Electrochemical Carbonitriding Mechanism And Corrosion Resistance For 316L Stainless Steel Bipolar Plates

Bipolar plate is a key component of PEMFC,accounting for a significant part of total weight and cost of the PEMFC stack.Metals exhibit higher mechanical strength,no gas permeability,much superior manufacturability,and can be machined to be thin plates to achieve the higher power density when compared to graphite.The main challenge of metals,however,is the corrosion in PEMFC environments,which causes considerable power degradation,and this is also an important factor restricting the development of its commercialization.Therefore,it is necessary to enhance the corrosion resistance of metallic bipolar plates.In this dissertation,316 L stainless steel bipolar plate is modified by electrochemcally carbonitriding technique in molten salt electrodeposition.The effects of the parameters for electrochemically carbonitriding on the microstructure and corrosion resistance of the carbonitriding layer are investigated.To obtain non-defective Cr/Cr?CN?,the microstructure of chromium coating on 316 L stainless steel after electrochemical carbonitriding in molten salt were studied.The main achievements are as follows:The effect of deposition potential(-1.0V_Ag/AgCl,-1.9V_Ag/AgCl)on the structure and properties of the carbonitriding layer was studied,with graphite as the counter electrode in the Li Cl-KCl-KNO2 molten salt system.When the deposition potential is-1.0V_Ag/AgCl,a compact layer with a thickness of 4⁵?m is obtained after polarization for 1.5 hours.There existed some micro holes in the layer and the carbon and nitrogen elements are available.When the deposition potential is-1.9V_Ag/AgCl,a compact layer with a thickness of 6⁷?m formed after polarization for 1.5 hours.The size of micro pores became smaller and the content of carbon and nitrogen in the layer increased.In 0.5mol/L H2SO4+2ppm F-solution.The corrosion potential of the carbonitriding layer obtained at-1.0V_Ag/AgCl and-1.9V_Ag/AgCl are higher than that of the substrate.The carbonitriding layers at-1.0V_Ag/AgCl are subject to failure in corrosive solution because of the formation of a discontinuous chromium nitride or carbide on the surface of stainless steel.In contrast,the carbonitriding layer at-1.9V_Ag/AgCl is more dense.In corrosive solution,its open circuit potential keeps high stability after immersion for 600 h,suggesting that the carbonitriding layer exhibited high chemical stability and could protect the steel effectively.The effects of polarization time?1.5h,2.5h?on the microstructure and properties of carbonitriding layer of 316 L stainless steel were studied.The thickness of the carbonitriding layer has been improved after polarization for 2.5h at the potential of-1.0V_Ag/AgCl and-1.9V_Ag/AgCl.The content of carbon and nitrogen increased significantly.The open circuit potential of stainless steel after carbonitridation for 2.5h at-1.0V_Ag/AgCl and-1.9V_Ag/AgCl,existed fluctuation in the corrosive solution and it maintained a high open circuit potential for the total immersion time obtained for that conducted at-1.9V_Ag/AgCl potential for 2.5h,which indicating that the corrosion resistance of 316 L stainless steel is significantly improved by electrochemically carbonitriding in molten salt.The effect of deposition temperature?480?,600??on the structure of the carbonitriding layer was studied.When the deposition potential is-1.0V_Ag/AgCl,compared with a temperature of 600?,the thickness of diffusion layer obtained at 480? decreased,accompanied by an increasing the concentration of carbon and a decreasing holes.At-1.9V_Ag/AgCl,there is no obvious variation for the content of carbon and nitrogen in the carbonitriding layer obtained at 480? and 600?.Chromium coatings on 316 L stainless steel were prepared by electrodeposition in molten salts and magnetron sputtering technique.The chromium coatings were carbonitrided electrochemically in the molten salts to obtain a Cr/Cr?CN?composite coating.The results showed that there are a large number of C,N and O elements on the surface of chromium coatings deposited in the molten salts.While there is no N element in the carbonitriding layer for chromium coatings deposited by magnetron sputtering technique.