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Keywords:

  • chromia;
  • chromium;
  • grain boundary diffusion;
  • kinetics;
  • nitridation;
  • water vapour

High purity chromium was reacted at 950 °C in flowing mixtures of N2-H2-H2O, N2-O2-H2O and Ar-O2. All gases produced an outer scale of Cr2O3. In addition, an underlying layer of Cr2N grew during reaction with N2-O2. As a result, the overall chromium consumption rate was significantly faster in N2-1O2 than in Ar-1O2 (gas compositions in vol%). Additions of water vapour to N2-1O2 slowed the rate of nitride growth, and nitridation was completely suppressed at a level of 10% H2O(g). Conversely, water vapour additions accelerated chromia growth rates. Reaction in N2-H2-H2O led to chromia scaling at rates faster than in dry N2-O2, but no nitride formation occurred. The results are interpreted in terms of inward oxygen transport via a hydrogen bearing species, and uptake by the scale of this species in a way which excludes nitrogen.