MARAD_Marine_Coating_Systems.pdf

RRF TE-1

SECTION 2 – VESSEL MAINTENANCE OPERAITONS

2.5.4 Extended Service Life (ESL)

ESL is defined as maintenance actions on ships to ensure the vessels achieve a service life as specified in the Annual Business Plan Guidance. The vessel specific service life is subject to change based on annual guidance provided to MARAD by USTRANSCOM. The SMGR shall complete ESL project annual in accordance with the business protocol

The status of ESLs is often reviewed by our external customers and auditors. MAR 610 keeps a ESL status database. The Area divisions with the assistance of the ship managers will be requested to provide information to update the date base as requested and at a minimum annually.

2.6 Marine Coating Systems

The technology of Marine Coating systems is an every changing environment. The type of coatings available have increased in sophistication to the point that it is often difficult to determine the "perfect" coating system for a vessel’s needs without considering ship managers’ experience, MARAD coating experience, various international and professional standard “societies” and coating system manufacturer's recommendations. The purpose of this publication and associated reference materials is to provide the ship manager guidance for use in maintenance and preservation on RRF vessels.

Since MARAD's primary maintenance goal is preservation, be it of a ship's hull or machinery, the type of preservation system employed is of great importance. MARAD believes its ship managers and MARAD personnel should seek to minimize preservation costs by ensuring coating systems are inspected and recorded in NS5 on a regular, continuous basis; and where possible allow for maintenance and repair of localized coating systems, rather than wholesale replacement. For this reason, the marine coating policies and practices outlined herein should be followed by the ship managers and MARAD personnel when inspecting, determining condition or status of coating systems, developing preservation work specifications, procurement of coating systems and their application as closely as possible. As well as, using their own preservation and coating experience, regulatory inspectors, and outside expert consultants (as approved).

Given the emphasis on coatings in the various mandated Rules and Regulations and the unprecedented acceleration of coating technology, ship manager personnel involved with assessing coatings and specification development should have factual knowledge of coating systems and the applicable standards.

2.6.1 MARAD References

To accomplish these goals the ship manager and Marad personnel shall utilize the following guidelines/reference materials:

1) Guidance Notes on the Inspection, Maintenance and Application of Marine

Coating Systems 3rd Edition Copyright © 2007 American Bureau of Shipping.

2) Society of Naval Architects and Marine Engineers Technical Research Bulletin 4-15, Coating Systems Guide for Exterior Surfaces of Steel Vessels, which is available from the Publications Department, S.N.A.M.E., 601 Pavonia Avenue, Jersey City, NJ, 07306.

3) The Steel Structures Painting Council (SSPC) manual, Vol. 2, Systems and

Specifications, which is available from the Steel Structures Painting Council, 4400 Fifth Avenue, Pittsburgh, PA 15213.

4) Society of Naval Architects and Marine Engineers Technical Research Bulletin

4-21, Abrasive Blasting Guide for Aged or Coated Steel Surfaces, which is available from the Publications Department, S.N.A.M.E., 601 Pavonia Avenue, Jersey City, NJ, 07306.

5) International Standards Organization (ISO), Standard 8501- Parts 1-4:2007

— Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.

— Part 2: Preparation grades of previously coated steel substrates after localized removal of previous coatings.

— Part 3: Preparation grades of welds, edges and other areas with surface imperfections.

— Part 4: Initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting.

2.6.2 Assessing Coating Conditions Aboard MARAD Vessels

Many factors contribute to the assessment of existing coating conditions aboard MARAD vessels, in particular corrosion and coating failure. Many types of corrosion (by oxidation is the most prevalent) exist including uniform, pitting, crevice, galvanic, deposition, impingement, and microbiological corrosion which are all found aboard marine vessels. The rate of corrosion itself is influenced by several factors including diffusion, temperature, conductivity, types of ions, acidity and alkalinity, and electrochemical potential.

