Oxidation and corrosion behaviour of Fe–Cr and Fe–Cr–Al alloys with minor alloying additions

The oxidation and corrosion properties of alloys based on Fe–40Cr and Fe–35Cr–5Al were studied using gravimetric and potentiodynamic techniques. The properties were modified by microalloying with ruthenium and rare-earth metals (REM). A high resistance to breakaway oxidation was characteristic of all the high chromium alloys assessed. Good oxidation resistance in the Fe–35Cr–5Al alloys was found to be contingent on (i) the rapid establishment of a stable protective aluminium oxide layer in the early stages, and (ii) the development of a secondary chromium oxide layer for long-term stability. Microalloying with 0.2 wt.% Ru promoted the formation of a chromium-rich layer at the substrate interface. The consequence of this differed in each case. In the Fe–35Cr–5Al alloy, the result was improved oxidation resistance, accompanied by segregation of the Ru to the aluminium oxide layer. In the Fe–40Cr alloy, the Ru addition was associated with an initially higher oxidation rate and an increased tendency to spalling. REMs, added as 0.05 wt.% mischmetal to the Fe–35Cr–5Al alloy, also lowered the oxidation rate, as expected. Relative to the Fe–40Cr composition, the substitution of 5 wt.% Al for 5 wt.% Cr had a negligible effect on the aqueous corrosion resistance in 10% H2SO4, but led to inferior pitting resistance in a 3.5% NaCl solution. As previously shown with stainless steels based on Fe–40Cr, small additions of ruthenium can also enhance the corrosion and pitting resistance of Fe–35Cr–5Al.