CORROSION AND ITS PREVENTION

Corrosion is an electrochemical reaction between a metal and the environment(oxygen & water), which results in destruction of metal.

The Corrosion Triangle :

That the ship looks good after painting is a matter of consequence and should not be the main reason for painting. There must be a clear understanding of the difference between “Decorative” and “Protective” painting. Marine paints are expensive and must be properly utilized so as to obtain the maximum value out of them.

Erosion is purely mechanical. It removes protective coating by mechanical means/ abrasion and exposes the metal surface to the atmosphere, resulting in corrosion.e.g. Abrasive effect of dust or salt particles carried by windUse of bulldozers, shovels or scrapers on deck or in the holds.Abrasive effect of Tug coming hard along-side the vessel orVessel going hard against the berth.

The Corrosion Reaction : It involves flow of electrons & is therefore called electrochemical reaction. Whenever a metal is placed in an electrolyte, there is a reaction because of tendency of metal to return to their stable state.

The maximum number of electrons in orbits is given by 2n2 (where n is the number of orbit counting from nucleus onwards).However the last orbit of any element can not hold more than 8 electrons. Galvanic or Reactivity Series:

1. Platinum2. Stainless Steel3. Copper4. Mill Scale More Noble5. Brass (Can be used as Impressed current 6. Nickel Anodes, generally used Silver or lead)7. Tin8. Cast Iron9. Iron & Mild Steel - Ship side 10. Aluminium11. Zinc Less Noble12. Magnesium (Can be used as Sacrificial Anodes)

The Galvanic Cell :

Anode gets corroded Cathode gets protected

The combination of electrodes, electrolyte & electron flow is called a galvanic cell.

This is the principle used in “Sacrificial Anode System” and the “Impressed current System”.

Factors Affecting Corrosion :

1) Humidity : Corrosion increases with humidity. At < 65% humidity, corrosion hardly takes place.

2) Availability of Oxygen : More corrosion takes place at water surface (than underwater or in enclosed spaces due to less oxygen available).

3) Temperature: Higher Air temperature means greater corrosion. Underwater as temperature rises, the solubility of oxygen decreases, so they tend to cancel out each other.

4) Velocity of Ship : greater velocity means plentiful oxygen supply resulting in greater corrosion.

CORROSION CELL

Corrosion always develops at the anode, where current leaves the metal and enters the electrolyte, whilst a protective effect occurs at the cathode. Thus if the whole metal surface is made sufficientlycathodic, corrosion will not occur. This is the basic principle of Cathodic Protection. In marine structures, such corrosion cells may result from the use of dissimilar metals. Usually, however, localised anodic and cathodic areas arise on the surface of the same metal through differences in the metal itself, variations in protective films, variations in stress concentrations or changes in the electrolyte. i.e. aeration, temperature and salinity.Corrosion may be prevented by removing one or more of these corrosive elements and for marine structures, the most practicable method is to apply a protective coating, thus introducing an electrical resistance between the metal and the electrolyte. Paint invarious forms normally provides the first level of protection. However, even the most efficient coatings are subjectto defects during application or service, with inevitable corrosion of the exposed metal.It is therefore generally accepted that cathodic protection, in conjunction with a high performance paint system provides the most effective and economic safeguard against corrosion.

The Sacrificial Anode System :

An anode is a block of relatively pure metal, normally Zinc, Aluminium or Magnesium. Certain alloys may be added to increase the life and make it more economical. Anodes should never be painted over.

The Anode corrodes protecting the steel and so it is made of less noble metal. The flow of electrons is from anode to the ships structure. Zinc or Aluminium anodes are most common. Magnesium anodes can not be used internally in oil tanks as they may cause a spark if they become detatched and fall or are struck against.

Anodes should be periodically inspected for contact damage & weardown. A few spare should be kept on board for replacement.

Areas to be most protected are in the stern region, in the vicinity of the propeller and rudder, the bow propeller tunnel, sea chests, DB tank drain plugs, bilge keel etc.

Advantages :1) No capital outlay for power equipment2) Simple installation, requiring no skilled labour.3) Power can not be supplied in the wrong direction.