Coating failure is another concern in assessment of coating systems. Examples of coating system failures include blistering (due to osmosis or electro osmosis through coating), jacking (due to rust or calcareous deposits under coating), shop primer failure, through film breakdown (common on under of deck plating or tank tops), edge breakdown, weld corrosion, reverse impact damage and stress related coating failures.

Coating failure can also be attributed to poor coating applications such as, sagging, runs, cissing, orange peel, cracking, holidays, over-thickness of coating applied, under-thickness of coating applied, over spray, grit inclusions, poor surface preparation, and human error.

Ship managers and Marad personnel shall utilize Reference 1 above in assessing coating system conditions aboard MARAD vessels in particular Chapters 1, 2, 12, 13, and 14. As well as, using their own preservation and coating experience, regulatory inspectors, and outside expert consultants (when approved). As stated above, ship managers and MARAD personnel should seek to minimize preservation costs by ensuring coating systems are inspected and recorded in NS5 on a regular, continuous basis; and where possible allowing for maintenance and repair of localized coating systems, rather than wholesale replacement.

2.6.3 Coatings

The following is taken from Reference 1 above:

Paint can be described as a liquid material capable of being applied or spread over a solid surface on which it subsequently dries or hardens to form a continuous, adherent, film. Paints basically consist of three major components and many additives which are included in minor quantities. The major components are:

• Binder (also called vehicle, medium, resin, film or polymer)

• Pigment and extender

• Solvent

Binders Binders are the film forming components of paint which determine the principal characteristics of the coating, both physical and chemical. Paints are generally named after their binder component (e.g. epoxy paints, chlorinated rubber paints, alkyd paints, etc.). Binders used in the manufacture of paints fall into two classes, thermoset and thermoplastic.

Cross-linked (thermoset) coatings These paints are usually supplied in two separate packs which are mixed together immediately before application. These types of coatings include epoxy resins, polyurethane resins, alkyd resins, and inorganic resins.

Thermoplastic Coatings These types of paint binders are simple solutions of various resins or polymers dissolved in suitable solvent(s) and are usually supplied as one pack products, making them especially suitable for maintenance work. These types of coatings include chlorinated rubber resins and vinyl resins.

Pigments and extenders Pigments and extenders are used in paints in the form of fine powders. These materials can be divided into the following types:

Type Purpose Anti-corrosive pigments To prevent corrosion of metals by chemical and electrochemical means, in above-water areas

Barrier pigments To increase impermeability of the paint film

Coloring pigments To give permanent color

Extending pigments To help give film properties required.

Anti-corrosive pigments These include zinc, aluminum pigments and zinc phosphate.

Barrier pigments The most common types of these pigments are aluminum (leafing aluminum) and Micaceous Iron Oxide (MIO).

Coloring pigments These pigments provide both color and opacity and can be divided into either inorganic or organic types. The most common coloring pigment is titanium dioxide, which is white.

Extender pigments As the name suggests, they basically adjust or “extend” the pigmentation of the paint until the required pigment volume concentration (PVC) is achieved.

Solvents Solvents are used in paints principally to facilitate application. Their function is to dissolve the binder and reduce the viscosity of the paint to a level which is suitable for the various methods of application, such as brush, roller, conventional spray, airless spray, etc. Solvents include true solvents, latent solvents and diluent solvents.

In some countries, certain types of solvents are not allowed. This is especially true in the USA, where the Hazardous Air Pollutant Substances Act, (HAPS) dictates a timeline for removing many solvents and extenders from coatings.

Anti-corrosion paints With few exceptions (such as anti-fouling paints, cosmetic effects, fire retardants, etc.), the majority of coatings applied to a vessel are used for anti-corrosion protection. There are many types of anti-corrosion coatings, but epoxy paints generally cover the greatest area on a vessel, particularly when they are used in sea water ballast tanks. Types of epoxy coatings include pure epoxy, modified epoxy, coal tar epoxy and solvent free epoxy.

