When the temperature of molten zinc is very high, a large amount of iron is dissolved in it. For example, when heated to 510°C, 0.10% of iron is dissolved, which reacts with 1.6% of the total molten zinc in the galvanizing pot to form zinc dross. Once the temperature of the molten zinc drops to 435°C, 0.02% of iron still remains in the molten zinc. However, during the cooling process, iron precipitates out from the molten zinc as tiny iron-zinc compound crystals and slowly settle to the bottom of the galvanizing pot. To minimize these tiny crystalline zinc dross (iron-zinc alloy) in the molten zinc, it is necessary to maintain the molten zinc at around 435°C for about a day after high temperature treatment. However, this is absolutely not allowed in practical operations, so the only option is to reduce the galvanizing temperature.
Meanwhile, when the temperature of the molten zinc rises, convective heat transfer intensifies, bringing zinc dross to the surface of the galvanizing pot and contaminating the molten zinc at the immersion depth, thereby deteriorating the quality of the galvanized layer. The presence of zinc dross worsens the flow of molten zinc, which can erode the iron-zinc alloy layer on the walls of the galvanizing pot, causing accelerated corrosion due to the loss of protection, and an increase in zinc dross.
If zinc dross remains in the galvanizing pot for an extended period, it will bake into a solid block, which intensifies as the temperature rises. This not only makes removal difficult but also obstructs the heating of the galvanizing pot, potentially causing overheating and perforation of the pot wall (steel plate), leading to zinc leakage.
In a normally operated hot-dip galvanizing process, the iron content near the surface of the molten zinc should be minimal, generally not exceeding 0.05%. If it reaches or exceeds 0.2%, hot-dip galvanizing should not be continued. Since the typical immersion depth is around 400 mm, where the iron content may be even higher, it should be well controlled.
