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The solidification of hot dipped galvanized coatings on steel Fasoyinu, Festus Awoyemi


Galvanized sheet is produced commercially by hot dipping steel sheet in a galvanizing bath to produce a thin zinc rich protective coating on the steel. This investigation is concerned with the solidification of the galvanized layer with particular reference to the growth of large "spangles" in the galvanized layer. The manner in which the galvanized layer solidifies was found to be dependent on a number of factors including melt supercooling, nucleation rate, dendritic growth, bath composition, bath impurities, and cooling rate. The present results show that the supercooling necessary to activate nucleating sites in a typical galvanizing bath is less than 1°C. The supercooling which occurs in the galvanized layer as it solidifies is also generally less than 1°C, contrary to results reported in the literature. This suggests that grains nucleate in the bulk of the galvanized layer, and not necessarily at the air or iron surfaces of the melt as has been reported. No clear evidence was obtained which shows that spangles, with large dendrite spikes, are associated with large melt supercooling. Accordingly, spangle formation cannot be attributed to higher dendritic velocities resulting from large thermal supercooling. The surface topography of the galvanized layer is a property of major consideration in industrial applications of the galvanized product. Present measurements show that the surface topography is strongly dependent on the solidification structure of the galvanized layer. Large spangles solidify dendritically with large variations in the surface topography. The spangles have shiny and frosty sectors, which are relatively smooth and rough respectively, and inclined to the steel sheet surface. Large depressions are present at the boundaries between adjacent spangles, termed "pulldown", which can markedly reduce the effective thickness of the galvanized layer, and cannot be removed by subsequent treatment of the galvanized sheet. Increasing the concentration of lead in the bath increased the pulldown. The mechanism of pulldown formation is not clear. Volume shrinkage during solidification cannot account for the large depressions observed. The hot dipped galvanized samples prepared in this investigation used galvanized sheets as starting material. Observation of the distribution of bath alloy additions in the samples, using microprobe analysis and radioactive tracers, clearly showed that the original galvanized layer was replaced by the metal from the bath. Solute segregation in a galvanized layer containing spangles is directly associated with the dendritic growth of the spangles. Solute is depleted in the dendrite spikes and concentrated between the spikes. No solute concentration was observed at the grain boundaries between adjacent spangles. Some preferential surface segregation, associated with shiny and frosty sectors of a spangle, was observed. The growth of large spangles in the galvanized layer is directly related to the galvanizing bath composition. Large spangles are obtained with alloying additions which have very limited solid solubility in zinc and relatively low liquid surface tensions. The diameter of the spangles decrease as the surface tension of the alloy addition increases. Spangle growth is associated with dendritic growth. Dendritic growth occurs as a result of constitutional supercooling at the dendrite tip due to solute segregation during growth at the solid/liquid interface. It is proposed that the presence of a thin layer of highly concentrated solute at the dendrite tip changes the curvature of the tip. The change is related to the liquid surface tension of the solute. Solutes with lower values of surface tension decrease the tip curvature which results in an increase in dendrite velocity and larger spangles. The orientation of the spangles is shown to vary appreciably. The basal plane of a spangle is observed to be inclined to the surface of the steel sheet at angles between 8 and 80 degrees. This differs from reports which indicate that the basal plane is nearly parallel to the steel sheet. The growth of a spangle is primarily associated with dendritic growth. Small (0001) platelets of solid form at heterogeneous nucleating sites in the melt from which <1010> spikes grow. Each spangle forms from a single nucleating source which is randomly oriented with respect to the surface of the steel sheet. As the dendrite spikes grow they shortly encounter the melt/air interface or the melt/steel interface. The spikes will continue to grow along the surfaces at a high velocity in a direction defined by the initial <1010> direction of the growing spikes. As the primary spike grows, secondary and tertiary spikes form, generally inclined to the melt surface and in <10l0> directions when possible. The secondary branches of a primary stalk which grow along the melt/air interface form shiny spangle segments. The secondary branches on the opposite side of the primary spike grow along the melt/steel surface and form frosty sectors. The difference is not due to an orientation difference between the spangles as reported in the literature.

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