Prevention of Corrosion in Sewage Holding Tanks

Sewage holding tanks constitute a highly aggressive chemical and biological environment in which degradation of internal structural materials may occur. The decomposition of organic matter generates gases and compounds which, in the presence of moisture, form corrosive media on steel surfaces.

The risk is significantly increased where sewage is retained in the tank for extended periods, where ventilation is inadequate, and where tanks are not emptied, flushed and cleaned at regular intervals. The consequences may include wastage of material, cracking and leakage, and, in the case of structural tanks, potential compromise of hull integrity.

Regular and thorough inspections of the internal surfaces of sewage holding tanks should be carried out, including complete emptying, flushing and cleaning in accordance with the maintenance plan. Particular attention should be paid to structural steel tanks, where the condition of protective coatings should be maintained and any identified defects rectified in a timely manner.

The installation of non-structural holding tanks constructed from corrosion-resistant materials eliminates the risk of tank penetration and associated consequences. Where conventional designs with structural steel sewage tanks are in use, systematic monitoring, proper maintenance and early detection of corrosion are essential to prolong service life and prevent incidents onboard.

Recipients: This safety lesson is addressed to shipowners and operators, shipbuilders, maritime administrations, classification societies, and seafarers.