Breakaway oxidation of austenitic stainless steels induced by alloyed sulphur

• Austenitic stainless steel with high S amount presents breakaway oxidation behaviour.
• MnS precipitates formed during the steel process are partially dissociated during oxidation.
• Mn is incorporate in the oxide scale, S is trapped in Mn,Cr-rich oxysulphides.
• Cr uptake in sulphide disrupts the bulk chemistry which promotes breakaway oxidation.

This work focuses on the effect of alloyed sulphur as MnS inclusions in austenitic stainless steels. AISI 304L and AISI 303 were oxidized at 1000 °C in synthetic air. The high sulphur grade, AISI 303, presents a breakaway oxidation with formation of nodules with an inner part composed of alternated layers of Fe-rich and Cr-rich oxides. During oxidation, MnS inclusions near the metal/oxide interface are partially dissociated. Manganese is incorporated in the oxide. The sulphur is trapped in Cr,Mn-oxysulphide aggregates which formation leads to a local decrease of the Cr concentration and consequently to the formation of non-protective Fe-rich oxide.

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