Preparation And High Temperature Corrosion Resistance Of Cr-Al Coating On The Surface Of Ni Based Superalley

One of the most effective ways to improve the high temperature corrosion resistance of the high temperature structural materials is the application of surface coatings. The protective coatings are being improved towards high performance, complicated structure and multi-functions in order to meet requirements for materials used in aggressive environments. The Cr-Al coating is fabricated by two-step packing diffusion method on the surface of K38G alloy. The protective effects of coatings were studied.The microstructure, thickness and anti-spalling property of the two-step fabricated Cr-Al coating are related to the fabrication parameters such as diffusion temperature, diffusion time and the content of the diffusion reagent.In the present study,the optimized fabrication steps are diffusion Cr at 1080℃for 9h using the diffusion reagent content:NH4Cl:1wt.%,Al2O3: 59wt.%,Cr:40wt.%;and then diffusion Al at 750℃for 9h using the diffusion reagent content: NH4Cl:1.5wt%, Al2O3:88.5wt%, Cr:10wt%.After heat treatment at 800℃for 15h,dense and homogenous inner diffusion Cr-Al coating consisted ofβ-NiAl and y'-NiAl can be fabricated.During the oxidation process, Al2O3 layer was formed on the surface of the Al coating and the inner diffusion Cr-Al coatings, which grows more slowly than that of Cr2O3 layer formed on the surface of the Cr coating.Therefore, the oxidation resistances of the Al coating and the inner diffusion Cr-Al coating are better than that of the Cr coating.Compared to the inner diffusion Cr-Al coating, the worse oxidation resistance of the Al coating can be attributed to the heavy interdiffusion between the Al coating and K38G alloy.Cr coating,inner diffusion Cr-Al coating can be the hot corrosion resistance coatings of K38G alloy at 950℃.Because of the producing of Cr2O3 which is stable in the hot corrosion environment, the hot corrosion resistance of Cr coatings,inner diffusion Cr-Al coatings and K38G alloy are better than that of Al coatings.And the hot corrosion resistance of Cr coating is better than that of the inner diffusion Cr-Al whose hot corrosion resistance is better than that of K38G alloy.In the hot corrosion environment,the Cr element in the coating would form protective Cr2O3 scales even if the Al2O3 layer initially formed lost its protection.In the present study, the inner diffusion Cr-Al coating performed the best high temperarure corrosion resistance.