Effect of temperature and dissolved hydrogen on oxide films formed on Ni and Alloy 182 in simulated PWR water

• Published in: Journal of nuclear materials 2016-08, Vol.477, p.280-291
• Main Authors: Mendonça, R., Bosch, R.-W., Van Renterghem, W., Vankeerberghen, M., de Araújo Figueiredo, C.
• Format: Article
• Language: English
• Subjects: Alloy 182; Austenitic stainless steels; Exposure; High temperature aqueous corrosion; Hydrogen; Mathematical models; Nickel; Nickel base alloys; Oxides; Passive films; Pressurized water reactors; Stress corrosion cracking; Welding
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2016.05.022

Alloy 182 is a nickel-based weld metal, which is susceptible to stress corrosion cracking in PWR primary water. It shows 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 hypothesis is that the oxide layer at this condition is not properly developed and so the material is not optimally protected against SCC. Therefore the oxide layer formed on Alloy 182 is investigated as a function of the dissolved hydrogen concentration and temperature around this Ni/NiO transition. Exposure tests were performed with Alloy 182 and Ni coupons in a PWR environment at temperatures between 300 °C and 345 °C and dissolved hydrogen concentration between 5 and 35 cc (STP)H2/kg. Post-test analysis of the formed oxide layers were carried out by SEM, EDS and XPS. The exposure tests with Ni coupons showed that the Ni/NiO transition curve is at a higher temperature than the curve based on thermodynamic calculations. The exposure tests with Alloy 182 showed that oxide layers were present at all temperatures, but that the morphology changed from spinel crystals to needle like oxides when the Ni/NiO transition curve was approached. Oxide layers were present below the Ni/NiO transition curve i.e. when the Ni coupon was still free of oxides. In addition an evolved slip dissolution model was proposed that could explain the observed experimental results and the peak in SCC susceptibility for Ni-based alloys around the Ni/NiO transition. [Display omitted] •Exposure tests with Ni-coupons showed that the Ni/NiO transition curve shifted to more oxidizing conditions.•The Ni specimens tested in PWR water were free of oxides at all temperatures.•The exposure tests with Alloy 182 showed that oxide layers were present at all temperatures.•The Alloy 182 surface morphology changed from spinel crystals to needle like oxides when the Ni/NiO curve was approached.•An evolved slip dissolution model was proposed that could explain the experimental results and the peak in SCC susceptibility.