Keizo Nishida1, Toshio Narita1, Norio Ohya2 and Tadaharu Yamauchi3
1Metals Research Institute, Faculty of Engineering, Hokkaido University, Sapporo
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, and kinetics, internal oxidation, surface morphology, and concentration profiles in the cross-section were clarified by means of thermogravimetry, X-ray diffraction, optical microscopy, and electron probe microanalysis.
Results obtained are as follows:
(1) Oxidation was measured by batch-type measurements of weight gain and scale thickness. Both of the rate constants were exponentially changed with nickel contents in the alloy and the scale. Accordingly, the over-all oxidation rates seemed to be determined by cation diffusivities which decreased with increasing NiO content in the oxide.
(2) The grown oxide was mainly the solid solution of CoO and NiO, except for the precipitation of Co3O4 particles in high Co-Ni alloys.
(3) In all cases, NiO and CoO were found to concentrate in the alloy-scale and scale-gas interfaces, respectively.
(4) Concentration profiles in the scale and an alloy substrate were shown by one master curve at a specified temperature when the concentration was plotted against x/√ . This result shows that the concentration profiles are determined by the diffusion process in both of the scale and the alloy.
(5) Internal oxidation and Co-depletion in the alloy substrate were found in the nickel-rich alloys, resulting in the enrichment of CoO in the scale. The growth rate of the Co-depletion zone depended not on the alloy composition but on the temperature, and its activation energy was about 72 kcal/mol which is close to those of oxygen- and inter-diffusivities in the Ni-Co alloys.
From the results of the present and other authors' studies, the composition-temperature diagram of the oxidation behaviors was schematically shown.
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