Hot Corrosion of Plasma Sprayed Coatings for Waste Gas Turbine Blades

High temperature corrosion tests in 75%Na₂SO₄-25%. NaCl molten salt have been conducted on waspalloy specimens plasma sprayed with Co-Cr-B-Si, Ni-Cr-B-Si-hCr₃C₂, X-40, X-40 + TiC and Ni/Al + Al₂O₃ coatings, test coupons were exposed isothermally at 750℃up to 200 h. After the corrosion tests, each sample was examined by OM, SEM and EMPA. It was found that X-40, X-40 + TiC and Ni/Al + Al₂O₃ were violently corroded by the molten salt due to cracking and spalling of the coatings, whereas Co-Cr-B-Si and Ni-Cr-B-Si + Cr₃C₂ exhibited superior corrosion resistance, which were thought to be related to the formation of chromium and silicon oxide film. A possible corrosion mechanism had been proposed and it is suggested that the excellent corrosion resistance and high hot-hardness of the two B-Si containing coatings may be of significance for applications in petroleum refining industry as protective coatings of power generating turbine blades which are used under severe corrosion/erosion conditions.