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Epoxy & Zinc Coatings for chemical tankers - Tanks maintenance procedure

The tanks of Chemical Tankers may be constructed or coated with various different types of materials and it is important to check with the P&A manual and the Paint Manufacturers Coating Resistance list prior to commencing Tank Cleaning Operations in order to ascertain the tank coating materials and any limitations with regards to temperature, use of cleaning chemicals etc which may be applicable to the vessel.

Tanks are generally: Stainless; Epoxy / Phenolic Coating or Zinc Coated / Marine Line Coating

A modern chemical tanker has all her cargo tanks coated unless they are made of stainless steel. The main reasons for coating are: easier cleaning and less risk for cargo contamination. The durability of properly applied and maintained coatings may be ten years or more. On the other hand one mistake in cargo selection may totally ruin a coating.

It is a matter of knowing the limitations and possibilities of each type,or even of each make, of coating. Here the chemical build up of coatings shall not be elaborated, only the physical properties as regards resistance and application.

Every paint manufacturer has his own resistance list stating approved cargoes, temperature and time limitations etc. Contact the maker if in the slightest doubt; mistakes may turn out to be very costly. Many coating manufacturers give some sort of guarantee for the first two years after application.

Use of Epoxy coatings

  1. Epoxy coatings generally possess a good resistance against alkalies, seawater, wine, vegetable oils, crude oils, gas oils, lub oils, jet fuels, gasoline and also weak acids (as in free fatty acids in vegetable oils, but acid value should not exceed 20-40).

  2. Epoxy has limited resistance against aromatic hydrocarbons ("solvents" such as benzene, toluene), certain alcohols (e g methanol), ketones (acetone) and some esters. Epoxy is sometimes indicated as resistant also to stronger acids. This may be correct, but as an applied coating one must count on "holidays" in the film, thus making epoxy unsuitable for really corrosive liquids.

  3. Epoxy coatings which have been stressed beyond their chemical resistance with strong solvents tend to soften; test with your nails. In such a case the coating must be given ample time to "weather out" trapped solvents and recover its hardness before being subjected to cargo or water again.

  4. Do not try to speed up the recovery by application of heat! The top skin of the coating may then first harden, leaving trapped solvent underneath, with flaking as a consequence.

  5. Ventilate with a good turbulence in all corners of the tank. Hardness of epoxy coatings can be established by means of a standardized test using pencils of different hardness as a reference (Sw standard SIS 184 187).

  6. Adhesion of a coating to the steel is also reduced if it is overstressed by a cargo of strong solvents. There are standard test methods for the determination of adhesion by means of glued-on tablets, which are then pulled off with a recording of the necessary force.

  7. Epoxy coatings should normally not be heated above 60 -(80) degr.C during tank washing, steaming etc, During the loaded voyage lower temperatures should be kept.

  8. Epoxy tar coatings. are made up from epoxy with an addition of coal tar. They are excellent against seawater and crude oils but should never be used in chemical tankers, Light hydrocarbons of moderate solubility such as jet fuels, gas oils, gasoline may cause the tar to leak out, which may contaminate the cargo.

  9. Salt water ballasting of cargo tanks should be avoided except in emergency situations/heavy weather conditions. Epoxy type coatings are best for this purpose.

  10. If time permits, touch up of defects should be carried out before ballasting to avoid severe pitting caused by cathodic effect of the stainless steel fittings in the tank, the water temperature and its chloride contents.

  11. If epoxy type coated tanks are not available then the next best choice is zinc. Note that prolonged use of zinc tanks for ballast will shorten the life of the coatings.
Zinc silicate coatings for cargo tanks

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

  2. Caustic Acid or Caustic Soap mixtures must not be used for cleaning as they will damage the coating and possibly complicate the cleaning process.

  3. 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).

  4. 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.

  5. 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.

  6. 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.

