The Effect of Tungsten and Molybdenum on Ferritic Stainless Steels for SOFC Interconnects

GDS results suggested that Cr content was decreased and spinel layer became thicker in Fe-22Cr-0.5Mn-4W. The chi phase particle seems to alter the diffusion process of metal cations, but the second phase neither blocks the fast diffusion path nor captures silicon from matrix since Fe-22Cr-0.5Mn-4W FSS with low Si content has large grain size. Formation of thicker Cr-Mn spinel layer by increased Mn content in the outermost oxide scale and thinner Cr₂O₃ oxide layer of Fe-22Cr-0.5Mn-4W and Fe-22Cr-0.5Mn-6W improved the electric conductivity and reduced Cr evaporation rate. However, formation of void by chi phase can have detrimental effect on the oxidation such as spallation of oxide scale. The addition of 0.1 and 2 wt% Mo to Fe-22Cr-0.5Mn improved the oxidation resistance. TEM observation in Pt marked specimens showed that the inward oxygen diffusion was suppressed by Mo addition. In the 4 wt% Mo alloyed steel, rapid growing Fe-rich spinel was formed after 300 h due to the damage in protective chromia layer caused by the evaporation of volatile Mo species. Alloying Mo up to 4 wt% was also effective in enhancing the electric conductivity