Characterisation of the oxide scale on a Ti–2Al–2.5Zr alloy with and without pre-oxidation in an alkaline steam at 300°C

The oxide scales on a Ti–2Al–2.5Zr alloy with and without pre-oxidation oxidised at 300°C in an alkaline steam for approximately 13 000 h were studied by SEM, XRD, XPS and in situ AES techniques. Some of the samples were subjected to 400°C deionised water steam at a pressure of 10 MPa for 72 h for pre-oxidation. The result showed that the oxide scales on the Ti–2Al–2.5Zr alloy were composed of Al2TiO5 (TiO2Al2O3), TiO2, Ti3O5, Ti2O3, Al2O3 and TiO. A comparison study has been carried out between the pre-oxidised and without pre-oxidised Ti–2Al–2.5Zr to determine the influence of pre-oxidation upon the properties of oxidation resistance. The oxidation kinetics showed that the weight gains of the pre-oxidised samples were much less than that of the controlled samples, which was consistent with the results by in situ AES analysis. The AES results showed that the whole thickness of the pre-oxidised sample (approx. 33 000 Å) was thinner than that of the controlled one (approx. 40 000 Å), in contrast the pattern for the stable oxide scale was reversed, i.e. the pre-oxidised sample was much thicker. Thus pre-oxidation is an effective method to improve the oxidation resistance of Ti–2Al–2.5Zr.