The Role of Copper on the Crevice Corrosion Behavior of Nickel-Chromium-Molybdenum Alloys in Aggressive Solutions

The effect of Cu on the localized corrosion of Ni-Cr-Mo alloys has been investigated in hot saline solutions by comparing the behavior of N06059 and N06200 alloys, using electrochemical and surface analytical techniques. No measurable effect of copper on anodic film growth kinetics and passive film properties was detected and the breakdown and repassivation potentials of the alloys were very similar. Angle-resolved x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy demonstrated that the copper segregated to the oxide/solution interface during anodic film growth and that this process was enhanced as the pH decreased, as would occur in a crevice prior to initiation. Galvanostatically controlled crevice corrosion experiments demonstrated that this surface accumulation suppressed the metastable breakdown events that preceded initiation on the Cu-free alloy. Dynamic secondary ion mass spectrometry showed that copper accumulated with molybdates in crevice corroded locations but could not confirm any influence of copper on crevice propagation.