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-Mb-1B and Ti-48Al-2Nb-2Cr-1B (at.%) - was studied in an environment of H-2/H2S/ H2O yielding p(s2) similar to 6.8 x 10(-1) Pa and p(O2), similar to 1.2 x 10(-15) Pa. potentials at 850 degrees 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. (C) 2006 Elsevier Ltd. All rights reserved.