Factors affecting isothermal oxidation of pure chromium in air

Highlights: • Oxidation behavior of Cr is studied in Ar-O₂, N₂-H₂, and air (N₂-O₂) at 950–1200 °C. • Effects of oxide volatilization, abrupt mass-gains, and nitridation are investigated. • A large part of the observed mass gain in the air is contributed by the subscale nitride formation. • The activation energy of nitridation is markedly lower than that of oxidation. • Local oxide scale failures during isothermal oxidation cause mass-gain steps during the isothermal oxidation test. - Abstract: Isothermal oxidation kinetics of pure chromium at 950–1200 °C in purely oxidizing (Ar-20%O₂), purely nitriding (N₂-5%H₂), and oxidizing-nitriding (synthetic air, N₂-20%O₂) atmospheres were studied. Effects of factors such as oxide volatilization, multiple mass-gains, and nitridation on oxidation kinetics of chromium were investigated using thermal analysis, acoustic emission analysis, and electron microscopy. Results obtained in single oxidant atmospheres compared to those obtained in air clearly showed the transparency of chromia against nitrogen in air. The abrupt mass gain steps observed during isothermal oxidation were appeared due to the local oxide scale failure resulting from growth stresses in the thermally grown chromia scale.