Evaluation of a Novel Perovskite-Based Conversion Coating for Corrosion Protection of 13Cr Ferritic Stainless Steels under Relevant SOFC Oxidizing Conditions

Abstract

Reduction of operational temperatures from 800°C towards an intermediate temperature (IT) range of 600-750°C is one major trend in current Solid Oxide Fuel Cell (SOFC) research as a cost-effective approach to higher reliability and durability of fuel cell components and systems [1]. Intermediate temperatures expand, in fact, enormously materials selection in SOFC design allowing the possibility of using conventional ferritic stainless steels for the realization of important components such as cell housings, gas manifolds, support structures and plate interconnects (IC). In particular, 18Cr ferritic stainless steels are currently the most evaluated materials to realize cost-effective metallic IC plates due to their excellent mechanical stability, ease of fabrication and high formability characteristics. However, excessive contact resistance due to oxide scale growth and evaporation of the protective chromia scales are two primary sources of cell performance degradation that are related to an insufficient long-term corrosion resistance of 18Cr ferritic stainless steels. In order to alleviate these problems, protective ceramic oxide coatings with spinel or perovskite layers are usually applied to provide better electrical contact and reduced Cr volatility [2]. For further cost reduction, 13Cr stainless steels have received some attention in the recent literature as a potential substitution alternative at the low end of IT-SOFC operation temperatures [3,4]. At temperatures below 700°C, the oxidation of these steels gives raise to protective oxide films mainly composed of mixed (Fe,Mn,Cr) spinel oxide phases [4]. As compared to chromia-forming alloys, stainless steels that form protective (Fe,Mn,Cr) spinel oxides have an expected advantage of lower Cr volatility and contact resistance issues with only a small trade-off in corrosion resistance properties, under appropriate temperature conditions. Recently, we have proposed a novel approach for high temperature corrosion protection of stainless steels, which is based on producing LaFeO3-based perovskite layers by chemical conversion reactions taking place in a molten carbonate salt bath. Our previous results indicated that LaFeO3 perovskite coatings were able to protect stainless steels with various Cr content in typical Molten Carbonate Fuel Cell (MCFC) environments [5,6]. In order to extend the application areas of this novel coating method, the aim of this work was to evaluate the suitability of LaFeO3 perovskite coatings also for the improvement of corrosion resistance of a 13Cr ferritic stainless steel under typical IT-SOFC conditions. Techniques such as XRD and SEM/EDX analysis were used to characterize morphology and structure stability of the perovskite coating after prolonged exposure at 700°C in ambient air atmosphere. Detailed results from these studies will be presented at the conference venue. Bibliography [1] M. C. Tucker, Progress in metal-supported solid oxide fuel cells: A review, . J Power Sources 195, 2010, 4570-4582; [2] N. Shaigan, W. Qu, D.G. Ivey, W. Chen, A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources 195, 2010, 1529-1542. [3] J. Fergus, Y. Zhao, Low-Chromium Alloys for Solid Oxide Fuel Cell Interconnects. ECS Trans 25, 2011, 2447-2453; [4] S. Frangini, A. Masci, S.J. McPhail, T. Soccio, F. Zaza, Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications. 144, Mater Chem Phys 2014, 491-497; [5] S. Frangini, A. Masci, F. Zaza, Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells. Corros Sci, 53, 2011, 2539-2548; [6] S. Frangini, F. Zaza, A. Masci, Molten carbonate corrosion of a 13-Cr ferritic stainless steel protected by a perovskite conversion treatment: Relationship with the coating microstructure and formation mechanism. Corros Sci, 62, 2012, 136-146.