Exploring De-alloying in Fe-Ni-Cr Alloys and its Relationship to Stress Corrosion Cracking in Nuclear High Temperature Water Environments

Most stress corrosion cracking (SCC) mechanisms initiate from localised corrosion (pitting, intergranular attack, de-alloying), which provides local stress concentration. Alloys are generally more susceptible to SCC than pure metals because selective dissolution or oxidation is possible. De-alloying involves the selective dissolution of the less noble (LN) component from an alloy. The more noble (MN) component enriches on the surface forming a brittle, metallic, nanoporous layer. In noble metal alloys and brass, SCC shows correlation with the threshold LN content below which de-alloying stops (the parting limit). In Fe-Ni-Cr engineering alloys de-alloying may be responsible for Cl-SCC, although this has not been proven explicitly. Initial indications show that de-alloying causes SCC in hot, caustic environments. In some cases, Ni enrichment and porosity are associated with cracks in stainless steel after long-term service in nuclear high temperature water environments, but it is unclear if this plays a causal role in cracking. Here the de-alloying mechanism (primarily the effect of Ni (MN) content) and its relationship to SCC in Fe-Ni-Cr materials (Fe10Ni, 310SS and Alloy 800) is examined using a hot caustic environment, and compared to classical de-alloying systems. De-alloyed layers formed on all materials, although Alloy 800 required a higher temperature. Increasing Ni content improved de-alloying resistance according to classical theory. Unlike classical systems, de-alloying occurred with concurrent MN dissolution and, at open circuit potential (OCP), the layers retained significant Fe and Cr (LN) instead of being ‘almost pure’ MN. Layers formed with applied anodic potential were friable and highly LN depleted. This behaviour was successfully modelled in Kinetic Monte Carlo simulations. Recently, it has been shown that SCC in noble element alloys depends on the mechanical integrity (quality) of the de-alloyed layer; a finding that was reflected here. At 140 °C at OCP the layer on 310SS was too thin to promote SCC and Alloy 800 did not de-alloy significantly. Layers formed with anodic potential did not result in SCC. In 50% NaOH at 280 °C, severely stressed 310SS cracked where thick de-alloyed layers formed. However, the thin layer formed on Alloy 800 was associated with SCC, even with low residual stress.Ph