High temperature corrosion mechanisms of certain new TiAl-based intermetallic alloys in an aggressive H_2/H_2O/H_2S environment at 850℃

摘要

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 x 10~(-1) Pa and p_(O_2) ～ 1.2 x 10~(-15) Pa potentials at 850℃. 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 > TM4Al-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.