How does silicon lead the kinetics of the galvanizing reaction to lose its solid-solid character

Abstract
The galvanizing reaction is very sensitive to alloying elements of steel. Therefore when industrial processes are considered the method is trying to counterbalance the influence of alloying elements in the steel by adding Al, Ni, Mn in the zinc bath. Although Si is often considered by steelmakers as necessary for killing and improving the mechanical properties of steel, it is usually undesirable by galvanizers because the reactivity of steel with zinc dramatically changes with % Si. This phenomenon is best illustrated by the thickness of the coating versus % Si which presents a sharp maximum near 0.08% Si and a minimum near 0.20% Si. This phenomenon discovered by Sandelin (1941) has not yet received any uncontroversial interpretation. In the present studies, we compared the morphologies and kinetics obtained in the same conditions for different silicon contents and we specially considered the coating obtained for small times of immersion. It is shown that in all cases, the first intermetallic phase which appears is [MATH]. The experimental analysis of the compounds as well as thermodynamical calculations prove that the solubility of Si in [MATH] is negligible but of about 1 at.% in [MATH]. Moreover, thermodynamical evaluation of Gibbs free energy of Fe-Zn liquid shows that, if zinc diffusion in substrate is negligible, [MATH] is in a metastable equilibrium at 450°C with the liquid containing about 7% Fe, which corresponds to the [MATH] compound. Our modelling shows that for Sandelin steels, the fist stage of the reaction is not driven by diffusion of Fe in solid phases but in the liquid, leading to a linear variation of the reaction with time t. In a second stage the kinetics obeys a [MATH] law and corresponds to the solid state diffusion observed for hypo-Sandelin steel. The diffusion paths across liquid domains introduce supplementary degrees of freedom in the morphologies of the coating which then corresponds to numerous equilibria and pseudo-equilibria.