The resistance of different alloys exposed at high temperature environments depends upon their mechanical resistance as well as their corrosion/oxidation properties. When the mechanical requirements are not in compromise, austenitic stainless steels may play a role in substituting the more expensive Ni and Co base alloys. However, at temperatures close to 950°C, the chromia scale usually grown to protect the alloy may be further oxidised into CrO₃, which is a volatile oxide and thus, the naked material may undergo a catastrophic oxidation. Fe-Cr-Al (Kanthal-like) alloys have been shown to be oxidation resistant at high temperatures (up to about 1000°C). These alloys rely on the formation of alumina scales to protect the alloy, having a chromium reservoir so as to reduce the aluminium amount needed to maintain the oxide scale.

Typically, aluminide coatings have been produced on different substrates by the pack cementation method normally leading to an improvement of the oxidation resistance of the coated alloys. In this case, we are trying to develop aluminium diffusion coatings at shorter times and lower temperatures than even in the high activity/low temperature pack cementation processes. We have recently presented how to grow these coatings on an AISI 304 stainless steel and it is the purpose of this work to keep on the same track by studying the properties of the oxide layers grown on the same substrate steel. Thus, aluminium diffusion coatings obtained by Chemical Vapour Deposition in Fluidised Bed Reactors (CVD-FBR) at 525°C for 1.5 h on Fe-18wt%Cr-8wt%Ni austenitic stainless steel plates have been exposed to isothermal experiments at 950°°C for up to 200 h at both discontinous and continuous intervals under atmospheric pressure. Muffle furnaces and thermogravimetry apparatus were used to oxidise both kinds of specimens whereas XRD and SEM/EDX were employed to investigate the morphologies and compositions of scales grown after exposure at high temperature.

The results will show quite different kinetics between the oxidised parent material and the Al-coated one. Loss of protective behaviour on the uncoated substrate will be attained, due to Fe incorporation into the oxide scale, initially grown as Cr₂O₃. However, the coated specimens will present lower kinetics, which may be divided into different steps related to the αAl₂O₃ formation, its transition into θ-Al₂O₃ and final step with a certain contribution of Cr and/or Fe from the substrate. Overall, the Al-coated alloy will be considered as a potential candidate for high temperature applications.