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

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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.

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Authors and Affiliations

  1. the Research and Development Department, VOEST-ALPINE STAHL LINZ GmbH, A-4031, Linz, Austria

    J. Strutzenberger (Postgraduate Student) & J. Faderl (Senior Engineer, Metallic Coatings)

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  1. J. Strutzenberger
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  2. J. Faderl
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Strutzenberger, J., Faderl, J. Solidification and spangle formation of hot-dip-galvanized zinc coatings. Metall Mater Trans A 29, 631–646 (1998). https://doi.org/10.1007/s11661-998-0144-8

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  • DOI: https://doi.org/10.1007/s11661-998-0144-8

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