Corrosion Resistance And Electrical Conductivity Of Metal Bipolar Plate By Heat-Assisted Forming For Fuel Cells

Stainless steel and titanium are considered to be two promising candidate materials of metal bipolar plates（BPPs）in proton exchange membrane fuel cell（PEMFC）for transportation in the future for the many advantages they have.However,there are still big challenges in formability,forming quality,corrosion resistance and electrical conductivity for stainless steel and titanium BPPs under the goals and requirements of PEMFC with high performance,high reliability and long durability.Among various technical routes,heat-assisted forming is regareded as a potential solution to improve the formability and quality of metal BPPs,and to realize mass production of metal BPPs with fine channel structure and high performance.For the moment,there are relatively few scientific reports on preparation of BPPs by heat-assisted forming.Besides,most of them are focused on the effects of heat-assisted forming processes on formability and quality of metal BPPs.The influences and mechanisms of forming processes on corrosion resistance and electrical conductivity of metal BPPs during heat-assisted forming processes remain unclear.In addition,there is a lack of research on adding coatings directly to metal BPPs by heat-assisted forming processes.Moreover,the effects of different woring conditions on corrosion resistance and electrical conductivity of metal BPPs in PEMFC are not studied comprehensively,which leads to no unified or standard method for corrosion resistance evaluation of materials for metal BPPs.In this paper,the effects of different working environments,such as temperature,pH and atmosphere,and voltage magnitude and its dynamic change under different working conditions on corrosion resistance and the related mechanisms of metal BPPs were studied comprehensively by combination of electrochemical analysis and microstructure characterization.In the meanwhile,the principle and methods for testing and analysis of corrosion resistance of metal BPPs was proposed.On this basis,electrochemical tests,such as potentiodynamic test,potentiostatic test,simulated working condition test and electrochemical impedance spectroscopy（EIS）analysis,as well as microstructure characterizations,such as optical microscopy（OM）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and back-scattered electron diffraction（EBSD）were conducted to reveal the evolutions of corrosion resistance and microstructure of heated-assisted stainless steel and titanium substrate materials under different forming processes and parameters.In addition,variations of corrosion resistance and electrical conductivity of directly coated stainless steel BPPs by heat-assisted forming were analyzed.This work aims to provide a theoretical basis for preparation and application of metal BPPs by heat-assisted forming.The main contents and results of the work are as follows.（1）A combination of electrochemical tests and surface morphology characterization method were utilized to study the influences and related mechanisms of different working environments,such as temperature,pH and atmosphere,and voltage magnitude and its dynamic change under different working conditions on corrosion resistance and electrical conductivity of 316L stainless steel（SS316L）BPPs.On this basis,the principle and methods that may be more suitable for testing and evaluating the corrosion resistance of metal BPPs are proposed.The results indicate that pH and voltage in PEMFC have the most significant effects on corrosion resistance of SS316L BPPs.Besides,the interactions between pH and voltage are strong.In the meanwhile,the influences of pH and voltage on corrosion resistance of SS316L BPPs vary with atmosphere in the solution.In the accelerated corrosion solutions with pH=0～1,intergranular corrosion and uniform corrosion are the main corrosion mechanisms for SS316L BPPs under dynamic voltage or high static voltage of 1.0～1.4 V（vs.Ag/AgCl）.However,pitting will be the main corrosion mechanism in simulated solutions with pH=3～5 under potentiodynamic and potentiostatic tests with different voltages.Based on the results,the principles for corrosion resistance evaluation of metal BPPs are proposed.First,the corrosion resistance of metal BPPs in anode and cathode of a PEMFC should be tested separately.Second,the corrosion mechanism should remain unchanged when the accelerated corrosion solutions are used for testing corrosion resistance of metal materials for BPPs.Potentiodynamic and potentiostatic tests are appropriate for corrosion resistance evaluation of metal materials for BPPs in the anode side of a PEMFC while potentiodynamic,potentiostatic and simulated working condition tests are suitable for corrosion resistance evaluation of metal materials for BPPs in the cathode side of a PEMFC.In addition,the simulated solutions with pH=3～5 is more appropriate for corrosion resistance evaluation of metal materials for BPPs if high voltage in PEMFC needs to be considered.