J. Foct et al., HOW DOES SILICON LEAD THE KINETICS OF THE GALVANIZING REACTION TO LOSE ITS SOLID-SOLID CHARACTER, Journal de physique. IV, 3(C7), 1993, pp. 961-966
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.