The formation of diffusion coatings on some low-alloy steels and their high temperature oxidation behaviour: Part 2. Oxidation studies

The oxidation of aluminised coatings on Fe---Ni, Fe---Si and Fe---Cr alloys and of chromised coatings on Fe---Ni and Fe---Si alloys were studied at temperatures in the range 700 to 1000°C and at oxygen pressures from 10−8 to 1 atm. and related to appropriate phase diagrams. Up to 800°C Al2O3 or Cr2O3 scales were formed leading to good oxidation resistance. At higher temperatures (not, vert, similar1000°C) the oxidation resistance was frequently lost due to oxide-scale failure and formation of iron oxides. Only aluminised Fe-19.2 wt%Cr alloy remained fully protective up to 1000°C in pure oxygen. Parabolic rate constants for the initial period of oxidation were a linear function of oxygen pressure. Activation energy data for the oxidation of chromised steels gave values in reasonable agreement with those obtained from the literature for the oxidation of pure Cr and Fe---Cr alloys. The effect of substrate structure on the oxide-scale failure mechanisms was studied in detail. The cracking of the scales has been shown to be associated with substrate grain boundaries and the formation of nodular oxides has been shown to be associated with grain boundaries and in particular with the triple points of grain boundaries.