Electrochemical characterization of the oxide layer on nickel-base alloys in simulated PWR primary water

The long-term exposure of austenitic Ni-base alloys in PWR primary water leads to the development of an oxide corrosion film which is generally considered to influence the stress corrosion cracking (SCC) behavior of the alloys. The proper understanding of the formation and structure of the oxide layers would eventually lead to a better understanding of the SCC process. The structure, composition and thickness of the film depend on the chemical composition of the alloy and the exposure conditions. In this work the influence of material composition, exposure time, dissolved hydrogen and the addition of zinc were evaluated. The nickel alloys 600, 182 and 52 were exposed to simulated PWR chemistry. The electrochemical behavior of the different materials was investigated by electrochemical impedance spectroscopy (EIS) and electrochemical polarisation measurements. The in-situ EIS measurements allowed to monitor the oxide layer formation/change. Chromium is responsible for the compact protective oxide layer on alloy 52 as well as on Ni alloys in PWR environment. The results showed no important influence of the chosen exposure time on alloys 182 and 600 under PWR relevant conditions but alloy 52 showed a increased resistance with time. Also hydrogen affected alloys behavior decreasing the resistance of alloys 600 and 182 but apparently not altering the resistance of alloy 52. (authors)