Non-protective and protective scaling of a commercial 1¾% silicon-iron alloy in the range 800°C-1000°C

Abstract

During an investigation, by a thermobalance technique, of the oxidation of a commercial silicon-iron alloy containing 1·74% Si and 0·3% Al in the range 800–1000°C it was found that the mode of scaling was dependent on the atmosphere used. In dry air the initial scale formed is protective and inhibits further increase in weight. In CO 2 the scale formed is not protective and oxidation proceeds according to a linear rate law. On substituting CO 2 for air as the oxidizing atmosphere the protective scale breaks down and oxidation then follows a linear law, but on substituting air for CO 2 the linear rate of scaling is replaced by a much slower rate which decreases with time. If a sequence CO 2-air-CO 2 is used the appropriate change in scaling behaviour is observed at each of the two changes in atmosphere. Metallographic and X-ray investigations showed that in air the initial scale formed consists of hematite and silica and that in CO 2 the oxidation products are magnetite, wüstite and fayalite. The apparent reversibility of the mode of scaling is explained in terms of the breakdown of the protective scale at a number of isolated points. This breakdown, which occurs readily in CO 2 but not in air, is discussed in terms of the thermodynamic instability of the higher oxides of iron in CO 2 and of the phase-boundary reaction at the scale/gas interface.