Oxidation of low carbon steel in N ₂-H ₂-H ₂O gas mixtures with fixed h ₂O/H ₂ ratio

The oxidation kinetics of a low carbon steel at 900°C in various N ₂-H ₂-H ₂O gas mixtures with a fixed pH 2 O/pH 2 ratio were studied. The water vapour partial pressure varied in a range from 0.05 to 0.25 atm. The experiments were designed to achieve equilibrium pO 2 =6.32×10 ¹⁷ atm at 900°C, under which conditions wüstite is the only thermodynamic ally stable iron oxide product. Parabolic oxidation kinetics were observed in all cases, indicating solid-state diffusion to be the rate controlling step. The value of the parabolic rate constant, k _p, was approximately proportional to pH 2 _O. According to Wager's diffusion model, when local thermodynamic equilibrium is established at both gas/scale and scale/metal interfaces, the oxide vacancy concentrations at the two interfaces are fixed. In terms of this theory, when the temperature and oxygen partial pressure are fixed, the parabolic rate constant should also be fixed. The observed values of k _p were about two orders of magnitude smaller than predicted from diffusion theory. At higher water vapour levels, agreement was closer. Reaction kinetics are discussed in terms of oxide scale morphologies and phase constitutions, as determined by scanning electron microscopy, cross-sectional metallography and X-ray diffraction.