Impact and abrasion-resistant coatings This type of coating is generally applied to the areas of ships which are most susceptible to damage, such as boot-tops and decks, and are sometimes used for the holds of bulk carriers.

MARAD Environmental Concerns and Coating Prohibitions In 1988, Congress enacted a partial ban on TBT antifouling paints, eliminating the need for EPA action. The OAPCA banned the application of antifouling paint containing organotin to vessels less than 25 meters in length. Organotin is defined as “any compound of tin used as a biocide in an antifouling paint.” The prohibition does not prevent the application of organotin antifouling paints to the aluminum hull, outboard motor, or lower drive unit of a vessel less than 25 meters in length.

Lead based paints due to their toxic nature and regulatory policies of the United State are also prohibited for use aboard MARAD vessels. Ship managers shall ensure that any coating systems or paints they utilize aboard MARAD vessels are certified lead free.

Coatings and paint containing polychlorinated biphenyls (PCB’S) shall not be utilized aboard MARAD vessels. Ship managers shall ensure that any coating systems or paints they utilize aboard MARAD vessels are certified PCB free.

Ship managers and MARAD personnel overseeing projects requiring the removal of existing coating systems shall ensure that coatings are tested for lead and PCB’s prior to removal and that all remaining materials (spent grit or water from surface blasting, removed coating materials, paint cans, etc.) are properly disposed of in accordance with applicable Federal, State and Local regulations.

Coal tar paints shall not be utilized aboard MARAD vessels. Coal tar pitch volatiles are a suspected carcinogen by OSHA standards, and are presently being studied and considered for exposure limitations.

Vinyl chloride coatings shall not be utilized aboard MARAD vessels. This type of paint vehicle is also listed as cancer suspect agent by OSHA. However, some of our vessels may have these coating systems and OSHA safety standards must be observed when removing vinyl chloride coatings.

In addition, the EPA established regulations regarding mercury- containing fungicides prohibiting their use in solvent-thinned paints. They can be used only as a preservative in water-based interior paints, or as a fungicide in water-based exterior paints. The EPA has not set limits on the amount of mercury that may be used; however, the Federal Hazardous Substances Act limits the use of mercury to 0.2% of the total weight of the paint.

In addition, zinc chromate coatings are also listed as suspected carcinogens and have exposure controls set by OSHA. Special precautions should be taken to protect personnel when applying or removing these paints. Refer to OSHA exposure limits before use.

Solvent Emissions Limits on volatile organic compounds (VOCs) created during the application of some coatings is also a consideration in coating procurement and specification development. Ship Managers and MARAD personnel should bear in mind the VOC’s contained in some paint products and their limitations for being applied as per Federal, State and Local regulations.

Background The passage of the Clean Air Act in 1970 involved the EPA in legislating solvents to be used in coatings. The original Act set emissions limits, but left the specifics of attaining these limits to each state. In California, the California Air Resources Board (CARB) developed a proposed model regulation restricting solvents, because studies had proven that all Volatile Organic Compounds (VOCs) contribute significantly to the formation of oxidants (smog). A modified version of the model ruling was adopted by the South Coast Air Quality Management District in southern California in 1979, and similar regulations will be adopted on September 1, 1989, in all other parts of California. Legislation is being considered in Congressional Committee to adopt the California standards nationwide. These regulations drastically reduce the allowable solvent content of most coatings.

Requirements The California regulations state that no person may sell, offer for sale, or apply any coatings manufactured after September 1, 1989, which contain more than 420 grams of volatile organic compounds (VOC) per liter of coating for an air- dried single-component alkyd or vinyl coating, or 340 grams/liter for a two-component coating as applied. The term "as applied" has significance in that the VOC content is measured at the point of application, which includes any thinners added to the paint for ease of application. These limits are dropped even further to 340 grams/liter for alkyd- type paints effective September 1, 1991.

