Knowledge about Zinc Flowers on Hot-Dip Galvanized Surfaces

What is zinc bloom?

The presence of beautiful zinc flowers is a key characteristic of traditional hot-dip galvanized steel sheets. The most perfectly formed crystalline structure of zinc closely resembles snowflakes or hexagonal star shapes; therefore, the zinc crystals condensed on the surface of the strip steel during the hot-dip galvanizing process are most likely to take on the form of snowflake- or hexagonal-star-shaped zinc flowers.

High-purity zinc liquid cannot solidify into zinc flowers during cooling because it lacks the necessary crystal nuclei. To obtain attractive zinc flowers, a prerequisite is to add an appropriate amount of two types of alloy elements with different properties to the zinc liquid: one type completely dissolves in the liquid zinc but is almost entirely immiscible in solid zinc—for example, lead and titanium; the other type exhibits some solubility in both liquid and solid zinc—for example, aluminum, tin, and antimony. If only one type of alloy element is added, although the surface condition of the zinc layer will change after solidification, this alone is still insufficient to produce typical zinc flowers.

Methods for forming zinc flowers

The method for forming zinc flowers involves deliberately generating a large number of crystal nuclei at an early stage and lowering the solidification temperature of the zinc liquid, thereby extending the growth period of the zinc flower crystals and allowing them to grow larger.

(1) Adding other elements to the zinc bath

After adding a certain alloying element to the zinc bath, the crystallization process can be prolonged, meaning that the surface zinc liquid will not solidify before reaching its eutectic temperature.

Pure zinc solidifies at 419.5℃. However, if tin is added (at less than 0.5%), the tin-zinc eutectic alloy will not solidify until the temperature drops below 198℃. If cadmium is added, the cadmium-zinc eutectic alloy will not solidify until the temperature falls below 264℃. Similarly, if antimony is added (at less than 0.3%), the antimony-zinc eutectic alloy will not solidify until the temperature drops below 409℃. And if lead is added, the lead-zinc eutectic alloy will not solidify until the temperature falls below 317℃.

Therefore, adding alloy elements to lower the melting point of the zinc bath results in a varying degree of lengthening of the solidification time compared to pure zinc. This significantly prolongs the growth period of the zinc flower crystals, facilitating their enlargement and enabling the formation of larger zinc flowers.

(2) Surface Gas Blowing Method

When the surface zinc layer is about to solidify, passing a steam flow or a sulfur dioxide gas flow over its surface will facilitate the smooth growth of zinc flower crystals, resulting in larger patterns. Another purpose of blowing sulfur dioxide gas is to form a dense oxide film, thereby enhancing the corrosion resistance of the galvanized layer.

(3) Steel Wire Mesh Method

This method can only be applied to galvanized thin sheets. It works by using magnetic rollers to bring the wire mesh into contact with the surface of the zinc layer. Since the points of contact between the wire mesh and the zinc layer are cooled first, crystalline solid particles form at these locations, serving as crystal nuclei. The remaining liquid zinc that has not yet solidified then uses these nuclei as starting points for outward crystallization, thereby forming zinc flowers.

(4) Water mist spraying method

While the pure zinc liquid on the surface of the galvanized steel part is still molten, a mist of water (a mixture of water and steam or water and air) is sprayed onto it to form crystal nuclei. The molten zinc liquid, still in a liquid state, uses these nuclei as starting points and spreads outward, thereby forming zinc flowers.

(5) Other methods

In addition to the methods mentioned above, larger zinc flowers can also be obtained if the surface of the plated part is smooth, the galvanized layer is relatively thin, the temperature is uniformly reduced, and sufficient cooling time is allowed in the air.

The shape of zinc flowers

The shape of zinc flowers depends on the type of alloy added to the molten zinc. For example, if lead and antimony are chosen, you’ll obtain phoenix-tail-shaped zinc flowers; whereas selecting lead and tin will yield fern-like zinc flowers, and so forth.

The crystal orientation of zinc blooms on the coating surface varies depending on their appearance, and consequently, their machinability differs during processing. For instance, during simple stretching and deep drawing, feather-like zinc blooms exhibit the best machinability, followed by fern-like zinc blooms; leaf-like zinc blooms have the worst machinability. However, in bending and tensile processes, leaf-like zinc blooms demonstrate the best machinability, followed by feather-like zinc blooms, while fern-like zinc blooms have the worst machinability.

