Preparation And Properties Of Nano-scale Cr And CrN Coated Stainless Steel Bipolar Plates

Proton exchange membrane fuel cell?PEMFC?is considered as one promising power sources for mobile power and potable application, accounting for high power density,fast start-up, low temperature and environmental friendliness. Bipolar plates are a key multifunctional component of PEMFCs as a result of a significant part of total weight and cost of the PEMFC stack. The bipolar plate materials include graphite, metals and composites. Metals exhibit higher mechanical strength, no gas permeability, much superior manufacturability, and can be machined to be thin plates to achieve the higher power density compared to graphite. The main challenge of metals, however, is the corrosion in PEMFC environments, which causes considerable power degradation. Therefore, it is necessary to enhance the corrosion resistance of metallic bipolar plates, In this dissertation, nano-scale Cr and CrN coatings were prepared on the 316 L stainless steel by DC magnetron sputtering process. The main achievements are as follows:The effects of substrate temperature on the microstructure and properties of nano-chromium coatings were investigated. The chromium film deposited at 300? presented a three-dimensional island structure with an average grain size about 50 nm and there existed obvious pores in it. However, the microstructure of chromium film deposited at 500? was dense with an increasing grain size. In 0.5mol/L H₂SO₄+2 ppm F- solution, the chromium coatings deposited at 300? and 500? increased significantly the free corrosion potential of the steel, and its corrosion current density was one order of magnitude lower than the bare steel. Moreover, the coating exhibited high stability during exposure up to 624 h.The effects of sputtering currents on the microstructure and properties of nano-chromium coatings were investigated. The microstructure of nano-chroumim of chromium deposited at 0.25 A, 0.35 A and 0.4A respectively, grow with a similar characteristic. Compared with the chromium deposited at 0.25 A, the coatings deposited at 0.35 A and 0.4A are more compact. Chromium coatings for 0.35 A and 0.4A exhibited high stability during long exposure to corrosive solution, indicating that both coatings can inhibit effectively the inward penetration of corrosion species, and offer the alloy excellent protection.The effects of Ar/N₂ flow ratio on the on the microstructure and properties of nano-scale CrN coatings were investigated. The grain size of three CrN is about 20nm-40 nm, and all of them present a three-dimensional cone structure. There existed obvious micro-defects in nano-CrN coatings with flow ratio 6:5 and 6:8. Therefore, the coatings were subject to failure in H₂SO₄+2ppmF- solution. In contrast, the CrN deposited with flow ratio 6:6 was more dense. In corrosive solution, its impedance spectra was diffusion-controlled and kept high stability after immersion for 672 h, suggesting that the CrN coating could protect the steel effectively.