Coating Systems Procurement and Application As marine coatings are an ever evolving specialized field, the following shall apply to application of coatings on any MARAD vessel:

1) For any major coatings project with a value of $100,000 (including procurement of coatings) or more, the drafter (ship manager or MARAD personnel) of the statement of work shall contact a recognized marine coatings manufacturer or a recognized independent coatings specialist for their recommendations as to what preparation method and what coatings they recommend for the surface areas that are to be coated. The ship manager or MARAD personnel responsible for drafting the coating system procurement specification shall ensure that the vendor guarantees the compatibility of its coating system for use with existing coating system. In addition the drafter of the coating specification shall ensure that the specification incorporates the coating vendor’s surface preparation and application requirements.

2) For any major coatings project with a value of $100,000 or more, a coatings manufacturer’s authorized representative or a recognized independent coatings specialist shall be hired by the Ship Manager or MARAD, if a direct procurement, to be in attendance during the surface preparation and coating of specified areas to be coated. The statement of work for the application of coatings shall require the contractor applying the paint to following the independent coatings specialist or coatings manufacturer’s authorized representative guidance for the preparation and application of any coatings in the statement of work. The coatings manufacturer’s authorized representative shall maintain a daily log noting all pertinent data associated with the proper application of coatings (i.e. date, time, ambient temperature, surface temperature, humidity, weather conditions, surface profile, humidity, paint type and batch number, etc.)

3) Procurement of coatings is left up to the discretion of the individual ship manager, as they may be able to use economies of scale based on their commercial association with paint manufacturer’s to save the Government money when purchasing paint as a GFM verses having it provided as part of the preparation and application contract.

4) Complete coating system data (paint type, batch number, etc.), coating specifications, paint representative’s daily log and any final paint reports shall be entered into NS5.

5) Ship managers and MARAD personnel shall utilize References 1 and 2 above in determining the proper coating systems and their compatibility with existing ship board coating systems. The coating manufacturer shall be consulted if information on a specific product, or coatings suitable for particular areas on the vessel, is required. As well as, using their own preservation and coating experience, regulatory inspectors, and outside expert consultants (when approved).

The Steel Structures Painting Council (SSPC) also publishes information relative to various types of paint systems and their suitability for different uses.

Typical MARAD Color Scheme (Exterior Only) (Deviations/Clarifications shall be requested prior to application)

• Superstructure and Decks: Haze Grey

• Top-side Hull Coating: Haze Grey

• Underwater Hull: Red

• Boot-top: Black

• Stack Bands Colors: Insignia Red, Insignia White and Insignia Blue

• Miscellaneous : International Orange and Safety Yellow

• Decks and Cargo Gear: Haze Gray fwd., upper works black aft of stack

• Kingposts: Haze Gray fwd., upper works black aft of stack

• Ro/Ro Ramp structure: Haze Gray fwd., upper works black aft of stack

• Markings: Black lettering on gray surface

White lettering on black surface

2.6.4 Surface Preparation

As stated above, ship managers and MARAD personnel overseeing projects requiring the removal of existing coating systems shall ensure that coatings are tested for lead and PCB’s prior to removal and that all remaining materials (spent grit or water from surface blasting, removed coating materials, paint cans, etc.) are properly disposed of in accordance with applicable Federal, State and Local regulations.

The following is taken from Reference 1 above:

General Good surface preparation may be considered to be the most important part of the entire coating process in that the greatest percentage of coating failures can be traced directly to poor surface preparation. All paint systems will fail prematurely unless the surface has been properly prepared to receive the coating. If contaminants such as loose rust, oil, grease, dirt, salts, chemicals, dust, etc. are not removed from the surface to be coated, adhesion will be compromised and/or osmotic blistering will occur in addition to premature failure of the coating in service.

No paint system will give optimum performance over a poorly prepared surface.

