A comparison of the oxidation behaviours of Al2O3 formers and Cr2O3 formers at 700 °C – Oxide solid solutions acting as a template for nucleation

The oxidation behaviour of a number of ferritic iron based commercial steels and model alloys containing 6 and 9 wt% Cr and 0–2.5 wt% Al have been studied at 700 °C. The oxidation time ranged from 5 min to 500 h and the atmosphere consisted of flowing dry synthetic air. The oxide layers formed were analysed by SEM, GI-XRD and ToF-SIMS. The material without Al formed a (Cr,Fe)2O3 film with an Fe enrichment in the outer part of the layer. The Al containing alloys showed more complex oxidation behaviour. The oxidation started initially by formation of (Cr,Fe)2O3 with an Cr enriched inner part. With time Al was oxidized and dissolved in the inner Cr rich part of the oxide. This process continued until it eventually was transformed into α-Al2O3 with minute amount of Fe in the outer and Cr in the inner part of the oxide. The thickness of all oxide films ranged from 20 to 400 nm apart from the material that contained 9% Cr and no Al, which experienced breakaway oxidation after 500 h at 700 °C. This means that materials alloyed with small amounts of Al must also be considered to be protective at 700 °C, as the thicknesses of the Al2O3 oxides was comparable with the ones not containing Al, and as they do not experience breakaway corrosion.