Effect of Microstructure and the Interface on Localized Corrosion in Metallic Nitride Coated Metallic Flow Field Plate in Simulated Low-Temperature Fuel Cell Conditions

Abstract

Proton Exchange Membrane (PEM) fuel cell offers clean energy on par with renewable energy generations. Graphite bipolar plate, engraved with flow field which is a key component in the PEMFC assembly suffers from volumetric and machining issues. Metallic plate is as an alternative, among which stainless steel and titanium are looked upon as promising candidates, but it suffers from the aspect of corrosion. Inherent corrosion current density (Icorr) of SS316L is 0.5 µA/cm2 which is well within the international standards (Department of Energy (DoE), US) but that of Ti-6Al-4V is 47 µA/cm2 which is not meeting the DoE targets under hydrogen purge conditions. Under fuel cell operating conditions, both the material tends to fail due to corrosion aspects. Various surface treatments are imparted to enhance corrosion in the PEMFC environment, but most of the corrosion protection coatings fail over a period of time due to various aspects.An in depth understanding of the microstructure of the coating prior and post, subjecting it to simulated fuel cell conditions and study of the interface will elude failure at earlier stages of real time operation. In this study, local investigations were carried out on metallic nitride coated SS316L flow field plate to juxtapose the changes induced in the microstructure due to change in pH of the simulated electrolyte, the effect of the interface on the adhesion of the coating. Titanium nitride particles (as seen in Fig.1) adhered to the base material and qualified with 5B rating in ASTM D3359 adhesion studies. Pre-processing and post-processing treatments resulted in the roughness of the coating (Ra) as low as 11nm. Corrosion current density of the coated specimen, Icorr is 0.455 µA/cm2, is within DoE target values and the Ecorr was positive, thus enhancing the corrosion resistance fuel cell operating conditions. Localized corrosion studies reveal the role of pitting agents, present in the simulated electrolyte, which enhance the mechanical cracking of coating layer thus undermining the durability of the coating when tested over a period of 6 months. Figure 1