The Zone of Metal Phase Consumption in Gas-Metal Reactions

Abstract

It is shown by an analysis of the scale growth process that the scale of even approximately uniform composition formed in the reaction between gas and metal must consist of at least two layers. One layer is formed by the outward migration of metal ions and electrons through the scale; the other forms under the scale in the zone of consumption of the metal phase. Both layers form under completely different conditions. The inner layer is vital for the maintenance of metal transport to the bulk of the scale and grows at a gas pressure of the order of the dissociation pressure of the scale substance. It is demonstrated that the inner layer will develop cavities. A third layer may be formed in some secondary process, for example, that of recrystallization. It is therefore evident that the concept of a uniform process of scale growth is an oversimplification and fundamentally incorrect. This conclusion is supported by an investigation on copper iodide scale. The protective film formation on aluminum and the accelerated growth of oxide films on metals at the beginning‐of oxidation are explained on the basis of the considerations presented above.