Metallic foam formation during continuous hot-dip galvanizing of steel sheet

This study examined skimmings formation in the continuous hot-dip galvanizing process. Skimmings are metallic foam that is produced near the strip exit region of the galvanizing bath due to the high degree of mixing and oxidative conditions in this area. Industrial operating data from ArcelorMittal Cleveland’s HDGL was examined and it was found that the production of skimmings per coil surface area increased with wiping pressure and line speed but more so with the former. Moreover, galvanizing (GI) baths were found to produce more skimmings per unit time than galvannealing bath (GA) and air wiping produced significantly more skimmings than nitrogen wiping. Dispersive X-ray mapping was found to be a reliable technique for identifying the elemental distribution and morphology of skimmings samples from the galvanizing lines of ArcelorMittal Cleveland and U.S. Steel Hamilton. Two different bench scale experiments were conducted as well. The first attempted to simulate skimmings formation in a galvanizing bath using a shrouded crucible loaded with liquid zinc, a steel impeller for agitation and an impinging jet of gas. The results showed that the rate of skimmings generation increased with impeller rpm. Also, GI alloys and air jetting produced more skimmings per unit time than GA alloys and nitrogen jetting. Finally, shrouding the crucible had an inhibiting effect on skimmings formation especially when comparing the two cases of a shrouded crucible under a nitrogen jet and an open crucible under an air jet. The second experiment attempted to simulate the mass transfer between liquid zinc and air in a galvanizing operation by top submerging a jet of air into a crucible loaded with liquid zinc. The liquid side mass transfer coefficients for aluminum and iron in liquid zinc were evaluated and compared favourably with the literature. Therefore, the assumption that the correlations for bubble flow in water can be applied to liquid metals was satisfactory.