Ships and offshore assets have areas with an inherently higher corrosion risk than other areas of the same structure. These areas merit special attention during their initial coating and subsequent maintenance activities.
Corrosion Risk at the Waterline and Splash Zone
As far as the ships are concerned, the waterline is the horizontal line where hull of a vessel meets the water's surface. The waterline is also the international load line, i.e. the limit to which the vessel may be legally loaded for a specific temperature and water type. In addition, there is a splash zone just above water line, the height of which depends upon the sea's turbulence. The risk of marine corrosion in the splash zone is high because this zone is continuously assaulted with salt-laden air and corrosive and polluted saltwater. The internal surfaces of ballast tanks also face the risk of saltwater corrosion.
Protective Marine Coatings
Marine coatings are defined as the protective coatings used for saltwater or freshwater environments that protect offshore structures, tankers, ships and other vessels from corrosion and abrasion induced by the unfavorable environments and unique operational conditions.
Challenges Faced by Marine Coatings Above the Waterline
Challenges faced by above the waterline marine coating projects include:
- Aggressive and rapidly changing marine environments
- Severity of corrosion risk based on the location of the surface in question, accessibility for the surface preparation, coating application and on-going inspection and maintenance activities
- Climatic condition such as temperature and relative humidity
- Worker safety and occupational health concerns as well as regulatory compliance needs
- The ship owner’s requirement to minimize the dry docking period by optimizing the time spent on inspection and essential repair activities, driving the need for better coating products and methods
The marine environment primarily consists of salt-laden atmospheric air with high humidity, which means that condensate accumulating on the substrate above the waterline will contain corrosive salts and pollutants to some extent. The coating material selection process, surface preparation and application processes must take these factors into account. (Avoid problems before they start with a better design; read Engineering Ships for Better Coating Performance.)
Considerations for Unfavorable and Variable Climate Conditions
For best results, surface preparation and coating processes should be performed under controlled humidity and temperature conditions. Coated surfaces must be protected from moisture condensation to prevent premature failures such as delamination due to poor adhesion, splitting, thermal contraction and warping.
Climate controller systems should be able to adjust the parameters quickly based upon changes in the weather. (For more information, see 9 Things to Keep in Mind when Performing a Marine Coating.)
Precautions Related to Surface Preparation
Surface preparation activities must be performed diligently. The iron oxide that forms on steel surfaces in the form of mill scale and formed during the rolling of steel plates can create corrosion cells and cause peeling when scale comes out, resulting in unprotected patches on the substrate.
The blast cleaning equipment must be chosen carefully so that clean abrasive can be recirculated and any steel dust that is produced is collected. The use of sand as an abrasive is prohibited in some countries such as UK. Discharging spent abrasive and steel dust to the environment must be avoided.
The use of oxy-acetylene flame, though less effective, may be the only alternative under severe unfavorable weather conditions. However, lead-based coatings should not be burnt off because it is a hazardous substance.
General Safety Concerns
Workers must understand the hazards involved and use appropriate personal protective equipment and safety procedures to mitigate the hazards. If pickling is used then it is important to provide skin protection from acids and other chemicals.
Coatings can contain irritants and toxic constituents, and the solvents can involve explosion risk or flammability. Dry surfaces must be wetted first before scraping off corrosion products and old coatings so that harmful dust is not inhaled by workers. Enclosed spaces and other interior spaces should be adequately ventilated.
Considerations for Choosing an Appropriate Coating
Zinc silicate primers are popular for above the waterline applications due to their exceptional corrosion prevention properties. However, zinc epoxies are more suitable during dry dock applications if near-perfect surface preparation is practically impossible. (Learn more about surface preparation for these types of coatings in Surface Preparation for Inorganic Zinc Silicate Coatings.)
Wherever epoxies are chosen for above the waterline surfaces, care should be taken by the formulator to choose the correct ratio of hardener to epoxy resin, based upon the required film thickness, humidity and substrate temperature. This ratio affects the curing time. With a too small ratio (i.e., a smaller quantity of hardener for a given amount of epoxy resin), curing will take too long and the uncured coating could become damaged, thus negatively affecting the aesthetics of the top surface.
Because polyurethane is a single component coating, it doesn’t require a hardener (it is cured by moisture present in the atmosphere). Like epoxies, polyurea coatings also requires accurate formulation and precise surface preparation. (These coatings are discussed in The History and Industry Adoption of Polyurea Coating Systems.)
When the substrate temperature falls below 3°C (37°F), curing is a critical consideration. Carefully formulated vinyl products can be cured around 3°C. Chlorinated rubber is another alternative. To remove condensed moisture from the surface, either a roller or brush method of application can be used.
Whenever temperatures above 30°C (86°F) are encountered, the spray nozzle should be held as close to the substrate as possible and the formulator should add the correct amount of thinner to ensure that coating doesn’t dry in the nozzle itself.
In the case of food grade cargo tanks, the coating of interior surfaces must meet regulatory requirements. Other liquid cargo tanks must be coated to ensure chemical compatibility of the coating as well as corrosion protection. Ballast tank interiors need stronger corrosion protection of coating layers free from porosity and disbanding in order to resist saltwater corrosion attack.