When clean steel is immersed in molten zinc at around 450 ° C, a sophisticated metallurgical reaction immediately begins:
1. Initial dissolution and reaction: The surface of the steel substrate first undergoes iron dissolution, and molten zinc immediately infiltrates the tiny lattice gaps on the substrate surface. At the same time, strong diffusion occurs between iron atoms and zinc atoms at the interface, forming a brittle intermediate compound layer - the iron zinc alloy layer (mainly composed of FeZn ₇ in the δ 1 phase and FeZn ₁ in the Zeta phase). This layer is dense and forms a strong "metallurgical bond" with the steel substrate, which is the core guarantee for the adhesion of the coating.
2. Alloy layer growth and coverage: With the passage of zinc immersion time, the alloy layer continues to grow and thicken. Subsequently, when the workpiece is extracted from the zinc liquid, a layer of molten zinc liquid will adhere to the surface. During the cooling and solidification process, this layer of liquid zinc crystallizes on top of the already formed dense alloy layer, forming a visible pure zinc layer (η phase) on the outer layer. Its crystals often exhibit a unique "zinc flower" morphology.
Modern technology often adds trace amounts of aluminum to zinc solution, which preferentially reacts with iron to form a thin layer. This can effectively inhibit the excessive growth of brittle Zeta phase, thereby significantly improving the ductility and appearance of the coating.
The final hot-dip galvanized layer is a perfect composite of a strong inner layer of iron-zinc alloy and an outer layer of sacrificial anti-corrosion pure zinc. This layer of "armor" is firmly attached to steel through metallurgical bonding, using multiple barrier mechanisms to resist environmental erosion, achieving its position as the cornerstone of century old protection.
