Corrosion and Its Prevention

 Learning Objectives

After completing this topic you will be able to:

•  Explain

1. Corrosion, Erosion and corrosion triangle
2. How a corrosion cell forms
3. Galvanic/ periodicity series of metals
4. Stress related corrosion cells
5. Formation of corrosion cells due to difference in surface conditions

• Describe

1. Cathodic protection using
      Sacrificial anodes
      Impressed current
2. Measures to minimize corrosion
3. Treatment of steel in a shipyard

• Explain the structure of paint and purpose of each constituent
• Use Material Safety Data Sheets (MSDS) effectively. Safety precautions when using paints
• List and state suitability of the following paint vehicles along with their application

1. Drying oils
2. Oleo resins
3. Alkyd resins
4. Polymerizing chemicals
5. Bitumen

• Make paint schemes for

1. Underwater areas
2. Boot topping
3. Top sides
4. Weather decks
5. Superstructures, Deckhouses and Interiors
6. Tank interiors

• Understand and plan the surface preparation of surfaces to be coated
• Understand the mechanics and manner in which anti fouling , anti corrosive  works
• Calculate wetted surface area using   of ship. Spread rate.

Corrosion, Erosion and the Corrosion Triangle

Corrosion

Metal are usually the product of a turbulent metallurgical process whereby ore is converted into metal. Corrosion can be termed to be a process whereby this metal that is born out a highly thermodynamic process tends to revert to its original form be it an oxide, sulphide or hydroxide. It is the slow destruction of the metal by chemical or electrochemical action, in some casesit is accelerated by erosion.

Erosion

Erosion is defined as the wear caused by particles carried by a stream of fluid be it liquid or gas passing over a surface. Erosion of the parent material is dependent on many properties that include the hardness of the particle being carried by the fluid and also the mechanical properties of the substrate material.

Cavitation is an extreme form of erosion and occurs in the vicinity of propellers, damaging the blade edges.

 

Erosion-Corrosion

Erosion-corrosion is the interactive process involving both erosion and corrosion, in which each of these processes is affected by the other, usually accelerating the solo effect of each component.

Corrosion Triangle

For corrosion cell to form it requires three things, an Anode, Cathode and Electrolyte. These form the three sides of a triangle and form the Corrosion triangle. Removal of any one of the constituents breaks the chain and stops the corrosion process.

Formation of corrosion cell

Types of Corrosion

Basically all corrosion is of an electrochemical nature, having said that we can now separate corrosion into different types. However due to the prevalence of electrochemical corrosion on ships we will deal with it in some detail.

Electrochemical or Galvanic corrosion

When two dissimilar metals are immersed in a conducting solution (electrolyte) and connected electrically, galvanic action commences. One of the metals becomes the anode, which releases electrons and corrodes, and the other acts as a cathode and releases ions. It should be noted that the galvanic circuit can also be completed in the same metal itself. E.g. Mill scale (oxidation) that coats steel and the underlying metal. The conducting connection between the metals completes the circuit and electricity is generated.

 

Which metal acts as the anode depends upon its periodicity. It should be noted that this periodicity is different from the general periodic table, since it gives the periodicity of the metal when immersed in salt water. More active metals i.e. those that are most likely to corrode are placed at the top of the list and will form the anode in the presence of another metal lower down the list that forms the protected cathode. Metals at the lower end exist in metallic form in nature and are termed noble. Eg Gold is found naturally in metallic form but iron (Fe) never is.

Since ships are primarily built of mild steel, we will consider how corrosion affects the metal. The galvanic cell described above can start at molecular level, and this cell is called a Corrosion cell.

Corrosion, and the rate at which corrosion occurs in seawater for example is dependent upon many factors, including oxygen levels, pH, temperature and salts dissolved in seawater such as chloride, sulphate, and sulphites.

Galvanic series of metals

Galvanic Series in Sea Water

Stress concentrations leading to corrosion cell formation

Stress corrosion

Corrosion and stress interact to produce Stress Corrosion Cracking (SCC). The corrosion cell is the precursor to the crack formation.

Stress corrosion fractures link a particular metal to a certain environment. E.g. High Tensile Steels with Hydrogen. They occur in pipes, boilers and stress junctions when the stress exceeds a particular value. Controls of stress and corrosion inhibitors are known to reduce instances of SCC.

