Antifouling Coatings

One of the most important coatings used in marine applications is antifouling coatings. These coatings are applied to the hulls of ships and boats to prevent biofouling, which is the unwanted accumulation of microorganisms, algae, plants, and animals on wet surfaces. Biofouling can have significant detrimental effects if left untreated as it increases drag, decreases fuel efficiency, and affects maneuverability.

Traditional antifouling coatings contained biocides like tributyltin (TBT) which leached from the coating into the surrounding seawater to deter biological growth. While very effective, these TBT-based antifoulings posed environmental and health risks. As a result, the International Maritime Organization banned the use of TBT-based antifoulings worldwide in 2008.

Modern antifouling coatings now rely on alternative biocides or biocide-free mechanisms to prevent slime and organism accumulation. Copper-based antifoulings are now one of the most common alternatives as copper is a naturally occurring element that marine life encounters. The copper leaches slowly from the coating and acts as a biocide at low concentrations safely. Other non-biocide mechanisms involve foul-release and fouling-release coatings, which create very slick surfaces that marine organisms struggle to adhere to. The coating periodically "sloughs off" any organisms that start to attach. Overall, the goal of modern antifouling coatings is to provide effective hull protection while minimizing environmental impact.

Anticorrosion Coatings

In addition to preventing biofouling, marine coatings play a critical role in protecting a ship's steel hull and other metal components from corrosion due to seawater exposure. Left uncoated, steel readily corrodes when submerged in seawater which contains dissolved oxygen, carbon dioxide, and chloride salts.

The main types of marine anticorrosion coatings are anticorrosive primers and topcoats. Epoxy and polyurethane coatings are commonly used anticorrosive primers that form a protective barrier on the metal surface, disrupting the corrosion process. Topcoats are then applied over primers to provide additional physical, mechanical, and aesthetic protection. Popular topcoats include further epoxy coats, chlorinated rubber, and acrylic coatings.

For optimal protection, anticorrosion coatings require multiple coats applied at the proper thicknesses per manufacturer specifications. Coatings must adequately cover edges and welds while maintaining an intact film. Any defects that allow water ingress can lead to corrosion initiating underneath the coating. Additionally, as coatings age their protective properties degrade over time from weathering and damage, so periodic inspection and overcoating is needed. With proper application and maintenance, high-performance marine coatings can extend a ship's drydocking period significantly by protecting its steel structure from corrosion for decades.

Specialized Coatings

Beyond generic hull and anticorrosion coatings, specialized marine coatings serve various other functions. Tank coatings applied to cargo oil tanks, ballast tanks, and fuel tanks assist in corrosion protection and help prevent contamination. Due to cargo compatibility concerns, different tank coating chemistries may be selected depending on the types of fluids being stored or transported.

Additionally, many vessels require scrubber coatings. These physically robust coatings line exhaust gas desulfurization (EGD or "scrubber") systems that remove sulfur oxides from engine exhaust to comply with emissions regulations. Scrubber coatings must withstand the corrosiveness of seawater and chemical cleaning agents used in the scrubbing process. Epoxy and silicon-based coatings tend to work well in these highly demanding environments.

In marine applications requiring fireproofing, intumescent coatings can provide protection. Intumescent coatings foam up when exposed to fire, swelling to form an insulating char layer that slows heat transfer and fire propagation on coated surfaces like steel decks. Some modern warships and offshore oil rigs utilize specialized camouflage coatings as well. These low-visibility coatings minimize radar, infrared, and visual detection signatures to disguise ships and structures against detection.

Testing and Certification

Given the critical role of protecting expensive maritime assets, marine coatings undergo rigorous testing to validate their performance claims. Key characteristics evaluated include:

Adhesion: Coatings are tested for adhesion to various substrates after humidity/UV conditioning using crosshatch or pull-off adhesion methods.

Abrasion/Impact Resistance: Coatings ability to withstand abrasive forces and impacts is quantified using fall handlers, sand wheel abrasion testers, orTaber abrasers.

Corrosion Protection: Coated metal samples are exposed to salt spray chambers, QUV weathering devices, or immersion tanks to assess corrosion inhibition.

Chemical/Solvent Resistance: Coatings undergo exposure to challenge chemicals like acids, alkalis, solvents to check non-degradation.

Successful coatings receive certification from classification societies like Lloyd's Register verifying they meet procurement specifications. Examples are Lloyd's Register Certificates for fitness of antifouling coatings or NORSOK M-501 for offshore structures. With thorough R&D testing and certifications, marine coatings manufacturers can provide assurance their products will maintain maximum performance and lifetime when used for their intended purpose.