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 cofferdam 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.
Keywords: corrosion, cathodic protection, steel, seawater, potentiostatic polarization