1.What is the essential difference in the principle of zinc spangle formation?
Spangle Formation in Hot-Dip Galvanizing
During hot-dip galvanizing, the steel strip is immersed in molten zinc (approximately 450°C). The molten zinc infiltrates the substrate surface, forming a coating. The strip then cools to room temperature. Spangle is the macroscopic manifestation of the directional growth of zinc grains (dendrites) during the solidification of the liquid zinc. Dendrites in the molten zinc originate from nuclei on the substrate surface and grow in the direction of heat conduction (from high to low temperatures), ultimately forming "spangles" with regular geometric shapes. Their size is directly related to the dendrite growth time and the distribution of alloying elements.
Spangle Formation in Electrogalvanizing
Electrogalvanizing is a process in which zinc ions (Zn²⁺) migrate toward the cathode (substrate) under the influence of an electric field and undergo reduction deposition. The coating is formed at room temperature or a lower temperature (20-60°C). Spangles are mainly aggregated forms of electrodeposited crystal particles, and their size is determined by the number of crystal nuclei and the grain growth rate. If there are few crystal nuclei and the grains grow disorderly, small spangles may be formed. If additives are used to inhibit grain growth or promote uniform nucleation, spangles can be eliminated and a bright surface can be formed.
2.How to control the size of zinc flowers in hot-dip galvanizing?
Precise Control of Alloying Elements
Adding elements such as Sb, Pb, and Sn modifies the solidification characteristics of the zinc melt:
Sb and Pb lower the freezing point, prolong the solidification time, and promote dendrite growth;
High Al content inhibits dendrite growth and refines the spangles.
In actual production, spangle size is directly controlled by adjusting the element ratio.
Graded Cooling Rate Control
The cooling rate determines the dendrite growth time:
Slow cooling: Long solidification time allows for full dendrite growth, resulting in large spangles;
Fast cooling: Short solidification time limits dendrite growth, resulting in small spangles.
Typical parameters: Cooling rate ≤ 5K/s for large spangles; cooling rate ≥ 15K/s for small spangles.
Optimizing Substrate and Zinc Wetting Properties
Substrate surface roughness and cleanliness affect zinc melt wetting uniformity: Excessive roughness can lead to localized aggregation of the zinc melt, resulting in unevenly sized spangles; insufficient cleanliness hinders nucleation, resulting in abnormal spangle distribution.
3.How to control the size of zinc flowers in electrogalvanizing?
Electrolyte Composition and Additive Control
Base Electrolyte: Zinc ion concentration and pH affect ion migration rates. Low zinc ion concentration combined with a high pH value promotes fine grain formation and reduces spangle formation.
Organic additives are key to controlling spangle formation. Brighteners adsorb on the crystal surface, inhibiting disordered grain growth and promoting uniform nucleation, directly eliminating spangle and creating a bright, spangle-free surface. Displacement agents improve current distribution to prevent localized grain coarsening.
Current Parameters and Temperature Control
Current Density: At low current densities, grain growth is slow and coarsening is likely, potentially leading to the formation of fine spangles. At high current densities, nucleation is rapid, resulting in finer grains and even finer spangles.
Temperature: Increasing electrolyte temperature accelerates ion diffusion, potentially leading to grain coarsening, requiring additive adjustments to counteract this effect.
Post-Plated Auxiliary Treatments
Electrogalvanizing rarely relies on the inherent aesthetic value of spangles; spangle removal is often achieved through post-plating treatments such as passivation, bright plating, or light rolling to eliminate minor surface irregularities.
4.What are the typical differences in zinc flower shape and size?
Zinc flower morphology: Hot-dip galvanizing presents obvious dendritic structure, clear boundaries and strong three-dimensional effect;There is no obvious dendrite in electroplated zinc, mostly small particles aggregate, or no zinc flowers (bright)
Size range: Hot-dip galvanizing can be controlled within 1-15mm (large zinc flowers of 8-13mm are common); electroplated zinc is naturally deposited within 0.5mm, and no zinc flowers are visible under the action of brighteners.
Difficulties in uniformity control: Hot-dip galvanizing is affected by the flow of zinc liquid and uneven cooling, which can easily lead to local size differences; electroplating is affected by current distribution, and tiny zinc flowers are easily formed on the edges due to current concentration.
Appearance orientation: Hot-dip galvanized spangles themselves are a "functional appearance" (such as the large spangle corrosion-resistant mark on architectural panels); electroplated galvanized spangles are regarded as a "defect", and the goal is a uniform, bright, spangle-free surface.
5.How does application orientation determine differences in control objectives?
Hot-dip galvanizing: Primarily used in applications such as buildings and outdoor facilities, spangle formation is not only a visual feature but also indirectly reflects coating quality (for example, the absence of spangles may indicate excessive aluminum content, affecting weldability). Therefore, it is necessary to proactively control spangle size and ensure uniform distribution.
Electrogalvanizing: Primarily used in applications such as automobiles and home appliances that require a high surface finish, spangle formation can lead to uneven coating adhesion and varying reflectivity. Therefore, the goal is to eliminate spangle formation. Brighteners and high current densities are used to achieve a smooth, spangle-free surface.