Alloy degradation in a co-firing biomass CFB vortex finder application at 880 °C

•For alloys 310S and 800H/HT; N penetrated the surface scales and formed internal Cr nitrides.•For alloys 310S and 800H/HT; the nitride formation lowered the Cr activity and inhibited the internal Cr permeation.•For alloys 310S and 800H/HT; Na2SO4 and NaCl reacted with Cr2O3 to form chromate and elemental S and Cl.•For alloys 310S and 800H/HT; Cl attacked the alloy interiors in a developed stage of corrosion.•For alloy 600; no internal nitrides or major sodium chromate formation were found.