CARGO TANK COATING

A chemical tanker defined as a tanker which carries liquid cargoes in bulk. There is a wide variety of cargoes carries by these ships, not only chemical products but also commodities (vegetable oil, fats, wine) and inorganic substances (sulfuric acids, phosphoric acid, and caustic soda).

About 30% of liquid trading cargoes (chemical and chemical products) are carried by chemical tankers. Due to the remarkable growth of chemical industry, shipping industry carries more than 50,000 different chemical substances. Accordingly the demand for more sophisticated and efficient chemical tankers shall be increased too.

About Cargo Tank Coating

The chemical tanker is a very special type of ship due to the complexity and the particularity of the cargo. So most of the times, much attention is given to the cargo tanks and to their ability to ensure the quality and purity of the cargo throughout the voyage.

Cargo tank coating has two main roles, first to create a separation barrier to avoid direct contact between the mild steel (tank construction) and the corrosive cargo substance. Secondly, it must have smooth/slippery surface to provide easy tank cleaning operation.

Cargo tank coatings can be categorized into two main groups:

1.Inorganic coatings- zinc silicates and ethyl silicate types.

Generally, the life of this coating is proportional to the thickness of the coat. This coating is one-layer coating, comprising of inorganic silicates pigmented with high percentage of zinc powder.

2.Organic coatings – epoxy and modified epoxy systems.

This type of coating consists an organic resin system, which form strong chemical bonds between the resin molecules. Those types of coating have the ability to resist in more strong acids or alkalis than inorganic coatings. And they tend to absorb significant quantities of cargo and contamination problems can occurs

Coating Systems and Types

Numerous types of coating have been used for cargo tank service in sea trades. Some of these coating have stopped to being used. And more reliable and flexible coating has been developed. Typical coating system can be categorized as Zinc and Epoxy coating

Zinc Silicates

Zinc silicates are formulation of zinc powder plus organic or inorganic binder, and designed to be porous films, which can create problem in the tank cleaning process especially when vessel carry non-volatile cargoes.

Main Characteristic:

·Not resistant to strong acid or bases, including sea water which has a slow weakening effect

·High resistance and tolerance to aromatic hydrocarbon solvent, alcohols and ketones

·Volatile cargoes are desorbed very fast, and retain non-volatile oil like cargoes

·Residues can result in contamination of next or after next cargo

Epoxy

Generally suitable for the carriage of alkalis, animals fats and vegetable oils but they have limited resistance to aromatics such as benzene and toluene, alcohols such as ethanol and methanol

Main Characteristic:

· Resistant to most strong acids and bases

· Do not retain oil like cargoes. Solvent cargoes are absorbed

·Water wash before thorough ventilation and desorption of residues could result in serious damage of the coating

·Residues can result in contamination of next or after next cargo

·Suitable for carriage of animal fats and vegetable oils provided the free fatty acid content of 5%

Coatings are required for any cargo tank which constructed from mild steel. Most of BLT Chembulk Group modern chemical fleet is SUS cargo tank. SUS are good materials for chemical tanks, because of their ability to create a passive layer on their surface. This passive layer is mainly consisted by chromium oxide,which is very resistant to corrosive environment.

However, in some environments like strong hot acids, chloride solutions and generally solutions which contain halogens, the passive film can break down locally and new film formulation can be disrupted. Generally, SUS is considered to be the ideal material of construction because it’s non-corrosive and easy to clean.