The basic purpose of coatings used in the marine environment is protection of the mild steel construction from corrosion, be it dry corrosion from a reaction between the steel and the surrounding vapour, or by wet corrosion from the oxidation and reduction of the mild steel in a liquid solution.

Additional articles on coating are listed below:

When carrying chemicals in cargo tanks of mild steel, there is now a secondary purpose of a coating which is to provide an insulation barrier between the mild steel and the cargo, mostly organic and inorganic substances, plus of course, to insure the integrity and purity of the cargo.

An ideal tank coating should have the following charateristics:

  • A smooth surface for easy tank cleaning
  • Good absorption/desorption characteristics i.e a minimum and maximum in each case.
  • Minimise any penetration of the coating by a product
  • Provide good corrosion resistance
  • Provide a continuous coating film with a high level of integrity
  • Perform excellent adhesion of the coating to the steel

Ignoring stainless steel for the moment as the ideal material of construction for cargo tanks, but which comes in many forms and variations, many of which are unsuited to a sea-going environment, cargo tank coatings can be categorised into two main groups:

  • Inorganic coatings – zinc silicates and ethyl silicate types
    • Alkaline zinc silicate
      • a water-based zinc silicate in which the curing occurs through the reaction between the zinc powder and the silica gel binder.
    • Ethyl zinc silicate
      • a solvent-based coating in which the curing is by evaporation of the solvent, as opposed to water. (simplistically put to say the least)
  • Organic coatings – epoxy and modified epoxy systems
    • Epoxy phenolics
    • Epoxy isocyanates
    • Cyclosilicon epoxy

Each of which have good and bad characteristics, namely:

  • Inorganic coatings:
    • A cargo, following discharge, can be completely removed from the coating by evaporation
    • High density products cannot be evaporated
    • Extremely resistant to aromatic hydrocarbons such as benzene, toluene, alcohols & ketones.
    • Not resistant to acids or alkalies, including sea water.
    • Not suited to vegetable oils or animal fats, but halogenated compounds are suitable, provided the surfaces are moisture free.
  • Organic coatings (epoxy phenolics & epoxy isocyanates):
    • Able to resist more acidic or alkaline products than inorganic coatings
    • Tend to absorb significant quantities of a product, causing contamination problems.
    • Suitable for the carriage of alkalies, glycols, animal fats & vegetable oils (with limitations)
    • Suitable for molasses (with limitations)
    • Limited resistance to aromatics – benzene, toluene and alcohols – ethanol, methanol.
  • Cyclosilicon epoxy

Essentially a cyclic silicon structure with five epoxidised phenol groups, cured by means of a catalyst to give a highly crosslinked polymer.

Put simply, these coatings are a two-component paint based on Siloxirane, a patented polymer with an organic/inorganic matrix.

Manufacturers, Advanced Polymer Coatings, claim that MarineLine 784, developed for the maritime chemical tanker, is the only high performance lining that withstands all IMO approved chemical cargoes with very low absorption characteristics, high chemical resistance and good mechanical properties.