Cathodic Protection can only be used to protect the underwater hull or ballasted tanks as seawater forms the electrolyte. In ballast tanks, the sacrificial Anodes are placedClose to the bottom of the tank.

The weight of anodes required to protect a given area of the ships hull is given byW=(IxAxH) / (l000xK)where,W = Weight of the anodes in kilograms,I = Current in milliamperes per square metre (ma/m2)A = Area to be protected in square metres,H = Duration of protection in hours,K = Constant of the material used.

for Zinc,K = 760 aH/ kg @ 90% efficiencyfor aluminium, K = 2400 aH/kg @ 90% efficiency

Potential measurements can be made around the ship by means of a reference cell. If the paint condition is poor, then more current will be required to protect the hull by providing extra anodes.Some current requirements for various structures are:Well painted hull : 1 ma/ m2

Underwater hull : 10-20 ma/ m2

Ballast tanks 50-70 ma/ m2

Oil rigs : 130 ma/ m2

Impressed Current System :Sacrificial Anodes are unsuitable for large ships because of extra turbulence and because it is more costly & cumbersome to fit them on large ships. Therefore for such vessels Impressed Current system is used.

Electrical connection with the hull via slip rings & brushes on the rudder stock & propeller shaft ensures the protection of the rudder & propeller.

As the underwater paintwork deteriorates, higher currents are required for protection.However too high a current can result in damage to paintwork &Chalky deposit on areas of bare metal, which has to be removed Before repainting can be carried out.

A protective shield of epoxy resin is applied for about 1 metre around the anodes to withstand the alkaline conditions there.

Also in ICP system the Anodes are well insulated from the hull & it’s appendages, simply because the Anodes are made of noble metals & when ICP system is switched off, if these Anodes are in direct contact with the hull they may cause corrosion of hull.

Mill Scale

Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, iron oxides consisting of Iron(II,III) oxide, hematite and magnetite.

Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are being produced by rolling red hot iron or steel billets in rolling mills. Mill scale is composed of iron oxides mostly ferric and is bluish black in color. It is usually less than 1 mm (0.039 in) thick and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating.

Because it is electro-chemically cathodic to steel, any break in the mill scale coating will cause accelerated corrosion of steel exposed at the break. Mill scale is thus a boon for a while, until its coating breaks due to handling of the steel product or due to any other mechanical cause.

It is a nuisance when the steel is to be processed. Any paint applied over it is wasted since it will come off with the scale as moisture laden air get under it. Thus mill scale has to be removed from steel surfaces by flame cleaning, pickling or abrasive blasting. All tedious operations wasteful of energy. This is why shipbuilders used to leave steel delivered freshly rolled from mills out in the open to allow it to 'weather' till most of the scale fell off due to atmospheric action. Nowadays most steels mills can supply their produce with mill scale removed and steel coated with shop primers over which welding can be done safely.

TYPES OF CORROSION1. Electro-chemical :

For an electro-chemical reaction, there must be anodic and cathodic areas on the metal and a path i.e. electrolyte, for the current to flow. On a single metal plate, different areas on the plate act as anodes and cathodes. The anodic area is usually surrounded by the cathodic area. This anode and cathode in the presence of an electrolyte makes up a ‘CORROSION CELL’Thousands of such corrosion cells exist all over the plate. The part which acts as the anode gets destructed while the part which acts as the cathode is protected. When atmospheric conditions are uniform such as high temperatures when accompanied by high humidities ashore, the corrosion cells are very small so that the plate corrodes uniformly all over forming a thin continuous layer of rust called ‘MILL SCALE’ When conditions are severe and non-uniform such as at sea, the corrosion cells are large and the plate gets severely corroded at places in what is known as ‘PITTING’.

Oxygen gets absorbed in the water forming hydroxyl (OH) ions which react with the free iron ions liberated by the anode to form rust. The complete reaction is,

Rust is formed in between the anode (where the free iron ions are present) and the surface of the water drop (where the hydroxyl ions are present).

Here rust is formed close to the anode and as the reaction proceeds, it may cover the anode and stifle the reaction. Rust being porous, the reaction does not completely stop but proceeds at a slow pace. However if the rust is removed, corrosion will again resume, hence the rust should not be removed unless it intended to coat the steel with paint.

