High-Temperature Oxidation of Co-10 w/o Cr Alloys I. Microstructure of Oxide Scales

Oxidation of Co-10 w/o Cr alloy results in an outer CoO layer, an inner CoO layer with inclusions of Cr-,O: ~ and CoCr.,O~, and an internal oxidation zone. Both oxide layers contain appreciable porosity and the morphology of the inclusions is a function of temperature. The microstructure of scales formed during oxidation is presented. It is assumed that the rate-control l ing step is the solid state diffusion of Co-ions in the CoO phase, while gaseous oxygen is rapidly transported across pores. The pores short-circuit he solid state diffusion process and enhance oxidation, while Cr-_,O3 and Co-Cr spinel inclusions inhibit the oxidation. Cobalt-chromium alloys serve as a basis material for production of a family of Co-base superalloys for high-temperature applications. To increase the ser-vice l ifetime and use temperatures of these alloys, an understanding of their chemical stability and, partic