Oxidation Kinetics and Scale Structures of Co–Ni Alloys in the Temperature Range from 1000 to 1200°C

Co–Ni alloys over the whole composition range were oxidized in the temperature range from 1000 to 1200°C under 1 atm of pure oxygen. Kinetics, internal oxidation, surface morphologies, and concentration profiles in the cross sections were investigated by means of thermogravimetry, X-ray diffraction, optical microscopy, and electron probe microanalysis.Results obtained are as follows:(1) The oxidation rate constants, which were determined by batch type measurements of weight gain and scale thickness, decreased exponentially with nickel content in the oxide scale.(2) In all specimens Ni and Co in the grown oxide scale were concentrated near the alloy-scale and scale-gas interfaces, respectively.(3) Concentration profiles of Co and Ni in the scale and alloy substrate can be represented by one master curve when the concentration was plotted against x⁄.(4) Internal oxidation and depletion of cobalt in the alloy substrate were found in the nickel-rich alloys, resulting in the enrichment of cobalt in the oxide scale. The growth rate of the depletion zone of cobalt depended not on the alloy composition but on the temperature. The apparent activation energy of the growth was about 72 kcal/mol which was close to those of oxygen- and inter-diffusivities in the Ni–Co alloys.From the present results and other authors’ studies, the alloy composition-temperature diagram of the oxidation behavior was schematically shown.