Generally, aluminum is not added to the zinc bath when performing wet hot-dip galvanizing. If aluminum is added to the zinc bath, due to its higher density compared to zinc, it will float on the surface of the zinc bath. The flux used in wet hot-dip galvanizing is directly placed on the surface of the zinc bath. Therefore, the flux will first come into contact with the aluminum layer on the surface, leading to a vigorous reaction between the two. This reaction produces volatile aluminum trichloride, with the reaction formula being:
3ZnCl₂ + 2Al → 3Zn + 2AlCl₃↑
From the above formula, it can be seen that the less reactive zinc is replaced by the more reactive aluminum from its compound (ZnCl₂), forming aluminum trichloride (AlCl₃). Aluminum trichloride can boil even at a low temperature of 123°C and therefore evaporates rapidly from the flux. At the same time, 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 assisting galvanization, leading to missed galvanizing spots on the surface of galvanized steel pipes.