Corrosion cells on a steel plate

2. Cavitation (Erosion / Corrosion)In a liquid, where there is turbulence-a due to rotation, vibration, etc., vacuum cavities are formed by the generating force e.g. in the propeller region. These vacuum forces act on the steel hull and paintwork only momentarily after which they collapse with astounding force exerting opposite force on the hull. The vacuum cavities are constantly generated due to movement of the propeller and as a result of the immense alternating forces being exerted on the hull, it moves slightly in and out like a bellows. This causes the paint to crack and break off as it is also subject to these forces. Thus the surface of the metal gets eroded and opens the way for electro-chemical corrosion to take place. This type of corrosion is also observed in pumps and in pipeline flow.

3. Impingement attack When the protective coating is removed, the surface is exposed as the anode and becomes susceptible to corrosion. Impingement corrosion is promoted by excessive local turbulence which results in anodic and

cathodic areas being formed, due to differing liquid velocities and/or retention of a surface layer. If the anode which is the impingement site is large as compared to the cathode, the corrosion will be light. However if the cathode is large as compared to the anode, the corrosion will be severe. If the turbulence is uniform over the whole area, the rate of corrosion will be less. This form of attack often occurs at the waterline especially at the bow and at inlets and outlets on the shipside. It can be reduced by designing the system to keep turbulence to a minimum.

4. Bimetallic Corrosion

Aluminium superstructure joined to a steel deck

Bimetallic corrosion occurs when two metals are in direct contact with each other or indirectly in contact through an electrolyte. To prevent corrosion, the metals must be adequately insulated from each other or protected by a good paint film. The more noble metal must be used for fasteners to join the two metals. The rate of corrosion will depend on the relative sizes of the anodic and cathodic materials. A small anode will corrode rapidly when attached to a large cathode, e.g. a steel flange on acopper pipe. Areas most susceptible to bimetallic corrosion are:

a. Welded areas as the weld is different from the parent metal.b. Inlets and outlets to tanks and on the shipside.c. Valves as they are composed of different metals.d. Aluminium superstructures on steel decks, steel lifting hooks in aluminium lifeboats, aluminium portholes in steel bulkheads.e. Bow and stern propeller areas.

5. Stress Corrosion : When a metal is subject to stresses such as bending, cold working or heat treatment, the alignment of the grains in the metal are disturbed. Thus anodic areas are set up which corrode and failure takes place along these areas. This type of corrosion was quite common in rivetted ships where the rivets used to corrode and fail. It can also occur in brackets and pipes which are cold flanged.

6. Crevice corrosion : When an electrolyte gets trapped within a crevice, either between a joint, or between close-fitting components or within rust, due to a chemical reaction, corrosive ions are formed which saturate the crevice. This results in the localised areas around the crevice becoming the cathode and corroding the crevice.

Crevice Corrosion

Composition of Paint

A) Vehicle or Binder

The binder, commonly referred to as the vehicle, is the actual film forming component of paint. It is the only component that must be present; other components listed below are included optionally, depending on the desired properties of the cured film.

The binder imparts adhesion, binds the pigments together, and strongly influences such properties as gloss potential, exterior durability, flexibility, and toughness.

Binders include synthetic or natural resins such as acrylics, polyurethanes, polyesters, melamine resins, epoxy, or oils.

Binders can be categorized according to drying, or curing mechanism.

1) Alkyd Paints – Oxidation Curing vehicle or binders –These require oxygen to cure. When applied to a surface, the solvent evaporates & oxygen penetrates the film helping the binder chains to interlink. Such paints remain liquid as long as they are in a closed paint drum – not exposed to air.These paints can not be applied as thick coats as it will prevent oxygen reaching bottom layers.

2) Chlorinated Rubber, Vinyl, Coal Tar & Bituminous Paints, drying oils, oleo resins – Physically drying vehicle – cure by evaporation of Solvent-There is no chemical process involved- therefore these are not sensitive to temperature conditions during applications. When touching up an old coat, the fresh paint will dissolve the edges of the old paint making the two coats fuse into each other.

