Temperature effects on the oxidation of low carbon steel in N ₂-H ₂-H ₂O at 800-1200 °c

A low carbon, low silicon steel was reacted with flowing N ₂-H ₂-H ₂O gases at temperatures of 800-1,200 °C, to produce scales of fast-growing wü stite, the only stable iron oxide under these conditions. Scaling kinetics were parabolic, after an initial period of linear reaction. The parabolic rate constants measured in the range 800-1,100 °C were two orders of magnitude lower than values predicted from Wagner's diffusion theory, and activation energies were higher than expected. At 1,200 °C, however, the measured parabolic rate constant was in agreement with prediction. The initial period of linear kinetics was extensive at 1,100 °C, and rates are in agreement with those predicted for surface reaction rate control. Because the surface reaction is much less sensitive to temperature than the solid-state diffusion process, the extent of the linear kinetic regime is smaller at lower temperatures.