The Protective Properties of Thin Alumina Films Deposited by Metal Organic Chemical Vapour Deposition against High-temperature Corrosion of Stainless Steels*

Coatings of Al20 3 were deposited on Incoloy 800H and A I S I 304 by means of metal organic chemical vapour deposition. Diffusion limitation was the rate-determining step above 420 °C. Below this temperature, the activation energy of the reaction appeared to be 3 0 k J m o l t . Coating with Ale0 3 increases the sulphidation resistance by at least 4 -10 times. The sulphidation resistance is influenced by the growth rate of the coating and by the thermomechanical properties of the coatingsubstrate combination. The sulphidation resistance also depends on the growth rate of the coating. The weight gain of coated specimens is about 10-14 times less than that of uncoated specimens. Treatment of the coated samples in air at 850 °C results in a still higher sulphidation resistance. This is due to the chromium oxide grown into the cracks in the coating during the treatment. These cracks are the result of mechanical stresses at irregularities.