Metallurgy of galvanized coatings

Abstract

In this review the authors cover both the scientific and practical aspects of the metallurgy of galvanized coatings. They begin the review by considering the current iron—zinc equilibrium phase diagram; special emphasis is placed on the correlation between the actual phases found in galvanized coatings and those which could be expected from the diagram. Data are given on the composition, concentration gradient, and hardness of the layers formed in different temperature ranges in all the types of Fe—Zn Interactions (s—s, s—l, s—v). This is followed by a section on the kinetics of interaction and the correlation between the nature of the galvanized layer and the type of kinetics, including the effect of stresses on the surface reactions. The most up-to-date theories based on thermodynamics, regarding the inconsistencies between the equilibrium phase diagram and the phases actually formed, are reviewed. The energies of activation and Arrhenius constants obtained in different researches are presented. A critical appraisal of the effect of alloying additions in steel (C, Si, P, S, Ti, Nb, V, 0₂ and N₂) and additions to the zinc bath (Fe, AI, Pb, Cu, Cd, Mn, Mg, Cr, Ni, Ti, V, and Zr) on the interaction of Fe(s)—Zn(l) is also included. This is followed by a review of the effects of surface preparation, the shape of an article, and of stresses on galvanizing. Galvannealing is briefly discussed. Finally, an attempt has been made to identify areas which. appear to have an established framework and those on which further work is required. Since the amount of work reported is so large only the more important references have been included.