HOW DOES SILICON LEAD THE KINETICS OF THE GALVANIZING REACTION TO LOSE ITS SOLID-SOLID CHARACTER

The galvanizing reaction is very sensitive to alloying elements of ste el. Therefore when industrial processes are considered the method is t rying to counterbalance the influence of alloying elements in the stee l by adding Al, Ni, Mn in the zinc bath. Although Si is often consider ed by steelmakers as necessary for killing and improving the mechanica l properties of steel, it is usually undesirable by galvanizers becaus e the reactivity of steel with zinc dramatically changes with % Si. Th is phenomenon is best illustrated by the thickness of the coating vers us % Si which presents a sharp maximum near 0.08% Si and a minimum nea r 0.20% Si. This phenomenon discovered by Sandelin (1941) has not yet received any uncontroversial interpretation. In the present studies, w e compared the morphologies and kinetics obtained in the same conditio ns for different silicon contents and we specially considered the coat ing obtained for small times of immersion. It is shown that in all cas es, the first intermetallic phase which appears is zeta. The experimen tal analysis of the compounds as well as thermodynamical calculations prove that the solubility of Si in zeta is negligible but of about 1 a t.% in delta(1). Moreover, thermodynamical evaluation of Gibbs free en ergy of Fe-Zn liquid shows that, if zinc diffusion in substrate is neg ligible, alpha-Fe is in a metastable equilibrium at 450 degrees C with the liquid containing about 7% Fe, which corresponds to the zeta comp ound. Our modelling shows that for Sandelin steels, the first 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 root t law and corresponds to the solid state diffusion observed for hypo-Sandelin steel. The diffus ion paths across liquid domains introduce supplementary degrees of fre edom in the morphologies of the coating which then corresponds to nume rous equilibria and pseudo-equilibria.