Difference in surface conditions leading to formation of corrosion cells

Mild steel is poor in resisting corrosion and its high position on the galvanic scale is because it is a heterogeneous metal, with lots of impurities that lend themselves to the formation of a corrosion cell. We will now consider a mild steel plate that has a coating of mill scale on it with a small pit or break in the mill scale which allows the oxygen laden seawater to come in contact with the steel. Here we have all the three ingredients of the triangle and a cell forms.

The plate becomes the anode and the anodic reaction consisting of the Iron (Fe) atom becomes Ferrous with the release of free electrons into the electrolyte which proceed to the cathode (Mill scale).

At the cathode these electrons trigger a reaction in which they combine with the Oxygen (O²) dissolved in the water, the Water (H₂O) to produce Hydroxyl ions that travel to the ferrous ions (Plate) and complete the flow of electrons and also combine to form Ferrous Hydroxide, which in turn combines with the O₂ present to give us the familiar “Rust” (Ferric Oxide.

The reaction can be summarised as follows

4Fe (Iron Plate) + 3O₂ (Oxygen in Seawater) + H₂O (Seawater) = 2Fe₂O₃.H₂O (Rust)

Pitting corrosion

Localised, but deep penetrating corrosion, caused by surface imperfections or spot breakdowns in coating. Observed mainly in ballast tanks which have not been maintained for a long period.

Bacterial or Anaerobic corrosion

This type of corrosion manifests itself mainly in oil, ballast tanks and cargo pipelines. This corrosion happens in the absence of oxygen, and is also termed microbiological corrosion. Research seems to indicate that sulphate-reducing bacteria (SRB) consume hydrogen through the action of their hydrogenase enzymes, and accelerate this type of corrosion. The conditions in which it occurs are

• Presence of sulphates, hydrocarbons and bacteria
• Temperature range of 20 – 40 degrees Celsius.

Cathodic protection using sacrificial anodes

Cathodic Protection

When corrosion occurs in metals, it creates its own potential known as corrosion potential. This is a very important in the fight against corrosion. In Cathodic protection systems, a metal with corrosion potential that is more negative than the structure is used, thereby converting the structure which was an anode into a cathode and the more negative metal as the anode where corrosion takes place. Care should be taken that the environment is not made excessively negative since that will accelerate corrosion by creating an excessively alkaline environment. It can also cause detachment of coatings.

Sacrificial

As the name implies, cathodic protection is provided, with the anodic metals getting corroded (sacrificed) in lieu of the steel structure. A ship’s structure is primarily made of steel, but various other metals in smaller quantities are used in certain places of the structure. Even the steel used may not be exactly the same in quality in every part of the ship’s structure. Such a structure consisting of dissimilar metals, when immersed in an electrolyte like salt water, can form a cell where one metal will be an anode and the other a cathode. This will result in flow of electrons from anode to cathode, which will corrode the anode. Metals used for anodes are usually magnesium, aluminum or zinc. The preference for shipboard use being zinc. It should be noted that for cathodic protection to be effective, the protected areas should be immersed in an electrolyte. E.g. Hull and tanks. The anodes are carefully distributed around the target area to evenly distribute the negative potential. The anodes themselves are welded to the steel structure, and have a normal life of 4-5 years, after which they have to be renewed. Needless to say they are used in conjunction with well coated systems.

The following media explains about the cathodic protection using sacrificial anodes

Sacrificial systems have the advantage of being

   a) Simple to install 

   b) Independent of a source of external electric power

Disadvantages being that they cannot self regulate themselves. Eg., if the coating state deteriorates and an increase in electron flow is required, that increase cannot be provided.

Impressed current system

Impressed Current

An impressed current system uses an external DC power source to provide a continuous supply of electrons to the anode. The anode itself is permanent, made of either titanium and in some rare instances platinum. High input impedance voltmeter and suitable half-cell, Silver alloy sensors continuously measure the electric potential of the hull, and supplies the data to a control unit which adjusts the supply of current to the permanent anodes. Being self adjusting, the system can adjust itself to deterioration in hull coatings, though this will increase the electricity consumption.

Monthly data from the system is analyzed usually by the manufacturer to check data so that the condition of the system and condition of the coatings can be ascertained.

Impressed-current installations have the advantage of being 

• Able to supply a relatively large current 
• Enables it to be used in most types of electrolytes 
• Able to provide a flexible output that may accommodate changes in, the hull coating.

Impressed current systems are to be switched off when divers are at work in and around the hull.

