1.How temperature affects zinc ions?
Low temperature (<20°C):
The electrolyte viscosity is high, and the zinc ion diffusion rate is slow. At this point, using a high current density will cause a "localized shortage" of zinc ions on the cathode surface due to rapid consumption. This results in:
The growth rate of existing nuclei slows due to a lack of surrounding ions;
The number of new nuclei decreases, and the remaining ions tend to concentrate in a few "active grains," causing them to continue growing and forming tiny zinc spangles measuring 0.1-0.5mm.
In addition, uneven ion diffusion at low temperatures exacerbates the "edge effect," making the spangles more visible at the edges.
Medium temperature (20-50°C):
The ion diffusion rate is moderate, allowing zinc ions to be evenly replenished to the cathode surface. Combined with a normal current density, the "nucleation rate ≈ growth rate" can be achieved:
The number of nuclei is sufficient and evenly distributed, leaving individual grains with insufficient space to grow. The resulting fine grains are free of visible spangles.
This is also the standard temperature range for most acid zinc plating processes, balancing ion supply and crystallization uniformity. High temperatures:
The ion diffusion rate is extremely rapid, resulting in an "oversupply" of zinc ions on the cathode surface. At this point:
The "material constraint" on grain growth is removed, allowing the formed nuclei to quickly acquire ions, accelerating growth.
If additive failure also occurs, the grains can overcome the inhibitory effect and continue to coarsen, forming irregular zinc spangles larger than 0.5mm**, which are particularly noticeable at low current densities.
2.How does temperature affect the "inhibitory activity" of organic additives?
Low temperatures (<20°C):
Organic additives have reduced solubility in the electrolyte and their molecular motion is slow, making them more susceptible to strong adsorption on the cathode surface.
Excessive adsorption can cover most of the active nuclei, inhibiting the formation of coarse grains and potentially hindering proper deposition, leading to thinning and hazy coatings.
If the temperature is too low, the additives may even crystallize and adhere to the strip surface, forming "spots" and disrupting the continuity of the coating.
Medium temperature (20-50°C):
The additive has moderate solubility and active molecular motion, enabling a dynamic adsorption equilibrium on the cathode surface.
It selectively adsorbs only on the active sites with the fastest grain growth, precisely suppressing coarse grains while not hindering overall nucleation, ultimately resulting in a spangle-free, bright coating.
High temperatures (>60°C):
Organic additives are prone to thermal decomposition.
The decomposition products lose their adsorption capacity and are unable to inhibit grain growth.
Undecomposed additives, due to excessive molecular motion, are unstable on the cathode surface, significantly reducing their inhibitory effect.
Both factors contribute to uncontrolled grain growth, forming noticeable spangles.
3.How does temperature affect crystal coarsening?
Low Temperature:
The electrolyte has poor conductivity. Maintaining a high current density will result in:
Uneven current distribution on the cathode surface. Localized "current overload" can cause rapid zinc ion reduction, resulting in coarse grain formation.
Therefore, the current density should be reduced at low temperatures, but this can lead to insufficient nucleation and the formation of microscopic spangles.
High Temperature:
The electrolyte has good conductivity, allowing for higher current densities, which theoretically promote nucleation. However,
At high temperatures, the additives have already decomposed. High current densities, however, accelerate zinc ion reduction, leading to rapid, uninhibited grain growth and more pronounced spangles.
Thus, spangles are likely to form at high temperatures, regardless of current level.
4.What is the effect of high temperature on hydrogen evolution?
The hydrogen overpotential decreases (for example, from -0.8V at 20°C to -0.6V at 70°C), accelerating the hydrogen deposition rate and consuming more current (up to 10%-20%).
The actual zinc deposition current decreases, slowing the nucleation rate.
When hydrogen escapes, it forms "bubble tracks" on the coating surface. This uneven zinc ion replenishment near these tracks can lead to the formation of localized coarse grains (zinc spangles are often distributed along the bubble tracks).
5.What is the "three-stage effect" of temperature on zinc spangles?
Low temperature: Zinc ion diffusion is slow, and local supply is insufficient; the additive has strong adsorption activity (easy to be too strong); the crystal state is fine and spangled, but may be foggy, with occasional tiny spangles on the edges;
Medium temperature: zinc ion diffusion; zinc ion diffusion; zinc ion diffusion; crystallization state and zinc flower nano-scale fine crystals, no visible zinc flower
High temperature: zinc ions diffuse quickly and are in excess; additives actively decompose thermally, inhibiting failure; crystallization and coarse zinc grains form obvious zinc spangles