Investigation of the temperature effect on the oxide layer formed on Alloy 182 in simulated PWR environment.

Ni-based alloys show a peak in SCC susceptibility at a certain temperature and hydrogen concentration. This peak is related to the electrochemical condition where the Ni to NiO transition takes place. One assumption is that the oxide layer at this condition is not properly developed and so the material is not optimal protected to SCC. Therefore the oxide layer formed on Alloy 182 is investigated as a function of the water chemistry and temperature. The main aim of this work is to correlate the behaviour and composition of the different oxide layers formed in high temperature water. Exposure tests have been performed with Alloy 182 and Ni coupons in a PWR environment as a function of temperature for a dissolved hydrogen concentration of 35 cc/kg. The temperature ranged from 300 to 345°C as in this range the Ni/NiO transition is located based on thermodynamic calculations. Post-test analysis of the formed oxide layers have been carried out by Scanning Electron Microscopy and Energy Dispersive Spectroscopy. The first results showed that there is a large difference between the low (300°C) and high (345°C) exposure temperature w.r.t. the oxide layer morphology.