High temperature corrosion mechanisms of certain new TiAl-based intermetallic alloys in an aggressive H2/H2O/H2S environment at 850 °C

High temperature corrosion behaviour of three TiAl-based intermetallic alloys – Ti–44Al–8Nb–1B, Ti–46Al–8Nb–1B and Ti–48Al–2Nb–2Cr–1B (at.%) – was studied in an environment of H2/H2S/H2O yielding pS2pS2 ∼ 6.8 × 10−1 Pa and pO2pO2 ∼ 1.2 × 10−15 Pa potentials at 850 °C. The kinetic results obtained by a discontinuous gravimetric method indicate that increase in Al and Nb concentrations led to enhanced high temperature corrosion resistance, the corrosion resistance decreasing in the order: Ti–46Al–8Nb–1B > Ti–44Al–8Nb–1B > Ti–48Al–2Nb–2Cr–1B. The scale development studies using SEM, TEM, EDX, WDS and XRD confirmed the formation of a multilayered scale on all materials. An outer layer consisting of TiO2 existed beneath which an Al2O3 layer was present. Then a layer of TiO2 formed again, below which an Al-enriched NbAl3 was observed. A TiS layer was found beneath the NbAl3 layer. The formation of TiS led to the development of a NbAl3 band between the multilayered scale and the substrate.