Tank Tests

The Salvage Association (SA) said last week that it has pinpointed phosphoric acid as another chemical cargo causing some anxiety to marine underwriters, following the controversy surrounding the carriage of calcium hypochlorite chemical cargoes in containers. The SA said in a statement its Singapore office has been involved in two cases where stainless steel tanks in chemical tankers have been attacked by the acid, leaving large black stains on the surface and in the worst case, pitting and grain boundary attack. The structural strength of the tanks is not affected but it is detrimental to other sensitive cargoes, and its unsightly appearance means the vessel could be rejected by cargo surveyors. The SA said samples of the acid were analyzed at a number of laboratories and the results, although varied, showed that the acid contained a high percentage of "destructive constituents," with an imbalance of chlorides to iron and aluminum. Tank washing with seawater and later, nitric acid, can also worsen the situation, the statement said. The chlorides which remain in the roughened stainless steel surface then form hydrochloric acid, which is detrimental to the metal. The SA said it is not possible to say how many cases there have been but added that one major carrier has recently suspended carrying any more cargoes until the investigation into the nature of the attack is completed. The cost of refurbishing tanks is high.