Regulate And Control Of Corrosion Properties Of GH3625 Alloy Based On Grain Boundary Engineering

GH3625 alloy is a solution strengthened nickel base superalloy with Nb and Mo as the main strengthening elements.It has excellent oxidation resistance and corrosion resistance at high temperature.In this paper,we mainly studied the high temperature corrosion behavior of GH3625 alloy in SO₂ acid environment.Based on the grain boundary engineering,we carried out the thermomechanical treatment of GH3625 alloy to adjust the ratio of low?CSL grain boundary in the alloy,so as to improve the corrosion resistance of GH3625 alloy and provide a theoretical basis for more extensive use of GH3625 alloy.The corrosion behavior of GH3625 alloy in high temperature SO₂ acid environment was studied.In this paper,GH3625 alloy which was oxidized for 48hours in air at 900?and corroded for 48 hours in three atmosphere with volume fraction of 2%SO₂-98%Ar,3%SO₂-97%Ar and 100%SO₂ at 900?was studied.The conclusion is as follows:After 48 hours oxidation in air at 900?,the oxidation rate is 0.2861 g·m^-2·h^-1,and a uniform and compact oxide layer with main components of Cr₂O₃ and NiCr₂O₄ was formed on the surface of the alloy.The corrosion rate of the GH3625 alloy is 0.3028 g·m^-2·h^-1?0.3306 g·m^-2·h^-1 and 0.3750 g·m^-2·h^-1 after 48 hours of corrosion in the atmosphere of different concentrations of SO₂ at 900?.A uniform,dense and relatively complete surface corrosion layer was formed on the alloy surface,mainly composed of Cr₂O₃ and NiO.There is no obvious peeling off or fault phenomenon in the corrosion layer,but there are sulfides at the contact interface between the surface corrosion layer and the alloy matrix and inside the alloy,whic h mainly exist in the form of CrS,Ni₃S₂ and low melting point eutectic Ni-Ni₃S₂.After analysis and summary,it is determined that the main corrosion mechanism is:Cr element with its own height content is firstly combined with O element to form a layer of Cr₂O₃ oxide layer on the alloy surface,which can well block the further oxidation and corrosion of O and S elements on the alloy surface,thus inhibiting the penetration and diffusion of O and S elements to the matrix,and protecting the alloy from further corrosion damage;inside the alloy matrix,because of the strong thermodynamics driving force of sulfurization of Cr element and the strong stability of corresponding sulfide,Cr element can fix s element in the form of sulfide,so as to slow down the corrosion of S element to alloy matrix.It shows that GH3625 alloy has excellent corrosion resistance in high temperature SO₂ acid environment.The effect of grain boundary engineering on the grain boundary ratio of GH3625alloy with low?CSL was studied.In this paper,GH3625 alloy was treated by solid solution treatment at 1150?/1 h,rolling deformation at 0%,5%,10%,15%and 20%,and then annealed at 1150?/5 min.It is found that the microhardness and grain size of the alloy can be improved by the thermomechanical treatment,and both in crease with the increase of rolling deformation.When the rolling deformation is 20%,the grain size of the alloy is the largest.The proportion of special grain boundaries in GH3625 alloy can be increased by the thermomechanical treatment.When the rollin g deformation is 5%,the proportion of special grain boundaries in GH3625 alloy reaches the highest value,which is 35.75%.It shows that the grain boundary ratio of GH3625alloy with low?CSL is increased based on grain boundary engineering.The effect of grain boundary engineering on the corrosion property of GH3625alloy was studied.In this paper,intergranular corrosion experiment of H₂SO₄+Fe₂SO₄solution and electrochemical experiment was carried out on GH3625 alloy samples after deformation heat treatment.It is found that the ratio of special grain boundary is the lowest and the average corrosion rate of the alloy is the highest without rolling deformation,which is 0.52833 g·m^-2·h^-1.When the alloy is deformed by 5%rolling deformation,the ratio of special grain boundary reaches the highest value,and the average corrosion rate of the alloy is significantly reduced,which is 0.3588 g·m^-2·h^-1.In the same way,the self corrosion potential of the alloy with high special grain boundary ratio is significantly higher than that of the alloy with low special grain boundary ratio.The analysis mechanism is as follows:the corrosion is easy to occur along the general large angle grain boundary,and the higher the ratio of low?CSL grain boundary of the sample,the less the general large angle grain boundary,thus reducing the number of grain boundaries that can be corroded,so the alloy is less likely to be corroded;the low?CSL grain boundary can block the connectivity of the general large angle grain boundary,and the corrosion spreads along the large angle grain boundary to the alloy matrix.When the low?CSL grain boundary is encountered,the corrosion channel is cut off,and the corrosion is easier to be prevented.It shows that the corrosion resistance of GH3625 alloy can be improved by grain boundary engineering.