Generally, aluminum is not added to the zinc bath when employing the "wet process" for hot-dip galvanizing. If aluminum is added to the zinc bath, due to its lower density compared to zinc, it will float on the surface of the zinc bath. The flux used in the "wet process" hot-dip galvanizing is directly placed on this zinc bath surface. Consequently, the flux first comes into contact with the aluminum layer on the surface, leading to a vigorous reaction between the two, which produces volatile aluminum trichloride. The reaction equation is as follows:
3ZnCl₂ + 2Al → 3Zn + 2AlCl₃↑
From the equation above, it can be seen that the less reactive zinc is replaced by the more reactive aluminum in its compound (ZnCl₂), forming aluminum trichloride (AlCl₃). Aluminum trichloride can boil even at a low temperature of 123°C, thus it evaporates rapidly from the flux. Meanwhile, unevaporated aluminum trichloride can generate insoluble acids and AICl₃·NH₃. AICl₃·NH₃ can boil and evaporate at 400°C. As mentioned above, the evaporation of aluminum compounds results in a significant loss of chlorine content in the flux, which plays a role in aiding galvanization, leading to missed galvanizing spots on the surface of galvanized steel pipes.
