The oxidation of dilute iron-silicon alloys ([Si] ⩽ 1 wo) in carbon dioxide

The manner in which silicon, present as a minor alloy constituent, modifies the oxidation of iron in CO₂/1 %CO at 500°C has been studied. Increasing amounts of silicon progressively reduce the oxidation rate within the range $\text{Si}\text{] = 0–1}\text{w}\text{o}$ and a variety of physical techniques have been used to examine oxidized specimens in pursuit of the origins of this beneficial influence. The scale forming on the alloys is composed of two layers in each of which iron is present as Fe₃O₄. The inner layer of scale contains silicon at approximately the same level (on a vol. % basis) as the original metal while the outer scale appears to contain no silicon. The boundary between the two layers is also marked by an abrupt change in grain size and in texture of the Fe₃O₄. At the boundary between the alloy and the scale there develops a thin layer of non-ferrous material in which the concentration of silicon is increased by more than an order of magnitude above that in the bulk alloy. This layer also includes a substantial accumulation of carbon which is thought to derive from carbon oxide gases which have penetrated to the base of the scale before taking part in the oxidation reaction. The observation of the layer of non-ferrous material between the alloy and the scale constitutes a qualitative agreement with theoretical studies which indicate that the reduction in oxidation rate conferred by the presence of silicon in the alloy is at least partly due to the impeded passage of Fe³⁺ ions from the alloy into the scale.