Surface cleanliness The extent to which a surface is made clean before the coating is applied, is a balance between the expected performance of the coating, the paint manufacturer’s recommendations, the time available for the job, the relative cost of the various surface preparation methods available, access to the area to be prepared and the condition of the steel prior to surface preparation. In many instances, coatings cannot be applied under ideal conditions, especially under repair and maintenance conditions. The quality of surface cleanliness which is achieved (or it is possible to achieve) will be very different for an un-corroded high quality steel plate with tightly adherent mill-scale, arriving at a new building shipyard and steel on a vessel which has been in service for ten years, with poorly adherent coating, loose rust scale and heavy pitting. Any substance which prevents a coating from adhering directly to the steel can be considered a contaminant. Major contaminants at the new building stage include:

• Moisture or water

• Oil and grease

• Salt and other Ionic species from the nearby sea and industrial areas

• White rust (zinc salts from weathered zinc silicate shop primers)

• Weld spatter

• Weld fume

• Cutting fume

• Burn through from welding on the reverse side of the steel

• Dust and dirt from the yard site and from neighboring industrial processes

In maintenance and repair situations, the presence of pitting, corrosion products, cathodic protection products, aged coatings and trapped cargoes, etc., must also be considered, particularly if only localized surface preparation of the most severely affected areas is being carried out prior to re-coating.

Surface cleaning and conditioning There are many methods available for cleaning and preparing steel surfaces prior to painting. The choice of preparation method will depend upon the areas of the vessel to be prepared and the equipment available. For example in a dry dock, the outer hull may be prepared by abrasive or water blasting before paint is applied.

At new building, water ballast tank blocks may be prepared for coating by thorough abrasive blasting to a pre-agreed standard such as ISO Sa 2.5, or by sweep blasting (a light blast which does not remove all of the intact shop primer) or by power tools.

On board maintenance may involve abrasive or water blasting, power and/or hand tool preparation depending upon the size and location of the area to be prepared and painted. As with edge preparation, a higher standard of surface preparation will result in a longer and more effective coating performance.

One of the major causes of coating blistering is the presence of retained soluble material, such as salt, on metal surfaces before painting. For some types of paint, such as chemical, cargo and ballast tank coatings, the level of soluble salts present is crucial to the long term performance of the coating.

While salts are easily removed from flat surfaces by water washing, it is the salts which become trapped in cracks in the coating, under old paint and rust and in pits in the steel surface that are more difficult to remove and can be problematic. Such residual salts will cause blistering or detachment of the new coating, if not removed.

High pressure water washing will remove the majority of these trapped salts, if carried out effectively.

Paint manufacturers specify the maximum level of soluble salts which may be present on the surface before the coating application and these will vary depending upon the paint itself and its service environment.

Ship managers and MARAD personnel have several different types of surface preparation methods available to them dependent upon the situation. These include solvent cleaning, abrasive blasting, spot blasting, sweep blasting, hydro-blasting/water jetting and power tool cleaning and pickeling.

Surface Preparation Standards

METHODS OF PREPARING STEEL FOR PAINTING ARE VERY IMPORTANT,

AND SHOULD BE GIVEN SPECIAL EMPHASIS IN THE COATING

SPECIFICATION. In addition to Reference 1 cited above, ship managers and MARAD personnel shall utilize Reference 3 above which includes:

SSPC-SP 1 SOLVENT CLEANING

SSPC-SP 2 HAND TOOL CLEANING

SSPC-SP 3 POWER TOOL CLEANING

SSPC-SP 4 FLAME CLEANING

SSPC-SP 5 WHITE METAL BLAST CLEANING

SSPC-SP 6 COMMERCIAL BLAST CLEANING

SSPC-SP 7 BRUSH-OFF BLAST CLEANING

SSPC-SP 8 PICKLING

SSPC-SP 10 NEAR-WHITE BLAST CLEANING

Also, Reference 5 above which includes:

— Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.

— Part 2: Preparation grades of previously coated steel substrates after localized removal of previous coatings.

— Part 3: Preparation grades of welds, edges and other areas with surface imperfections.

— Part 4: Initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting.

