| Abstract: | ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the chemical composition of passive films formed on chromium steels with 4–30% Cr at 25 and 70°C in oxygenated water for different times. The composition-depth profile of the films has been investigated by successive stripping of the film by ion etching.The composition of the film varies with exposure time: short time yields an Fe---Cr ratio corresponding to that of the alloy; prolonged exposure produces increasing enrichment of chromium through the whole layer, with the highest concentration at the outer surface. The passive film generally seems to consist of two layers: outermost one of (Cr(OH)3) and underneath one of iron-chromium oxide (Fe1+xCr2−xO4. The low alloy steels (3·9 and 7·8%Cr) were passive for a limited time during which the Cr enrichment was high (8 times); after breakdown of the chromium-rich passive layer, it was replaced by FeOOH.When a dry-formed oxide film, which consists mainly of iron oxide, is exposed to water, its composition slowly changes until that of a water-formed film is reached. |
