Microstructures, tensile properties and corrosion behaviors of Fe20Cr25NiNb stainless steels with different Al contents in supercritical carbon dioxide

The microstructures, tensile properties and corrosion behaviors of the Fe20Cr25NiNb stainless steels with different Al contents were investigated in the supercritical carbon dioxide. Results show that the Fe20Cr25NiNb stainless steels transform from single-phase austenitic structure to duplex austenitic–ferritic structure and then to ferritic structure with Al increasing from 0 to 8 wt.%. The strengths increase while the elongation deteriorates with Al increasing. The addition of Al can significantly improve the corrosion resistance. The steel without Al exhibits a relatively high weight gain as a result of the formation of thick chromia scale and larger magnetite nodules. Three steels with Al have ultra-low and similar weight gains. The oxide scale formed on the steels with Al is very thin and its components are the mixtures of chromia and alumina. More alumina flakes are generated on the oxide scale and more σ phases are formed in the matrix of the steels with higher Al content.