Friday, March 27, 2020

GLS TANKS: NEAR COLLAPSE DUE TO SERIOUS CORROSION

GlS TANKS: NEAR COLLAPSE DUE TO SERIOUS CORROSION


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GLS TANKS INDIA - Rostfrei Steels Private Limited |During an owner’s annual inspection, serious corrosion attacks were found in integrated GLS Tanks, which required extensive repair.
Brackets and stiffeners were found partly corroded through, as shown in figures 1 and 2. The corroded areas in the GLS Tanks had to be partly renewed and repaired to an acceptable standard.

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Probable Cause
The process for cleaning the GLS Tanks was considered to have been a major factor in the corrosion process. The GLS Tanks were sterilized annually, which exposed the tanks to a solution of hypochlorite for 12-24 hours. In addition, the shore-supplied water was frequently chlorinated, which meant that the tanks were frequently exposed to chlorides.
Even small amounts of hypochlorite can be very dangerous for aluminum, irrespective of the Al-alloy chemical composition, resulting in the development of serious pitting. Once pitting corrosion has started, it may continue to develop for a certain period if the tank is not properly washed with water after the application of hypochlorite. The filling and emptying of the tank are not sufficient to remove all the corrosion products. The reason for the corrosion attack is considered to be that the thin aluminum film on the surface is depolarised and locally broken down, in connection with the formation of chlorine released from the hypochlorite.
Lessons to be Learned
  • Hypochlorite or other chloride-containing chemicals should not be used for sterilization of aluminum freshwater tanks if the tanks have not been fully coated;
  • In the case of uncoated aluminum freshwater tanks, other means of sterilization than chloride-containing chemicals is needed, e.g. by using ultra-violet light;
  • If hypochlorite is used in coated tanks, it is essential that the tanks are thoroughly cleaned after the process. This is to minimize the change of corrosion occurring in the event of the coating breaking down.

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