Abstract
Color changes during oxidation of a 12Cr ferrite/martensite stainless steel were investigated in-situ by using a high vacuum (5 × 10−3 Pa) heating stage where the high vacuum is provided by an oil diffusion pump. The color of the oxide film changed from initial metallic silver to brown, then consecutively to purple, liberty blue, blue-gray and finally to azure blue during the 90 min heat treatment at 690 °C in high vacuum. In contrast to the above changes, during oxidation at 1130 °C, the color changes from metallic silver to brown, then to various blue and finally changed back to metallic silver. The color as a result of thin-film interference mainly depended on the oxide film thickness. The significant reduction of the thickness of the oxide film on the sample vacuum heat-treated at 1130 ℃ is owing to the volatilization of oxide, which is a result of the reaction of the already formed Cr2O3 with residual water vapor and oxygen to generate CrO2(OH)2 and CrO3 gas, which evaporated from the surface at 1130 °C. Ferrite grains have a much lower oxidation kinetics than that of martensite.
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