Oxidation and electrical behavior of AISI 430 coated with cobalt spinels for SOFC interconnect applications

Stainless steel can be used as interconnect plates in solid oxide fuel cells (SOFCs) below operating temperature of 800 °C. Unwanted reactions between the alloy and other SOFC components decrease the efficiency of these energy convertors. One approach to improving interconnect properties is to apply a surface coating to them. In this study, AISI 430 ferritic stainless steel interconnect is coated in a cobalt-base pack mixture using the pack cementation method. Isothermal oxidation, cyclic oxidation and oxidation at different temperatures (400–900 °C) are applied to evaluate the role of the coating layer during oxidation. Area-specific resistance (ASR) of the coated substrates has also been tested as a function of temperature and time. The surface morphology was examined by SEM, the chemical composition and structure of oxide formed were analysed by EDS and XRD. Results showed that the coating layer transforms into MnCo2O4, CoCr2O4 and CoFe2O4 spinels during isothermal oxidation. This scale is protective and acts as an effective barrier against chromium migration into the outer oxide layer and prevents weight gain. The mass gain and spallation indicated that the formation of spinel significantly improved the high temperature oxidation. These spinels also cause a reduction in ASR for coated substrates (9.7 mΩcm2) as compared to uncoated substrates (36.1 mΩcm2) after 200 h of isothermal oxidation at 800 °C.