Kinetics of oxide formation on various steel surfaces in the presence of oxygen-nitrogen mixtures

Abstract

The formation of oxides at 400°C and 90°C with different oxygen concentration conditions on various metal surfaces; carbon steel, stainless steel and nickel-alloy was studied. From 400°C tests of carbon steel surfaces, oxide films were observed in both conditions; the oxide formed on the surfaces which were exposed to an atmosphere with no change in O2 concentration was Hematite. On the surfaces which were exposed to an atmosphere with changing O2 concentration, Magnetite and Hematite were found due to the different conditions of apparatus used. The thickness of the oxide film formed on each membrane was calculated from the weight gained by the membrane and the kinetics of oxide formation on each condition was modelled. From the experiment of carbon steel at 400 °C exposure, the reaction rate was controlled by diffusion of O2 through the oxide layer. The diffusivity of oxygen through the oxide layer, Do2, was calculated from the rate of oxide formation on the surfaces which were exposed to an atmosphere with no change in O2 concentration and was found that Do2= 7.05545 × 10-14m2/s and the parabolic rate, kp =1.089×10-17m2/s. Do2 was used in the prediction of oxide form in the atmosphere with changing O2 concentration. The layer of oxide formed on stainless steel and nickel-alloy surfaces, however, was very thin and the weight gains on these metals were not significant. Carbon steel and stainless steel were also tested at 90°C, weight gains on these membranes were not significant and no oxide films were observed.