The effect of oxygen partial pressure and temperature on the oxidation behaviour of unstabilized and Nb-stabilized AISI 430 ferritic stainless steels were investigated over the temperature range 850 -950 C in air, Ar þ 1 vpm O 2 or Ar -H 2 -H 2 O atmospheres. Isothermal thermogravimetric analyses were performed for 50 h. The microstructure of the oxide films grown on the steels were characterized by SEM and chemical analyses were performed by EDS, grazing X-ray diffraction and secondary ion mass spectrometry (SIMS). Nb-stabilized AISI 430 steel is more resistant to oxidation than unstabilized AISI 430 under all tested conditions, except above 900 C in Ar-H 2 -H 2 O. While the oxidation behaviour of unstabilized AISI 430 is strongly affected by the atmosphere composition, Nbstabilized AISI 430 oxidation rates do not depend strongly on the atmosphere. For both steels, the chemical analyses show Cr 2 O 3 as the main phase in the oxide scales, but solid solutions such as FeFe 2Àx Cr x O 4 and MnCr 2 O 4 formed in almost all scales for atmospheres of Ar þ 1 vpm O 2 or in Ar -H 2 -H 2 O, and Fe 2 O 3 and Mn 1.5 Cr 1.5 O 4 , are also observed for oxidation in air.

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