Influence of annealing treatment on wetting of steels by zinc alloys

In continuous hot-dip galvanizing process, steel strip is immersed in a molten zinc bath containing small amounts of aluminium (less than 1 wt%). Before immersion in the molten metal, the steel strip is annealed at 800 °C in an atmosphere of nitrogen and hydrogen (5 vol.%) with low partial water pressure (20–50 Pa). Under these conditions, the most oxidizable alloying elements in the steel (e.g. Si, Mn) segregate to the surface where they form oxide particles. The presence on the steel surface of such oxide particles sometimes results in problems of wettability by liquid zinc–aluminium and may lead to the formation of surface defects. In order to gain a better understanding of this problem, the wetting by liquid zinc–aluminium of heterogeneous surfaces composed of metallic iron and silica particles or films was investigated at 450 °C using the dispensed drop technique. The liquid zinc–aluminium wets oxidised surfaces to a lesser extent than the iron substrates. With the operating conditions used in the laboratory experiments, the transition from wetting to non-wetting was found to occur when the surface area fraction covered by silica was equal to 34 ± 6 %. Mechanisms are proposed to explain the formation of the surface defects sometimes observed in galvanized coatings, based on the wettability experiments and characterization of surface defects from industrial coatings.