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HaiGang Yunyi
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Tianjin Haigang Steel Coil
Service first, quality assured. More dedicated and focused on crafting even better boards.
04
2021
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02
What is the hot-dip galvanizing reinforcement treatment?
Have any of you friends ever heard of or mastered the process for strengthening and enhancing the structure of hot-dip galvanized coatings? I didn’t know much about it before, but after looking into it, I finally understood. So today, I’d like to share with everyone the details of this process for reinforcing and improving the structure of hot-dip galvanized coatings.
Zinc passivation solution
Zinc is a highly reactive chemical element with an inherently active nature, making it a restless metal. Consequently, when exposed to air and in close contact with atmospheric gases, zinc readily oxidizes and darkens, eventually forming “white rust” and becoming susceptible to corrosion. To address this issue, after hot-dip galvanizing, the zinc surface is passivated using a mixture of three acids—chromic acid, hydrochloric acid, and sodium cyanide—which forms a thin, chemically stable film on the zinc layer, effectively putting the active metallic zinc into a “semi-inhibited” or “passive” state. This passivation process not only enhances the color and aesthetic appeal of the zinc coating’s surface but, more importantly, creates a fine, protective passivation film that significantly improves the corrosion resistance of the zinc coating and extends the service life of the product. Additionally, passivation helps prevent fingerprints from being left on the zinc coating.
Types of Passivation
At this stage, China's passivation processes include: passivation (covering low-concentration, medium-concentration, and high-concentration chromic acid color passivation, as well as single-pass three-acid color passivation); light green passivation using five acids; colorful passivation; and bright red passivation.
The Great Principle of Passivation
The passivation of chromium trioxide in hot-dip galvanized coatings is largely a chemical reflection process that occurs at the interface between the solid and liquid phases (the metal material's passivating aqueous solution). The reaction process itself is highly complex. Whether it involves high-chromium or low-chromium passivation, the key reaction is the redox reaction between zinc and hexavalent chromium.
The compositional principle of hot-dip galvanized coatings
Hot-dip galvanizing is a metallurgical process that encompasses the entire production cycle. From an external economic perspective, the hot-dip galvanizing process involves two stable equilibria: thermal circulation and the equilibrium of zinc-iron exchange. When steel workpieces are immersed in molten zinc at temperatures around 450°C, the workpieces at room temperature absorb heat from the molten zinc. As their temperature rises above 200°C, the interaction between zinc and iron gradually becomes more pronounced, and zinc begins to penetrate the surface of the steel workpiece. As the workpiece temperature continues to approach the temperature of the molten zinc, its outer surface forms an alloy layer with a specific ratio of zinc to iron, thereby establishing a well-defined layered structure for the zinc coating. Over time, the different alloy layers within the coating exhibit varying growth rates. From a macroeconomic standpoint, this process is primarily characterized by the immersion of galvanized grating panels into the molten zinc; initially, the zinc level experiences boiling. Once the zinc-iron reaction reaches equilibrium, the zinc level gradually stabilizes. When the workpiece is removed from the zinc bath and its temperature slowly drops below 200°C, the zinc-iron reaction ceases, and the hot-dip galvanized coating is fully formed, with its thickness precisely determined.
The specified thickness of hot-dip galvanized coatings—factors affecting the thickness of zinc coatings
The factors that affect the thickness of hot-dip galvanized coatings include: the chemical composition of the base metal, the surface roughness of the steel sheet, the content and distribution of specific elements—silicon and phosphorus—in the steel sheet, thermal stresses in the steel sheet, the geometric dimensions of the workpiece, and the hot-dip galvanizing process itself. Both current international and national hot-dip galvanizing standards classify steel sheets according to their thickness ranges and specify that the average thickness and local thickness of the zinc coating must meet certain minimum requirements to ensure the corrosion resistance of the zinc coating. For workpieces of different steel thicknesses, the time required for thermal cycling and the equilibrium of zinc-iron exchange varies, resulting in different coating thicknesses. The specified average thickness of hot-dip zinc coatings is based on industrial production experience derived from the aforementioned hot-dip galvanizing principles, while the local thickness takes into account the uneven distribution of zinc coating thickness as well as the practical requirements for the coating's corrosion resistance.
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We primarily produce various products including galvanized strip steel, pickled strip steel, and cold-rolled strip steel. These products are widely used in industries such as construction, mechanical manufacturing, automotive, and household appliances.
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