1

ELECTRON FL OW

MILD STEEL(ANODE)ACCELERA TEDCORROSION

COPPER(CATHODE)PRO TECTED

SEA W ATER


36 A B S

In the same way that the corrosion of copper is reduced, as shown above, the corrosion rate of mild steel can be significantlylowered by connecting it to zinc, which is near the end of the galvanic series. The zinc is referred to as the sacrificial anodeand the prevention of corrosion of the mild steel is by cathodic protection.

Figure 6-2

As an alternative to sacrificial cathodic corrosion protection, it is also possible to suppress the corrosion of mild steel inseawater by using impressed current cathodic protection. In the same way that coupling mild steel to zinc results in a flow of electrons to the mild steel to prevent metal loss, an auxiliary anode made from a non-consumable material, platinised niobium, replaces the anode of the sacrificial system.

The arrangement is shown schematically in figure 6-3.

Figure 6-3

Generally the structure would be coated, otherwise the current requirement for protection would be too expensive. Cathodic protection systems are generally used in conjunction with painted steel, protection being afforded to gaps or breaks in the coating.

ELECTRON FL OW

MILD STEEL(CATHODE)PRO TECTED

ZINC(ANODE)

ACCELERA TEDCORROSION

SEA W ATER

D C POWER SOURCE

+

Ð REFELECTRODE

ANODE

CURRENTFLOW

PRO TECTEDSTRUCTURE


37A B S

AIR DRYING PAINTS: are paints which dry and form a film when exposed to air, without any external heat being applied.Oil and alkyd paints are usually air drying.

AIRLESS SPRAY: is a method of paint spraying which does not use compressed air to atomize the paint. In this method,the paint is put under great pressure (up tp 5000 psi - 360 kg/sq.cm) and is atomized by being forcedthrough a small nozzle. Airless spray is a very fast and efficient method of application since the paintis forced into the surface at very high speed, which assists in wetting the surface.

ALKYD: is a synthetic resin made by reacting two chemicals in the presence of a natural or processed oil.Because of the wide variety of possible constituents, alkyds can be ‘tailor-made’ to meet conditionsfound in practice.

ANODE: a piece of metal fixed to steel to provide cathodic protection. Anodes must be fixed so that they are inelectrical contact with the steel they have to protect, and must not be greased or painted.

ANTIFOULING: for underwater use on hulls. Contains agents which prevent the adhesion and growth of organisms onthe hull. Antifoulings are formulated so that the control agents migrate into water closest to the hull,making it repel organisms.

BINDER: the component in paint or varnish which binds the constituents to the surface. Depending on the typeof paint, most manufacturers use binders based on alkyds, chlorinated rubbers, epoxies, etc.

CATHODIC PROTECTION: a method of altering the electrical characteristics of steel so that it is less liable to rust in water. Steelprotected in this way has to be painted with particularly resistant paint systems.

COAL TAR EPOXY: a combination of epoxy resins and tar which, in a paint, give a very water resistant film. A curingagent must be added if curing is to take place.

CONVENTIONAL PAINTS: a collective description of paints based on naturally occurring binders, such as bitumen, alkyds, andoils. They are all one pack types, and usually react with air to dry and cure.

EMULSION PAINTS: paints in which the binder is dispersed in water (emulsified) e.g. polyvinyl acetate (PVA), acrylics etc.The paints dry as soon as the water evaporates and the emulsified droplets of resin join together toform a solid film.

EPOXY: epoxy resins which are cured by chemically reacting with a curing agent such as amines, amineadducts, and polyamides. Properties can be tailored to meet a wide range of needs.

FILM THICKNESS: is the thickness of the paint or system. The recommended film thickness for each product are given inthe Technical Data Sheets. The protection given by a paint depends on the applied thickness.Specialized equipment is available to measure the film thickness.

FLASHPOINT: the temperature at which the vapor of a material will be ignited by a spark or open flame. It ismeasured under standardized conditions.

HARD COATINGS: is a coating which chemically converts during its curing process, normally used for new construction,or non-convertible air drying coating which may be used for maintenance purposes. Hard coating canbe either inorganic or organic.

LATEX: a resin used in emulsion paints.

PIGMENTS: powders, insoluble in resins, which give the paint its color, finish, and protective properties.

