Effect of chromium on the oxides and stress corrosion cracking behavior of nickel alloys in high-temperature water

Nickel alloys such as Alloy 600 undergo Stress Corrosion Cracking (SCC) in pure water at temperatures between about 260 C and the critical point. Increasing the level of Cr in Ni-Fe-Cr alloys increases SCC resistance in aerated and deaerated water. The mechanism is not understood. The effect of Cr composition on the in-situ oxide rupture strain and corrosion kinetics of Ni-9Fe-Cr alloys was determined experimentally, to evaluate whether the anodic dissolution model, for SCC can account for the effect of Cr on SCC. Oxide films were characterized by Auger Spectroscopy and Analytical Electron Microscopy. The alloy corrosion rate and corrosion product oxide microstructure and mechanical properties are strongly influenced by Cr composition. Chromium incorporates into the corrosion film gradually with time, concentrating near the metal-oxide interface. As Cr concentration increases from 5 to 30%, oxide rupture strain in pressurized water at 288 C increases from about 8 × 10−4 to 2 × 10−3 cm/cm. Corrosion kinetics are parabolic; corrosion rate first increases and then decreases as Cr increases from 5 to 39%. These observations are qualitatively consistent with an anodic dissolution SCC mechanism. However, parametric modelling of the SCC growth and initiation processes, applying available creep, oxide rupture strain and corrosion kinetics data, indicates that the Ford-Andresen formulation of this mechanism accounts for only a fraction of the effect of Cr on SCC resistance. A model refinement which accounts for the grain boundary sliding component of creep deformation as the process for oxide rupture and crack initiation, rather than tensile-mode failure of the oxide, is developed. This process can continually expose fresh grain boundary material to the environment and explain the intergranular nature of SCC in these materials. It is suggested that creep driven grain boundary sliding of low-Cr grain boundaries is a viable SCC initiation mechanism.