High-temperature oxidation and thermal cycling of aluminum-electroplated stainless steels

An alumina coating, produced from the oxidation of an aluminum-electroplated deposit, improved the oxidation resistance in air of a ferritic, AISI-type 446 stainless steel, Fe-24Cr-1.2Al containing 0.15% of mischmetal, and an austenitic AISI 321 stainless steel containing 0.53% Ti, at least up to 1100°C. In thermal-cycling tests from 1000°C to room temperature, the alumina coating was adherent on the ferritic and austenitic steels, for at least 1000 and about 700 cycles, respectively. The addition of rare earths to the ferritic steels and titanium to the austenitic, provided good adhesion between the coating and substrate. The porous nature of the coating was found to be very beneficial by causing the coating to be more resistant to thermal and growth stresses. Oxidation mechanisms are discussed in the light of results obtained from the thermogravimetric tests and metallographic observations by SEM-ED analysis.