Measures to minimise corrosion

• Barrier coats: Shielding the metal from the atmosphere/ salt water by applying a coating film on it. This effectively destroys one leg of the corrosion triangle, either stopping existing corrosion or preventing corrosion cells from forming.
• Cathodic protection: As explained in this section, preventing the steel structure from becoming an anode.

It should be noted that these measures are adopted singly or together, depending on the area to be protected.

Treatment of steel in shipyard

Treatment of steel in a shipyard:

1. Removal of mill scale

A thin layer of Iron Oxide (I, II, III) that forms over hot rolled steel as the sheets/ plates come out of the rolling mill. They are Cathodic to steel, but perform the task of a barrier against the atmosphere in the stock yard where plates are kept prior to their being used.

2. Application of barrier coats in the form of shop/ holding primers

Prior to being used, the mill scale is removed by shot blasting/ pickling in a humidity controlled oxygen lean atmosphere and then immediately coated with shop or pre fabrication primers. These form the base coat for subsequent final painting schemes. Other than the obvious anti corrosion properties, pre fabrication or shop primers should be:

• Short drying time
• Compatible with cutting/ welding systems
• Not emit noxious or toxic vapors during welding/ cutting
• Have high abrasion resistance

3. Applying barrier coats to sections damaged in the cutting/ welding/ bending process while building section blocks.

Damages to the pre fabrication primer are made up by blasting the seams and applying coatings of paint. This is also done carried out in rooms that have humidity and temperature control. Finally after the blocks are put together, the ship owners chosen paint scheme is applied.

4. After assembly of the sections/ blocks of the ship,

The blocks are painted so that the shop primer does not get weathered/ damaged. Sometimes the “paint and then weld” process is used. The welding damage to the final paint is made up by stripe coats.

The main disadvantage of block painting is a risk of mechanical damage and burns during transport of the sections to the slipway and during fitting-out. However, this damage can be minimized by adequate planning during the construction phase

5. Surface defects

Removed by grinding keeping within the allowed tolerances.

6. Cathodic (Fitting out dock)

Before the hull is floated out/ launched, cathodic protections in the form of sacrificial anodes are used. Impressed current systems replace sacrificial anodes only in the final stage of ship building.

The following media explains about the Treatment of steel in shipyard

Structure of paint and purpose of each constituent

Paint structure

We can define paint by stating that it is an opaque pigmented coating, which could be in liquid or powder form, which gives corrosion protection, decoration and in the case of specialty coatings specific technical properties.

It consists of:

Film formers/ Resins: Molecular products which react to form macro molecular structures e.g. chlorinated rubber/ epoxy paints. Most film formers are also known as resins. They form a continuous adhering film to the metal substrate.

Plasticizer: They are additives, usually organic oils with high evaporation temperature and help the paints flow and flexibility i.e. the spread rate. Vinyl acetate polymers are also used as plasticizers.

Film formers/ Resins and Plasticizers together combine to form the Binder of a paint. This acts as a adhesive which bonds the paint to the surface.

Pigments & Extenders: Finely ground materials which give the paint colour, special properties eg corrosion control. They can be organic or inorganic. Extenders do not add to colour but give special properties to paints e.g. Gloss, abrasion resistance

Solvents: Volatile liquids which make most paints very inflammable. They are used in paint as a solvent for dissolving resins and other pigments. They evaporate soon after application and pay no role in any of the paints properties.

Purpose of Material Safety Data Sheet (MSDS)

Material Safety Data Sheets (MSDS)

As the name suggests this document which is unique for every product contains safety information regarding the Paint. It contains information regarding:

• Handling & storage:

1. Temperature
2. Spill handling methods
3. Ventilation requirements
4. Physical & Chemical properties eg Flash point, Density

• Pre and post application methods:

1. Mixing ratios
2. Precautions for use with spray painting systems

• Medical First Aid & Pollution potential

1. Toxicological information
2. Environmental data & Disposal information

• Fire fighting information.

1. Extinguishing agents

Common paint vehicles - Drying oils, oleo resins, alkyd resins, polymerizing chemicals, Bitumen and suitability of each for various applications.

Paint vehicles along with their application

Drying oils

Belong to the Film former/ Resin category. A category of vegetable oils which could dry at normal ambient temperature. Eg Linseed oil. They have now become obsolete having been replaced by Resins and Binders.

