1.What is the core principle of the cooling phase?
When a workpiece is removed from the hot-dip galvanizing bath (approximately 440-460°C), the surface zinc layer is in a liquid or semi-liquid state, and metallurgical reactions between the zinc and the base iron continue (e.g., forming Fe-Zn alloy layers: Fe₃Zn₁₀, FeZn₇, etc.). If cooled naturally, a slow cooling rate can result in:
Excessive grain growth within the zinc layer, forming coarse columnar crystals, which reduces the hardness and toughness of the coating;
Continuous thickening of the Fe-Zn alloy layer ("over-alloying"), which is inherently brittle and prone to cracking and detachment;
Liquid zinc flows due to gravity or surface tension, resulting in uneven coating thickness (e.g., accumulation at corners and thinning on flat surfaces).
Therefore, the cooling stage requires controlled rapid cooling (common cooling media are air, water mist or circulating water) to forcibly interrupt the further reaction between zinc and iron, while allowing the zinc layer to solidify in a "fine-grained" form, locking in the uniformity of the coating and the stability of the microstructure.
2.What is the core purpose of cooling?
Controlling coating thickness and uniformity: Rapid cooling prevents the flow of zinc liquid, preventing localized over-thickness or under-thickness of the coating, and ensuring that the zinc coating thickness on the workpiece meets standards (such as the minimum thickness required by GB/T 13912).
Optimizing coating mechanical properties: Zinc layers with finer grains are tougher than coarser grains, improving impact and bending resistance (e.g., the coating is less likely to crack when the workpiece is bent).
Preventing alloy embrittlement: Interrupting the Fe-Zn reaction prevents the alloy layer from becoming too thick, reducing the risk of coating brittleness and ensuring the feasibility of subsequent processing (such as cutting and stamping).
3.What is the core principle of passivation?
Pure zinc layers react easily with oxygen, water, and carbon dioxide in air, forming loose zinc oxide (ZnO) or zinc carbonate (ZnCO₃・3Zn(OH)₂). These products cannot form a dense barrier and will continuously flake off, exposing new zinc layers and accelerating corrosion (the so-called "white rust" phenomenon).
During the passivation phase, the cooled workpiece is immersed in a passivating agent solution (commonly used chromate passivators, such as CrO₃ and Cr₂(SO₄)₃; or environmentally friendly passivators, such as chromium-free passivators). A dense, stable inorganic/organic composite film (passivation film) is formed on the zinc surface through chemical or electrochemical reactions:
Chromate passivation: Zn²⁺ on the zinc surface reacts with Cr₂O₇²⁻ and H⁺ in the passivating agent to form Cr³⁺ (forming Cr(OH)₃ or Cr₂O₃). The Zn²⁺ composite oxide film has a dense structure, isolating the zinc layer from external corrosive media (water, oxygen, salt spray, etc.).
Chromium-free passivation (such as silane passivation and titanium-zirconium passivation) involves the hydroxyl groups of silane molecules bonding with the hydroxyl groups on the zinc surface to form a cross-linked organic silicon film. Alternatively, titanium-zirconium ions react with the zinc layer to form an oxide ceramic film, which also acts as a corrosion barrier.
4.What is the core purpose of passivation?
Significantly Improved Corrosion Resistance: The passivation film blocks contact between the zinc layer and corrosive media, significantly delaying the onset of "white rust" (for example, white rust on unpassivated workpieces in a humid environment can be delayed to months or even years after passivation).
Enhanced Surface Adhesion: The surface polarity or roughness of the passivation film improves the adhesion of subsequent coatings (such as paint and plastic spraying) and prevents coating detachment.
Improved Appearance and Stain Resistance: The passivation film typically exhibits a uniform bluish-white, colored, or black color (depending on the type of passivation agent), enhancing the consistency of the workpiece's appearance. The smooth film resists dust and oil stains, making it easier to clean.
5.What is the core value of the cooling and passivation stage?
The cooling stage is the prerequisite for "ensuring the basic performance of the galvanized layer" (uniformity and toughness), and the passivation stage is the key to "giving the galvanized layer long-term protective capabilities" (corrosion resistance and adhesion). The combination of the two enables hot-dip galvanized workpieces to not only adapt to complex processing requirements, but also to serve for a long time in harsh environments such as outdoor, humid, and salt spray, ultimately achieving the protection goal of "low cost and long life".