From defence to damage: the impact of seawater passivation on microbially influenced corrosion in CuNi 70/30 alloy†

The work examined the effect of seawater passivation on microbially influenced corrosion (MIC) of CuNi 70/30 alloy when tested with sulfate-reducing bacteria (SRB). The experiments were performed in two stages. In stage I, CuNi 70/30 samples were passivated in natural filtered and pasteurized seawater for 35 days. Electrochemical tests showed that passivated samples had improved corrosion resistance compared to non-passivated samples, as demonstrated by higher linear polarization resistance, lower corrosion current densities, and higher charge transfer resistance values. In stage II, the MIC performance of 35 days seawater passivated samples was investigated and compared with control non-passivated samples. Samples were immersed in modified Baar's medium with and without SRB for 28 days under anaerobic conditions. The results showed that the passivated samples experience greater MIC susceptibility as compared to the non-passivated samples. This unexpected susceptibility is attributed to the existence of a copper oxide film on the surface of the passivated samples, which converted into copper sulfide film in the presence of SRB, leading to film cracking driven by structural changes at the oxide/sulfide film interface. The defective and porous surface film significantly contributes to the accelerated corrosive attack of the exposed base metal.