On Potential Application of Coated Ferritic Stainless Steel Grades K41X and K44X in SOFC/HTE Interconnects

K41X is a ferritic stainless steel grade which was successfully developed in exhaust gas manifold where the temperature could reach 950°C. It contains about 18% wt of chromium and it is stabilized with both titanium and niobium to warranty a good weldability, formability and high temperature corrosion resistance. Moreover, an addition of niobium improves high temperature mechanical properties, in particular the creep resistance. K44X, an enhanced version of K41X with 19%-wt. of Cr plus niobium and molybdenum, was recently developed to be used up to 1000°C. It exhibits better high temperature properties and oxidation resistance. Thanks to their high temperature resistance and their cost competitiveness, these two grades were recently considered as potential candidates to be used as interconnects for Solid Oxide Fuel Cells (SOFC) and High Temperature Electrolysis (HTE), either bare or more certainly coated in order to increase the life duration of the SOFC or HTE systems. This paper will present the high temperature properties of K41X and K44X, in particular oxidation behavior in isothermal and cyclic conditions under operating atmosphere. The positive effect of the addition of a protective coating on these steel grades in terms of oxidation resistance will then be presented. Most of the studied coatings are Mn-Co spinels deposited by sol-gel, atmospheric plasma spray or electroplating, their aim being to limit the chromium evaporation and to fit the severe performance requirements. They lead to low and stable contact resistance, which is a requirement necessary for long-term SOFC/HTE operation: for example a contact resistance of 22 mΩ.cm2 was obtained after 3500 h at 800°C in air with MnCoFe spinel coating. In this respect, K41X was recently chosen to be tested for the 3rd generation stacks of SOFC in the European project “REAL SOFC” or the prototypes in French ANR projects.