CHAPTER SEVEN:Glossary of Frequently Used Coating Terms


38 A B S

POLYMER: a high molecular weight material created from lower molecular weight constituents by chemicalreaction. Polymers with resinous characteristics are frequently used in paints.

POLYURETHANE: a resin with special characteristics. Paints based on polyurethane are either one or two pack, areextremely hard wearing, and are generally resistant to chemicals. They may be formulated to beexceptionally color stable and weather resistant.

POTLIFE: the time for which a two pack paint or varnish can remain mixed before it should be discarded. Thepaint should be used within this time, since the curing will be so far advanced by then that the paintwill not behave in the normal manner. Paint must never be allowed to remain in spray equipmentafter the expiry of it potlife

PVA PAINT: see Emulsion Paints

RESIN: a material used as a binder constituent which forms a noncrystalline film when dried.

SEMI-HARD COATING: coating which dries in such a way that it stays flexible, but still hard enough to touch and walk upon.These coatings do not appreciably erode with the usual ballast water movement. Such coatingsprovide temporary protection of existing structures.

SHOPPRIMER: a rust preventing paint for temporary protection of blasted steel immediately after blasting.Shopprimers will protect the surface from corrosion during construction and until the final paintsystem is applied.

SOFT COATINGS: coating that remains soft so that it wears off at low mechanical impact or when touched by hand;often based on oils (vegetable of petroleum) or lanolin (sheeps wool grease), these coatings aregenerally used to give temporary protection to existing structures.

SOPHISTICATED PAINTS: are paints which are based on unconventional binders, such as epoxies, chlorinated rubbers, vinyls, etc.Generally, sophisticated paints give a higher level of protection than conventional paints, but are notso simple to handle.

SPREADING RATE: the area which is covered by one liter of paint.

TAR EPOXY: see Coal Tar Epoxy

THERMOPLASTIC PAINTS: are paints which dry by evaporation of solvent only. The binder is unreactive.

THIXOTROPIC PAINTS: have a semi-solid or gel consistency when undisturbed, but flow readily when stirred or shaken, orwhen being applied. The process is reversible, and a fluid paint reverts to a gel consistency when thedisturbance ceases. When applied, thixo-tropic paints will flow easily as long as they are being worked,but quickly regain a gel consistency which assists in preventing runs and sags.

TWO PACK PAINTS: used to describe paints which are supplied in two separate containers and which have to be mixedtogether before use.

ZINC PHOSPHATE: a pigment with corrosion preventing properties.

ZINC SILICATE PAINTS: zinc-filled paints based on an inorganic binder. Zinc silicates.

ZINC-RICH PAINTS: zinc filled paints based on a large proportion of metallic zinc in powder form. They usually containmore than 85% zinc in the dry film and provide very hard films, and are resistant to solvents.


ASSESSMENT OF EXISTING SURFACE COATING SYSTEMS

For the purpose of consistent assessments of the ‘degree of effectiveness’ of an existing surface coating system, it issuggested that the following ‘rating’ be used:

• GOOD condition with only minor spot rusting.• FAIR condition with local breakdown at edges of stiffeners and weld connections and/or light rustling over 20% or more

of areas under consideration, but less than as defined for POOR condition.• POOR condition with general breakdown of coating over 20% or more of areas or hard scale at 10% or more of areas

under consideration.

Tanker Structure Co-Operative Forum has tabulated the above definitions as follows:

DEFINITION OF COATING CONDITIONS

Rating/Condition Good Fair Poor

Spot Rust MinorLight Rust Minor >20%

EdgesWeld <20% >20%

Hard ScaleMinor <10% >10%

GeneralBreakdown Minor <20% >20%

Other References

ISO RI3 RI4 RI5

European RE3 RE5 RE7Rust Scale

Note: The lowest rating within any category shall govern the final rating.

An ‘Assessment Scale for Breakdown’ of coatings is shown in Examples of coating system condition categorized under theabove rating system. These are shown on the following pages.

39A B S

CHAPTER EIGHT:Assessment Scale for Breakdown


40 A B S


The coating condition should normally be judged over large areas. For classification purposes, it is normal to judge the complete tank. However, if the conditions vary to a great extent between the various main parts (bottom, deck, longitudinal bulkheads, and transverse bulkheads) of the tank, then an evaluation of the various parts

may be advantageous.

