Computational Study of Stress Corrosion Cracking on Nickel Based Alloy Exposed to Water Environment

Department of Nuclear EngineeringAs the design life of nuclear power plants approaches, continued operation of nuclear power plants is emerging as a problem. For long-term operations, it is imperative that the safety should be guaranteed first. To this end, we need to be able to understand the underlying mechanisms of material degradation to predict the degradation of the components of nuclear power plants. The nickel based alloys are pivotal structural materials used in nuclear industry because of corrosion resistance and mechanical properties that tolerate harsh and extreme environments. However, nickel based alloys undergo stress corrosion cracking (SCC) in high-temperature high-pressure environments. Stress corrosion cracking is a generic problem in high temperature waters, with most alloys employed suffering from cracking under some conditions. Among these conditions, the oxidation of nickel-based alloys can be regarded as the origin of the initiation of SCC, which is the main degradation process that induces failure of nuclear power plants. However, due to the difficulty of experiments and theoretical analysis on high temperature and high pressure conditions, there remains uncertainty about over the underlying mechanism despite many decades of research. It is difficult to justify the continuous operation of nuclear power plants because the conditions in which these uncertainties exist lead to possible component problems due to degradation of materials. In this thesis, computational approaches and X-ray experiments have been used to overcome traditional experimental and theoretical limitations on the primary water stress corrosion cracking of nickel based alloys to investigate the atomistic mechanism of phenomenon. The first principle method and molecular dynamics techniques have been applied to Ni/H2O and Ni-15Cr-8Fe/H2O systems as one of the ways to predict complex interactions between water and alloy elements. By the computational approach, the behavior of atoms including adsorption energy and barrier energy are evaluated and compared with results derived from conventional experimental methods. Previous studies have shown that PWSCC is affected by a number of factors, but it is affected by alloy elements and changes in the composition of alloying elements change the oxidation behavior that occurs on the surface. However, a discrepancy between the various phase boundaries have been found in the oxide layer of the nickel based alloy surface and many consequences was effected by this phenomenon are not yet clear. Because, the oxidation layer properties on the oxidation of nickel in primary water is important to understand PWSCC, oxidation behavior analysis provides reliable oxidation layer phase boundary properties and compares with experimental data in this thesis. First principle method, molecular dynamics and X-ray experimental techniques were used to analyze the early stage oxidation process on the nickel surface. In these early stage oxidation process models, potential is identified as a key parameter that should be verified in reactions caused by water on metal surfaces to assure reliable results. Developed for nickel-based alloys, the reactive force field potential is suitable for early stage oxidation process calculations and is used in many calculations where chemical reactions should be considered. The molecular dynamics techniques are used to identify discrepancies between phase changes and boundaries on surfaces and to verify them through X-ray experiments. In this thesis, it has been proposed that the oxide layer was formed on the nickel substrate and structural changes occurred in the surface while the nickel was exposed to high temperature water environment. In the molecular dynamics results, oxygen atoms penetrate into the nickel substrate and interaction with nickel and chromium atoms. In addition to, this oxide layer is formed two types of the oxide layer. One is distorted crystal system and the other is amorphous layer by the interaction between oxygen and alloying elements atom. In X-ray experiments result show direct experimental evidence of co-existing amorphous and crystalline oxide phases within the film. This structural mismatch region can provide a diffusion path in which continuous phase change occurs. The oxygen atom can easily penetrate into the substrate and base metal suffer from accelerated mass transport due to vacancy or molecular structural change on the metal surface. This environment is promoted in which crack can be formed by providing conditions for creating a brittle oxide film inside the oxide layer. These damage processes are believed to play a major role in the initiation of cracks.ope