Influence of silicon and aluminum additions on the oxidation resistance of a lean-chromium stainless steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe-16Cr-16Ni-2Mn-1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800degreesC for a duration of 1000 hr. For comparison, conventional 18Cr-8Ni type-304 stainless steel specimens were also tested. The results showed that at 700degreesC, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800degreesC, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800degreesC, showed zero weight gain in subsequent testing for 1000 hr at 700degreesC. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800degreesC were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide-base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.