Corrosion behavior of tantalum coatings on AISI 316L stainless steel substrate for bipolar plates of PEM fuel cells

12 figures, 1 table.-- Supplementary information available.Corrosion resistance of tantalum coatings 30 μm thick deposited by chemical vapor deposition on SS316L coupons has been evaluated by electrochemical impedance spectroscopy (EIS). To this end, anodic and cathodic operating conditions of proton exchange membrane fuel cells (PEMFC) have been simulated in a three-electrode heated corrosion cell. Interfacial contact resistance (ICR), contact angle and durability tests have been performed in long-term tests (>100 h) polarizing the electrode to 1.193 V vs. Ag/AgCl. Results obtained by different experimental techniques show a dense coating structure with a high polarization resistance, mainly formed by surface crystals of α-Ta (bcc), TaO and carbon. An atomic ratio (in %) of oxide to metallic species (Ta/Ta) of 4.8 was verified from XPS spectra, which is slightly increased to 6.23 after the anodizing treatment. The modified surface composition yielded a coating capacity higher than the amorphous oxide, favoring the in-plane electrical conduction. After the treatment, no noticeable changes were observed neither in surface morphology nor in contact angle (>90°). ICR values in the range of 22.3–32.6 mΩ cm were obtained for a clamping pressure of 140 N cm. No morphological changes or loss of coating adherence were observed during the long-term tests.Authors acknowledge the financial support of the Secretariat of State for Research of the Spanish Ministry of Economy and Competitiveness under project DPI2015-69286-C3-1-R, and the Spanish Ministry of Science, Innovation and Universities under the project RTI2018-096001-B-C31. Support of the Regional Government of Aragon to the Fluid Mechanics for a Clean Energy Research Group (T01_17R) of the LIFTEC is also acknowledged, as well as the technical and human support of the Materials and Surfaces Service provided by SGIker (UPV/EHU/ERDF, EU).Peer reviewe

Evaluation of the corrosion resistance of Ni(P)Cr coatings for bipolar plates by electrochemical impedance spectroscopy

In the present research, the corrosion resistance of Ni–P and Ni–P–Cr coatings on AA7075-T6 aluminum plates under simulated anodic and cathodic conditions of polymer electrolyte membrane fuel cells (PEMFC) has been studied by electrochemical impedance spectroscopy (EIS). Three Ni–P coatings 20 μm, 30 μm, and 40 μm thick applied by electroless deposition were tested. Besides, a two-layer Ni–P–Cr coating 30 μm thick was also analyzed. It was formed by an inner Ni–P layer, and an outer 10 μm thick chromium one added by electroplating. Corrosion tests were combined with interfacial contact resistance (ICR), roughness, contact angle, and SEM-EDX measurements. The best results were obtained for the 20 μm Ni–P and the two-layer Ni–P–Cr coatings, although the latter showed a high ICR value due to the high electrical resistivity of the chromium oxide surface formed. It was verified that coating degradation occurs when the electrolyte penetrates the micro-cracks and the nodular surface interfaces, reaching the base metal and causing the coating delamination. This behavior is associated with a sharp decrease in the polarization resistance (Rp) of the equivalent circuit model fitted to the EIS results.Authors acknowledge the financial support of the Secretariat of State for Research of the Spanish Ministry of Economy and Competitiveness under project DPI2015-69286-C3-1-R, and the Spanish Ministry of Science, Innovation and Universities under the project RTI2018-096001-B-C31. Support of the Regional Government of Aragon to the Fluid Mechanics for a Clean Energy Research Group (T01_17R) of the LIFTEC is also acknowledged, as well as the technical and human support of the Materials and Surfaces Service provided by SGIker (UPV/EHU/ERDF, EU)