These primary references shall be utilized in determining the proper surface preparation for repairing and renewing coating systems and their application with existing ship board coating systems. In addition, the coating manufacturer shall be consulted and their commendations incorporated into the work specification to ensure warranty, as required. As well as, using their own preservation and coating experience, regulatory inspectors, and outside expert consultants (when approved).

Another excellent reference for older vessels is The Society of Naval Architects and Marine Engineers (SNAME) Technical and Research Bulletin No. 4-21, ABRASIVE BLASTING GUIDE FOR AGED OR COATED STEEL SURFACES, which is available from the Publications Department, The Society of Naval Architects and Marine Engineers, 601 Pavonia Avenue, Jersey City, NJ 07306.

2.6.5 Coating Application

As per Reference 1 above:

The objective in applying coatings is to provide a film which will give protection and/or decoration to the structure being painted. The variables which govern the success of any application and subsequent performance are:

• Surface preparation (see Section E above)

• Film thickness of the paint system

• Methods of application

• Conditions during application

Film Thickness Measurement An adequate film thickness is necessary for a coating system to perform to expectations and provide good anti-corrosion protection or achieve the expected anti-fouling lifetime, etc. Under thickness will result in premature failure. However, over application can also cause problems, such as solvent entrapment and subsequent loss of adhesion, cracking of the paint (including mud-cracking) or splitting of primer coats. Ideally the coating thickness should be that specified, allowing for practical application variations.

The wet film thickness (WFT) of the coating is measured and can be converted to a dry film thickness (DFT) following the paint manufacturer’s guidelines for that product. The wet film thickness measurement can serve as an aid in determining how much coating should be applied to reach the specified DFT. The dry film/wet film ratio is based on percentage of solids by volume of the coating being used.

Coating manufacturer’s data sheets sometimes list solids by weight as well as solids by volume. The basic formula using solids by volume is:

WFT = DFT/% solids by Volume

The actual DFT specified will depend upon the type of paint being applied and the nature of the substrate surface. To determine whether a coating thickness is acceptable, there are guidelines produced by the coatings manufacturers, such as the 80-20 and 90-10 rules. For example, the 90-10 rule means: no measurement may be below 90 percent of the DFT specified without repair being undertaken and not more than 10 percent of the measurements may be in the range 90-100 percent of the DFT specified without repair being undertaken. The figures will depend upon the type of coating being applied and the area of the vessel being coated.

Measurements of the dry film thickness are influenced by the profile of the substrate, particularly when abrasive blasting has been used. Thin films (of less than 25m) cannot be measured accurately over blasted surfaces using commercially available paint thickness gauges. Measurements of DFT at edges and corners are not accurate due to the measurement techniques used by these gauges. Paint companies can advise on suitable methods for these circumstances.

Methods of Application The normal methods of application of paint coatings are by:

• Brush

• Roller

• Conventional air spray

• Airless spray

Curing Conditions There are a number of factors which must be considered when marine coatings are applied. The major ones include:

• Condition of the substrate

• Temperature

• Relative humidity

• Weather conditions

• Condensation

• Ventilation

• Ultraviolet light (UV)

2.6.6 Cathodic Protection

Ship managers and MARAD personnel inspecting, determining condition or status of coating systems within tanks and on the underwater hull, developing preservation work specifications, procurement of coating systems and their application should always incorporate the appropriate cathodic protection requirements for these areas, as required. Underwater hull coating specifications should always address maintenance and repair of impressed current systems whenever possible.

As per Reference 1 above (visual aids and examples are available within the reference):

General The dissolution of steel is an anodic process which takes place when metallic iron loses two electrons and becomes an iron ion in solution. If the potential of a corroding object is lowered (i.e. it is made more electronegative) to the reversible potential of the anodic reaction, then the metal dissolution will stop, since the rate of dissolution will exactly balance the rate of re-deposition.

