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

Abstract

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°C of AISI 441 under 5%H₂O in O₂ following the cathodic SOFCs conditions. The typical duplex oxide scale, composed of a (Mn,Cr)₃O₄ 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.

Keywords:

• Ferritic stainless steel
• Oxide nodules
• Niobium intermetallic
• Niobium-titanium carbonitrides
• 3D-reconstruction

Acknowledgements

The authors would like to acknowledge the Franco-Thai scholarship (W.W.) supported by the French Government and also the Faculty of Engineering, King Monkut’s Institute of Technology Ladkrabang for funding supports.