Growth of oxide scales upon isothermal oxidation of CVD-FBR aluminide coated stainless steel

The performance 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 critical, 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 alloys have been shown to be oxidation resistant at high temperatures. This relies 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. In this work, aluminide coatings were deposited by means of the CVD-FBR technique on AISI 304 substrates, at 525°C for 1.5 h. A subsequent heat treatment up to 900°C was applied to the coated specimens to enhance interdiffusion of the species, which led to substoichiometric NiAl phases. Uncoated as well as coated plus heat-treated specimens have been oxidised at 950°C, up to 200 h, under continuous and discontinuous isothermal conditions. The results indicate that aluminide coatings provide a much higher beneficial effect under continuous oxidation than in discontinuous tests, the latter undergoing breakaway oxidation. Growth morphologies and compositions will show an iron enrichment during the latter oxidation stages, which will be responsible for oxidation resistance failure in the discontinuous tests.