Some of the pictures shown in this guide are not from tankers but from ballast tanks of dry cargo ships. However, since thecoating condition is in focus here, it was decided to use them is this context.

The photographs should be considered a tool to assist the Surveyor in the performance of his duties, and this GuidanceManual is intended to be used as such. It does not set a standard. It is not part of the Rules for Building and Classing Steel Vessels.

41A B S

CHAPTER NINE:How to Use This Guide


A small handbook is also provided for the following pages for easy reference.

43A B S

CHAPTER TEN:Examples


45A B S

Coating Condition ............................................................................. GOOD

Example Number: 1

Notes:

1. Blisters

2. Surface discoloration

1%

TSCF Assessment Scale:

less than 1%



Example Number: 2

46 A B S

Notes:

1. Coating intact

2. Filmy surface contaminant peeling

3. No corrosion

4. No scale

®

1%


less than 1%


47A B S


Example Number: 3

Notes:

1. Minor rusting on weld seams

2. Spot rusting

3. Filmy deposit much of surface

1%


less than 1%



Example Number: 4

48 A B S

Notes:

1. Line of sediment

2. Note absence of rust stains

®

1%less than 1%



49A B S


Example Number: 5

Notes:

1. Visible coating repairs

2. Minor rust stains

1%less than 1%




Example Number: 6

50 A B S

Notes:

1. Coating discolored from surfacecontamination

2. Organisms

3. No corrosion

4. No scale

®

1%less than 1%



51A B S


Example Number: 7

Notes:

1. Good coating

2. Dirty surface

3. Sediment on structure

1%less than 1%




Example Number: 8

52 A B S

Notes:

1. Minor corrosion

2. Surface contamination(oily and sediment)

®

1%less than 1%



53A B S


Example Number: 9

Notes:

1. Dirty coating

2. Discolorations due to oilycontaminants and sediments

1%less than 1%




Example Number: 10

54 A B S

Notes:

1. Scattered corrosion

2. Coating repaired

®

1%less than 1%



55A B S


Example Number: 11

Notes:

1. Sediment

2. Drip marks on lower frames

1%less than 1%




Example Number: 12

56 A B S

Notes:

1. Rust stains

2. Moist coating or stiffeners

®

1%less than 1%



57A B S


Example Number: 13

Notes:

1. Scattered corrosion

1%less than 1%




Example Number: 14

58 A B S

Notes:

1. Marine organisms

2. Dark stripe coat

3. Spots are contaminants

®

1%less than 1%



59A B S


Example Number: 15

Notes:

1. No corrosion

2. Significant surface discoloration

1%less than 1%




Example Number: 16

60 A B S

Notes:

1. Top coat failure

2. Blisters

3. Detachment

4. No corrosion

®

1%less than 1%



61A B S


Example Number: 17

Notes:

1. Filmy surface contamination

2. Discolored coating - note drippattern

3. No corrosion

1%less than 1%




Example Number: 18

62 A B S

Notes:

1. Scattered corrosion on edges and welds

2. Surface discoloration

®

1%less than 1%



63A B S


Example Number: 19

Notes:

1. Staining - scattered over >20%

2. No edge breakdown

3. Note absence of texturered/orange areas

1%less than 1%




Example Number: 20

64 A B S

Notes:

1. Light rusting >1%

2. Scattered rusting

3. Extensive rust staining >20%

®

1%less than 1%



65A B S


Example Number: 21

Notes:

1. Coating discolored from surfacecontamination

2. Organisms

3. No corrosion

4. No scale

1%less than 1%




Example Number: 22

66 A B S

Notes:

1. Corrosion on edges and somewelds - minor spot rusting

®

2%1%



67A B S

Coating Condition................................................................................ FAIR

Example Number: 23

Notes:

1. Corrosion

2. Localized breakdown on edges

3. Intact coatings on welds

4. Extensive staining

20%1%




Example Number: 24

68 A B S

Notes:


2. Touch-up, (light areas) repairing

3. Isolated breakdown at corner

®

20%less than 20%



69A B S


Example Number: 25

Notes:

1. Anode working

2. White deposits 3%

3. Corrosion on edges

4. Top coat loss

10%5%




Example Number: 26

70 A B S

Notes:

