Process of Scale Formation for the TiAl Alloy during High-Temperature Sulfidation

Abstract

Sulfidation behavior of γ-TiAl alloy was investigated at 1173K for up to 360ks in H₂S-H₂ gas mixture. The process of scale formation was discussed from measurement of corrosion amount, observation of cross-sectional microstructure, identification of reaction products by X-ray diffraction and calculation of dissociation pressure of sulfides. The scale became duplex structure. In the outer scale, titanium sulfides (TiS, Ti₃S₄, TiS₂) and aluminum sulfide (Al₂S₃) were identified, while in the inner scale, titanium sulfides (TiS, Ti₃S₄) were identified. And the TiAl₃ phase was formed at scale/matrix interface. Corrosion rate changed at the test time of approximately 5.4ks, and at same time, inner scale and TiAl₃ layer began to form. Titanium sulfide (TiS) was clarified to be more stable than aluminum sulfide (Al₂S₃) by calculation of dissociation pressure. From these results, the scale was considered to be formed in two steps. That is, the outer scale was formed during transient period, and then titanium was sulfidized alternatively, forming of the inner scale which mainly consists of titanium sulfide and the TiAl₃ occur during steady state.