Control of Silicon Reactivity in General Galvanizing

Abstract

In general galvanizing Si dissolved from steels destabilizes the C and C 1 phases in the alloy layer of the coating. It also alters the morphology of f crystallites in the alloy layer thereby exposing the d layer to the melt. The incompatibility of the liquid phase and the d phase cause the latter to disintegrate. These changes result in an excessive steel reactivity in general galvanizing. This reactivity can be easily controlled by alloying the Zn bath with metals, such as Ni, if the Si content of the steel does not exceed 0.20%. To combat a fully developed reactivity problem, one has to stabilize the C and d sublayers in the coating. Alloying the bath with suitable metals can generate new equilibrium states to allow the co-existence of the liquid phase and the d phase thereby preventing its disintegration; the C and C 1 phases can be stabilized through solid solution of the alloying elements in these phases. The introduction of Al to general galvanizing baths generates new equilibrium states thereby ushering a total new approach to Si reactivity control.

Effect of Silicon in Dual Phase Steel on the Alloy Reaction in Continuous Hot-dip Galvanizing and Galvannealing

“…In the present study, the chemical potential gradient of Fe and Zn governs the alloy reaction and can provide the basis to explain the fact that the Fe-Zn interdiffusion is markedly retarded on the steel containing 0.44 wt% Si. Su et al and 31) Tang et al 24) calculated the FeZn-Si ternary phase diagram at 450°C and experimentally verified the phase boundaries associated with the various Fe-Zn phases. It was found that the solubility of Fe in liquid Zn decreases significantly with increasing the Si concentration in the liquid, as shown in Fig.…”

“…7,10,12,24) According to the nucleation mechanism, there are two possible routes to nucleate the ζ phase in the coating. First, the ζ phase can nucleate and grow at the area without or with lower Si contents.…”

“…In addition to the external selective oxidation that have been proposed as a retardant factor on the formation of a compete Fe-Zn IMCs layer, 17,22,23) the built-up Si at the steel/molten Zn interface has been shown to perturb the thermodynamic equilibrium of the Fe-Zn IMCs and influence the formation and thickness uniformity of the Zn coating in batch type galvanized steel. 24,25) Figure 2 shows the cross-sectional optical micrographs of color chemically etched GI and GA specimens. Some FeZn IMCs presented at the interface and no bare spots were observed on all the GI specimens, implying that external oxidation of Si and Mn hardly influences the wettability and galvanizability.…”

Effect of the silicon content in steel on the hot-dip zinc coating microstructure formation

“…When the silicon content of about 1% (high-silicon steel) the FeSi phase amount becomes significantly greater ( Figure 6) [9]. а -95% Zn, b -98% Zn [9] Foreign studies have also confirmed the existence of an equilibrium state between the liquid, the ζ-phase, and FeSi-phase, which prevents the equilibrium between the liquid phase and the δ-phase [10,11]. This phenomenon destroys the integrity of the coating layers; liquid zinc has the direct contact with the steel base, which accelerates the interdiffusion of iron and zinc.…”

Experimental Investigation of the Zn-V-Sb System at 450 °C

“…In the galvanizing industry, the phenomenon is commonly referred to as the Sandelin effect or Si reactivity. [1,2] Adding alloy elements into zinc bath are a reliable effective method to counter the phenomenon. In the primitive galvanizing technology, lead (Pb) was frequently added to the zinc bath to improve the fluidity of molten zinc and the development of spangles at the coating surface.…”