“Blossoming of Zinc Flowers” | Knowledge of Zinc Flower in Aluzinc Coating

What is Zinc Flower?

Zinc flower is a characteristic feature of traditional hot-dip galvanized/aluzinc-coated steel plates. When the steel strip passes through a zinc bath, the surface is coated with liquid zinc. During the natural solidification process of the zinc layer, zinc crystals grow to form zinc flowers.

The crystalline structure of zinc, in its most complete state, resembles snowflakes or hexagonal stars. Therefore, through hot-dip technology, zinc crystals on the steel surface have a chance to form snowflake or hexagonal star-shaped zinc flowers.

Zinc Flower

Appearance of Zinc Flower

High-purity liquid zinc cannot solidify into zinc flowers during cooling because it lacks necessary crystallization nuclei. To obtain typical zinc flowers, two different types of alloy elements with distinct properties need to be added in appropriate amounts to the zinc liquid. One type of alloy element should be completely soluble in the zinc liquid and nearly insoluble in solid zinc, such as lead and titanium. The other type should have certain solubility in both liquid and solid zinc, like aluminum, tin, antimony, etc.

If only one type of alloy element is added, although the surface condition of the zinc layer changes after solidification, it is not sufficient to form typical zinc flowers.

Methods to Form Zinc Flowers

  1. Addition of Other Elements to the Zinc Liquid: Adding certain alloy elements to the zinc liquid can prolong the crystallization process, preventing the surface zinc liquid from solidifying until it reaches the eutectic temperature. This lowers the melting point of the zinc liquid, extending the growth time of zinc flower crystals.
  2. Surface Blowing Gas Method: Blowing steam or sulfur dioxide gas through the zinc layer surface during its solidification process allows smooth growth of zinc flower crystals. This method also forms a dense oxide film, enhancing the corrosion resistance of the galvanized layer.
  3. Steel Wire Mesh Method: This method, applicable only to thin galvanized sheets, uses magnetic rolls to bring a steel wire mesh in contact with the zinc layer surface. The points where the steel wire mesh touches first experience cooling, generating solid particles as crystallization nuclei. The remaining un-solidified zinc liquid then crystallizes outward from these nuclei, forming zinc flowers.
  4. Water Mist Spray Method: Spraying water mist (mixture of water and steam or water and air) on the surface of aluzinc-coated steel plates before the pure zinc liquid solidifies creates crystallization nuclei. The un-solidified zinc liquid then diffuses outward from these points, forming zinc flowers.
  5. Other Methods: Besides the mentioned methods, achieving a smooth surface of the coated part, a relatively thin coating, uniform temperature descent, and sufficient cooling time in the air can also result in larger zinc flowers.
Galvalume Steel Coil

Classification of Zinc Flowers

According to the European standard EN 10346:2015, the size of zinc flowers can be categorized as Normal Spangle and Minimized Spangle.

Normal Spangle (大锌花): Refers to the size and appearance of zinc crystals formed after the normal solidification of zinc liquid.
Minimized Spangle (小锌花): Obtained by controlling the solidification process of zinc to achieve a specific size and appearance of zinc crystals.

In the hot-dip aluzinc process, uneven sizes of zinc flowers often occur.

Uneven zinc flowers

Factors affecting the size and uniformity of zinc flowers are numerous, and steel plants need to strictly control each factor to produce qualified products.

  • Cooling Speed: Faster cooling leads to shorter crystal growth time and smaller zinc flowers.
  • Number of Crystallization Nuclei: More zinc crystal nuclei result in smaller zinc flowers. The water mist spray method utilizes this principle.
  • Raw Material Substrate: Different surface roughness of the substrate leads to differences in coating adhesion. Larger substrate surface roughness results in better coating adhesion and smaller zinc flowers.

Leveraging extensive experience in steel products, Shangang Steel accumulate a vast amount of data on coating applications. We aim to create an organic integration of coating application experience with science, art, and technology.

The formulation of the aluminum-zinc liquid ensures better purity and fluidity. Our unique technology, utilizing air cooling with variable diameter inlet and increased pressure for outlet air, as well as water cooling, ensures the crystallization of the aluminum-zinc layer on the steel strip surface is more uniform, allowing zinc flowers to bloom more beautifully. Contact us for more information.