Oxidation behavior of ferritic stainless steel containing Nb, Nb–Si and Nb–Ti for SOFC interconnect

The effect of Nb on the oxidation kinetics, electrical conductivity and Cr evaporation behavior of FSS has been discussed depending on the Nb content and oxygen active element such as Ti and Si. Nb in ferritic stainless steel is saturated during heat treatment as NbO2 at the outermost oxide scale and as both Nb2O5 and Laves phase near the oxide scale/alloy interface. Excess Nb (>4.7 wt%) suppresses precipitation of Nb2O5, because of rapid Laves phase growth. Nb enhances selective Ti oxidation, whereas Ti retards Nb2O5 precipitation near the scale/alloy interface. On the other hand, Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb2O5 and Laves phase. Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface. Co-addition of Nb and Ti is effective to decrease the electrical resistance and Cr evaporation rate of oxide scale.

► Interfacial enrichment of Nb enhances the precipitation of Nb2O5 and Laves phase.
► The precipitation of both Nb2O5 and Laves phase enhances the oxidation resistance.
► Nb enhances Ti oxidation but suppresses Si oxidation at the scale/alloy interface.
► Co-addition of Nb and Ti reduces the Cr evaporation rate.
► Nb can replace La in FSS SOFC interconnect and remove excess steel making process.