High temperature corrosion mechanisms of certain new TiAl-based intermetallic alloys in an aggressive H 2/H 2O/H 2S environment at 850 °C

Abstract

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 H 2/H 2S/H 2O yielding p S 2 ∼ 6.8 × 10 −1 Pa and p O 2 ∼ 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 TiO 2 existed beneath which an Al 2O 3 layer was present. Then a layer of TiO 2 formed again, below which an Al-enriched NbAl 3 was observed. A TiS layer was found beneath the NbAl 3 layer. The formation of TiS led to the development of a NbAl 3 band between the multilayered scale and the substrate.