Oxidation of 304L Stainless Steel by Steam and by Air^* Available to Purchase

The oxidation of 304L stainless steel has been measured in steam and in air in the temperature range from 1000 to 1375 C (1832 to 2507 F) using a thermogravimetric technique. This temperature range is of interest in the analysis of a reactor loss-of-coolant accident. Experiments in steam showed initial linear weight gain with time for the first 6 to 28 minutes, followed by parabolic rate behavior. The linear reaction rate constant W/t was found to be 1.1 × 10⁵ exp (-44,350/RT) while the parabolic constant W²/t was found to be 2.4 × 10¹² exp (-84,300/RT), where T is in °K and W is in mg/cm² and t is in sec. The parabolic rate controlling process is diffusion through an Fe-Ni-Cr spinel layer observed in the inner layer of the scale. The oxidation of 304L stainless steel by air from 1100 to 1360 C (2012 to 2480 F) was slow compared to the oxidation by steam in this same temperature range, parabolic rates being lower than those in steam by a factor of ~ 10³. Although X-ray diffraction observed Fe₃O₄ (major) and Fe₂O₃ (minor) on the surface of the coatings formed in air at temperatures of 1360 C and below, Cr₂O₃ was found at the metal-oxide interface. The low rates in air thus reflect the formation of a protective Cr₂O₃ film. The rate in air at 1375 C is exceptionally rapid compared to those at the lower temperatures possibly because of the Fe-FeO liquidus at 1370 C (2498 F).