The mechanism of accelerated corrosion of copper-nickel alloys in sulphide-polluted seawater

Copper-nickel alloys are reported to suffer accelerated corrosion in sulphide-polluted seawater. In the present study, the corrosion rates of 90 : 10 Cu : Ni alloy tube specimens and the characteristics of the surface corrosion products were determined for several environmental exposure histories. Some specimens were exposed only to de-aerated, sulphide-polluted seawater, some were exposed only to aerated, unpolluted seawater, and others were exposed first to polluted then to unpolluted seawater. In addition, preliminary experiments were performed to determine the effect of pH (7.0–8.2) on the corrosion rates of both 90 : 10 Cu : Ni and 70 : 30 Cu : Ni alloys in sulphide-polluted de-aerated seawater. The results support a previous prediction that the presence of dissolved sulphide in seawater does not lead immediately to accelerated corrosion but rather that the porous cuprous sulphide corrosion product formed in the polluted water interferes with the normal growth of the protective oxide film on subsequent exposure to unpolluted seawater. This interference with the oxide growth allows the initially high corrosion rate in aerated seawater to be maintained for an abnormally long period.