Spangle formation in galvanized sheet steel coatings

Very large grains, termed “spangles,” are produced on galvanized sheet steel coatings when lead is added to the zinc bath. The spangles have been attributed to melt undercooling prior to solidification. The present results indicate this is not the case, undercooling being less than 1 °C. The spangle diameter is shown to be dependent on the alloy addition to the bath, large spangles being obtained with Bi and Sb as well as Pb. The spangle size is related to the surface tension of the alloying addition, the size decreasing as the melt vapor surface tension of the alloying element increases. It is proposed that spangles form dendritically from a nucleus in the melt. Alloy additions with low interfacial energies and very limited solid solubility are highly concentrated ahead of the dendrite tip. This decreases the tip radius and increases the dendrite velocity, producing large grains. The basal plane orientation of the samples varies between 17 and 80 deg with respect to the steel sheet surface, which is inconsistent with basal plane dendritic growth in Zn along (1010) directions. It is proposed that solute additions to the melt and growth in a thin liquid layer can modify the dendrite growth direction, accounting for the spangle orientation. On leave from Obafemi Awolowo University, lie Ife, Oyo State, Nigeria