72. Why is aluminum prohibited in the zinc bath of hot-dip galvanizing?

In conventional "wet" hot-dip galvanizing processes, aluminum is typically excluded from the zinc bath. When aluminum is introduced, its higher density causes it to float on the zinc surface. The flux, being positioned directly above the zinc bath, first reacts with the molten aluminum layer, generating volatile aluminum trichloride (AlCl₃) through the following equation: 3ZnCl₂ + 2Al → 3Zn + 2AlCl₃↑. This reaction demonstrates how less reactive zinc (ZnCl₂) is displaced by more reactive aluminum, forming AlCl₃. Remarkably, AlCl₃ boils at 123°C, causing rapid evaporation from the flux. The remaining AlCl₃ then reacts with unvolatilized aluminum to produce insoluble acids and aluminum chloride (AlCl₃) and ammonia (NH₃), which boil at 400°C. These evaporation processes deplete the flux's chlorine content essential for plating assistance, ultimately resulting in mottled coating patterns on galvanized steel pipes.