| Summary: | 博士 === 國防大學理工學院 === 國防科學研究所 === 102 === In this study, we successfully prepared the trivalent chromium of Cr-C layer has excellent properties of electrical conductivity and corrosion resistance by electroplating process. The main purpose of this study is to investigate the role of organic salt which comes from the plating solution of trivalent chromium on the process of coating growth. The effects of differently operated parameters such as concentration of chelating agent, pH value, temperature, current density and plating time during Cr-C coatings plating were discussed. The structure composition, surface morphology, corrosion resistance and conductivity of Cr-C coating formed at different operated parameters were studied. Moreover, the development of the Cr-C coating technology with high conductivity and corrosion resistance and how to apply at the fuel cell bipolar plates was an important matter.
The results found that the stable hydrate [Cr(H2O)6]3+ will be formed when chromium is dissolved in water and that could react with the chelating agent. The generated [Cr(H2O)5HCOO]2+ may be substituted water molecules from [Cr(H2O)6]3+ and destabilize stability of structure and make chromium ion easier to be reducted. The electrical conductivity of bath will be raised by adding conductive salts and it reduces the resistance of the reduction to make reaction faster.
The buffer moderates the pH value of reaction interface and increases the overall reduction reaction and makes the coating more complete.
The operating parameters of electroplating Cr-C bath such as the concentration of chelating agent, pH, temperature, current density and plating time will affect the distribution of current density, efficiency of plating, corrosion resistance and carbon content of coating. The carbon content could be raised by adding chelating agent and promoted formation of Cr-C coating. The carbon content will be increased and then decreased when the concentration of the chelating agent increased. Among that, the highest carbon content will be at 46 at% when concentration of chelating agent is 2M. With the increase of pH value, the distribution of the current density will wider and the better one is at pH4. The thickness of coating will be thicker with raised temperature and the maximum thickness is 27 μm at 40 ℃. With an increment of current density and plating time, the holes and particles produced on surface of Cr-C coatings. The coating surface would have the best character when current density and plating time are at 10 A/dm2 and 10 minutes, respectively. The higher carbon content within coatings, the better hardness, hydrophobic and electrical conductivity we have. However, the larger residual stress of coating will result more cracks on Cr-C surface when an increase of carbon content. With plating time and current density increases, the cracks will become larger and reduce the corrosion resistance of coatings.
According to the normal procedure, the reduction process of trivalent chromium ions is completed with two steps as Cr3+→ Cr2+→ Cr. Therefore, the intermediate Cr2+ plays an important role in the process of electroplating. When transition product Cr2+ receives electronic and return to metal Cr or oxidized back Cr3+, it will in a very unstable state. So that the thickness of the coating has not been accumulated when Cr2+ could not increase to a certain amount in the initial stage. In addition, a regular reaction will occur such as electrolyze water that makes the water be desorbed from hydrate and consume part of energy.
The Cr-C coating was applied to a metallic bipolar plate of PEMFC as coating of conductive and protective. Based on excellent properties of electrical conductivity, thermal conductivity, mechanical strength, processing characteristics, corrosion resistance and small volume, copper alloy was more suitable than traditional graphite to apply at metallic bipolar plates of PEMFC. In this study, we obtained the lower contact resistance (6.31 mΩ) when current density and carbon content are 10 A/dm2 and 46 at%, respectively. Moreover, the open circuit voltage of single cell was 0.98V and the large differences of battery output power was obtained at low current density. The maximum output power was about 320 mW/cm2 and be closer to that of graphite plate (350 mW/cm2). We found that the better performance can be obtained at current density of 10 A/dm2.
Keywords: Electroplating Trivalent Chromium, Cr-C Coating, Corrosion Resistance, Conductivity, Hydrophobicity, PEMFC, Metallic bipolar plates
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