The reduction behaviour of single crystals of wüstites containing Ca++, Li+, Na+, K+, Be++, Mg++, Sr++, Ba++, Al+3 and Si+4 has been examined in controlled CO-CO2 gas mixtures at 800°C, 960°C and 1200°C. The catastrophic swelling of wüstites on gaseous reduction coincides with the formation of an unusually open iron structure in which the iron appears as whiskers or needles as previously reported. However, the present investigations have also demonstrated that the whiskers do not necessarily constitute the only product form of the iron and under the certain conditions there is a transition from needle growth to porous growth of iron.
Metallographic examination has revealed that the exact shape of the reaction interface between wüstite and iron phases can vary considerably and is dependent on the rates of chemical reactions, rates of iron ion diffusion ( surface diffusion and bulk diffusion), the oxygen potential in the reducing gas and composition of wüstite. A number of previously unreported product morphologies have been observed which are shown to have significant effects on the macrostructural changes on reduction. A general theory of the decomposition of inorganic solids, consistent with the present observations, is proposed.