（2）Heat-assisted forming SS316L BPP samples under different parameters of heat-assisted forming process and with different surface conditions were prepared by means of tube furnace heating,heat-assisted stretching and electrochemical vibration cleaning.In addition,the effects and related mechanisms of temperature and deformation on corrosion resistance and electrical conductivity of surface and inner substrate of heat-assisted forming SS316L samples were studied carefully by electrochemical testing and microstructure characterization.The results show that the self-corrosion current density and corrosion current density under the voltage of 0.6 V（vs.Ag/AgCl）of SS316L BPP samples heated at the temperature range of 850～900℃in the atmosphere can be reduced to 0.03～0.04μA·cm^-2and 0.04～0.05μA·cm^-2,respectively,in simulated testing solutions with pH=3.However,the contact resistance between the heat-assisted forming SS316L BPP samples and commercial carbon paper under the compression pressure of 1.4 MPa will increase to 30000～40000 mΩ·cm².The formation of oxides,such as Fe₂O₃,Fe Cr₂O₄,Cr₂O₃,as well as increase in the thickness of oxide film is main reason for the higher corrosion resistance and larger contact resistance.Besides,pinholes and micro cracks along the grain boundary appear on the surface of the oxide layer when the heating temperature is above 600℃.The influences of temperature and deformation on corrosion resistance of internal substrate of SS316L BPPs are relatively small.However,the passivation current density in the voltage range of-0.2～0.2 V（vs.Ag/AgCl）of the heat-assisted forming SS316L BPP samples is higher than the cold formed SS316L BPP samples when temperature and deformation are 750～900℃and 20～45%,respectively.The increase in grain heterogeneity and number of small angle grain boundaries in 2～5°can lead to an increasing tendency of galvanic corrosion of the internal substrate of stainless steel.This may be the main reason for the increase of passivation current density of internal substrate of SS316L BPP samples formed under the aforementioned conditions.（3）Heat-assisted forming SS316L BPP samples under different forming temperature and holding time were prepared with a box resistance furnace,and a high Cr metal coating was directly added to the surface of heat-assisted forming SS316L BPPs by magnetron sputtering deposition（MSD）process.After this,the changes and related mechanisms of corrosion resistance and electrical conductivity of coated heat-assisted forming SS316L BPP samples were studied by electrochemical test and electron microscopy analysis methods.In addition,the process parameters of coated heat-assisted forming SS316L BPP samples were optimized while the technical route of adding coating directly to the surface of heat-assisted forming SS316L BPPs was proposed.By added a high chromium coating directly with magnetron sputtering process to the surface of heat-assisted forming SS316L BPPs under 900℃in the atmosphere for 10 min,the self-corrosion current densities for the coated heat-assisted forming SS316L BPPs in simulated anodic and cathodic environment with pH=3 of PEMFC are 0.043μA·cm^-2and 0.021μA·cm^-2,respectively.In addition,the contact resistance between the coated heat-assisted forming SS316L BPPs and commercial carbon paper under the pressure of 1.4 MPa is close to that between gold-plated copper plate and commercial carbon paper under the same pressure.The atoms of coated metal with high electrical conductivity can penetrate into the stainless steel substrate through the pinholes and microcrack in the oxide layer during the process of MSD.As a result,a conductive network between the coating material and metal substrate can be formed.This is the main reason for the improvement of electrical conductivity of coated heat-assisted forming stainless steel bipolar plate.In addition,the heat-assisted forming SS316L BPPs with better corrosion resistance as compared to the cold formed SS316L BPPs can further contribute to a higher corrosion resistance of by adding a coating on its surface.（4）TA2 titanium BPP samples heated under different heating temperature and holding time were prepared in a box resistance furnace.Besides,the influences and related mechanisms of heating temperature and holding time during heat-assisted forming process on corrosion resistance of TA2 titanium BPP samples were studied by electrochemical test and microstructure characterization.Based on this,the optimized parameters of heat-assisted forming process for titanium BPPs were obtained.The results indicate that both corrosion resistance and plastic formability of TA2 titanium bipolar plate substrate may be improved when temperature and holding time of heat-assisted forming process are about 600℃and 10 min,respectively.In simulated solutions with pH=3,the self-corrosion current density and corrosion current density at 0.6～1.4 V（vs.Ag/AgCl）for TA2 titanium BPPs formed under the aforementioned conditions are 0.1μA·cm^-2and 0.01～0.03μA·cm^-2,respectively.In addition,the self-corrosion current density of heat-assisted forming TA2 titanium BPPs is further reduced to about 0.02～0.03μA·cm^-2,which is significantly lower than that of the original TA2 titanium BPP sample.The results also show that heat-assisted forming TA2 titanium BPPs have superior corrosion resistance under high voltage and working conditions in PEMFC.The results of microstructure characterization and electrochemical impedance analysis indicate that the formation of TiO₂,increase in the thickness of oxide film and improvement of the compactness of surface oxide film can all improve the corrosion resistance of surface oxide film.Moreover,the improvement of uniformity of its internal grain size can also contribute to the improvement of the corrosion resistance of internal metal substrate.These are the main reasons for better corrosion resistance of heat-assisted forming titanium BPPs.