Mechanism of Iron Oxide Scale Reduction in 5%H2-N2 Gas at 650-900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H-2-N-2 gas at 650-900 degrees C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 degrees C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 degrees C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650-750 degrees C differed from that at 750-850 degrees C, with the former being oxygen diffusion in alpha-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite-steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron-wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite-iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.