Microstructure Characteristics And Corrosion Failure Mechanism Of Joints Of Copper/Steel Direct-connected Cathode Plate For Copper Electrolysis

With the rapid development of science and technology,lightweight,energy-saving and multifunctional materials are becoming more and more popular.The single performance of metal materials can hardly meet the actual industrial needs any more,dissimilar metal composite materials have been widely used in industrial production such as aerospace,power electronics,automobile manufacturing,and metallurgical engineering due to their multiple excellent properties at the same time.In the production process of our country's stainless steel cathode plate for copper electrolysis,compared with the traditional copper-steel clad process structure,if the copper/steel dissimilar metal direct welding structure is selected,the copper directly serves as the conductive rod to conduct electricity,and the stainless steel plays the bearing role.This new structure not only makes the manufacturing process short,but also has only one weld between copper and steel.Compared with the original complex process,the new one produces lower resistance,and the conversion efficiency of electrical energy to chemical energy is improved.In addition,the new structure can also reduce costs,which is of great significance to the development of copper electrolysis refining industry.Aiming at the permanent stainless steel cathode plate used for copper electrolysis with a direct connection of copper/steel structure,the welding process of Cu-316 L stainless steel dissimilar material by Tungsten Inert Gas(TIG)Welding was carried out in this paper.Besides,the welding process,the microstructure and performance of welding joint were studied.The results show that there is no intermetallic compound inside the T2 copper/316 L stainless steel dissimilar metal weld joints(DMWJs),but due to the decrease in temperature,the Cu-Fe alloy phase separates during the solidification of the joint,thus two types of copper/steel mutual dissolution zone are formed,including the region of banded ?-Cu phase dispersed in a two-phase solid solution(? + ?),dominated by ? phase,and the region of spherical granular ? phase dispersed in a two-phase solid solution(? + ?),dominated by ?-Cu phase.The interface of mutual dissolution zone is quite clear.The average Vickers hardness of the weld area is up to 110 HV,and the tensile strength of the joint at room temperature is 206 MPa.The fracture locations of the joints all appear in the heat-affected zone of the copper base metal,and the fracture form is ductile fracture.It fully meets the requirements of mechanical properties of stainless steel cathode plate under actual working conditions.Since the cathode plate for copper electrolysis should have high electrical conductivity and mechanical properties as well as corrosion resistance in the actual service,the galvanic corrosion behavior of Cu/316 L stainless steel DMWJs under service condition were studied.The results show that the macroscopic corrosion couple with 316 L stainless steel as cathode,Cu as anode,and the microcosmic corrosion couple with ? phase as cathode,?-Cu as anode are easily formed in the DMWJs.The immersion corrosion failure of Cu/316 L stainless steel DMWJs is caused by the double effects of macroscopic and microscopic galvanic corrosion.In addition,the dissolution of ?-Cu and the shedding of ?-phase in the Cu-rich region caused by micro-coupling corrosion are the main reasons for the failure of Cu/316 L stainless steel DMWJs.Finally,the evolution behavior of the Cu/316 L stainless steel DMWJs during the corrosion process was studied.The Cu/316 L stainless steel DMWJs including copper/steel base metal area and weld area were immersed in solution for different periods,the corrosion morphology and electrochemical characteristic parameters of each micro-zone were observed and analyzed after soaking for different periods.The results show that the corrosion resistance of 316 L stainless steel is enhanced after soaking,which is caused by the formation of dense and continuous passivation film on the surface.As the immersion process progresses,the corrosion resistance of copper and welds first increases and then decreases.The corrosion resistance of the weld before immersion is better than that of copper,but as the immersion time increases,the corrosion resistance of the weld metal decreases faster and more significantly than that of copper,resulting in the most corrosion resistance of the weld.