CTM 9.7 A4	Chemical Tanker Manual CTM Care & Maintenance of Zinc Silicate Coatings	Doc No.: CTM 9.7 A4 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

1. GENERAL

Zinc silicate is an inorganic paint. This automatically means it is insoluble in organic solvents. It is durable, heat resistant, and abrasion resistant. It has outstanding mechanical strength and provides galvanic protection.

Cargoes tend to be adsorbed rather than absorbed. ("Absorb" refers to a situation where something is taken into a medium, and disappears. "Adsorb" refers to a situation where something gets stuck onto.)

It has no reaction with solvent/water mixtures. Dedicated methanol carriers are zinc silicate coated. They can be loaded with methanol again and again without ventilation. Venting is not so crucial as for epoxy tanks whose coatings softens. However, it is better if tanks are ventilated. As sometimes wall wash will show HC/colour. Vent for 1 or 2 days to eliminate this problem.

Zinc Silicate salts are produced throughout the active life of the coating which appear as white patches. This is normal and will not cause a problem unless it is extracted by a wall wash surveyor. Even if these white salts are removed they will be regenerated over a period of time. Small damages to coating are self-healing by electrochemical reaction.

Since the coating cannot handle pH values between 9 and 14, remember LAC and caustic are extremely harmful. So in zinc tanks use 0.05% Teepol/ Genepol. Special acid cleaners like bufferclean 5.5 (citric acid of pH 5.5) can be used. Bufferclean 5.5 removes all zinc white salts and brings it down like snowfall. Buffer solution resists any change of its pH value due to dilution or an addition of solution of acid/ alkali. The process by which H+ or OH- ions are removed so that pH remains constant is known as buffer action.

Please remember dried detergent can also cause white patches. For tank cleaning 80°C water temp can be used. Zinc can tolerate higher temperatures than epoxy. However, due to its porosity, zinc tanks should first be cleaned at moderate temp and then only cleaning temp can be increased. Tallow/ animal fat, when heated, increases in acidity near heating coils. Also, esters, alcohols, amines, chlorinated, and brominated compounds should not have moisture content greater than 0.01% (100 ppm). Molasses when diluted, especially by bad quality fresh water from lakes, etc., create acids within 24 hours which attack coating. Continuous exposure to salt/ FW reduces life of coating. Toluene and Xylene are good degreasers.

Zinc paint is resistant to: Phenols and cresols, aromatic hydrocarbons BTX, alcohols and ketones, CTC, EDC, Trichloroethylene, and esters like amyl and ethyl acetate provided tank surfaces are free of moisture. Resistant to Acetone, Acrylonitrile, Ethanol, Methanol, IPA, MEK, Methyl Acrylate, VAM, Butyl acetate, Ekta Solve, Adiponitrile etc.

Zinc paint is not resistant to: Acetic acid, Alkaline Glycol Monoalkyl Ether, Diethanolamine, Diethyl Suphate, Brake fluid, Vinyl Acetate, Pentanoic acid, Triethanolamine, Nonyl Phenol, Ethyenediamine, Ethyl Hexanoic acid, Polyethylene Polyamine, Ethatholamine, Aviation turbine fuel, alkalies greater than pH 9, acids less than pH 6.

Fatty acids of veg/ animal oils attack zinc silicate paint which can cause zinc to shed contaminating cargo. Amines water content if >500 ppm (0.05%) can cause alkali attack. So to prevent water contamination, the tank must be dry and nitrogen blanketed. Never use hot water for pre-cleaning veg/ animal oils in zinc tanks because they have FFA (free fatty acids). The manufacturers of zinc silicate coating describe a max FFA content of 2.5 as the limit. In practice, this limit is way too high. Zinc-silicate-coated tanks are not used for crude edible fats and oils, unless the acid value is 1 or under.

Recirculation of veg/ animal oil in coated tanks is a must to prevent sedimentation and also to equalize the FFA content between the upper and lower layers. Since zinc lining is porous, prolonged pre-cleaning with ambient water is advisable after drying oils before introducing chemicals.

