Impact of Zinc Dross on the Galvanized Layer of Galvanized Steel Pipes

During hot-dip galvanizing, zinc dross in the molten zinc becomes mixed within the zinc bath and subsequently incorporated into the galvanized layer when applied to steel pipes. Surrounded by pure zinc, these zinc dross particles become anchored within the pure zinc layer. Additionally, an increase in iron content in the molten zinc reduces its wettability to the surface of the steel pipe billet, resulting in an uneven distribution of the galvanized layer. This leads to a rough and uneven surface of the galvanized layer, accompanied by dull spots, and in severe cases, the formation of zinc tumors of various sizes. The zinc dross increases the brittleness of the pure zinc layer, causing the galvanized layer to peel off when bent. During the copper sulfate test, it may cause false termination points. As we know, the more impurities in the pure zinc layer, the less corrosion-resistant it becomes. Similarly, zinc dross included in the pure zinc layer can produce a micro-battery effect, leading to the preferential corrosion of the surrounding pure zinc layer. An increase in the amount of zinc dross results in a thicker galvanized layer, thereby increasing the consumption of zinc. For example, when the immersion time in molten zinc is 30 seconds and the temperature of the molten zinc is 450°C, the weight of the galvanized layer on the steel pipe is 330 grams per cubic meter when the iron content in the molten zinc is 0.06%. When the iron content in the molten zinc increases to 0.25%, the weight of the galvanized layer on the steel pipe increases to 450 grams per cubic meter.