Intermetallic FeAl based coatings deposited by the electrospark technique: corrosion behavior in molten (Li+K) carbonate

Abstract

In the strongly corrosive environment of the molten carbonate fuel cells (MCFC) the protection of the wet-seal areas of the bipolar plates is accomplished by aluminum diffusion coatings. The coating layer is usually produced by depositing metallic aluminum onto a stainless steel surface with the ion vapor deposition (IVD) technique followed by a diffusion annealing treatment in order to transform the as-deposited Al into a corrosion resistant FeAl-based intermetallic surface alloy. In an attempt to find cheaper alternatives, in this work we report the results obtained depositing directly a layer of FeAl intermetallic alloy onto a 316L austenitic stainless steel by using the electrospark deposition (ESD) technique. ESD can apply metallurgical bonded coatings in ambient conditions without the need of post-deposition annealing treatments. Structure, chemical composition and morphology of the FeAl coating has been analyzed and compared to a standard IVD aluminized coating. By electrochemical measurements and long-term immersion tests it is inferred that the corrosion resistance of the electrospark coating is comparable to that of the IVD coating. Some sign of coating degradation after 1000 h immersion in the (Li+K) carbonate mixture at 650 °C was due to coating microcracking and Al depletion.