Two unusual and severe cases of shipbottom pitting were encountered within the space of a few years. Although the two incidents occurred in polluted harbors 3000 miles apart, the vessels involved showed the same symptoms. These symptoms were rapid pitting, inactivation of the blackened antifouling paint and good electrical conductivity of the black surface film. The black conversion product, found only in the upper layer of the antifouling paint, was shorted to the steel hull.
A study of the painting history of the vessels involved and of the conditions to which they were exposed in service indicated two common factors were present on ships that pitted. First, there was contact between the antifouling paint and the steel hull and secondly, there was exposure to sulfide forming waters. When one of these factors was missing, vessels exposed to the other condition did not show unusual pitting.
To confirm a theory as to the cause of pitting a series of test panels duplicating conditions experienced in service were prepared. The tests, with suitable controls tended to substantiate the theory advanced. From information so obtained it was possible to recommend simple measures that would prevent the recurrence of the severe underwater corrosion described.
