Various explanations have been suggested for the effect of water vapor at low oxygen activity. At low oxygen activities, evaporation of chromium (VI) oxide hydroxide is negligible and the accelerated oxidation of stainless steels in H 2 + H 2O environment must have other causes. Addition of small amounts of sulfur dioxide has been shown to prolong the incubation time for breakaway oxidation in the presence of KCl and to slow down scale growth after breakaway. Thus, the breakdown of the initial protective oxide in the presence of water vapor/alkali was explained by the formation of chromium(VI) oxide hydroxide/alkali chromate in a reaction with the protective scale temperatures. It has been suggested that, at high oxygen activity, water vapour and alkali cause breakaway oxidation of stainless steels in a similar way. O 2 + H 2O + KCl - (high oxygen activity) The four corrosive environments selected were: Breakaway oxidation is induced in alloy 304L by exposure to different corrosive environments at the same temperature and then the microstructural evolution in the different environments is studied in detail. A comparative investigation of the microstructural evolution post-breakaway oxidation is therefore performed. The present study is designed to study that underlying mechanism using different corrosive factors to trigger breakaway oxidation in Fe, Cr, (Ni) alloys. However, the similarities in the scale structures observed after breakaway hints that there is an underlying mechanism which is generally applicable and that does not require a specific combination of corrosive factors. Different authors have attributed this characteristic breakaway behavior to a variety of causes, which are more or less specific for the type of environments studied. A similar behavior has been reported for commercial alloys in different corrosive environments and temperatures, see e.g. The microstructure of the inward growing spinel scale has been shown to be complex, including both fully oxidized regions and regions of internal oxidation as reported for Fe–Cr model alloys at 600 to 900 ☌. Breakaway oxidation of stainless steels in various environments and at different temperatures have been reported to result in a similar microstructure consisting of an outward growing Fe-rich oxide and an inward growing Fe, Cr, (Ni) spinel oxide, see e.g. The rate of further oxidation then relies on the properties of the scale formed after breakaway. In demanding high temperature applications the ((Cr xFe 1−x) 2O 3) scale may lose its protective properties resulting in breakaway oxidation. A great number of publications concerning the oxidation properties of steels and stainless steels are summarized by Kofstad, Birks and Meier and Young. The corundum-type Cr rich ((Cr xFe 1−x) 2O 3) scale is known to provide protection against oxidation and corrosion. Depending on the corrosive environment and the alloy microstructure, different concentrations of Cr in the alloy are necessary for corrosion protection. In order to form such a protective scale the supply of chromium to the scale has to be sufficient. Stainless steels are designed to form a dense slow-growing corundum-type Cr-rich ((Cr xFe 1−x) 2O 3) scale in high temperature applications.
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