A computational chemistry study of oxidation mechanism at the random grain boundary of Fe-Cr binary alloy

Stress corrosion cracking (SCC) is a life limiting factor for nuclear power plants. It deals with many variables directly and indirectly at different levels that need to simplify the problem. SCC initiation in LWR environments are localized and accelerated oxidation at interfaces considering microstructure, surface condition, and local environment. Alloy oxidation is an important issue for SCC initiation. Moreover, alloy grain boundary oxidation differs from surface oxidation. Several studies have concluded that grain boundaries are attacked preferentially during oxidation, which can have an adverse effect on the overall oxidation kinetics. Analytical transmission electron microscopy (ATEM) observation of tight crack have found that the crack propagated along the grain boundary where the crack tip 1~5 nm wide. Atomic level analysis of the interaction of grain boundary with the environment becomes important. In the present study, tight-binding quantum chemical molecular dynamics (QCMD) have been considered to study the oxidation mechanism of Fe-Cr binary alloy random grain boundary at boiling water reactor (BWR) environment. The slab model consists of eighty metal atoms including sixty-nine iron and eleven chromium atoms. The center part of the metal slab rotate 25 degree about the vertical axis in order to consider the random grain boundary. Single layer nine water molecules embedded on the metal surface which corresponds to the water coverage of 0.33. One percent uniaxial strain applies to the surface along the X-axis to analyze the effect on intergranular stress corrosion cracking (IGSCC) oxidation phenomena. The metal water interaction causes diffusion of environmental species and segregation of metallic atoms in the surface. Water molecules diffuse through the grain boundaries which can act as fast diffusion path due to areas of lattice mismatch. It is difficult to diffuse water molecules in the solid surface. It reveals that the grain boundary oxidation differs from the alloy surface oxidation. Applied strain increases the diffusivity of environmental species along the grain boundary which can influence the rate at which oxidation occurs at grain boundary. So, the process can enhance the alloy intergranular solid state oxidation.