Aluminum (Al), which appears silvery white, belongs to the face-centered cubic structure with 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 does not naturally occur in commercial zinc. However, aluminum is deliberately added to the molten zinc during hot-dip galvanizing. The purposes are to enhance the gloss of the zinc coating on steel pipes, improve its flexibility, alter the structure of the iron-zinc alloy layer, and counteract the influence of iron in the molten zinc. A detailed breakdown is as follows:
(1) Aluminum enhances the gloss and flexibility of galvanized steel pipes.
Theoretically, to achieve this, only 0.02% aluminum in the molten zinc is sufficient. However, since aluminum is easily oxidized at the surface of the molten zinc, based on experience, approximately 0.2% aluminum must be added to maintain a 0.02% aluminum content in the molten zinc. Due to the high affinity between aluminum and oxygen, which forms an alumina layer, this layer effectively prevents the diffusion of oxygen, protecting the underlying molten zinc and molten zinc from oxidation. Similarly, other metallic elements in the molten zinc are also shielded from oxidation. As we know, zinc oxide formed after the oxidation of molten zinc is yellow, and oxides of lead and cadmium are also yellow. Without the effect of aluminum, the galvanized layer's surface would be significantly tarnished with yellow components, greatly affecting its gloss. Therefore, a certain amount of aluminum is added during hot-dip galvanizing to obtain a bright galvanized layer. Meanwhile, when the molten zinc contains 0.2% aluminum, the best pattern can be 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 metallic additive, and if used, it should be limited to below 0.01%.
(2) Modifying the structure of the galvanized layer
Theoretically, to achieve the purpose of modifying the galvanized layer's structure, an aluminum content of 0.2 to 0.3% in the molten zinc is sufficient. However, in actual production, aluminum in the molten zinc easily reacts with oxygen and is consumed. Therefore, to maintain an aluminum content of 0.2 to 0.3% in the molten zinc, approximately 1.5% to 3.5% aluminum must be added. To illustrate the effect of aluminum content on modifying the galvanized layer's structure, let's observe the changes in the galvanized layer's structure as the aluminum content gradually increases from low to high:
An increase in aluminum content to 0.05% in the molten zinc is intended to enhance the surface gloss of the galvanized layer but does not affect its structure. Therefore, the galvanized structure is the same as that plated from pure molten zinc, consisting of an adhesion layer (phase a), an intermediate layer (phase γ), a slightly cracked layer (phase δ₁), a drifting layer (phase S), and a pure zinc layer (phase η). The difference from the galvanized layer plated from pure molten zinc is in the crystalline shapes of the phases.
When the aluminum content in the molten zinc is 0.1%, the crystals of the drifting layer (phase δ₁) exist in large blocks and are no longer arranged in a continuous layer but as detached inclusions.
When the aluminum content in the molten zinc is 0.15%, the distribution of the drifting layer (phase δ₁) is also not continuous but consists of larger, mutually detached 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 inhibiting effect on etching (alloying) is strong. If the immersion galvanizing is maintained at a temperature of 440°C for 1 hour in this molten zinc and then inspected, no reaction is found to have occurred. Therefore, only a pure zinc layer exists on the galvanized layer of the sample. This is because the reaction between aluminum and the steel pipe forms a thin film of FeAl₃ (or Fe₂Al₅ according to some sources) compounds, hindering the diffusion of iron ions towards the zinc.
From the above, it can be seen that the amount of aluminum is an important factor in changing the structure of the galvanized layer. When the aluminum content is fixed, process parameters such as immersion time, fluidity (as shown in Figure 3-5), and immersion temperature also influence the change in the zinc layer's structure. Therefore, in hot-dip galvanizing production, the relationship between these three factors is stipulated by the process specifications, and only under strictly regulated operating conditions can the desired galvanized layer be obtained.
(3) Counteracting the influence of iron in the molten zinc
Aluminum reacts with iron in the molten zinc to form three compounds: FeAl, FeAl₂, and FeAl₃, thereby reducing its impact on the galvanized layer.