Oleo resins

Used for thinning oil based paints, the most common being Turpentine. Has been largely replaced by paint thinners. Its use in varnishes continues but is being replaced by more efficient synthetic compounds.

Alkyd resins

These resins are useful as film forming agents in paint, varnish and enamels. Alkyd resins are one of the important ingredients in the synthetic paint industry having replaced Drying oils and oleo resins. Different types of alkyd resins, mainly characterized by their oil content, are used for different synthetic paint products.

Polymerizing chemicals

Polymerization is a chemical process whereby polymer chains are created. An example of Alkene Polymerization is Polystyrene. Paints that cure by polymerization are generally two-pack coatings that polymerize by way of a chemical reaction, and cure into a cross linked film. There are Alkyd Polymers used in high gloss paints, Epoxy polymers used for anti corrosive paints.

Bitumen

Used in coatings, used for weather proofing and anti corrosion. E.g. Coal Tar epoxy. It is resistant to harsh chemicals, UV light and high moisture levels, and is used on the hull and also in bilges. Owing to some toxic properties Bitumen paint is gradually being replaced by Polyurethane coatings.

Paint schemes for Underwater areas, boot topping, top sides, weather decks, super structures and tank interiors

Paint schemes for:

• Underwater areas

The properties required are

1. Corrosion inhibiting
2. Fouling resistant
3. Abrasion resistant
4. Smooth
5. Compatible with Cathodic protection

Two coats of chemically curing two-component epoxy or coal tar epoxy. Polyurethane /coal tar combinations, which cure at lower temperatures than epoxy/coal tar, can be used. A barrier coat is then applied to prevent the anti corrosive paint from bleeding into the antifouling coat. This sealer coat is followed by two coats of TBT free antifouling. Total film thickness will be 250 - 400 microns.

• Boot topping

The main requirement is an abrasion and scratch resistant surface. The paint scheme will be similar to the underwater hull except that the anti corrosive coatings should be Glass flake reinforced epoxies.

• Top sides, Superstructures, Deck houses

1. Cosmetic qualities
2. Gloss
3. Corrosion inhibition

Two coats of anticorrosive epoxy followed by a single topcoat of aliphatic aliphatic polyurethane/acrylic paint. Passenger ship uses a final coat of anti stain coat which contains ingredients which react with rust stains to produce a colourless water soluble compound. Total dry film thickness around 150 microns.

Increasingly water based topcoat finishes are being used.

• Weather decks

1. Corrosion inhibition
2. Abrasion and impact resistance
3. Anti skid properties
4. Resistant to seawater

Fuel and lube oil spills

Two pack epoxies, polyurethanes or zinc silicate paints with a DFT of 250 microns for epoxies and polyurethanes and 75 - 100 microns for zinc paints. The top coat can be made anti slip by adding fine aggregates to it.         

• Interiors

1. Corrosion inhibition
2. Cosmetic  qualities
3. Fire resistance 
4. Non toxic

Epoxy acrylic or silicon alkyd emulsions on a anti corrosive multi purpose epoxy primer.

          

• Ballast Tank interiors

The properties required are

1. Resistant to polluted sea water
2. Corrosion inhibiting
3. Free of Pores
4. Compatible with Cathodic protection and resistant to its by products

Two to three coatings of straight, modified or solvent free epoxy with a total film thickness of around 250 - 300 microns. The coats should be preferably light colored.

For cargo tanks, other than similar requirements is the important aspect of compatibility with the cargo to be carried.

Surface preparation for painting

Surface preparation of surfaces to be coated

Proper surface preparation prior to coating is essential and cannot be overemphasized. The steel surface should be completely free of oil, grease, old coatings and surface contaminants (such as mill scale and rust).

Standards of surface preparation are set by International Standards:

• ISO 8504-1:2000 Part 1: General principles. Preparation of steel substrates before application of paints and related products. 
• ISO 8504-2:2000 Part 2: Abrasive blast-cleaning. Preparation of steel substrates before application of paints and related products 
• ISO 8504-3:1993 – Part 3: Hand- and power-tool cleaning. Preparation of steel substrates before application of paints and related products
• Various national standards.

Oil and Grease

The presence of even a very thin layer of oil or grease can destroy or seriously impair adhesion of paint. Solvents can be used to dissolve the grease, but the solvent itself should be completely removed. Commercial chemical cleaners such as water rinsable detergents are available but before they are used it must be determined whether they are compatible with the paint and environment protection rules. When cleaning tanks which have contained crude oil before overcoating, it is likely that a combination of steam cleaning and degreasing will be necessary.

