Effect of Nb Additions on the High-Temperature Performances of NiFe2O4 Spinel Coatings Fabricated on Ferritic Stainless Steel

Abstract

Ni–Fe alloy coatings with Nb contents of 0, 4.2 at.%, 7.0 at.% and 9.0 at.% were electrodeposited on ferritic stainless steels for intermediate-temperature solid oxide fuel cell (SOFC) interconnect application. The oxidation behavior, scale-growth mechanisms and electrical performances of the Ni–Fe- and NiFe–Nb-coated steels were then investigated in air at 800 °C corresponding to the cathode environment of SOFC. It was found that the scales formed are mainly composed of an external NiFe2O4 layer and an inner Cr2O3-rich layer after oxidation, with some Nb-rich oxides also observed for the Nb-containing coatings. Doping 4.2 at.%Nb into the coating decreased the growth rate of Cr2O3 and suppressed the formation of voids at the scale/alloy interface, whereas excess Nb impaired the oxidation resistance of the steel. The Nb addition affects the oxide growth direction by suppressing the cation transport. All the coatings decreased the area-specific resistance (ASR) of the steel, with the NiFe–4.2 at.%Nb-coated sample exhibiting the lowest ASR value. The effects of Nb on the oxidation mechanism and electrical properties of the Ni–Fe-coated steels are discussed.