Protection in a sea of uncertainty

In fact, taking corrosion as a whole, it costs just over 3% of the global GDP – so quite apart from the safety aspects, corrosion is a serious business.

Some 40% of the total direct cost of corrosion onboard a ship can be attributed to lack of adequate protection from coatings. Some catastrophic failures of ships have been caused by corrosion breakdown of ballast tanks, and Hempel quotes a Lloyd’s Register principal surveyor as saying that effective corrosion control in segregated water ballast spaces is probably the single most important feature, next to the integrity of the initial design, in determining the ship’s effective life span and structural reliability.

Fortunately, there are procedures in place to raise standards, through the IMO PSPC standards. These are designed to achieve a target lifetime of 15 years for ballast tank coatings, and all coatings for ballast tanks must now have a type approval certificate and be subject to regular periodic inspections. PSPC coating standards are being expended to cargo tanks and other areas.

Mr Aamodt explained that when iron or steel corrodes, it tries to revert to its original natural state, by converting to red iron oxide, Fe2O3, otherwise known as rust). The process starts in the presence of water (H2O), oxygen (O2) & ions (salts, i.e sodium and chlorine). It is an electrochemical process, involving transport of ions from an anodic site to a cathodic site, and can occur either in the atmosphere or immersed. The presence of water and salts means that the marine environment is highly corrosive, with 1,000kg of rust reated every 0.2s, equating to a figure greater than 423,000 tonnes of rust per day.

Most areas of a ship are subject to corrosion – ballast tanks, cargo holds, cargo tanks, the outer hull both above and below the waterline, potable water tanks, decks, void spaces and cofferdams in all of which corrosion is caused in slightly different ways, and all require particular coatings to limit the effects of corrosion. In the case of ballast tanks, it is possible to use an epoxy coating which, properly applied, will not degrade in seawater. But we do, of course, see corrosion in ballast tanks, and this is because the steel may be exposed to seawater due to stress cracking in the coatings, poor protection of edges, too low film thickness and because of the reverse effects of impact on the outside surfaces of the ballast tanks.

Hempel says that it supplies various different coatings for newbuilding and for maintenance and repair purposes. Coatings for original application to ballast and cargo oil tanks have to be PSPC approved, while shipyards demand products that can be simply and quickly applied, and which are compatible with shop primers. Owners look for long recoating intervals, and keen pricing is important for both, with yards looking for low initial cost and owners considering total lifecycle costs.

Maintenance coatings are equally important. Looking at drydocking, again, extended maintenance intervals are important to yards and owners, while other desirable features include ease of application, high surface tolerance and fast drying. While for everyday onboard maintenance ship operators look for low cost, low waste (with products available in smaller packages), global availability and good surface tolerance.

For ballast tanks, Hempel offers its Hempadur Quattro Fibre 47604, an epoxy coating with fibre reinforcement designed to resist cracking and stresses and provide superior edge protection over long periods, as well as Hempadur Multi-Strength 45540, a high solids pure epoxy maintenance coating designed for fast immersion time and quick curing, high surface tolerance, good resistance to moisture, abrasion and corrosion, high cathodic resistance, and one-coat application for up to 350μm dry film thickness.

For the future, Mr Aamodt says that Hempel is considering various prospects for ship corrosion protection, including ‘smart’ self-healing coatings, and super-hydrophobic carbon nano-tube and nano-spring paints. He also sees a re-emerging of the ‘cradle to grave’ principle in controlling life-cycle costs, while design ideas such as the DNV Trident ‘ballast free’ ship could prove interesting.