FFA content may increase rapidly when cleaning is prematurely stopped. Semi-drying vegetable and animal oils react with oxygen to form a varnish-like polymeric layer which is very difficult to remove. If there is a delay in washing drying oils to prevent the oxidized varnish from forming on the bulkheads, the air should be removed from the empty tank.

If a zinc silicate tank has been attacked, you can see a thin layer of white dust on the surface OR the coating gives a porous appearance. White salts caused due to alkali PH > 9 attack are insoluble calcium and magnesium salts. Only bufferclean 5.5 can remove such white salts, and the tank gets a fresh appearance. Never allow salt to dry on the coating after sea water wash. Salt electrolyte makes the coating sacrificial.

Zinc silicate tanks are not considered suitable for continuous immersion in water. The surface is porous and cargo invades into pores and capillaries. Cleaning is similar to regeneration of an adsorber bed. This is done by heating to greater than BP. Unlike epoxy, desorption occurs quite fast. Methanol removes non-volatile matter from the pores/ capillaries.

Most Zinc Coatings are porous and have a tendency to trap or hold previous cargoes making them difficult to clean to a high standard.

2. LOADING CARGO IN ZINC COATED TANKS

Following should be kept in mind when loading cargo in Zinc Coated tanks:

• Caustic Acid or Caustic Soap mixtures must not be used for cleaning as they will damage the coating and possibly complicate the cleaning process.
• Not all Zinc coatings will become clean by the same method; this depends not only on the last cargo carried but other factors such as the age and condition of the coating (blistering, areas of open rust).
• Following the carriage of Acetate and EDC cargoes, proper ventilation must be carried out in order that any free liquid remaining is removed as such residues when mixed with water will result in coating damage. Comply with coating maker’s restriction for cleaning with water.
• Stowage of viscous, high melting point slops or dark coloured liquids should be avoided whenever possible due to the porous nature and difficulty of cleaning.
• Always comply with the Coating Maker’s requirements for each cargo.
• Zinc silicate coatings, particularly those of inorganic type, are very resistant against strong solvents and normally tolerate higher temperatures than epoxies. Typical products are: aromatic hydrocarbons (benzene, xylene etc.), alcohols, and ketones. Jet fuels may suffer zinc "pick up" from the coating to an extent which is considered contamination. Therefore, check with shipper’s requirements.
• Zinc silicates are not resistant against acids or alkalies. The pH value of the cargo should be within the range 5.5 - 10.5 (neutral pH 7.0). This means that some molasses (slightly fermented - low pH) may attack zinc silicates, as well as high contents of free fatty acids in vegetable or animal oils.
• Zinc silicate coatings may under such circumstances cause zinc pick up into the cargo. They are therefore not normally suitable for edible oils for human or animal consumption. Remember that the contents of free fatty acids, and thus also the aggressiveness and zinc pick up, may increase during transport.
• Certain coatings have Governmental approval for edible oils, check for pH-limitations in such cases.
• Zinc silicates are not suitable for long-term exposure to seawater, the lifespan will be unduly reduced.
• After carriage of molasses in zinc silicate coated tanks, a thorough cleaning should be carried out as soon as possible. Sour cargo remaining on the tank bottom may damage the coating.
• Zinc silicates are only partly resistant to chlorinated compounds (e.g., carbon tetrachloride, ethylene dichloride, trichlorethylene). If the water content is high, hydrochloric acid may develop, which will attack the coating. In a similar way, hydrolyzable hydrocarbons such as esters, acetates, and halogenated compounds may attack the cargo. If, however, the product is guaranteed dry and the cargo tanks and piping are completely drained and dried, these products can be carried.
• Alkaline tank cleaning agents (caustic) should never be used in zinc silicate coated tanks. Considerable damage can be done in one single cleaning operation. Zinc silicates stand well up against other cleaning agents such as "solvent cleaners" and "emulsifiers" unless they have alkaline additives: Check first with the maker of the cleaning product!
• If a zinc silicate coating has been attacked, one can often observe a thin layer of white dust on the surface, or the coating gives a porous appearance. Inform the office at once; it may be that the last cargo was off specification and caused the damage.