When measured against a reference electrode such as a Saturated Calomel Electrode (SCE), the reversible potential of corroding steel referred to above, is in the order of -860mV (SCE). The measured potential for mild steel in sea water (which is a combination of the potential of the anodic and cathodic reactions taking place) is in the order of –640mV (SCE) measured with the same reference electrode.

To protect steel successfully using cathodic protection, it is therefore only necessary to lower its potential by around a quarter of one volt (250mV).

Sacrificial Anodes Cathodic protection using sacrificial anodes produces a decrease in the potential of the ship by connecting the vessel to a metal which takes up a reversible potential of less than –850mV (SCE) and allowing the sacrificial metal to produce the electrons rather than the corrosion reaction of the steel.

Common choices for the sacrificial anodes are zinc, aluminum and their alloys. Both zinc and aluminum produce potentials of less than -1000mV (SCE). Aluminum has to be alloyed with other metals such as tin in order to make it corrode freely in service.

Magnesium anodes cannot be used in ballast tanks because they generate hydrogen in use and can have deleterious effects on some ballast tank coatings.

Aluminum anodes have limited use in tankers as they can produce a spark hazard if they fall or are dropped from significant heights.

The protection current available from the zinc and aluminum anodes are similar.

Zinc requires 11.3Kg (25 pounds) of material to produce one Ampere-year of current, while 8.2Kg (18 pounds) of aluminum is necessary.

In addition to protection via an electrochemical potential change, sacrificial anodes also protect by forcing the cathodic reaction to occur on the steel. The cathodic reaction reduces oxygen and water to generate alkaline hydroxyl ions as described previously. The production of calcareous deposits initially enhances the protection effect. Sacrificial systems in ballast/cargo holds and ballast tanks are not capable of working during the period when the tanks are empty. This may constitute 60 percent of the vessel usage pattern.

Unfortunately, steel corrodes most rapidly when it is damp with a conducting film over the surface rather than fully immersed and the sacrificial anode system cannot act directly at this time. The CP system does have an indirect effect during this period before the tank dries, as the retained hydroxyl ions work to keep the steel surface passive and the calcareous deposits act as a barrier to oxygen.

In very general terms, a steel surface which is coated with a good quality system of paint will drain in the order of about 5mA/m2 from a cathodic protection system and an exposed steel surface may require 110mA/m2 for protection. Consequently, the current requirement for a tank will increase with time as the extent of coating breakdown (and thus exposed metal area) increases.

The efficiency of a cathodic protection system can be assessed by examination of both the condition of the sacrificial anodes and the nature of the calcareous deposit produced.

The location of sacrificial anodes is also important as they must be placed clear of any overhanging stiffeners whenever possible, to aid the ionic flow around the anodes and prevent incorrect current flow. Wrongly positioned anodes can result in severe blistering of the coating of structures in their immediate vicinity, such as stiffeners, ballast, other pipes and brackets, and can prevent the current from being thrown to areas where it is required for protection.

Impressed Current Cathodic Protection On exterior hulls, it is also possible to use an Impressed Current Cathodic Protection (ICCP) system in the same way that coupling mild steel to zinc results in a flow of electrons to the mild steel to prevent metal loss. In the ICCP system, an auxiliary anode made from a non-consumable material, such as platinized titanium or mixed metal oxides, replaces the zinc or aluminum anode of the sacrificial system.

A reference electrode is also required to monitor the polarization voltage produced on the hull by the current flowing from the anode. It allows feedback control in the voltage rectifier that is used to produce the current.

Areas immediately around the impressed current anodes can suffer from blistering and a thick layer of anode shield material is applied to prevent the loss of the paint.

The electrodes in such an impressed current system are capable of drawing very high currents locally and thus producing copious quantities of oxygen, hydrogen and chlorine if the system malfunctions. This is totally unacceptable in holds and tanks and consequently impressed current systems are not used in these areas.

It is very important that the active anodes and reference electrodes are not painted over during re-coating work, as this will prevent the ICCP system from operating and could cause the paint to disband from the steel.

Table of MARAD General Preparation and Coating Guidelines