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AISI 304 austenitic stainless steel specimens are oxidised in laboratory air at 750 °C for 48 h. They are further subjected to the reduction test in carbon at 1350 °C for 30 and 60 min. The results show that the mass gain of the oxidised AISI 304 slighter increases to be 0.08 mg cm–2 after the reduction for 30 min and is unchanged at the longer reduction period up to 60 min. The oxide on AISI 304 is deteriorated after the reduction but its morphology tends to be unchanged when the reduction period is longer from 30 to 60 min. The results then indicate the superior performance of the AISI 304 to combat the corrosion under carbon at this high temperature.

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To facilitate the assessment of the oxide stability in H2-H2O or CO-CO2 atmospheres, auxiliary axes are constructed in the Ellingham diagram. Based on A. Ghosh’s approach, the geometrical interpretation of the diagram is proposed for the reaction 2X + O2 = 2Y, where X and Y could be originated from H2 and H2O or CO and CO2. Two cases are considered when oxygen partial pressures are lower and higher than one bar. By a geometrical method, it is proved that with an appropriate set-up of values relating to the auxiliary axes, the axes representing the ratio between the equilibrium partial pressure of hydrogen and that of water vapour, as well as the ratio between the equilibrium partial pressure of carbon monoxide and that of carbon dioxide, can be constructed. The geometrical method on the construction of axes using thermodynamic derivation is explained in the paper.