Effect of dissolved hydrogen on corrosion behavior of Ni-base alloy in high-temperature water

The corrosion behavior of Alloy 690 (UNS N06690) was investigated in a 500 h immersion test conducted in varying hydrogenated Li/B solutions at 330degC. The corrosion rates were determined by the chemical descaling of the oxides; rates were found to decrease as the hydrogen concentration increased from 35 to 100 cc/kg. Scanning electron microscopy and X-ray diffraction were used to characterize the morphology of the oxide layers. Two oxide layers were identified: an inner layer composed of very small grains of a few nm in diameter and; an outer layer composed of some plate-like crystals clustered together. The sizes of the crystals built up on the outer layer decreased with increasing of the hydrogen content. The difference in crystal size may be due to changes in the crystal growth environment. The composition of the oxide film built up on the specimen surfaces depended on the hydrogen concentration in the test environments, as determined by XPS depth analyses. The chromium and iron contents of the oxide films increased with increasing hydrogen levels from 35 to 65 cc/kg, whereas the nickel content increased with decreasing hydrogen concentration. Under a 35 cc/kg hydrogen level, related to the corrosion potential of the Ni/NiO equilibrium, a rapid change of nickel into nickel oxides containing chromium and iron occurs. Under high hydrogen level of 65 cc/kg, the nickel oxides reduced to metallic nickel, which was confirmed by XPS results of the nickel 2p spectrum. At lower potential with 65 cc/kg hydrogen content, there was a potential zone in which the chromium and iron oxides were stable at testing temperature and pH. As a result of this, it is thought that increasing the hydrogen content of the environment is likely to increase the amount of the chromium and iron oxides. (author)