Formation of reaction product layers on copper alloys in chloride solutions

Copper and copper alloys are widely used in marine applications. This is mainly due to their inherent corrosion resistance and antifouling capability. The corrosion resistance of copper and its alloys depends on the formation of a protective reaction product layer. Various opinions on reaction product layer formation mechanisms have been presented. In this paper the formation of reaction product layers on copper and copper alloys was studied in the laboratory and under field conditions in brackish sea water. The laboratory studies included polarization curves, cyclic voltammetry and Contact Electric Resistance measurements. During immersion tests passivation was monitored by polarization resistance and corrosion potential measurements. The polarization curves showed, that addition of chlorides in the solution will decrease corrosion potential and cause a rapid dissolution reaction at the same potential that is the corrosion potential in chloride-free solution. Cyclic voltammetry results showed, that without chlorides the layer may contain several oxide and hydroxide phases. The pH and copper chloride complex concentration at the surface must be high enough for nucleation of cuprous oxide. The Contact Electric Resistance results showed, that the first anodic current peak in cyclic voltammograms is not necessarily caused by reaction layer product formation. The CER results show that the reaction product layer resistance begins to increase at 100-200 mV higher potentials in chloride solutions than in chloride-free solutions. The immersion tests showed again, that the formation of the reaction product layer may take a long time. Especially corrosion resistant copper-nickel alloys in lean brackish sea water had a stable corrosion potential only after several weeks.