Corrosion of austenitic Fe-Ni based alloys with various chromium and aluminum additions in a carburizing-oxidizing atmosphere at 800ᵒC

The corrosion behaviors of Fe-19Ni-13/21Cr-xAl (x = 0, 2, 6 at. %) alloys in a carburizing-oxidizing atmosphere were compared with those in a purely carburizing atmosphere at 800oC. For alloys with 13 at. % Cr, 2 at. % addition of Al did not improve the corrosion resistance effectively but induced a slightly increase of the total mass gain. 6 at. % addition of Al produced a large decrease of the total mass gain, therefore the corrosion resistance was improved significantly. For alloys with 21 at. % Cr, additions of Al did not affect the total mass gain obviously. Fe-19Ni-21Cr-xAl (x = 0, 2, 6 at. %) showed similar mass gain. Increase of Cr content from 13 at. % to 21 at. % is effective for protecting the alloys from the carbon attack for Al-free alloys and alloys with 2 at. % Al. However, addition of Cr is not so helpful for alloys with 6 at. % Al. The addition of oxygen improved the corrosion resistance of all alloys significantly except the Fe-19Ni-13Cr-6Al. Pure external chromia scales on alloys without Al and with 2 at. % Al could not suppress the inward diffusion of the carbon atoms. Aluminum and chromium worked together to form mixed oxide scales inhibiting the carbon attack totally on alloys with 6 at.% Al.