Iron (Fe) is silvery-white in color, 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 by the reaction between zinc liquid and steel pipes, steel galvanizing pots, steel mechanical equipment falling into the zinc liquid;
(3) Zinc slag generated from the reaction between iron salts attached to steel pipes after pickling and the zinc liquid. According to data, 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, increasing the viscosity of the zinc liquid. This leads to poor fluidity during zinc flow, thicker coatings (primarily η-phase), and galvanized coatings become brittle, lacking flexibility, with a dull and rough appearance. Some sources indicate that iron content in zinc at the ppm level can increase the hardness of the zinc layer and hinder the recrystallization process. When the iron content reaches 0.02%, the lifespan of the galvanized coating is short (with zinc as the anode), and aluminum or silicon is typically added to remove iron. Therefore, in normal galvanizing operations, the iron content from the surface of the galvanizing bath to the working depth must not exceed 0.05% (equivalent to Zn-4 to Zn-5). If remelted zinc is used, an iron content of 0.2% is not allowed. 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 coating is 330 grams per square meter, and when the iron content is 0.25%, the weight of the galvanized coating increases to 450 grams per square meter. This demonstrates an increased consumption of zinc. Iron in molten zinc only affects the η-phase of the pure zinc layer and has no significant impact on the iron-zinc reaction.
