59. How does iron in zinc bath affect hot-dip galvanizing?

Iron (Fe), with a relative atomic mass of 56, appears silvery-white. Pure iron has a melting point of 1535°C and a boiling point of 3000°C. In molten zinc, iron primarily originates from three sources: (1) contamination from remelted zinc with higher iron content; (2) the ζ-phase generated from reactions between steel pipes, galvanized steel pots, and steel machinery with molten zinc; and (3) zinc slag produced when iron salts adhered to steel pipes after acid pickling react with molten zinc. According to reports, one part of iron salt can react with twenty-five parts of zinc.
The higher the iron content in zinc bath, the more zinc residue is produced, increasing the viscosity of the zinc bath. This reduces the fluidity of zinc flow, resulting in a thicker coating (primarily η-phase), a brittle zinc plating lacking flexibility, a dull gray surface, and a rough appearance. Notably, when zinc contains iron at levels reaching a few ten-thousandths, it increases the hardness of the zinc layer and hinders the recrystallization process. At an iron content of 0.02%, the lifespan of the zinc plating becomes significantly shortened (as zinc acts as the anode). To address this, aluminum or silicon is typically added to remove iron. Therefore, standard zinc plating procedures stipulate that the iron content in the zinc bath from the surface to the working depth must not exceed 0.05% (equivalent to Zn-4 to Zn-5). The use of remelted zinc is prohibited when its iron content reaches 0.2%. Experimental data shows that at 450°C, zinc plating weight increases to 330 grams per square meter at 0.06% iron content, and rises to 450 grams per square meter at 0.25% iron content. This demonstrates increased zinc consumption. Iron in the zinc bath only affects the pure zinc layer phase and has no significant impact on the iron-zinc reaction.