Aluminum (Al), with a silvery-white appearance and a face-centered cubic structure, has a lattice constant of 404959.6 nanometers, a relative atomic mass of 26.8, a melting point of 658°C, and a boiling point of 2000°C. Aluminum is not naturally present in commercial zinc; rather, it is intentionally added during hot-dip galvanizing processes. The purposes of adding aluminum are to enhance the gloss of the zinc coating on steel pipes, improve their flexibility, alter the structure of the iron-zinc alloy layer, and counteract the effects of iron in the molten zinc. These are detailed below:
(1) Aluminum Enhances the Gloss and Flexibility of Galvanized Steel Pipes
Theoretically, to achieve these goals, an aluminum content of only 0.02% in the molten zinc is sufficient. However, due to aluminum's susceptibility to oxidation at the surface of the molten zinc, an empirical aluminum addition of around 0.2% is necessary to maintain a 0.02% aluminum content in the molten zinc. Aluminum has a high affinity for oxygen, forming an alumina layer that effectively prevents oxygen diffusion, protecting the underlying molten zinc and molten zinc from oxidation. Similarly, other metal elements in the molten zinc are also shielded from oxidation. Oxidized zinc, lead, and cadmium are yellow, and without aluminum, the galvanized layer would significantly contain yellow components, adversely affecting its gloss. Therefore, a certain amount of aluminum is added during hot-dip galvanizing to obtain a bright galvanized layer. Additionally, when the molten zinc contains 0.2% aluminum, the best pattern is obtained, and the flexibility of the galvanized layer is particularly good.
However, the American Society for Testing and Materials recommends not using aluminum as a brightening metal additive, and if used, it should be limited to below 0.01%.
(2) Altering the Structure of the Galvanized Layer
Theoretically, to alter the structure of the galvanized layer, an aluminum content of 0.2 to 0.3% in the molten zinc is sufficient. However, in practical production, aluminum easily reacts with oxygen in the molten zinc and is consumed, so an aluminum addition of around 1.5% to 3.5% is necessary to maintain an aluminum content of 0.2 to 0.3%. To illustrate the effect of aluminum content on the galvanized layer structure, let's examine the changes in the galvanized layer structure as the aluminum content increases:
An increase in aluminum content to 0.05% in the molten zinc enhances the surface gloss of the galvanized layer but does not affect its structure. Therefore, the galvanized structure is the same as that obtained from pure molten zinc, consisting of an adhesion layer (phase a), an intermediate layer (phase γ), a slightly cracked layer (phase δ₁), a floating layer (phase S), and a pure zinc layer (phase η). The difference from the galvanized layer plated out from pure molten zinc is in the crystalline shape of the phases.
When the aluminum content in the molten zinc is 0.1%, the crystals of the floating layer (phase S) exist in large blocks and are no longer arranged in a continuous layer but as separated inclusions.
When the aluminum content in the molten zinc is 0.15%, the distribution of the floating layer (phase S) is also not continuous but consists of larger, mutually separated crystalline clusters, with only the layer (phase δ₁) showing a slightly denser structure.
When the aluminum content in the molten zinc is 0.24%, the effect of inhibiting etching (alloying) is strong. If the galvanizing is performed at a temperature of 440°C for 1 hour in this molten zinc and then inspected, no reaction is found. Therefore, only a pure zinc layer exists on the galvanized sample. This is because the reaction between aluminum and steel produces a thin film of FeAl₃ (or Fe₂Al₅ according to some sources), which hinders the diffusion of iron ions towards the zinc.
From the above, the amount of aluminum is an important factor in altering the structure of the galvanized layer. When the aluminum content is fixed, process parameters such as galvanizing time, fluidity (as shown in Figure 3-5), and galvanizing temperature also influence the change in zinc layer structure. Therefore, in hot-dip galvanizing production, the relationship between these three factors is specified by the process specifications, and only under strictly controlled operating conditions can the desired galvanized layer be obtained.
(3) Counteracting the Effects of Iron in Molten Zinc
Aluminum reacts with iron in molten zinc to form three compounds: FeAl, FeAl₂, and FeAl₃, thereby reducing its impact on the galvanized layer.
