The mechanism of oxide film formation on austenitic stainless steels in high temperature water

The oxide layers grown on an austenitic stainless alloy in high temperature water and the repassivation kinetics of the material have been studied with respect to the film formation mechanism. Examination of the morphology and composition of oxide layers reveals a double-layer structure. The inner Layer consists of a chromium-rich spinel and is covered by an outer layer of magnetite or iron-nickel spinel. An investigation of film growth after abrading samples with a corundum grinder at temperatures up to 250 degrees C shows that passivity is fully restored within several seconds. No significant influence of temperature can be detected. The mechanism of oxide growth is explained by applying the concepts of passivity of metals to the corrosion system studied and the major reaction sequences are outlined. Breakdown of passivity in high temperature water is related to crystallisation of the initially amorphous, thermodynamically metastable, passive film. The film grows and slowly transforms into a thick oxide layer. Consequences for other models of growth are discussed. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.