Preparation Of Micro-arc Oxidation Coating On Surface Of 316L Stainless Steel And Research On Its Corrosion Resistance To Liquid Pb-Bi

Liquid Pb-Bi alloy is considered to be candidates of the spallation target material and coolant of ADS and LFR on account of their excellent properties such as good neutron yield,high specific heat capacity and low melting points.However,the high-temperature liquid Pb-Bi alloy will corrode the pump body and other steel materials components of this system,thus affecting the service life of nuclear engineering components.Micro-arc oxidation technology can generate a ceramic coating in situ on the metal surface,which has good corrosion resistance.In this paper,micro-arc oxidation technology is used to treat the surface of steel materials to improve the service life of nuclear reactor components.Therefore,a micro-arc oxidation ceramic coating is obtained by combination process on the surface of 316L stainless steel,which is commonly used in nuclear reactor.The corrosion resistance of this coating in liquid Pb-Bi alloy was investigated,which could provide important theoretical basis for prolonging the service life of key components of nuclear reactor.In this paper,using CMT fusion brazing method,by optimizing the wire feeding speed and welding speed,the 6061 aluminum layer with excellent microstructure and certain bonding strength was obtained on the surface of 316L stainless steel in the form of lap joint.Then,the aluminum layer was used as the intermediate layer to conduct micro-arc oxidation treatment on its surface,and the surface morphology,roughness and thickness of the ceramic coating under different process parameters were compared and analyzed,and the parameters were optimized by orthogonal experimental method.Finally,the micro-arc oxidation specimens and the comparative specimens were subjected to static corrosion test for different time in liquid Pb-Bi at400℃.The results showed that:By using CMT fusion brazing method and adding ER4043 aluminum silicon welding wire,the effective connection between 6061 aluminum alloy and 316L stainless steel can be realized.The welding parameters have significant effects on the macroscopic morphology of welded joints.During the welding process,in addition to Al,Fe and Si,the elements of Cr and Ni in stainless steel also diffuse slightly.The intermetallic compound layer with FeAl₃,Fe₂Al₅ and Al_0.5Fe₃Si_0.5 as the main phases is formed in the brazed interface layer,and its thickness is greatly affected by the heat input.If the thickness of the intermetallic compound layer is too large or too small,the tensile strength of the welded joint will be affected.When the thickness is1.5μm,the strength of the specimen can reach 50 MPa.According to the microstructure and mechanical properties,it was found that the optimum welding parameters are welding speed 270mm/min and wire feeding speed 2.7 m/min.The ceramic coating prepared by micro-arc oxidation technology was composed ofγ-Al₂O₃andα-Al₂O₃.Current density,oxidation time,pulse frequency and duty cycle ratio have a certain influence on the surface morphology,roughness and thickness of ceramic coating,among which the influence of current density and oxidation time is more significant.Through the analysis of orthogonal test results,when the current density is 5 A/dm²,the oxidation time is 30 min,the frequency is 450 Hz,and the duty cycle is 8%,the coating thickness is moderate and the surface roughness is small.After 200h corrosion test on the weld specimens without micro-arc oxidation treatment,a large area of matrix was detached from the surface,showing obvious signs of corrosion.After1000 h corrosion test of 316L stainless steel,an oxide layer composed of Fe₃O₄ and FeCr₂O₄ was formed on the surface,which could not prevent the penetration of Pb and Bi.After micro-arc oxidation treatment,the ceramic coating was sound in liquid Pb-Bi as it was before the corrosion test.No obvious pitting or defects,as well as penetration of Pb and Bi,were found.Based on the comprehensive test results,the ceramic coating can effectively prevent the dissolution of the component of matrix material into the liquid Pb-Bi and the penetration of Pb and Bi.