84. How does zinc slag affect the galvanized coating of galvanized steel pipes?

During hot-dip galvanizing, zinc slag from the molten zinc mixture becomes embedded in the zinc layer. When steel pipes are coated, these slag particles remain trapped within the zinc coating. The increased iron content in the molten zinc reduces its wettability on the pipe surface, resulting in uneven zinc layer distribution. This causes the galvanized surface to become rough and develop dull spots, with severe cases forming irregular zinc nodules. The slag also increases the brittleness of the pure zinc layer, making it prone to peeling during bending. In copper sulfate tests, it may create false termination points. As we know, higher impurity levels in the pure zinc layer reduce corrosion resistance. The slag's presence triggers microcell effects, corroding the surrounding pure zinc layer first. Increased slag content thickens the galvanized layer, leading to higher zinc consumption. For example, at 450°C and 30-second immersion time, a 0.06% iron content in the molten zinc yields 330g/m³ of zinc coating on steel pipes. When iron content rises to 0.25%, the coating weight increases to 450g/m³.