Electrochemical Behavior of CrN/Cr Bilayer Coating on 316L Stainless Steel in Simulated Cathodic Environment of HT-PEFC
Authors: Li Ruiyu , Cai Yun , Wippermann Klaus , Lehnert Werner 
Published: February 2020

High-temperature polymer electrolyte fuel cells (HT-PEFCs) operate in the range of 120 – 180 °C and currently employ phosphoric acid doped polybenzimidazole membranes. This, in turn, represents a quite aggressive environment for the components of a fuel cell [1]. Due to this reason, bipolar plates are presently made of graphitic composite materials. Indeed, they show long-term stability, but otherwise, they exhibit high mass and volume as well as a high effort of manufacturing. Therefore metallic materials come into play as promising candidates for the bipolar plate [2]. They provide some crucial advantages, like enhanced volumetric and gravimetric power density for stacks, increased ductility, high mechanical stability and the capability of low-cost mass production. Due to the corrosion of the metals, it is not feasible to employ bare commercially available stainless steels or Ni-based alloys as metallic bipolar plates for HT-PEFC application. One possibility to reduce the corrosion rate and to keep the benefits of metals could be applying conductive and inert coatings [2]. For coatings without self-healing ability, localized corrosion should occur at the damaged site and continuous corrode the substrate under the coating. Self-healing ability can be given to the films by introducing a metallic component into the films: The oxidation of the metallic component will heal flaws in the films [3]. According to this idea, CrN/Cr bilayer coating has been studied. Using an electrochemical cell, which was specially designed for HT-PEFC experimental requirements, we analyzed CrN/Cr bilayer coated 316L and bare 316L by means of potentiodynamic, potentiostatic and free corrosion potential monitor tests combined with electrochemical impedance spectroscopy. By purging different gases (nitrogen, oxygen) into the electrolyte, we have also investigated the influence of oxygen on the corrosion resistance ability of CrN/Cr coating under simulated cathodic HT-PEFC environments. Additionally, ex-situ characterizations using SEM+EDX, XPS and ICP-OES were carried out in order to investigate the metallic surface, corrosion production and the corresponding dissolved ion species in the electrolyte. The CrN/Cr bilayer coating shows good anti-corrosion capability in the simulated cathodic environment of HT-PEFC with the existence of oxygen, i.e. no cracks or holes on the surface of the samples after potentiostatic tests at either 0.65 V or free corrosion potential for more than 4 hours at 130 ºC. Cathodic operation conditions including a relatively high potential and the presence of oxygen lead to the oxidation of chromium on the damaged site, therefore healed the damaged site and beneficial for providing sufficient corrosion resistance to the cathodic HT-PEFC environment.
References:
[1] C. Hartnig and T. J. Schmidt, Electrochimica Acta, 56, 4237 (2011).
[2] V. Weissbecker, U. Reimer, K. Wippermann, & W. Lehnert, ECS Transactions, 58(1), 693-704(2013).
[3] M.Yasuda , N. Akao, N. Hara, K. Sugimoto, Self-healing Corrosion Protection Ability of Composition-Gradient Al2O3⋅ Nb Nanocomposite Thin Films, J. Electrochemical Society, 2003, 150(10): B481-B487