Improvement In Corrosion Resistance And Surface Conductivity Of Stainless Steel316L By Ion Implantation

As an important material surface modification method, ion implantation is widelyapplied in science research and industry. Ion implantation technique can substantiallyalter the chemical composition and microstructure of materials surface, and thus changethe physical, chemical and mechanical properties of materials. This research is supportedunder financial support of National High Technology Research and DevelopmentProgram863(No.2006111A1112), National Natural Science Foundation of China (No.50820125506) and Key Program for International S&T Cooperation Projects of China(No.2009DFB50350). In this research, ion implantation was applied to improve thecorrosion resistance and surface conductivity of stainless steel316L (SS316L), which isused as bipolar plate materials in polymer electrolyte membrane fuel cell (PEMFC).In this dissertation, the chemical composition change, the electrochemical behaviorand interfacial contact resistance (ICR) of SS316L were systematically investigated in thesimulated and accelerated PEMFC solutions. Then, the effects of ion implantation of C,Ni and Ni-Cr co-implantation on the chemical composition and microstructure of SS316Lwere studied, and the corresponding effects on the corrosion behavior and surfaceconductivity were also investigated. The relationship among ion implantation parameters,surface chemical composition and microstructure, corrosion resistance and surfaceconductivity was revealed. At last, the optimized parameters for each kind of implantedelement were applied to modify the stainless steel bipolar plates, and the bipolar plateswere assembled into fuel cell stack to investigate the improvement on the single fuel cellperformance. The main research results are as below:Firstly, the electrochemical behavior, passive film change and surface conductivityof SS316L in simulated and accelerated PEMFC environments were investigated. Theresults indicated that the corrosion potential was higher and the passive current density was lower in the simulated solutions. The passive ability is relative weak and decreasingwith ascending pH value in the simulated solutions. SS316L undergoes pitting corrosionin pH3solution. In pH5and pH6solutions, SS316L released more metal ions due to theformation of porous iron oxide on the surface. EIS results indicated that the polarizationresistance of SS316L in the anode environment is higher than that in the cathode one, andthe electrochemical reaction is controlled by diffusion. The passive film on SS316Lchanged from Fe and Cr oxides to Cr oxide dominated structure due to selectivecorrosion of Fe. ICR increased due to this reason. ICR of SS316L increased greatly dueto much thicker passive film after polarized in pH5and pH6solutions cathodeenvironment.Secondly, the effects of C, Ti, Ni, Nb ion implantation and Ni-Cr co-implantation onthe surface chemical composition and microstructure of SS316L were studied by X-rayphotoelectron spectroscopy (XPS), Atomic Force Microscope (AFM) and high resolutionTransmission Electron Microscope (TEM). The results indicated that Gaussian-likedistribution of ion implanted element was formed on the surface of SS316L. Due tosmaller ion radius of carbon, the implanted layer is much deeper, while the implantedlayer of metal ion is usually thinner due to larger ion radius. AFM results implied that theinfluence of ion implantation on surface topography increases with the larger accelerationvoltage and implantation dose. HR-TEM results indicated that, as the accumulation ofirradiation damage and defect introduced by ion implantaion, amorphous layer would beformed at a certain implantation dose. The precipitated graphite nanophase with diameterof10nm was formed on the surface of C implanted SS316L at higher implantation dose.This phenomenon can be explained by the energy dissipation based ion-irradiationinduced phase transfer theory.Thirdly, due to the change in surface chemical composition and microstructure ofSS316L after ion implantation, the corrosion behavior and surface conductivity alsochanged correspondingly. In general, ion implantation will homogenize the surface ofSS316L, and the enrichment of implanted element is beneficial to corrosion, thus properimplantation dose can improve corrosion resistance. While excess implantation dose willintroduce more defect and deteriorated the integrity of surface structure leading to worse corrosion resistance. In specific, the corrosion current density of SS316L in the simulatedPEMFC cathode working environment is17.9μA/cm~2. While after C, Ni, Ni-Crimplantation, the corrosion current density are decreased to3~4μA/cm~2,7μA/cm~2,6.7μA/cm~2, respectively. The corrosion potential of SS316L in the simulated PEMFCanode environment is-0.3V, after C, Ni, Ni-Cr implantation, the corrosion potentialshifts to the range between-0.05V~0.1V. The potentiostatic test results indicate that allthe sample with proper ion implantation dose is stable in the corrosion environment. Afterpotentiostatic test, ICP measurement is used to identify the corroded metallic ionsconcentration in the solution. The results indicate that the corroded metallic ionsconcentration is decreased10,10and6times in the simulated cathode environment, forC, Ni and Ni-Cr implantation, respectively. And in the simulated anode environment, themetallic ions concentration is reduced by2.5and2.7times, for C and Ni-Cr implantation.As for ICR, the sputtering effect of ion implantation will reduce the thickness ofpassive film on stainless steel surface, and the element implanted into stainless steel isbeneficial to the improvement of surface conductivity. On the other hand, the formationof amorphous layer by ion implantation is detrimental. ICR of unimplanted316L is402.8m·cm~2. At the compaction force of150N/cm~2, ICR of C-2h, Ni-1and NiCr2sample is127.8,76.5m·cm~2and50m cm~2, respectively.At last, according to the results of corrosion resistance and surface donductivity, thestainless steel bipolar plates (SSBPs) were surface modified with optimized ionimplantation parameters. The ion implanted SSBPs were then assembled into fuel cellstack to investigate the single cell performance. The results indicated that the single cellperformance was greatly improved by ion implantation. In specific, the peak powerdensity of single cell assembled with unimplanted SSBPs was566.5mW/cm~2, and thepower density at0.6V output voltage was299.7mW/cm~2. While the peak power densityof single cell assembled with C, Ni, Ni-Cr implanted SSBPs was840.0,802.8and721.1mW/cm~2, respectively. The power density at0.6V output voltage was581.1,558.6and481.0mW/cm~2, increased by94%,86.4%and60%, respectively.