3) Epoxy & Polyurethane Paints – Chemically curing Vehicle-Consist of two or three components i.e. a hardener, an activator & the base. The curing process begins when the components are mixed together in the correct proportion. The time between mixing & curing is known as “potlife”. This

may vary from several minutes to a few hours, depending upon product type & temperature. Suitable atmospheric conditions are required for application of these paints. Such paint will harden and dry at temperatures > +100C & Relative Humidity < 80%. These paints are highly abrasion resistant.

B) Pigment :

Pigments are granular solids incorporated into the paint to contribute color, toughness, texture or simply to reduce the cost of the paint. Alternatively, some paints contain dyes instead of or in combination with pigments.

Pigments can be classified as either natural or synthetic types. Natural pigments include various clays, calcium carbonate, mica, silicas, and talcs. Synthetics would include engineered molecules, calcined clays, blanc fix, precipitated calcium carbonate, and synthetic silicas.

3) Solvent

The main purpose of the solvent is to adjust the curing properties and viscosity of the paint. It is volatile and does not become part of the paint film. It also controls flow and application properties, and affects the stability of the paint while in liquid state. Its main function is as the carrier for the non volatile components. In order to spread heavier oils (i.e. linseed) as in oil-based interior housepaint, a thinner oil is required. These volatile substances impart their properties temporarily—once the solvent has evaporated or disintegrated, the remaining paint is fixed to the surface.

This component is optional: some paints have no dilutant.

4) Extenders ( or Fillers)These enhance the weather durability and consistency of the paint.They increase the volume of paint & thereby reducing the price.

5) Auxilliaries These are used for imparting special properties.

e.g. Walnut Shell powder is used for imparting “Anti-Skid” property.

Antifouling Paints : are the most expensive type of paints which contain poisnous substances like cuprous oxides, mercury, arsenic, tin, lead or zinc compounds which are slowly released into water killing marine organisms such as slime(bacteria), Weed(Algae) & barnacles etc. before they attach themselves to the ship.These organisms destroy the paint film as well as increase the hull roughness causing extra fuel consumption.

The leach rate i.e. the rate at which the poison is released is governed by the type & amount of toxin in the paint, the solubility of the binder and the design of the paint. Today high performance Antifoulings can prevent fouling for upto 5 years.

Two basic types of Antifoulings :

Soluble Matrix Type :Toxins are contained in water soluble binder composed of acidic resin. This resin reacts with alkaline sea water and slowly dissolves, releasing the toxin.

Insoluble Matrix Type:Poison particles are in contact with each other throughout the film. Also known as contact Antifouling. The poison is removed by sea water as it reaches into the paint film.

Long Life/ High Performance Antifoulings – Two types

1) Reactivable Antifouling:Chlorinated rubber based, highly charged with cuprous oxide. When the toxin level in the outer layer becomes low, the surface can be scrubbed down using special automated equipment. The top

layer of the paint is removed, exposing a fresh film. Reactivation is carried out at intervals of 1-2 years.

2) Self Polishing AntifoulingsAcrylic binder with organo tin poisons. The binder slowly dissolves in sea water releasing the poison in a controlled manner.The abrasive action of sea water as the ship moves through it, keeps the hull smooth, reducing friction and thereby fuel consumption. Life of paint 3-5 years.

Antifoulings with TBT- Organotin Tributylin, have been proven to cause deformation in oysters & sex change in whelks(sea snails).

These antifoulings have been banned by “International Convention on the control of harmful Anti Fouling Systems on ships”.As recommended by the 21st session of the IMO assembly, the conference agreed to an effective implementation date for a ban on the application of organotin based systems.

By Jan 2008 ships either- shall not bear such compounds on their hull or external parts or surfaces

or

- shall bear a coating that forms a barrier to such compounds leaching from the underlying non-compliant anti fouling compounds.

This convention includes a clause in Article 12, which states that a ship shall be entitled to compensation if it is unduly detained while undergoing inspection for possible violations of the convention.

Treatment of steel in a shipyard

In the shipyard corrosion protection is of prime importance.

Pre-HeatingThe plates & profiles first pass through a pre-heater, this raises the temperature of the metal ready for blasting & removes any surface moisture.

