Solidification and spangle formation of hot-dip-galvanized zinc coatings

Abstract

Solidification of hot dip coatings was studied regarding thermal conditions. Optical phenomena occurring at the liquid zinc surface were documented and the solid zinc surface was characterized in respect to optical and microscopic appearance, distribution of Pb and Al, crystal orientation, and topography. Resulting from these observations, a solidification model can be derived: zinc nucleation occurs at the steel/zinc interface. Due to thermal conditions in the slightly undercooled liquid zinc film, solidification occurs by rapid sideways dendritic expansion of the nucleated grains along the steel/zinc interface.Dendritic growth is controlled by interaction of crystal orientation of the nucleated zinc grain and thermal conditions in the undercooled layer. This leads to formation of different shaped grains with thicker and thinner sectors. The mechanism of sideways expansion continues until the entire interface is covered with dendritic zinc grains. Even though the zinc outer surface is still a liquid phase, final spangle size, as well as surface appearance and shape of the grains, is already determined at that early stage of solidification. Further growth only leads to a thickening of the solid layer; however, its relief remains almost unchanged. Thickening occurs relatively slowly due to the fact that marginal heat flow toward the surface now represents the limiting factor.Growth of the solid zinc layer results in continuous enrichment of Pb and Al in the residual liquid. Then, outer surface solidification starts as segments of single grains emerge. Distribution of the enriched residual melt in between the already solid areas depends on the relief of the solid layer. Finally, eutectic Zn-Pb reaction with precipitation of Pb particles takes place, which defines the dull appearance of these regions. Solidification for ''lead-free'' coatings is essentially the same, except that the final eutectic Zn-Pb reaction is missing. Additional investigations of dendritic secondary arm spacing indicate that Pb does not act by suppressing zinc nucleation. Pronounced dendritic growth is proposed to be favored by a change in interfacial energy. The new solidification model is applicable for a wide range of processing conditions and explains the origin of the typical spangle structure.

Influence of Hot-Dip Galvanised Coating Morphology on the Adhesion of Organic Coatings Depending on the Zinc Bath Pb Content and the Postgalvanising Cooling Method

“…The method and rate of cooling are also of a great importance for the formation of dendrites, as during the process the external layer, formed in the diffusion process of the iron-zinc system, solidifies [19,21,27,30].…”

“…When the basal plane of the zinc nucleus is parallel to the plane of the substrate, all its six directions <1100> are parallel to the surface, and a flat hexagonal dendrite is formed with a 60 0 angle between the branches. The zinc nuclei are usually oriented is such a way that the basal plane angle is different than 0 0 and 90 0 [29][30][31].…”

Effect of Aluminium and Magnesium on the Corrosion Resistance of Zinc Coatings

“…Therefore, standard aluminium content in zinc-coating baths does not exceed 0.2%. However, tests prove that the obtained intermetallic Fe-Al phases are unstable and decompose during further zinc-coating [1,7,9,18,19], which means that the effect of restraining the coating thickness by aluminium is of the greatest importance for continuous zinc-coating. On steels with a high content of silicon, deliberate increasing aluminium content in the bath is proposed to limit the coating growth [9,13,15].…”

Growth Law For Peritectic Phases Formation In The Zinc Coating

“…The first layer, directly contacting the substrate, is an alloyed layer usually composed of a few sub-layers of the intermetallic phases/compounds. The external layer, solidifying on the surface of the alloyed layer, is formed when the galvanized product is emerged from the bath, [10][11][12][13][14][15].…”