The Effect Of Impurity Ions On Corrosion Behavior Of 316 Stainless Steel In Molten Salt Reactor For LiF-NaF-KF Molten Salt

The Molten Salt Reactor(MSR)is the one of the six nuclear reactor types of the IV nuclear power technical features,which uses fluoride salts of low melting point,high boiling point and high termal conversion efficiency as coolant and fuel carrier.However,molten fluoride salts are strong corrosive to the molten salt reactor structural materials.The stainless steel is one of the candidate materials for the major parts of the molten salt reactor in the future and was widely used in the most commercial nuclear reactor.Corrosion behavior of several different stainless steel in initial impurity ions LiF-NaF-KF salt by static corrosion immersion experiments have been studied.The high temperature molten salt corrosion resistance of 304,316,316L,316LN and 316H stainless steel were compared,and the factors of corrosion behavior of 304 and 316H stainless steels in molten salt were discussed.The influence of SO42-ions on the corrosion behavior of 316L stainless steel in molten salt was studied.The effect of adding Zr2+ in molten salt on the corrosion behavior of high temperature resistant stainless steel 316H was studied.The main results and conclusions are as follows:The order of the high temperature molten salt corrosion resistance of these stainless steels in the molten fluoride salts at 700? from low to high is:304(the worst),316,316L,316LN,316H(the best).Because of the existance of the Mo element,a Mo-containing compound can form on the grain boundary,and the diffusion rate of Cr on the grain boundary decreaced.So the corrosion resistance of the Mo-containing stainless steel is enhanced,and the high temperature molten salt corrosion resistance of 316 stainless steel is better than 304 stainless steel.Cr and A1 nitride precipitation phase layers are found on the surface of the stainless steel after corrosion.The precipitates will hinder the diffusion of Cr in the alloy,which can slow down the corrosion rate of the alloy in the molten salt.The corrosion rate of 316L stainless steel in LiF-NaF-KF salt increases almost linearly with the increase of SO42-ion content in salt.The investigation shows that Cr element in the alloy reacts with SO42-ions to form Cr2O3 and S element,then S element reacts with Mn element in the alloy to form MnS.C2O3 and MnS can melt in the molten salt to accelerate the corrosion of the alloy.The addition of Zr2+ ions in LiF-NaF-KF salt can effectively reduce the corrosion rate of 316H stainless steel in molten salt.Zr2+ is unstable in high-temperature molten salt and can easily form stable Zr4+.Zr is more active than Cr in the alloy,so Zr2+ is preferentially oxidized into Zr4+ in the molten salt,thereby consuming the impurity ions in the molten salt to slow down the corrosion rate of the 316H stainless steel.In this paper,the differences of corrosion resistance of different stainless steels in high temperature molten salt environment were concluded,and the corrosion mechanism of stainless steel in molten salt were discussed.The influence of the precipitation of Cr,Al nitride and grain boundary Mo compounds on the corrosion of stainless steel in the molten salt environment was revealed,and the effect of the addition of SO42-and Zr2+ in the salt on corrosion of stainless steel was clarified.This research is very important for stainless steel to be applied in molten salt reactor.