Tank coating systems require regular inspection and maintenance. Re-coating is typically necessary within the vessel’s trading life and is usually completed during dry dock periods.
According to West, vessels must carry the tank coating’s Manufacturer’s Cargo Resistance Guide onboard at all times and consult it when planning cargo sequencing and tank cleaning.
The bulletin provides a detailed comparison of zinc silicate and epoxy coatings, outlining their performance, limitations and compatibility with various cargoes from clean petroleum products to aggressive chemicals and food-grade substances. Today we take a look at zinc coatings.
Zinc:
Zinc silicate is an inorganic paint system. The silicate coating base is either water-based alkali or solvent, typically ethyl silicate. These coatings are very hard, corrosive and temperature resistant with galvanic protection.
Zinc coatings have good resistance against neutral pH cargoes, but are not resistant to strong acids and bases, which may cause the resins to dissolve. They exhibit excellent resistance to strong solvents and chemicals.
Cargoes carried in zinc coated tanks get adsorbed or entrapped into the porous structure of the coating, making cleaning between different cargoes essential. Zinc coatings can typically tolerate higher temperatures than epoxy coatings and have a shorter desorption time.
If cleaning is skipped, cargo residues can desorb into future cargoes, risking contamination
Credit: West P&I Club
Cargo can physically penetrate into the ‘gaps’ formed between the zinc particles and silicate structures. Importantly, the type of cargo will affect how easy it is for residues of the cargo to be removed from the coating:
(a) Volatile solvent-like cargoes can be rapidly and almost completely removed from the coating by evaporation / ventilation of the tanks following completion of discharge.
(b) Whereas, removal of heavier oil-like (residual) cargoes, which will also become retained in the coating system, will not be removed by evaporation/ventilation and will require more rigorous cleaning.
Such oil-like (residual) cargoes are therefore more problematic for zinc coatings.