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How Steel Grating is Hot-Dip Galvanized

Time:2021-11-23

Hot-dip galvanizing, also known as hot-dip zinc coating, involves melting zinc ingots at high temperatures, adding auxiliary materials, and then immersing fabricated steel grating components into the galvanizing bath. This process ensures a robust zinc coating adheres to the steel surface. The advantages of hot-dip galvanizing include superior corrosion resistance and excellent adhesion/hardness of the coating. After galvanizing, the weight of the steel grating increases; this added weight is commonly referred to as the zinc pickup.

 

Metallurgical Formation Mechanism

 

The formation of the zinc layer is a process of creating an iron-zinc alloy between the steel substrate and the pure zinc layer. Only when this alloy layer forms during immersion does a proper bond occur between the iron substrate and the pure zinc layer.

 

When the iron workpiece is immersed in molten zinc, a solid solution of zinc in alpha-iron (body-centered cubic) is formed at the interface. This is a crystal structure where zinc atoms dissolve into the solid base metal iron. At this stage, the atomic bonding between the two metals is relatively weak.

 

Once the zinc in the solid solution reaches saturation, the atoms of iron and zinc begin to diffuse mutually. Zinc atoms diffuse into the iron lattice of the substrate, gradually forming an alloy with the iron. Simultaneously, iron atoms diffusing into the molten zinc form an intermetallic compound, FeZn₁₃. This compound sinks to the bottom of the galvanizing kettle and is known as zinc dross (zinc slag). When the workpiece is withdrawn from the bath, a layer of pure zinc forms on the surface, which crystallizes into a hexagonal structure.

 

Temperature and Time Control

 

The iron dissolution rate (iron loss) varies significantly with temperature and holding time. Near 500°C, iron loss increases sharply with heating time. Below or above the range of 480–510°C, iron loss increases slowly over time. Therefore, the range of 480–510°C is termed the "Aggressive Dissolution Zone" (Malignant Dissolution Zone). Within this temperature range, the zinc bath severely corrodes both the workpiece and the kettle. Above 560°C, iron loss increases markedly again. At temperatures exceeding 660°C, zinc aggressively attacks the iron substrate, causing a rapid surge in zinc dross and making the galvanizing process unfeasible.