OXIDATION AND CORROSION BEHAVIOR 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 techniq ues. 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 oxidatio n resistance in the Fe-35Cr-5Al alloys was found to be contingent on ( i) the rapid establishment of a stable protective aluminium oxide laye r in the early stages, and (ii) the development of a secondary chromiu m oxide layer for long-term stability. Microalloying with 0.2 wt.% Ru promoted the formation of a chromium-rich layer at the substrate inter face. The consequence of this differed in each case. In the Fe-35Cr-5A l alloy, the result was improved oxidation resistance, accompanied by segregation of the Ru to the aluminium oxide layer. In the Fe-40Cr all oy, the Ru addition was associated with an initially higher oxidation rate and an increased tendency to spalling. REMs, added as 0.05 wt.% m ischmetal to the Fe-35Cr-5Al alloy, also lowered the oxidation rate, a s expected. Relative to the Fe-40Cr composition, the substitution of 5 wt.% Al for 5 wt.% Cr had a negligible effect on the aqueous corrosio n resistance in 10% H2SO4, but led to inferior pitting resistance in a 3.5% NaCl solution. As previously shown with stainless steels based o n Fe-40Cr, small additions of ruthenium can also enhance the corrosion and pitting resistance of Fe-35Cr-5Al. (C) 1998 Elsevier Science S.A.