High temperature sulphide corrosion of TiAlNb and TiAlAg alloys

The sulfidation behaviour of binary Ti-48Al and ternary Ti-52Al-2Nb, Ti-51.2Al-5Nb, TI-48 Al-10Nb and Ti-45.9Al-15Nb (at%) alloys were studied in sulphur vapour atmosphere at 900 oC in the pressure range 10-1000 Pa. Binary TiAl and ternary TiAlNb TiAlAg intermetallic alloys were prepared by mechanical alloying (MA) process. After annealing at 1250 oC for 2 hrs in vacuum all three kinds of alloys were crystalline including two main phases γ-TiAl and α2-Ti3Al with grain size in the range from about 0.2 to 1 μm. Some amounts of α-Ti, AlNb3 and Ag3Al were also detected. In the present study the influence of the niobium and silver additions to TiAl on the sulphidation kinetics, microstructure and chemical composition was investigated. Sulphidation of the TiAlNb and TiAlAg alloys in sulphur vapour atmosphere at 900 0C in the pressure range 10 - 1000n Pa followed parabolic or parabolic-linear rate law depending on the time of reaction. The amounts of sulfidation increased with increasing addition of niobium in the range from 2 - 15 at.%. In the case of Ti-49Al-2Ag alloy sulphidation kinetics was much faster in comparison with other alloy compositions, caused probably by an Ag-Al liquid phase formation. The sulphide scales formed on TiAlNb with 2 - 15 at.% of Nb and TiAl - 2 at.% Ag alloys had multi-layered structures of three different layers. It was observed that outermost sulphide layer is rich in Al2S3 with addition of some amount of Ti-sulphides and outer layer is rich in Ti-sulphides. The inner layer consisted of mixture of Ti-sulphides and Al2S3. Ti-sulphides were represented by TiS, Ti3S4, TiS2, Ti5S8 depending on the sulphidation conditions, p(S2) = 10-1000 Pa. The cross-sectional microstructure and phase composition of the reaction products were examined by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and X-ray diffraction analysis (XRD). (author)