60. How does aluminum in molten zinc affect hot-dip galvanizing?

Aluminum (A1) is a silvery-white metal with a face-centered cubic (FCC) crystal structure. Its lattice constant measures 404959.6 nm, atomic mass is 26.8, melting point 658℃, and boiling point 2000℃. Commercial zinc products do not contain aluminum, which is intentionally added during hot-dip galvanizing. This process serves three key purposes: enhancing the luster of the galvanized steel pipe surface, improving flexibility, modifying the microstructure of the iron-zinc alloy layer, and neutralizing the effects of iron in the molten zinc. The details are as follows: (1) Aluminum improves the surface luster and flexibility of galvanized steel pipes.
Theoretically, only 0.02% aluminum content in zinc bath would suffice to achieve this goal. However, since aluminum readily oxidizes on the zinc surface, empirical evidence suggests adding approximately 0.2% aluminum is necessary to maintain the required 0.02% level. The strong affinity between aluminum and oxygen forms an aluminum oxide layer that effectively blocks oxygen diffusion, protecting both the underlying molten aluminum and zinc from oxidation. This protective mechanism also prevents oxidation of other metal elements in the zinc bath. As is well known, zinc oxidation produces yellow zinc oxide, and lead and cadmium oxides exhibit similar yellowish hues. Without aluminum's protective role, the galvanized surface would become heavily stained with yellow compounds, significantly compromising its luster. Therefore, adding an appropriate amount of aluminum is essential in hot-dip galvanizing to achieve a bright finish. Moreover, a 0.2% aluminum content in the zinc bath not only yields optimal decorative patterns but also ensures exceptional flexibility in the galvanized layer.
However, the American Society for Testing Materials (ASTM) recommends that aluminum should not be used as a brightening metal additive, and if used, its content should be limited to less than 0.01%.
(2) Altering the Microstructure of Galvanized Layers Theoretically, an aluminum content of 0.2-0.3% in molten zinc is sufficient to modify the microstructure of galvanized layers. However, in practical production, aluminum readily reacts with oxygen in the molten zinc, leading to its consumption. To maintain the target aluminum content, approximately 1.5%-3.5% of aluminum must be added. To demonstrate how aluminum content affects the microstructure, we analyze the changes from low to high aluminum concentrations: An increase of 0.05% in aluminum content enhances the surface gloss of the galvanized layer but has no effect on its microstructure. Thus, the galvanized layer retains the same composition as that produced from pure zinc liquid, consisting of an adherent layer (Phase a), an intermediate layer (Phase Y), a slightly cracked grating layer (Phase 81), and a floating layer (Phase S) of pure zinc (Phase n). The key difference lies in the distinct crystalline morphology of the phases compared to pure zinc liquid.
When the aluminum content in zinc liquid is 0.1%, the crystallization of the floating layer (3 phase) is in the form of a large block, and it is not a continuous layer, but a kind of separated inclusions.
When the aluminum content in zinc liquid is 0.15%, the distribution of the floating layer (phase 5) is not a continuous layer, but some larger, separated crystalline clusters, and only the grid layer (phase 81) presents a slightly more dense structure.
When the aluminum content in the zinc bath reaches 0.24%, the alloying effect becomes highly effective in preventing corrosion. If the zinc bath is maintained at 440℃ for 1 hour of plating, no reaction is observed upon removal and inspection. Consequently, the galvanized layer on the sample consists solely of a pure zinc layer. This occurs because aluminum reacts with the steel pipe to form a FeAl₃ (or Fe₂AlO) compound film, which inhibits the diffusion of iron ions toward the zinc layer.
As demonstrated above, aluminum content is a key factor in altering the microstructure of the galvanized layer. When the aluminum content is fixed, other process parameters-including zinc immersion time, fluidity (as shown in Figure 3-5), and temperature-also influence the zinc layer's microstructure. Therefore, in hot-dip galvanizing production, the interplay among these three factors is governed by the process specifications. Only by strictly adhering to the specified operating conditions can the desired galvanized layer be achieved.
(3) The effect of iron in zinc bath is offset because aluminum can combine with iron in zinc bath to form three compounds, namely FeAl, FeAl2 and FeAl3, which reduces the effect on galvanized coating.