Study On Corrosion And Protection Technology Of 316 Stainless Steel In Molten Salts

It is considered to be an excellent high-temperature heat transfer or heat storage medium,since molten salt has the advantages of high boiling point,high specific heat,high heat conduction,high chemical stability,low vapor pressure,low melting point and other thermophysical properties.It is widely used in molten salt reactor,concentrated solar power system,nuclear waste treatment and comprehensive utilization of nuclear energy.316 stainless steel is a mature commercial structural material with good mechanical properties,high temperature performance,neutron irradiation resistance,and rich experience in using as fuel supporting structural material.It is a material approved by ASME standard engineering and widely used in the commercial nuclear reactors.The corrosion of materials in high temperature molten salt is a key problem that must be solved when 316 stainless steel is used in engineering.In order to predict and control the corrosion of 316 stainless steel in different molten salts,the study on the corrosion behavior and mechanism should be conducted.In this paper,the corrosion behavior and mechanism of 316 stainless steel in two molten salt systems of fluorine and chlorine salts are studied,and the corrosion protection technology is carried out.The research contents are summarized as follows:1.The effects of Cr and Cr F₂ on the corrosion behavior of 316 stainless steel and welded 316 stainless steel in FLi Na K molten salt under 700℃were studied.In FLi Na K molten salt,the corrosion of 316 stainless steel and welded 316 stainless steel is the selective dissolution of active element Cr and Fe from metal matrix into salts.Due to the use of purified molten salt,the corrosion of 316 stainless steel is light,and no obvious corrosion trace is observed.Without Cr,the weight loss per unit area of the 316 stainless steel samples is about 0.1 mg/cm².The depth of Cr diffusion layer of the base metal is about 9μm after corrosion at 700℃for 1000 hours,and the Cr diffusion layer of the weld area is about 14μm;With Cr in salts,the weight gain per unit area of the samples is about 0.3 mg/cm²,a layer of Fe-Cr alloy phase about4μm is deposited on the surface of the samples.By adding 1500 ppm Cr F₂,a layer of Fe-Ni coating about 10μm thick can be prepared on the surface of 316 stainless steel at higher temperature（greater than 700℃）,and the coating has good corrosion resistance after 100 hours immersion test.2.The effect of metal Mg on the corrosion behavior of 316H in Na Cl-KCl-Mg Cl₂ molten salt was studied.In chloride salt,the corrosion mechanism is the dissolution of the active element Cr into salts,which is similar to FLi Na K.The difference is that Mg Cl₂ is very easy to absorb water,and then a hydrolysis reaction occurs during the heating can produce Mg OHCl and HCl,which aggravates the corrosion.At the same time,the hydrolysate Mg O will also be deposited on the surface of the samples,reducing the accuracy of weight loss data.The temperature has a great influence on the molten salt corrosion of 316H.There is no obvious intergranular corrosion characteristic at 500℃,but serious intergranular corrosion occurres in 316H at 600℃and 700℃.The corrosion depth of the samples at the three temperatures is respectively 40μm,60μm and 300μm after 1200 hours corrosion.Mg O particles can intrude into the grain boundary pores at 600℃and especially at700℃.By adding metal Mg,the intergranular corrosion of 316H can be significantly inhibited.The samples generally show weight gains,and the cross-section morphology has no corrosion trace,and the elemental analysis shows that there is Mg O deposition on the surface of the samples.3.The effect of Mg on the corrosion behavior of 316H in dynamic corrosion circuit was studied.The loop is driven by the temperature gradient of 150℃（700-550℃）,the corrosion behavior of 316H is slightly different from the reported dissolution of active elements at the hot leg and deposition and precipitation at the cold leg.In fact,the corrosion depth of the sample is about 88μm,Cr,Fe elements diffuse and dissolve along the grain boundary,and Ni,Mo are enriched at the grain boundary in the hot leg of the loop,which shows serious intergranular corrosion.The corrosion is weak in the cold leg,without intergranular corrosion,and the corrosion depth is only 2μm.There are Cr and Fe depleted layers and Ni and Mo enriched layers,with a depth of about 5μm.The intergranular corrosion of 316H at the hot leg of the loop was inhibited by adding Mg.And the maximum corrosion depth of the sample was less than 10μm after 2000 hours test.According to the time law curve of corrosion depth,the corrosion depth of 316H can be reduced to less than 50μm/year,by corrosion control,which can match the material design requirements for molten salt energy storage demonstration device.