Type and Condition of Tank Linings
Generally speaking, tanks lined with stainless steel require less time and use of chemicals to obtain the desired result than coated tanks. Further polished stainless steel tanks hold or trap less product than the rougher surface which is more commonly used. It is important that all tanks are cleaned each time to a good Water White Standard. Any residue remaining within the stainless profile will harden up and will be very difficult to remove when preparing the tank for wall wash cargo. Remaining cargo can eventually cause pitting in the steel resulting in expensive repairs. Unlike Phenolic / Epoxy or Zinc lined tanks almost any cleaning medium can be used without damaging the tank lining. When cleaning stainless steel, it is important to take into consideration whenever salt water is used for part of the cleaning process, Salt water should be immediately and completely flushed with fresh water to remove any salts which can roughen and cause pitting if left on the surface. Salt water ballasting of stainless steel tanks should be avoided expect in emergencies.
Precaution: Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous passive chromium oxide, film. Pickling and passivation are both acid treatments and neither will remove grease or oil. If the steel is dirty, it may be necessary to use a detergent or alkaline cleaning before pickling or passivation.
E.g. Pure epoxy, phenolic epoxy and isocyanate epoxy form cross linkages to different degrees resulting in relatively good resistance to a greater range of cargoes. Epoxy systems are usually resistant to some weak acids and strong alkalies and do not absorb oil-like substances. Epoxy coatings tend to absorb, however, solvent-like cargoes (Resistant with limitations according coating resistance list). This absorption is caused by swelling and subsequent softening of the coating. After transporting aggressive cargoes, the coated tank has to be ventilated until the cargo has been desorbed (released) from the coating film, which results in hardening and decreasing swelling. This can take up to several days, depending on type of cargo, type of coating and film thickness. Water may not be used for cleaning until this ventilation process is finalized. Otherwise the water can lead to blistering and subsequent serious damage of the coating. The more solvency power a cargo has, the more cargo residues could still be present in the coating. This could lead to either contamination of the next or after next cargo or breakdown of the coating film.
Provided that the linings are in good conditions and free of cracks and blisters, they generally do not hold or trap very much previous cargo. Generally alcohol or acetate type cargoes, although permitted to be loaded and carried, may soften the lining and get trapped in the lining making it more difficult to clean. A good way to determine whether or not a cargo may soften the lining is to refer to the tank coating resistancy list. If the list restricts the carriage of subsequent cargo, it is a good indication that the lining may have soften trapping some of the cargo. The tank coating manufacturer’s guidelines must be adhered to.
A ventilation period or non aggressive subsequent cargo is required after certain cargoes. This must be followed to avoid blistering of coating and therefore failure of coating. Extensive and aggressive cleaning can also soften the lining. If the lining is not suitable for the carriage of the cleaning medium, the medium should not be used for cleaning. Should the lining become soft, ventilation for a period of 24 hours or more or re-circulation of caustic solution will usually help harden and cure the lining.
Zinc silicate coating is an anti corrosive paint system that is based on zinc dust (86% wt) with some additives and a binder. The high levels of zinc dust giving zinc-zinc metal contact resulting in cathodic protection similar to those obtained from galvanising. Zinc coatings are inherently porous, which is resulting in a variety of cleaning problems. It is believed that the cargo migrates into the pores and capillaries, similar to a fluid adsorption processes. Zinc coatings have a good resistance against solvents, but are not resistant against strong acids and bases. Most zinc lining surface are porous and have a tendency to hold or trap previous cargoes making them somewhat difficult to clean to a high (Wall Wash) standard. Caustic or caustic based soap mixtures cannot be used as they will damage the lining and possibly further complicate cleaning. Quite frequently, when the previous cargo is not water soluble,RXSOL surfactants (soap) or solvents will be needed as part of the cleaning process to remove all traces of the cargo. It should be remembered that not all zinc coating will become clean by the same method: this depends upon not only last cargo but other factors such as how old the coating is and whether there is delamination, blistering or areas of open rust. Cargos which are to be carried in zinc tanks and have FFA (free fatty acid) notations should be checked for the amount of FFA prior to loading. Some drying oils which are carried have significant quantities of moisture present which settles out during the voyage, soften the zinc coating in the lower areas of the tank and causing cleaning problems.
Following the carriage of Acetate cargoes, proper ventilation must be carried out so that any free liquid remaining is removed, as such residues mixed with water will damage the coating. Stowage of viscous – high melting point slops or dark colored liquids should be avoided whenever possible due to the porous nature and greater likelihood of coating damage.