Contemporary use of Ni and Bi in hot-dip galvanizing

Five series of commercially hot-rolled mild steels, with different silicon and phosphorus content, were tested in four industrial hot-dip galvanizing plants with different bath compositions: two innovative alloys containing low percentages of Ni and Bi (Ni ≈ 0.04% and Bi ≈ 0.1%), one alloy containing Ni (≈ 0.05%) and a traditional Zn-Pb (Pb ≈ 1.1%) alloy. The coatings obtained were studied by chemical analysis, thickness measurements and by metallographic technique observation. The Zn-Ni-Bi alloy gave very interesting results with all the steels tested. Indeed, the steel reactivity and the coating thickness in the two innovative baths were always lower than those obtained by the traditional Zn-Pb bath and similar to those obtained with the bath containing Ni (Technigalva). These results have been attributed to a synergistic effect of Ni and Bi on the reactivity control of the reactive steels; in fact, even a low Ni content gives the same or even better results than the classical Zn-Ni bath, except for high phosphorus steels. Furthermore, Bi is not detrimental to human health and the environment and can be considered a very good alternative to Pb to improve bath fluidity.