Salts

Sea salts are fairly easily removed by fresh water. The main problem is that of surface irregularities and porosity. Fine hair cracks can tenuously retain salts. A high pressure water spray is the most effective, and if it is not available then a thorough scrubbing is necessary.

Weed and Shell fouling

Can only be effectively removed with a high pressure water lance (2000 - 5000 psi)

Mill scale, Rust and Old paint

Manual with Power tools

May be removed by hand wire brushing, sanding, scraping and chipping. However, these methods are incomplete, and always leave a layer of tightly adhering rust on the steel surface. Power tools wire brushes, impact tools (such as needle guns and headers), grinders and sanders are all commonly used but none provide the ideal surface. One danger of indiscriminate use of wire brushes is to polish steel since this interferes with adhesion.

Blast Cleaning

The most effective method for removal of millscale, rust and old paint. Uses abrasives such as garnet, grit or shot under high pneumatic .

Blasting standards are graded eg ISO 8501- 1:1988, preparation of steel substrate before application of paints and related products – visual assessment of surface cleanliness. These along with other national standards (ASTM, SSPC, JSRA) are slowly replacing the old Swedish (SA) grades. There is no equanimity in these grades but SA 2.5 is taken to be same as (ISO 8501-1:1988) or SSPC-SP10 for Marine structures that are exposed to a severe corrosive environment.

Swedish standards:

• BRUSH OFF (Loose mill scale, loose rust and foreign particles are removed) - SWEDISH Sa.1
• COMMERCIAL (Mill scale, rust and foreign particles are substantially removed and grey metal is visible) SWEDISH Sa.2
• NEAR WHITE METAL (Mill scale, rust and foreign particles are removed to the extent that only traces remain in the form of spots or stripes. The cleaned surface will show varying shades of grey. SWEDISH Sa.2.5
• WHITE METAL Visible mill scale, rust and foreign particles are entirely removed. The cleaned surface should have a uniform metallic colour but may show varying shades of grey when viewed at different angles. SWEDISH Sa.3

Grey Metal

Before blasting, steelwork should be degreased, since the blasting does not remove these contaminants. It should be noted that blasting damages surrounding coating which is peppered by stray abrasive particles and the protective value of the scheme in the vicinity may be destroyed. 

Sweep Blasting

Sweep blasting is the treatment of a surface by the sweeping of a jet of abrasive across the surface. Its effectiveness depends on the nature and condition of the surface, the type and particle size of the abrasive and above all the skill of the operator. 

Abrasive/ Slurry Blasting

Wet blasting using a slurry of water and abrasive. This has the advantage that the hazards of dust and associated health problems are largely overcome. This process also removes salts from the surface.

Hydro blasting & Hydro jetting

A technique for cleaning surfaces, which relies entirely on the energy of water free of abrasives striking a surface to achieve its cleaning effect. Pressures are very high ranging from 10 - 25000 psi.

Non Ferrous Metals

Galvanised steel and Aluminium

Sweep blasting (At low pressure for Aluminium) preceded by degreasing is the preferred method, however when this is not possible then etch primers or acid etch solutions may be used to passivate the metal before application of the final coating. Etch primers are not recommended for below water structures.

Safety Precautions when Using Paints

Safety Precautions when Using Paints

Before we can understand the safety precautions to be taken we have to know the hazards associated with Paints and their usage. The MSDS should always be consulted for advise regarding safety measures.

1. Fire and Explosion

Most paints contain flammable Volatile Organic Compounds. VOC's as solvents. These are a potent explosion hazard when the Lower Explosion Limit (LEL) is reached. Paints should only be stored in the designated paint locker. No naked flame procedures should be strictly enforced when working with paints, more so in enclosed spaces and storage spaces.

Solvents float in water; hence water as a fire extinguishing medium should never be used. Foam, Chemical, CO₂ is the desired medium.

2. Skin & Eye contact

Paints contain hazardous chemicals, even though an International effort has reduced many of them, they have not been eliminated. Paint in contact with mucous membranes of the eye and mouth can have very severe effects.

Never us solvents to remove paint. Wash your hands and mouth after painting.

Barriers in the form of PPE should always be used. If inadvertent contact does happen use copious amounts of fresh water, consult the MSDS and seek medical attention.

