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COATINGS • OFFERING MORE THAN JUST PROTECTION, TODAY’S CARGO TANK COATINGS MAKE INSPECTION EASIER, REDUCE CROSS-CONTAMINATION AND ENHANCE SAFETY

THERE ARE MANY reasons for coating the cargo tanks of chemical tankers: it protects the tank, it protects the cargo and it makes tank washing simpler and more effective. More than that, though, it also provides greater flexibility in the employment of the tankers themselves, as Advanced Polymer Coatings, which supplies the widely used MarineLINE polymer coating, explains.

Vessel owners are continuously looking for ways to reduce operating expenses and chemical-resitant coatings must deliver opportunities to help meet that target. As a result, development in the coatings sector include new products that are resistant to the widest possible range of chemicals. This allows the owner the versatility to use the same asset to transport as wide a variety of chemicals as it can, responding to changes in market conditions and eliminating the need to either re-coat current vessels or offer different storage options for each chemical.

Obviously, tank coatings should not absorb cargo that can be released later to contaminate the next chemical cargo loaded onto the vessel. But, in both industrial and marine applications, coatings must offer a wide range of technical features in addition to this chemical resistance. They should, for instance, prevent the cargo being contaminated by the coating itself, provide easier inspection of the coating both during and after application, dissipate static electricity, offer improved abrasion resistance, and be easy to repair in the field.

As an example, two-component (2K) epoxy coatings show excellent resistance to sulphuric acid in most concentrations and transport and storage temperatures. They have been successfully used for years but unfortunately can darken on exposure to sulphuric acid. Recent work to improve this discoloration effect on exposure to sulphuric acid has shown that different chemistries can lessen coating discoloration while still maintaining the excellent level of chemical resistance to sulphuric acid.

TAKE A HOLIDAY Coatings need to be inspected immediately after application and ambient curing for ‘holidays’ (holes, voids, or discontinuities in the film) that can expose the substrate to corrosion. For non-conductive coatings, holiday inspection is typically done by spark testing as per NACE SP0188. This testing can only be done once the coating is ambient cured, after which any holidays detected must be repaired. Altogether, this is a very time-consuming process.

Likewise, coatings must periodically be inspected for damage to make sure that there are no cracks, holes, or chips. The entire vessel must be inspected after draining and cleaning, which can be difficult again because of the complex geometries along with the hazards associated with sending workers into the vessel to carry out the inspection.

Coatings should, therefore, facilitate the inspection process while also reduce the chances of coating failure. The use of a fluorescent pigment in the coating basecoat greatly facilitates inspection of the coating by making the detection of holidays easier using UV light.

THE COATING OF A CARGO TANK SHOULD PROTECT

THE VESSEL AND THE CARGO BUT ALSO OFFER

THE OWNER MORE OPPORTUNITIES FOR

With a fluorescent basecoat, the coating can be inspected immediately after applying the topcoat and during subsequent integrity inspections using a UV light.

STOP THE STATIC A final trend in coating development is anti-static linings. Static electricity presents a very significant hazard for the safety of people and property, as well as products that are sensitive to static electricity, such as electronics components. The importance of dissipating static electricity to the safety of people and property is shown in a case study from Barton Solvents in 2007, when naphtha was being unloaded from a tank truck into a bulk storage tank.

Naphtha, a non-conductive liquid, was unloaded from a tanker containing three separate compartments. During the unloading, air was introduced as the transfer hose was switched between compartments, creating a flammable mixture in the bulk storage tank’s headspace. The introduction of air also caused turbulence inside the tank, causing the metal float to lose its grounding and accumulate a static charge.

During the naphtha unloading, the accumulated static electricity was enough to produce a spark and ignite the flammable mixture above the liquid. The explosion was strong enough to throw the storage tank beyond the tank farm, causing two other vessels to explode, hurling metal fragments into nearby residences and businesses.

The ability for a coating to dissipate static electricity is usually measured in the surface resistivity per area, using a method such as ASTM D257. For typical 2K filled epoxy coatings, the surface resistivity is >1010 ohms/ square, making them insulating materials.

To reduce the surface resistivity of epoxy coatings, conductive additives can be used. These additives must be used at low levels so that they do not affect the chemical resistance of the coating. One such additive that has found use in static dissipative coatings is carbon nanotubes, which can be added to reduce the surface resistivity to 106 ohms/square.

Testing to date has shown that the addition of low levels of carbon nanotubes does not affect the coating’s chemical resistance. By adding the carbon nanotubes, a static dissipative coating with outstanding chemical resistance can be produced.

Developments in chemically resistant, polymer-modified epoxy coatings have met several market needs, including carrying sulphuric acid, coating inspection facilitation, and anti-static coatings These coating systems continue to be leaders in chemicalresistant marine and industrial applications and will meet future market needs through continued innovation. www.adv-polymer.com