Study On Corrosion Behavior Of GH3625 Alloy At High Temperture

GH3625 alloy is mainly composed of Nb and Mo,and belongs to solid solution strengthened nickel-base superalloy.The alloy has excellent oxidation resistance and corrosion resistance below 650?.In this paper,we mainly studied the change of oxidation resistance and corrosion resistance of GH3625 alloy under the high temperature.It provides a theoretical basis for the study of the corrosion mechanism of GH3625 alloy.In this paper,the corrosion resistance a nd corrosion results of GH3625 alloy are analyzed from three aspects.In the aspect of thermodynamics,the Gibbs free energy is simply calculated by the software,and the sequence of the corrosion reactions is determined by it.In terms of kinetics,the ox idation behavior and corrosion rate of alloy under the high temperature are calculated through the law of oxidation weight gain and thickness of oxide film in mixed atmosphere.On the other hand,the corrosion products and microstructure were analyzed to d etermine the effect of alloy composition and structure on the corrosion resistance of alloys.In order to understand the destruction mechanism of the oxide film of GH3625alloy,we studied the hot corrosion of GH3625 alloy at 800?,900?and 1000?.Finally,the corrosion mechanism is preliminarily determined.The conclusion is as follows:alkaline melting corrosion mainly occurs in hot corrosion of GH3625 alloy.TheCr₂O₃ on the surface of the alloy dissolves in the molten salt as Na ₂Cr O₄,which causes the protective oxide layer to lose its surface.With the decomposition ofCr ₂O₃on the surface of the alloy,Cr-depleted regions appear at the interface of the alloy matrix/corrosion layer,hindering the growth of theCr ₂O₃ oxide layer.As a result,the alloy cannot form a continuous oxide layer,so that O andS can invade and corrode the alloy matrix.After the alloy was etched at 800°C,900°C and 1000°C for 120 h,the corrosion rate was about 3 mg/cm²,6 mg/cm² and 8 mg/cm².After the alloy is hot-etched at 800°C,a small amount of molten salt is formed,and the corrosion layer is relatively complete.The alloy mainly consists ofCr ₂O₃ and spinel-like NiCr₂O₄.When the alloy is hot-etched at 900°C and 1000°C,the corrosion layer appears obvious shedding and faults,which are mainly divided into outer layer,middle layer and inner layer:the outer layer is composed of NiCr ₂O₄ and Ni O oxide layer;The middle layer is a denseCr₂O₃ oxide layer;the inner layer is composed of sulfides?Cr₂S₃ and Ni₃S₂?,oxides?Cr₂O₃ and Ni O?and telluride?Nb C?etc.In order to understand the corrosion resistance of GH3625 alloy in Na ₂SO₄+Na Cl molten salt,the corrosion behavior of the alloy in Na ₂SO₄+Na Cl molten salt at 900°C was investigated by the crucible hot corrosion metho d.The following conclusions are drawn as follows:the corrosion weight loss kinetic curve of GH3625 alloy shows an approximate parabolic trend with a parabolic rate constant of 0.00422.With the increase of corrosion time from 20 h to 120 h,the alloy loss rate increased from 0.028g/cm² to 0.044 g/cm².After etching,the corrosion layer can be divided into three layers:the outermost layer is a loose Ni O oxide layer.The middle layer is a denseCr₂O₃ oxide layer.The inner corrosion zone is mainly Ni ₃S₂.Ni in the alloy is easily reacted withS to form a low-melting eutectic Ni·Ni₃S₂,which accelerates corrosion.The denseCr₂O₃ oxide film on the surface of the alloy has a good inhibitory effect on the diffusion of the corrosion elementS.However,it volatilizes easily after reacting with the Na Cl salt to generate the volatile phaseCr O ₂Cl₂,resulting in the loss ofCr.Therefore,it is the damage of the Na Cl salt to the oxide film that promotes the acceleration of corrosion.In order to understand the corrosion resistance and corrosion mechanism of GH3625 alloy inSO₂+H₂O atmosphere,the corrosion behavior of GH3625 alloy inSO₂+H₂O atmosphere at 900?was studied.The conclusion is as follows:the corrosion rate of GH3625 alloy after etching in air andSO ₂+H₂O atmosphere at900°C for 120 h is 0.0010 mm/a and 0.0011 mm/a,respectively.After etching,the thickness of the corrosion layer is about 10?m.After analysis,it is determined that the outer corrosion product is mainly Ni O and NiCr₂O₄.The granular corrosion product is?Ni,Fe?Cr₂O₄,the corrosion layer is loose.The inner corrosion product isCr₂O₃,which is very dense,but there is a small amount of corrosion holes.Due to the precipitation of the second phase,which are:?''phase,white granular?'phase,block?''phase,bulk carbide Nb C phase,needle?phase,Cr-depleted regions and Ni-poor regions are mostly formed at the grain boundaries.Where the corrosion layer is preferentially destroyed is the place where the second phase in the matrix grain boundary preferentially precipitates.According to the Gibbs free energy level of the alloy corrosion reactants,the corrosi on mediumSO₂ and the oxides Ni O andCr₂O₃react with each other during the corrosion process.The destruction of the corrosion layer is mainly caused by the reaction of the sulfide and the oxide.After thermodynamic analysis,the most stable corrosion products are mainlyCr₂O₃,CrS,Ni O,and NiCr₂O₄.