Process modification for coating SnO 2:F on stainless steels for PEM fuel cell bipolar plates

Abstract

Our previous procedure for depositing SnO 2:F on stainless steels for interfacial contact resistance (ICR) reduction and as a protective coating for metal bipolar plates was modified by pre-etching and coating characterization. Only ferrite stainless steels acquired a good quality SnO 2:F coating. The modified SnO 2:F coating decreased the ICR over our previous coating results, confirming the beneficial effect of the pre-etching; however, the corrosion resistance we obtained was significantly reduced. For the pre-etched and coated AISI444 and AISI446, both dynamic and potentiostatic polarizations revealed that the corrosion resistance of the modified coated steels is not as good as that obtained with the original coating process. The pre-etched and coated AISI444 showed higher currents in the polymer electrolyte membrane fuel cell (PEMFC) anode environment, as compared with earlier coated (original) AISI444 results. It took 10–50 min to reach a stabilized current for the pre-etched and coated AISI446; moreover, the stable currents are higher than those for original coated AISI446. Inductive coupled plasma (ICP) analysis for dissolved metallic ions in the test solutions was in agreement with the polarization results. Additionally, both the polarization and ICP analysis results indicate that the PEMFC cathode environment is more corrosive to the coated steels (especially AISI444) than the PEMFC anode environment. The Auger electron spectroscopy (AES) investigation showed that the pre-etched and coated AISI444 had heavy dissolution in the PEMFC cathode environment, confirming that for this process it has enhanced corrosion over the PEMFC anode environment. Both coated AISI444 and coated AISI446 showed cracks and peel-off of the modified coating after the polarization. This is apparently the source for the higher anodic currents in the polarization curves as compared to our original process for coating the steels. The conclusion is that both the corrosion resistance in PEMFC environments and the adhesion of the modified SnO 2:F coating are challenges for further development.