The mechanism of whisker growth in the reduction of wüstite

Abstract

The growth of iron whiskers on wustite is explained on the basis of C. Wagner’s mechanism for the reduction of nonstoichiometric oxides. The first metal nucleus develops into a whisker if the iron accumulation in the supersaturated wustite particle is nearly uniform, a condition which is favored by chemical control. A mathematical model is proposed in which the “filling ratio” of wustite by iron prior to nucleation is expressed in terms of the reaction constant, the chemical diffusion coefficient of iron, the particle radius, the Fe/o ratio at equilibrium with the gas, and the critical Fe/o ratio for nucleation at the most favorable site. The morphology of iron in the early stages of reduction, which goes from a single cylindrical whisker, through sponge iron to a smooth layer, can be predicted by comparing at all times the map of the actual Fe/o ratio at the surface with the map of the local Fe/O ratio for nucleation. A satisfactory test of the theory is obtained through a survey of the experimental evidence available about the effect of several physico-chemical factors of reduction on the iron morphology,e.g. surface characteristics, particle size, composition of the reducing gas, solute cations in wustite, temperature, and gas transport.