Microstructure and Hot Corrosion Property of a Si–Co–Y Diffusion Coating Deposited on TiAl–Nb Alloy

Si–Co–Y diffusion coatings were deposited on TiAl–Nb alloy using pack cementation process. The influence of activators and deposition temperatures on the coating structures was investigated, alongside the coating formation process and hot corrosion performance of the optimized coating in molten salt of 25%NaCl + 75%K2SO4 at 850 °C. The results show that a dense and compact Si–Co–Y diffusion coating can be prepared on TiAl–Nb alloy, with a multi-layered structure including an outmost layer of (Ti, X)Si2 (X represents Co, Al, Nb, Y), an outer layer composed of TiSi2 + Ti5Si4 + Ti5Si3 mixtures, a middle layer of Ti5Si3, and an inner layer of TiAl2. The coating prepared with AlCl3·6H2O and NH4Cl had many pores. Increase in deposition temperature led to a higher coating growth rate within the range of 1050–1100 °C, but temperature exceeding 1100 °C caused the formation of intensive holes in the coating. Hot corrosion tests at high temperatures proved that the Si–Co–Y diffusion coating prepared on TiAl–Nb alloy exhibited excellent hot corrosion resistance in 25%NaCl–75%K2SO4 molten at 850 °C. A protective hot corrosion product scale composed of a TiO2 + Na2SiO3 + Na2TiO3 outer layer and an Al2O3 middle layer, formed on coating after hot corrosion for 50 h. The scale can effectively inhibit the inward diffusion of corrosion medial of O, Cl and S elements.