High temperature oxidation of model alloys Ni-25Cr, influence of minor elements Mn and Si on growth mechanisms of protective oxides

Nickel-based refractory alloys are mainly used in oxidizing atmosphere at high temperature. These alloys are generally capable to form chromia oxide scales at temperature between 700 and 1100 deg. C. A few relationships between composition, microstructure and kinetic are known, but the systematic study of the oxidation behaviour of new industrial alloys is required because the impact of minor elements and their possible synergy are not fully understood. Thus, this PhD thesis aims to refine the understanding of the relationship between the chemical composition and the chemical reactivity of metallic materials in an oxidizing atmosphere in order to contribute to the design of alloys with optimized oxidation properties. To reach this purpose, the oxidation behaviour of model alloys, simplified compared to those proposed today by industry, was studied. The techniques available nowadays (EBSD, SEM-FIB, MET-HR) were used in order to determine the oxidation mechanism and the effect of manganese and silicon additions on these mechanisms. The characterization of the first oxidation step was performed in-situ in an environmental SEM (Collaboration with the ICSN in Marcoule) and allowed the observation of the former oxide, the Mn_1+xCr_2-xO₄ spinel regardless the alloy Mn content. Manganese addition led to an increase of the oxidation rate in comparison to the one of a Ni25Cr (% wt.) and to a decrease of the chromia oxidation rate thanks to the presence of the spinel Mn_1+xCr_2-xO₄ at the oxide/gas interface. In opposite, the silicon addition decreased the parabolic constant by a factor 5. This dramatic effect was attributed to the presence of silica at chromia grain boundaries which decreased the chromium diffusion rate across the oxide scale. The addition of both manganese and silicon led to the same oxidation rate than the reference alloy in isothermal condition whereas, the lifetime in cyclic oxidation was considerably increased. The knowledge of the growth direction coupled to that of the semiconductivity type, determined by photoelectrochemical measurements at SiMaP laboratory in Grenoble, allowed the identification of the major point defects in formed oxide scales. Based on the collected results, the involved solid state diffusion mechanisms responsible of the oxide layer growth were proposed. (author)