On the effect of oxide scale stability on the internal nitridation process in high-temperature alloys

Abstract

AbstractInternal high-temperature corrosion of metallic materials is an important process often determining the service live of components used for high-temperature processes. As soon as a material looses the ability to form a dense and adherent oxide scale, non-metallic elements (e. g., O, N, C, and S) can diffuse from the atmosphere into the bulk material and react with alloying elements, forming different kinds of internal precipitates. The corrosion behaviour of two commercial alloys, Nicrofer 7520Ti and CMSX-4, and of the model alloy Ni – 20Cr – 6Ti was investigated in air and nitriding atmosphere at 1000 °C and 1100 °C under isothermal and thermal-cycling conditions. The study was performed using a continuous thermogravimetric method, which permits mass changes to be measured in different atmospheres and under thermal-cycling conditions. It was shown that damage in the oxide scale, caused by cracking or spallation, leads to an enhanced internal corrosion attack manifesting itself in the precipitation of oxides and nitrides. The experimental results were described in a mechanismbased approach by means of a simulation model that combines a numerical finite-difference treatment of the diffusion differential equations and the commercial thermodynamic program ChemApp. The model is able to predict the complex material behaviour during high-temperature exposure.