3. Inhalation & Ingestion

Signs of inhalation are dizziness, headaches and drunkenness. The culprit again is the VOC's. Ventilate thoroughly, check for LEL's if required. Use face masks or air hood if working in spaces where ventilation is not adequate.

Food and drink should never be consumed in the proximity of a painting operation.

Spray painting machines specially airless spray's require additional PPE and breathing protection, owing to the high pressures involved.

How Anti-fouling paint acts

Antifouling Paint

 

The main properties are that it should control Micro and Macro fouling, be smooth and comply with the AFS Convention. Severity of fouling follows the worlds temperature zones with the maximum being in Tropical and minimum in Polar zones.

Anti fouling paints do this by leaching Biocides into the water around the vessels underwater hull. With the AFS Convention the chemicals which are leached should not be harmful to the environment and contain either Organotins or TBT's.

TBT free paints use a seawater soluble binder called Rosin and the biocide is cuprous oxide. Hydrolyzing polymers are replacing Rosins in modern paint systems. The smoothness of an antifouling coat was obtained by self polishing high build coatings where the outer layer would erode as the water washed past the hull with the ships movement. With the banning of Organo tins and TBT's and the introduction of Rosin binders, the self polishing trait of paints took a hit, however with hydrolyzing polymers the selpolishing properties rival the old TBT based paints.

 

Silicon and fluoropolymer biocide free paints have also been introduced which have a very slippery surface due to which fouling is unable to attach themselves to the coating. Another property of a Antifouling coating is that it should last the 5 year dry docking cycle.

1. Wetted surface area and Spread rate.
   It is important to the ships officer or operator to know how much paint will be required for maintenance/ dry docking budgets. For this it is essential that we know how much area is to be coated. There are various methods to calculate the area eg Denny-Mumford, Holtrop-Mennen and Komsi.

AFS Convention

Salient features of International Convention on the Control of Harmful Anti-fouling Systems  on Ships, 2001.

• Adopted on 05-10-2001 ; In force from 17-09-2008.
• Anti-fouling system (AFS) means coating, paint, surface treatment, surface or device used on ship to prevent attachment of unwanted organisms.
• The convention applies to all ships except war ships and ships owned or operated by State and used only on government non-commercial service.
• No more favourable treatment shall be given to ships of non-Parties.
• All ships shall not apply Organotin compounds, which act as biocides, in AFS.
• Each State shall prohibit and/or restrict application, re-application, installation or use of AFS on its ships and on foreign ship whilst it is in the port of the State.
• Wastes from application or removal of AFS shall be collected, handled, treated and disposed of in an environmentally safe manner.
• If any violation is detected or reported, the Flag State shall take action in accordance with its laws, which shall be adequate in severity to discourage violations.
• Ships of GT at least 400 tons engaged in International voyages, excluding fixed or floating platforms, floating storage units (FSUs) and floating production storage and off-loading units (FPSOs), shall be subject to the following surveys :

• Initial survey.
• Survey when the AFS is changed.

• On completion of initial survey an AFS certificate shall be issued supplemented by a Record of AFS in the prescribed forms.
• Certificate shall cease to be valid if the AFS is changed and the certificate is not endorsed, or if the ship is transferred to another State.
• Ships of length at least 24m. but of GT less than 400 tons, engaged in International voyages, excluding fixed or floating platforms, FSUs and FPSOs, shall carry a Declaration on AFS signed by the ship owner in the prescribed form.

How Anti-corrosive paint acts

How do they work

Anti Corrosive Paint

As we have seen in the previous section, the properties of a Anti corrosive paint should be corrosion inhibition and compatibility with Cathodic protection systems and their by products. The main work of an anticorrosive coating is to form a barrier between the seawater (electrolyte) and the steel plate of the hull.

As we are aware, one of the prime movers of the corrosion triangle are ions. What anticorrosive coats do it to nearly block the movement of these ions through the paint. The coating are necessarily high build and do not contain any metals or molecules that will interfere with the cathodisation of the hull by Cathodic protection systems.

They consist of coal tar or Polyurethane epoxies, though Polyurethanes are getting out of favor due to the presence of toxic isocyanates.

Wetted surface area

The following formula gives:

Where

S = Wetted surface area

 = Volumetric Displacement

L = LBP

Once the area is calculated, the spread rate is obtained from the manufacturer’s data sheets. The spread rate varies according to the mode of application e.g. Roller, airless spray etc. The quantity of paint required can now be found.