Blast CleaningSheets & profiles are thoroughly blast cleaned.The blast chamber removes rust and mill scale and provides a finish to internationally recognised preparation grades.

Rust gradesFour rust grades are specified. These are defined by precise written descriptionsand photographic examples in ISO 8501-1documentation. They vary from A: mill scale,to D: where the mill scale has rusted away and general pitting is visible.

Preparation gradesSurface preparation by blast cleaning is designated by the international standard ISO8501. Four grades are specified, ranging from Sa 1: light blast cleaning, to Sa 3: blast cleaning to visually clean steel.

Paint spray chamber, voc-system and filterThe various plate and profile widths are automatically identified and are coated in a continuous process with a weld-primer coatingthickness of approximately 15 - 25μm.Paint dust and solvents (if no water based paint is used) are treated according to the local requirements in an automatic filter unit and aVOC-treatment plant.

Drying chamber & slat conveyorThis chamber can be heated by the exhausted air from the pre-heater . Additional circulation of high quantities of air acceleratesthe drying process.Lying on the support points of the conveyor cross slats

the wet primer remains undamaged as the plates pass through thedrying chamber.

MarkingEach sheet has its own unique identification to allow subsequent allocation and control. The mark is spray painted by computer controlled nozzles.

Edge cleaningTo ensure optimum weld quality, the edges of the profiles are blasted to remove paint from the weld area .Airblast units for this operation are suitable for smaller profiles and often precede a gascutting machine.

ConveyingThe plates are now conveyed to be fabricatedinto ship segments.

Surface Preparation Standards & their meaning:Steel Structures Painting Council / Swedish Standard/MeaningSSPC-SP-1 / - / Solvent CleaningSSPC-SP-2 / St 2 / Hand Tool CleaningSSPC-SP-3 / St 3 / Power Tool CleaningSSPC-SP-4 / - / Flame CleaningSSPC-SP-5 / Sa 3 / White Metal Blast CleaningSSPC-SP-6 / Sa 2 / Commercial Blast CleaningSSPC-SP-7 / Sa 1 / Brush Off Blast CleaningSSPC –SP-8/ - / Pickling(Removal of rust or millscale by electrolysis or chemical reaction or both)SSPC-SP-9 / - / Weathering followed by blast cleaningSSPC-SP-10/ Sa 2.5 / Near White blast cleaning

Safety Precautions for Painting

Typical Painting Scheme – Chugoku Paints

antifouling for underwater area and boottop

Type of system Name of paintNumber of coats

Recommended DFT

(in total) (6)Typical surface

preparationHolding primer (optinal)

NZ primer Sor

Epicon zinc rich primer B-2

1(1)

25 (u)25 (u)

Abrasion Resistant Pure Epoxy and Self-polishing

Antifouling Latest generation Tin Free

Hydrolisys Type

Bannoh 500 R 2 250 u

3+1Sea Grandprix

Series* (2)1-3 100-450 u

Abrasion Resistant Pure Epoxy and Self-polishing

Antifouling 1st generation Tin Free

Bannoh 500 R 2 250 u3

+1TFA Series orSeatender Series* (2)

1-3 100-450 u

Multipurpose Tar-free Epoxy Self-polishing

Antifouling 1st generation Tin Free

Umeguard SX HS 1 175 u

4+1

Hiper AC 1 50 u

TFA Series* orSeatender Series (2)

2-3(2-3)

100-450 u(100-450 u)

Chlorinated RubberRavax AC-P 2 120 u

3Ravax AF 2 80 u

Antifouling SealerHiper AC 1 50 u

Silvax SQ-K 1 50 u

underwater area

Type of system Name of paintNumber of coats

Recommended DFT

(in total) (6)Typical surface

preparationHolding primer (optinal)

NZ primer Sor

Epicon zinc rich primer B-2

1(1)

25 u(20 u)

Abrasion Resistant Pure Epoxy 1st Generation Foul Release Coating

Bannoh 500 1 125 u 1

Bannoh 500 1 125 u

Sea Grandprix Eco- 1 100 u

Speed Tie-Coat

Sea Grandprix 1 150 u

Abrasion Resistant Pure Epoxy 2nd Generation Foul Release Coating

Bannoh 500 1 125 u

1Sea Grandprix Eco-Speed C Tie-Coat

1 100 u

Sea Grandprix Eco-Speed C

2 150 u

boottop area

Type of system Name of paintNumber of coats

Recommended DFT

(in total) (6)Typical surface

preparationHolding primer (optinal)

