Iron (Fe) appears silvery-white, with a relative atomic mass of 56. The melting point of pure iron is 1535°C and its boiling point is 3000°C. The primary sources of iron in molten zinc are:
(1) Introduction from remelted zinc with high iron content;
(2) ζ-phase formed as a result of the reaction between steel pipes, steel galvanizing pots, steel machinery and equipment, and molten zinc, which then falls into the molten zinc;
(3) Zinc slag produced by the reaction between iron salts adhering to steel pipes after pickling and molten zinc. According to reports, one part of iron salt can react with twenty-five parts of zinc.
The higher the iron content in molten zinc, the more zinc slag is produced, and the viscosity of the molten zinc increases, resulting in poorer fluidity during zinc flow, thicker coatings (primarily η-phase), and a galvanized layer that becomes brittle, lacks flexibility, exhibits a dull surface, and is rough. Some data indicates that when the iron content in zinc reaches a few ten-thousandths, it increases the hardness of the zinc layer and hinders the recrystallization process. When the iron content reaches 0.02%, the lifespan of the galvanized layer is very short (with zinc as the anode), and aluminum or silicon is generally added to remove iron. Therefore, during normal galvanizing operations, it is stipulated that the iron content from the surface of the molten zinc to the working depth must not exceed 0.05% (equivalent to Zn-4 to Zn-5). If remelted zinc is used, it is not allowed when its iron content reaches 0.2%. Experimental evidence shows that at the same temperature of 450°C, when the iron content in molten zinc is 0.06%, the weight of the galvanized layer is 330 grams per square meter, and when the iron content is 0.25%, the weight of the galvanized layer increases to 450 grams per cubic meter. This demonstrates an increased consumption of zinc. The iron in molten zinc only affects the η-phase of the pure zinc layer and has no impact on the iron-zinc reaction.
