Haihao Group takes you to have an in-depth understanding of the hot-dip galvanizing process
As an expert in the field of piping systems, Haihao Group often undergoes some surface treatments on its piping system products. Among them, hot-dip galvanizing is the most commonly used process. In this article, we have an in-depth discussion of hot-dip galvanizing coatings. The fascinating process of formation reveals its protective properties and strict control over the formation of zinc dust and slag.
I. Formation of Hot-Dip Zinc Coatings: The process of hot-dip zinc coating involves the formation of an iron-zinc alloy between the iron matrix and the outermost layer of pure zinc. When the iron workpiece is immersed in molten zinc, a zinc-α iron (body-centered) eutectic is initially formed at the interface. This crystalline structure, where zinc atoms are dissolved in the solid-state iron, allows for a robust bond between iron and the pure zinc layer. As the zinc reaches saturation in the eutectic, the zinc and iron atoms diffuse into each other, gradually forming an alloy within the iron matrix. The iron that diffuses into the molten zinc forms a metallic intermetallic compound, FeZn13, which settles at the bottom of the hot-dip galvanizing kettle, referred to as zinc dross. When the workpiece is withdrawn from the zinc bath, a pure zinc layer, in the hexagonal crystal form, is formed on the surface.
Protective Properties of Hot-Dip Zinc Coatings: Hot-dip zinc coatings, with a thickness generally exceeding 65μm, offer superior coverage, dense layers, and an absence of organic impurities. Zinc’s corrosion resistance mechanisms involve both mechanical and electrochemical protection. In atmospheric corrosion conditions, the zinc layer’s surface develops protective films such as ZnO, Zn(OH)2, and basic zinc carbonate, known as white rust. This protective film mitigates the corrosion of the underlying zinc to some extent. When the zinc layer is severely damaged, endangering the iron matrix, zinc provides electrochemical protection. With a standard potential of -0.76V for zinc and -0.44V for iron, zinc acts as the anode, dissolving, while iron, as the cathode, is protected.
III. Control of Zinc Ash and Zinc Dross Formation in the Hot-Dip Galvanizing Process: The presence of zinc ash and zinc dross not only compromises the quality of the galvanized coating, resulting in rough layers and zinc tumors but also significantly increases the cost of hot-dip galvanizing. Effective control measures involve avoiding the peak iron dissolution temperature range of 480–510°C during the galvanizing and annealing process. The choice of low-carbon, low-silicon steel plates for the galvanizing kettle construction helps reduce iron dissolution. The temperature should be maintained within the ranges of 450–480°C and 520–560°C for optimal galvanizing.
Control of Zinc Dross Quantity: Reducing zinc dross involves minimizing the iron content in the zinc bath. Practical steps include:
Avoiding galvanizing operations in the critical iron dissolution temperature range of 480–510°C.
Selecting galvanizing kettle materials with low carbon and silicon content, such as 08F high-quality carbon steel plates.
Regular removal of zinc dross by raising the temperature to separate it from the zinc liquid, followed by lowering the temperature for zinc dross to settle at the bottom of the kettle.
Preventing the introduction of iron into the zinc bath through auxiliary plating agents, which may form iron-containing compounds over time.
Haihao Group’s hot-dip galvanizing process demonstrates its commitment to quality and innovation. By revealing the complexities of zinc coating formation and emphasizing the precise control of zinc ash and dross, Haihao Group ensures the delivery of galvanized piping system products that meet the highest industry standards. If you need these piping system products, please feel free to contact us.