1. Active corrosion on pipe

2. Discoloration on flats

3. Deposits on vertical structure

4. Scattered corrosion on flatblisters

®

10%less than 10%



71A B S


Example Number: 27

Notes:

1. Edge corrosion

2. White deposits

3. General corrosion 10% - 20%

20%10%




Example Number: 28

72 A B S

Notes:

1. Localized heavy corrosion besideanode - >50% on upper part oftank

2. White deposits

3. Dark scale

®

50%less than 50%



73A B S


Example Number: 29

Notes:

1. Edge corrosion


20%less than 20%




Example Number: 30

74 A B S

Notes:

1. Coating breakdown on welds

2. Spot rusting

®

0.5%less than 20%



75A B S


Example Number: 31

Notes:

1. Edges corrosion

2. Anode working

3. Coating generally intact

2%




Example Number: 32

76 A B S

Notes:

1. Edge corrosion

2. White deposits lower areas

3. Red rust on overheads

®

0.5%



77A B S


Example Number: 33

Notes:

1. Significant edge breakdown

2. Discoloration

10%




Example Number: 34

78 A B S

Notes:

1. Edge corrosion

2. Corrosion on welds

3. Generalized breakdown 10 - 20%

4. No hard scale

®

20%10%



79A B S


Example Number: 35

Notes:

1. Edge breakdown >20%

2. General breakdown

20%less than 20%




Example Number: 36

80 A B S

Notes:

1. Edge breakdown

2. Hard scale >10%

3. Light corrosion >20%

4. Abscence of red rust due towhite deposits from Anode action

®

20%10%



81A B S


Example Number: 37

Notes:

1. Light rusting - 20%

2. Staining at waterline

3. Rust stains

20%less than 20%




Example Number: 38

82 A B S

Notes:

1. Edge breakdown

2. White deposits >5%

3. Red rust noticeable

®

20%less than 20%



83A B S


Example Number: 39

Notes:

1. Local corrosion on edges ofstiffeneers and drain holes

0.5%



Coating Condition .............................................................................. POOR

Example Number: 40

84 A B S

Notes:

1. Edges breakdown

2. White deposits >5%

3. Red rust

®

5%



85A B S


Example Number: 41

Notes:

1. Edges breakdown on verticalstiffeneers

2. White deposits

3. Abscence of red rust

4. Surface contamination belowweep holes

20%less than 20%




Example Number: 42

86 A B S

Notes:

1. Edge breakdown

2. Corrosion on horizontal surfaces

3. Hard scale on horizontal surfaces

4. Discoloration

®

20%



87A B S


Example Number: 43

Notes:

1. General breakdown >20%

2. Topcoat detachment

20%less than 20%




Example Number: 44

88 A B S

Notes:

1. Corrosion >20%

2. Hard scale >10%

3. Deformed stiffeneer edges

®

10%20%



89A B S


Example Number: 45

Notes:

1. Corrosion 20%

2. Scale not visible

3. Discoloration

20%less than 20%




Example Number: 46

90 A B S

Notes:

1. Corrosion >20%

2. Discolorations

3. Intact coatings on welds

4. Top coat loss

®

20%less than 20%



91A B S


Example Number: 47

Notes:

1. Localized corrosion >20%

2. Steel cross section loss

20%




Example Number: 48

92 A B S

Notes:

1. Corrosion >20%

2. Hard scale >10%

®

10%25%



93A B S


Example Number: 49

Notes:

1. General breakdown on overhead >20%

2. Edge breakdown

3. Rust staining

20%less than 20%




Example Number: 50

94 A B S

Notes:

1. Corrosion >20%

2. Scale >5%

3. Topcoat detachment

4. Black staining

®

5%20%



95A B S


Example Number: 51

Notes:

1. Anode working

2. White deposits

3. Top coat delamination

4. Edge breakdown

5. Coating loss on verticalbulkheads on right 10%less than 10%




Example Number: 52

96 A B S

Notes:

1. Corrosion 20%

2. Hard scale >10%

3. Delamination of scale on bracket

®

10%20%



97A B S


Example Number: 53

Notes:

1. Hard scale

2. Rust stains

3. Surface discolorations

4. Note top surface of lowerlongitudinal

10%less than 10%


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