NZ primer Sor

Epicon zinc rich primer B-2

1(1)

25 (u)25 (u)

Abrasion ResistantPure Epoxy

Epoxy Finish

Bannoh 500 2 250-300 u1

Epicon Marine Finish 1 50 u

Surface Tolerant EpoxyEpoxy Finish

Umeguard SX 1-2 150-300 u4

Epicon Marine Finish 1 50 u

Surface Tolerant EpoxyAcrylic Finish

Umeguard SX 1-2 150-300 u4

Acri 700 Finish 1 40 u

topside area

Type of system Name of paintNumber of coats

Recommended DFT

(in total) (6)Typical surface

preparationHolding primer (optinal)

NZ primer Sor

Epicon zinc rich primer B-2

1(1)

25 (u)25 (u)

Abrasion ResistantPure Epoxy

Epoxy FinishPolyurethane Finish

Bannoh 500 2 200-300 u

1Epicon Marine FinishUny Marine Finish

1(1)

50 u(50 u)

Surface Tolerant Epoxy Umeguard SX (3) 1-2 125-300 u 4

Polyurethane FinishEpoxy FinishAcrylic Finish

+1Uny Marine Finish

Epicon Marine FinishAcri 700 Finish

1(1)(1)

50 u(50 u)(50 u)

AcrylicAcri 700 Primer 2 120 u

3Acri 700 Finish 1 40 u

OleoresinousLZI Primer HB 2 140 u

3Evamarine Finish 1 40 u

outside superstructure, fittings

Type of Paint

Existing System

Name of PaintNumbe

r of Coats

Recommended DFT

(in total)(6)

Typical Surface

Preparation

Holding Primer

(optional)

NZ Primer S orEpicon Zinc Rich

Primer B-2

1(1)

25 u(20 u)

Surface TolerantEpoxy

Epoxy or Acrylic

Chlorinated Rubber

Oleoresinous

Umeguard SX(3) 1-2 125-200 u

6Finish Coat (4) 1 40-50 u

AcrylicChlorinated

Rubberor Acrylic

Acri 700 Primer 2 120 u5

Acri 700 Finish 1 40 u

Oleoresinous Oleoresinous

Roswan QD HBLZI Primer HB

2(2)

140 u(140 u) 5

Evamarine Finish 1 40 u

weather deck coating

Type of system Kind of Paint Name of PaintNumbe

r of Coats

Recommended DFT (in

total)

Shop primer

Epoxy Non ZincEpoxy Zinc RichInorganic Zinc

Ultra Heat Resist

NZ primer S orEpicon Zinc rich primer

B-2Elbond HM

Cerabond 2000

1(1)(1)(1)

22 u(18 u)(15 u)(15 u)

Epoxy(Anti-abrasion)

Anticorrosive Bannoh 500 1-2 125-2000 u

Finish Epicon Marine Finish 2 100 u

Epoxy PrimerEpoxy Finish

Anticorrosive Umeguard SX 1-2 125-200 u

Finish Epicon Marine Finish 2 100 u

Epoxy PrimerAcrylic Finish

Anticorrosive Umeguard SX 1-2 125-200 u

Finish Acri 700 Finish 2 80 u

cargo holds

Type of Paint

Existing System

Name of PaintNumbe

r of Coats

Recommended DFT

(in total)(6)

Typical Surface

Preparation

Holding Primer

(optional)

NZ Primer S orEpicon Zinc Rich

Primer B-2

1(1)

25 u(20 u)

Pure Epoxy(Anti-

abrasion)Epoxy

Bannoh 500 (5) 2 200-300 u1

Finish Coat (4) 1 40-50 u

EpoxyEpoxy or Acrylic

OleoresinousUmeguard SX (5) 2 200-300 u 2

Oleoresinous Oleoresinous

LZI Primer HB 2 140 u

3Hold Silver orEvamarine Finish

(5)

1(1)

25 u(40 u)