Possible connection between nodule development and presence of niobium and/or titanium during short time thermal oxidation of AISI 441 stainless steel in wet atmosphere

AISI 441 ferritic stainless steel is a good candidate for metallic interconnects of solid oxide fuel cells (SOFCs). In this alloy, the minor elements Ti and Nb are used to stabilise the ferritic structure but their influence on steel durability is not well understood. This study focuses on the early stages of oxidation (24 h) at 800 degrees C of AISI 441 under 5% H2O in O-2 following the cathodic SOFCs conditions. The typical duplex oxide scale, composed of a (Mn, Cr)(3)O-4 spinel top layer and a Cr2O3 rich sublayer is observed, with oxide nodules growing in places. These objects, in the micrometre range in size, are studied by FIB tomography. The analyses reveal a complex structure and a development strongly linked to the presence of niobium and/ or titanium compound(s) in the subjacent substrate.