  7. Always comply with the Coating Maker’s requirements for each cargo

  8. 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, ketones. Jet fuels may suffer zinc "pick up" from the coating to an extent which is considered a contamination. Therefore: check with shippers requirements.

  9. 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:

  10. 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 aggressivity and zinc pick up, may increase during transport.

  11. Certain coatings have Governmental approval for edible oils, check for pH-limitations in such cases.

  12. Zinc silicates are not suitable for long time exposure to seawater, the life span will be unduly reduced.

  13. After carriage of molasses in zinc silicate coated tanks a thorough cleaning should be carried out as soon as possible. Sour cargo remains on the tank bottom may damage the coating.

  14. 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.

  15. 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!

  16. 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 Owners at once; it may be that the last cargo was off specification and caused the damage.

  17. Phenolic resins are a recent addition to the family of tank coatings. They have a wide resistance list including strong solvents which the epoxies do not tolerate- At the same time the phenolics accept about all of the products that zinc silicates tolerate. This type of coating is likely to gain further application on board.

  18. Polyester coatings have poor resistance to solvents but are fairly resistant to weak acids and alkalies. They are not used on board in chemical tankers to any extent.

Maintenance of tank coatings

Maintenance of tank coatings means, above all, not to subject the coatings to non-permissible cargoes. Check with maker's recommendations. Limitations as regard pH-values, max temperatures and max permissible storage time on board must be followed. Let epoxies regain their hardness if softened.

Generally it is not worth while to recoat an epoxy coated tank on top of an old coating, the risk for a poor bond is too great. Minor damaged areas can, with a certain degree of success be recoated. The area must first be degreased well.

The area should then be ground to a bright steel finish with a rotating grinder, grain 80 to 120, with an even transition to the coating. 1 - 2 coats of primer plus 3 - 4 finishing coats may be necessary in order to build up the proper film thickness (200-300 microns). The tank wall may have to be warmed up (preferably from behind) in order to insure that there is no risk for condensation on the surface. If possible the, tank wall should be warmer than the tank atmosphere.

For application of coating on a whole tank the steel bulkheads must be properly sandblasted (non marine origin sand).

If ballast is carried in Stainless Steel cargo tanks it should be discharged as soon as possible.

Whenever salt water is used for cleaning stainless steel tanks, fresh water must be used immediately afterwards for the removal of salts which can roughen and cause pitting if allowed to remain on the surface.

When deballasting coated cargo tanks, the ballast water must be “chased down” using tank cleaning machines. Do not allow the cargo tanks to dry off before fresh water washing as this can cause significant staining.

If ballast is carried in Stainless Steel cargo tanks, it should be treated with caustic flakes to achieve a ph of 11-12. This is equal to approximately 50kg of flake NaOH added to each 1000MT of sea water. Caustic must be well mixed within the tank.

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Pre-cleaning /washing of cargo tanks

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Testing of tanks and cargoes

Practical tank cleaning methods for various noxious liquid cargo

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Determining proper tank cleaning by acid wash method

Supervision of all tank cleaning and gas freeing operations

Disposal of tank washings, slops and dirty ballast - safe method

Following reference publications provide useful guidance and international regulations for carrying hazardous chemicals at sea.

Our detail pages contain somewhat larger lists of resources where you may find more useful information.

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Our detail pages contain somewhat larger lists of resources where you may find more useful information.

Main Info pages!

Home page ||| Chemical hazards ||| Cargo planning & Stowage ||| Cargo loading ||| Cargo documents ||| Safe stability ||| Cargo care ||| Preparation for unloading ||| Inert gas systems |||Gas freeing ||| Nitrogen handling ||| Chemical handling Safe practice |||Handling equipments ||| Cargo & Ballast pumps ||| Cargo tanks |||Tank cleaning |||Special cargoes |||Spills emergencies |||Fire protection is merely an informational site about various aspects of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage, Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and Chemical Processing. If you are interested in finding out more about chemical tanker guideline please visit IMO official website. For any comment please Contact us

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