Corrosion of a Steel-hulled Vessel by Cathodic Protection

A steel-hulled vessel was found to have suffered serious corrosion damage near one of its impressed current cathodic protection anodes. The damage was observed at breaks in the anode shield near a weld and in the coffer dam housing the anode. Underwater inspections revealed that all of the vessels in this class were experiencing similar problems. Several potential causes, such as the reversal of the power supply leads, were ruled out, as the cathodic protection system appeared to be functioning as designed. As a result, a theory was developed that involved an unusual corrosion mechanism that was dependent on the accumulation of alkaline by-products of the cathodic reaction, in conjunction with an overly “protective” potential. Laboratory simulations using potentiostatic techniques were performed in order to test this theory. Corrosion of the steel was observed within three days at a nominally “protective” potential.