| As an important structural material,316L stainless steel has been paid much attention in chemical,marine,energy and medical industries due to its excellent comprehensive corrosion resistance.However,with the increasingly severe application environment,316L stainless steel is also faced with serious corrosion failure threats.The corrosion forms mainly include pitting corrosion,cavitation erosion corrosion and crevice corrosion,which have serious impacts on the service performance and life of 316L stainless steel.Corrosion mainly occurs at the interface between metallic material and the corrosive medium,so the surface corrosion resistance is the key to affect the overall performance.In order to improve the corrosion resistance of 316L stainless steel without reducing its mechanical properties,surface modification is the best way.High-temperature element diffusion technology has the advantages of low cost,high efficiency,high stability and high bonding strength,which has a broad research and application prospect.In this paper,Cr element was diffused into 316L stainless steel by high-temperature pack chromizing technology,and Cr-rich modified layer was prepared on the surface of316L stainless steel.The microstructure characteristics and the microstructure evolution after heat treatment were analyzed.The corrosion behaviors of chromizing layer in PEMFC solution environment and dynamic marine cavitation erosion environment were systematically studied,and the mechanism of microstructure on its corrosion resistance.The main work and conclusions are as follows:（1）After high-temperature pack chromizing treatment at 1070℃,a chromizing modified layer with thickness of 75μm was prepared on the surface of 316L stainless steel,and its Cr concentration was over 50 wt.%.The phase composition of chromizing layer mainly includes Fe-Cr alloy phase and chromium carbide phase.The morphology analysis shows that there are three characteristic microstructures,i.e.grains,grain boundary Cr-depletion areas and grain boundary precipitates.The results of phase and element analysis show that both the grains and the grain boundary Cr-depletion areas are Fe-Cr alloy phase,while the precipitate is chromium carbide phase,which presents the characteristics of network distribution along the grain boundaries.The micro Vickers hardness of the chromizing layer is about 600-700 HV,which is more than three times higher than that of 316L stainless steel.（2）In typical acid environment,the corrosion resistance of chromizing layer in simulated PEMFC electrolyte solution was studied.The polarization curves show that the free corrosion current densities of the modified layer in simulated anode and cathode environments are2.64×10-7 A·cm-2 and 2.22×10-7 A·cm-2,respectively,which are 2-3orders of magnitude lower than that of 316L stainless steel.Simultaneously,the charge transfer resistance and polarization resistance are also significantly improved,which means the corrosion resistance is remarkably enhanced.Under simulated working conditions,after 4 h potentiostatic polarization test,the corrosion current density of the chromizing layer is lower than the standard requirement of 1μA·cm-2,which improves the corrosion durability.In addition,the interface contact resistance of the modified layer is only 1.4 mΩ·cm2 under 140 N·cm-2compaction force,which is much lower than the 105.2 mΩ·cm2 value of316L stainless steel used as bipolar plate and the international standard value of 10 mΩ·cm2.The analysis of corrosion resistance mechanism shows that the performance improvement mainly comes from two aspects:one is that the inherent chromium carbide phase in the modified layer has excellent corrosion resistance;the other is that the Cr2O3 oxide film formed on the surface of Fe-Cr alloy phase plays a significant role in corrosion protection.The improvement of surface conductivity is mainly due to the chromium carbide phase distributed at the grain boundaries,breaking the charge transfer barrier between the matrix and outside formed by Cr2O3 oxide film,and effectively reduces the interface contact resistance.（3）The regulation and control of microstructure by post heat treatment can significantly improve the cavitation erosion resistance of the modified layer.After the chromizing modified layer was kept at 1200℃for 30 min and quenched by water cooling,the chromium carbide phase decomposed,the elements redistributed,the Cr-depletion areas disappeared,and the phase structure changed into a single Fe-Cr alloy phase,realizing the homogenization of microstructure.The cavitation erosion test results show that the cavitation erosion resistance of the modified layer before heat treatment is not improved compared with316L stainless steel,while the cumulative mass loss of the modified layer after heat treatment is 3.97 times lower than 316L stainless steel,the erosion damage is remarkably reduced,and the cavitation erosion resistance is significantly improved.The cavitation erosion behaviors of the samples are understood from two aspects of static corrosion and mechanical impact:the free corrosion current density of the heat-treated chromizing layer in 3.5 wt.%Na Cl static solution is only 1.192×10-8A·cm-2,the charge transfer resistance and polarization resistance are significantly higher than those of 316L stainless steel,the surface Volta potential distribution is more uniform,and the tendency of galvanic corrosion is significantly reduced;the chromizing modified layer after heat treatment possesses excellent hardness and toughness,which can reasonably resist and absorb mechanical impact energy,thus,the comprehensive mechanical properties of the surface are significantly improved.The improvement of static corrosion resistance and surface mechanical properties is due to the homogenization effect of heat treatment on microstructure.Reasonable microstructure regulation and control can maximize the cavitation erosion resistance of chromizing layer in dynamic marine environment.Generally speaking,the chromizing modified layer has a great difference in service performance under different working conditions,mainly due to the adaptability of different microstructure to service requirements.Therefore,we should follow the characteristics of actual service environments,design and control the microstructure in order to give full play to the advantages of materials. |
