1.What is the core principle of zinc flower formation?
Crystallization Dynamics Foundation
Zinc flowers are essentially the macroscopic morphology of zinc crystals during solidification. Two conditions must be met for their formation:
Supercooling trigger: When zinc liquid cools from liquid to solid, it must reach a certain supercooling (the actual temperature is 2℃~10℃ lower than the theoretical solidification point) to promote the formation of crystal nuclei.
Anisotropic growth: Zinc crystals grow at different speeds along different crystal directions, forming radial or dendritic crystals, and finally presenting snowflake-like patterns on the surface of the coating.
2.What are the key influencing factors?
The role of lead (Pb): Traditional hot-dip galvanizing liquid often contains 0.1%~0.3% lead. Lead is insoluble in solid zinc and will segregate at the grain boundary, reducing the surface energy of zinc crystals, promoting uniform distribution of crystal nuclei, and forming coarse zinc flowers (up to 5~10mm in diameter).
Regulation of aluminum (Al): When the aluminum content in zinc liquid is controlled at 0.1%~0.2%, aluminum will preferentially form Al₂O₃ film on the surface of zinc liquid, inhibiting zinc liquid oxidation, while affecting the growth rate of crystal nuclei, forming medium-sized zinc flowers (1~5mm).
Cooling rate: Rapid cooling (such as air knife plating) will lead to increased supercooling, an increase in the number of crystal nuclei but a short growth time, forming fine zinc flowers (<1mm); slow cooling will promote the growth of crystal nuclei and form coarse zinc flowers.
3.What are the main causes of different zinc flowers?
Large zinc flowers (normal zinc flowers): The zinc liquid contains lead, cools slowly (air cooling), and the crystal nucleus grows fully.
Small zinc flowers (zero zinc flowers): Adding refiners such as antimony and bismuth to the zinc liquid, or using rapid cooling (such as water cooling) to inhibit crystal growth.
No zinc flowers: the aluminum content in the zinc liquid is ≥0.2%, forming a dense oxide film to prevent the growth of crystal nuclei; or through alloying treatment (such as zinc-iron alloy).
4.What are the practical application scenarios of zinc spangle control?
Large zinc flowers: steel structures, guardrails, pipes, etc., use their rough surface to enhance coating adhesion, and do not have high requirements on appearance.
Small zinc spangles / no zinc spangles:
Home appliance panels: The surface needs to be sprayed or printed, and no zinc spangles can avoid the influence of the pattern on the uniformity of the coating.
Automobile panels: The surface is required to be smooth and beautiful, and the stamping formability of the no zinc spangle coating is better.
Medical devices: The surface without zinc spangles is easier to clean and disinfect, and meets hygiene standards.
5.What are the challenges and trends in spangle control?
Environmental pressure: After eliminating the lead-containing process, it is necessary to control zinc spangles by replacing elements such as antimony and bismuth or optimizing the cooling process, and at the same time, it is necessary to balance corrosion resistance and cost.
High-end demand: The fields of electronics and aviation require extremely uniform coating surfaces (such as mirror effects), and it is necessary to combine vacuum coating, electrolytic galvanizing and other processes to replace traditional hot-dip galvanizing.