Study On Surface Modification Of Stainless Steel Coated With Carbonized Chromium As Bipolar Plate

Proton exchange membrane fuel cell (PEMFC) has many advantages of high energy efficiency, start fast, environmentally friendly, no noise, which is expected in many fields. The key to steeping up the commercial applications of the PEMFC is to reduce costs and improve the stability of the PEMFC stack. As one of the key components of the PEMFC, the performance of the bipolar plate has directly related to the size of output power and length of life of the fuel cell stack. Modification of chromium carbide coated membrane of stainless steel plate has been applied to a stainless steel bipolar plate of the PEMFC because of its excellent performance. However, the Chromium carbide coating has pores and defects inevitably due to the technical limitations. How to further improve corrosion resistance of modification of chromium carbide plate is the key to its large-scale applications and also has a great importance to accelerate the process of commercialization of PEMFC.In this paper, the process of heat treatment and rare earth passivation were operated on316L stainless steel plate coated with chromium carbide membrane in order to ulteriorly improve the corrosion resistance of stainless steel plate coated with chromium carbide membrane.The electrochemistry performance test of316L stainless steel plate coated with chromium carbide membrane after different temperature and different time heat treatment was proceed in0.5mol/L H2SO4+2mg/L F"(simulated PEMFC environment).The test result indicate:with the increasing temperature of heat treatment, the pores of the chromium carbide membrane are decreased, the grain boundary gap is reduced and the membrane tends to be dense. However, overheating can make the film appeared micro-cracking. The best heat treatment process:300℃insulation2h.After optimum heat treatment temperature processing, the corrosion current density of the chromium carbide plate dropped from20.283μA·cm-2to0.379μA·cm-2; from4.625μA·cm-2to0.115μA·cm-2in the context of analog cathode-anode environment (voltage of0.8V) which has improved its electrochemical performance. The results of EIS confirm that the heat treatment process not only improves the compactness of the film, but increases the energy barrier for electron transfer and electrochemical reaction. This kind of process inhibits and slows down the corrosion reaction and plays a good role in the protection of the bipolar plate.Potassium permanganate-rare earth salt passivation make the open circuit of bipolar plates increased by about400mv and the corrosion current density decreased by about two orders of magnitude in simulated PEMFC cathode and anode environment. The test results of EIS show that the passivation treatment not onl y improves the denseness of the film’s surface, but also enhances the electron transfer resistance of the coating layer, as well as increases the resistance of the electrochemical reaction of the matrix metal. Thereby,this kind of treatment has greatly improved the corrosion resistance of the bipolar plate.The optimum conditions for potassium permanganate-the rare earth salt passivation is: KMnO445g/L, Ce (NO3)3·6H2O10g/L, pH value of6, Mg (NO3)2150g/L, temperature of60℃and the heating time of4min. SEM and EDX tests indicate:the micro-pores of the film surface reduces and the density increases after passivation treatment, cerium involves in the course of the deposition of the passivation film.