Oxidation, carburisation and metal dusting of 304 stainless steel in CO/CO2 and CO/H2/H2O gas mixtures

The oxidation and carburisation behaviour of 304 stainless steel was studied during thermal cycling in CO/CO2 at 700 °C, and also in CO/H2/H2O at 680 °C. Thermal cycling caused repeated scale separation which accelerated chromium depletion from the alloy subsurface regions. The CO/CO2 gas, with aC=7 and pO2=10-23atm., caused internal precipitation of oxides and carbides, some surface damage, but no dusting. In contrast, the CO/H2/H2O gas, with aC = 19 and pO2=5.4×10-25atm. caused rapid graphite deposition and metal dusting. This was accompanied by internal oxidation and carburisation. The internal oxide was identified as spinel, which forms in the short term, but not at long reaction time. Its formation produced a significant volume expansion, which disrupted the material and resulted in surface damage in both gas atmospheres. In CO/H2/H2O, however, direct graphite deposition and metal disintegration into dust was the main reaction. The very different reaction morphologies produced by the two gas mixtures are discussed in terms of competing gas-alloy reaction steps.