| Although nickel-based superalloys have both high strength and hot corrosion resistance,with the rapid development of the aerospace and marine industries,the requirements for the performance of engine turbine blade materials are getting higher and higher.Moreover,the turbine blades working in the coastal environment are subjected to the combined action of Na2SO4 formed during fuel combustion and Na Cl in the marine atmosphere,causing thermal corrosion and accelerating blade failure.Therefore,the pure nickel-based superalloys still can not meet its performance requirements,high temperature protective coating must be added on the surface of the blade to improve its resistance to hot corrosion,which plays an important role in the defense and military fields.The silicon modified aluminide coating prepared by the slurry method has good hot corrosion resistance,which is related to the diffusion and distribution of Si element in the coating.At present,a lot of research has been done on the hot corrosion behavior of the coating covered with a salt film on the surface,but the corrosion mechanism is divided and there is still a need for further research.Moreover,there is a lack of research into the subsequent and long-term hot corrosion behavior of the coating by covering the salt film only once.In this paper,Al-Si coating was prepared on the surface of K438 nickel-based superalloy by vacuum thermal diffusion.The surface covered with the saturated salt solution was subjected to a cyclic hot corrosion test at 1050 °C for 120 h.Oxidation kinetics were studied using vernier calipers and electronic balances;Phase composition of coating and oxide layer were analyzed by X-ray diffractometry(XRD);Surface and cross-sectional microscopic morphology of samples were observed by scanning electron microscopy(SEM);The composition,element content and distribution of the coating and the oxide layer were analyzed by X-ray diffraction spectroscopy(EDS).The main findings are as follows:(1)The Na Cl-covered Al-Si coating has a large area oxide shedding on the surface,and a lot of holes appear in the cross section,resulting in the largest amount of oxide shedding per unit area,and the worst resistance to hot corrosion;The pre-oxidized Pt-modified Al-Si coating covered with 50% Na Cl + 50% Na2SO4(mass fraction)produced a thin layer of relatively pure and dense alumina with the best hot corrosion resistance.(2)The element of Pt is added to the alloy to promote the upslope diffusion of the element of Al and increase the outward diffusion rate of the element of Al.The Pt Al2 phase formed by the Pt particles on the surface of the coating and the extradiffused element of Al also accelerates the out-diffusion process of the element of Al,thereby reducing the internal oxidation.At the same time,the element of Pt reduces the critical Al content of the oxide layer,reduces the growth rate of the oxide film,and forms a thin oxide layer,so that elements such as S and Cl in the molten salt can easily diffuse into the interior of the coating through the oxide film.The coating that causes the coating of the salt film has the worst resistance to hot corrosion.(3)By pre-oxidation forms a protective oxide film before covered salt film and hot corrosion,which effectively prevents the molten salt from directly contacting the coating,making the interface of oxide layer and the coating smoother.At the same time,the thickness of the oxide layer is reduced,the density of the oxide layer is enhanced,the purity of the alumina in the oxide layer is improved,and the internal stress caused by the difference in thermal expansion coefficient between the oxides in the oxide layer is reduced,thereby improving the hot corrosion resistance of the coating. |
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