Zinc flower size

According to the European Standard EN 10346:2015, the size of hot-dip galvanized spangles can be classified into normal spangle and minimized spangle.

Regular zinc bloom refers to the size and morphology of zinc grains formed after the zinc melt has solidified normally. In contrast, “no bloom” (also known as “small bloom”) is achieved by specially controlling the solidification process of zinc, resulting in zinc grains with a different size and morphology. If a user requires a specific zinc bloom size, this should be discussed and agreed upon at the time of order placement.

Many factors influence the size of zinc blooms. When the composition of the molten zinc meets the conditions necessary for zinc bloom formation, the following factors will affect the size of the zinc blooms:

(1) Raw material: The thicker the raw steel plate and the lower its surface roughness, the larger the zinc flowers will be.
(2) Cooling rate: The faster the cooling rate, the shorter the crystal growth time and the smaller the zinc bloom.
(3) Number of Crystal Nuclei: The size of zinc flowers is closely related to the number of zinc flower crystal nuclei. When the steel substrate surface is rough or has poor cleanliness, the zinc flowers tend to be relatively small. Zinc grains form on the steel substrate; the more crystal nuclei there are, the smaller the zinc flowers will be. The spray-mist method used in the production of small zinc flowers exploits this principle.
(4) Other factors (such as the production environment).

To obtain small zinc flowers, during galvanizing, as the molten zinc approaches its solidification temperature, spray water droplets with a diameter of less than 0.1 mm onto the surface of the galvanized sheet immediately, thereby forming fine and uniform crystal nuclei. To produce smooth zinc flowers, subsequently perform leveling rolling with a reduction in thickness of less than 1%. Small zinc flowers are suitable for coating applications, while smooth zinc flowers are ideal for deep drawing.

Zinc coating adhesion

As the heated steel strip passes through the zinc bath, an iron-zinc alloy layer first forms on the substrate, and a pure zinc layer then covers the alloy layer. The iron-zinc alloy layer has an uneven thickness, is relatively loose, brittle, has poor ductility, and is prone to cracking.

If a certain amount of iron-based salts remains on the steel plate surface before galvanizing, the resulting iron-zinc alloy will exhibit poor adhesion, poor ductility, and a tendency to flake off easily. When the pure zinc coating contains impurities such as zinc oxide, zinc dross, cadmium, or lead, large zinc blooms may form, or the continuity of the zinc layer may be disrupted, making the pure zinc layer prone to minor cracking.

When the galvanized layer becomes too thick, its flexibility also deteriorates. When aluminum is added to the zinc bath, the aluminum in the bath reacts with the substrate, initially forming Fe2Al5 or Fe2Al3, which thins the iron-zinc alloy coating and thereby enhances the adhesion of the zinc layer.

Uneven Zinc Coating and Influencing Factors

Uneven zinc bloom on the coating surface is a common surface defect found in hot-dip galvanized products. Although it generally does not significantly affect the performance of most products, it does impair their aesthetic appearance and thereby limits their application in certain market segments. In some cases, products with uneven zinc bloom also exhibit poor adhesion of the zinc layer, directly compromising their usability.

The uneven surface appearance of zinc flowers on hot-dip galvanized sheets can be categorized into the following types:

(1) One side has zinc spangles, while the other side does not. The diameter of the zinc spangles on the side with spangles is approximately 6 mm.
(2) Along the longitudinal direction of the strip steel, the size of the zinc flowers exhibits regular fluctuations.
(3) Along the width of the strip steel, one side has large zinc flowers while the other side has small zinc flowers.
(4) The zinc flowers at the edges of the strip steel are small, while those in the middle are large.

The main factors affecting uneven zinc flower formation include: the ratio of two alloying elements in the zinc bath composition, the air knife control process, the strip steel temperature, the impurity content in the gas and combustion-supporting air, the cleanliness of the strip steel surface, the reduction state within the furnace, buildup on furnace rollers, scratches on the strip steel, oxide scale on the raw material surface, the temperature of the strip steel as it enters the zinc pot, the temperature of the zinc bath itself, and the cooling temperature after plating.