Ask the Expert: Aluminum Anodizing: From Process to Performance

Angela Heintel provides an overview of the key factors that impact zinc plating efficiency, including additive balance, zinc concentration, generator control, and carbonate levels in high-production rack systems.

Products Finishing

Expert Clinic/Plating
Angela Heintel, Lab Chemist ~ Columbia Chemical

Q. We operate a high-production rack alkaline zinc line processing complex geometries and have observed reduced plating rates and areas with little or no deposit. We use a zinc generator to maintain metal levels. What additional factors could be reducing efficiency?

A. In a high-production rack alkaline zinc system, chemistry imbalances are often amplified due to increased current densities and tight cycle times. A common misconception when troubleshooting issues such as poor coverage, dullness at low current densities or reduced plating speeds is that the brightener concentration is too low. When a small addition of brightener produces a slight improvement, it is common to assume that adding more brightener will continue to improve results. However, excessive brightener additions, without first considering bath parameters, impacts efficiency and destabilizes the system. Reduced efficiency is typically caused by an imbalance between zinc metal and the organic additive system used, inconsistent zinc generator control or high carbonate levels.

            Zinc metal should generally be maintained at 1.5-2.0 ounces per gallon (oz/gal), caustic soda at approximately 16-20 oz/gal and carbonates as low as possible, with a maximum of 12 oz/gal. Even when these values fall within range, Hull cell panels remain the most reliable tool for evaluating plating speed and distribution. Under controlled conditions, a 2-amp, 30-minute Hull cell panel should produce approximately 0.50 mils at 80 ASF and 0.25 mil at 4 ASF, yielding a 2:1 thickness ratio. Reduced thickness in high current density areas indicates loss of cathodic efficiency.

Maintaining the balance

It is helpful to understand a bit of the science behind the process. The organic additive system in alkaline zinc plating consists of a base (polymer carrier) and a brightener component. The base provides primary cathodic polarization, which controls current distribution and throwing power. The brightener refines grain structure, enhances surface brightness and contributes to secondary polarization. Cathodic polarization essentially reduces the rate of anodic dissolution and slows down corrosion. When operating a high-production rack system, maintaining a balance between zinc concentration and this additive system is critical. Within optimal operating conditions, polarization supports uniform zinc deposition across the full current density range. When the concentration of the additive system exceeds its optimal range — particularly the base/polymer component — cathodic polarization increases beyond the necessary range for efficient zinc deposition, leading to increased overpotential. As this overpotential rises, zinc ions are consumed faster than they are replenished and side reactions are activated, such as hydrogen evolution. The effects can be seen as embrittlement, pH shifts to acidic conditions and reduced overall efficiency.

Potential low plating rate, efficiency factors

If you are experiencing low plating speeds, evaluate the entire system before only adjusting brightener levels. A quick corrective step may include the addition of industrial-grade sodium hypochlorite at 1% per volume, followed by thorough filtration to remove any potential organic contamination and excessive brightener/base components.

            Zinc metal concentration is another factor that directly influences efficiency and polarization. Concentrations above 2.0 oz/gal provide minimal added efficiency gain, while concentrations below approximately 1.0 oz/gal significantly reduce thickness due to insufficient zinc metal available. Maintaining a stable zinc concentration will result in a consistent zinc-to-brightener ratio and predictable polarization behavior. Changes in zinc concentration will shift the polarization response and translate into measurable thickness variation and unstable plating rates.

            Zinc generator systems must be controlled to maintain stable zinc concentration in your main plating bath. Variations in zinc concentration in the generator tank, caustic strength or automatic feeder rates produce corresponding inconsistencies in the main bath. Both the generator tank and main bath should be routinely analyzed for zinc metal and caustic soda levels to ensure automated feed systems are properly calibrated to deliver solution at controlled, consistent rates.

            Sodium carbonate concentration also impacts efficiency and should be kept as low as possible. Elevated carbonate levels reduce bath conductivity and increase solution resistance, contributing to decreased cathodic efficiency. Routine monitoring prevents carbonate buildup and costly operational expenses.

            To address reduced efficiency in alkaline zinc plating, begin with effective troubleshooting through disciplined routine analysis, Hull cell verification and controlled additive adjustments. Thickness results should be documented and correlated with individual parameter changes to identify root causes. Remember, in high-production rack systems, process stability is the foundation for consistent plating speed, uniform coverage and high-quality alkaline zinc deposits.

 

This article was published in the May 2026 issue of Products Finishing magazine.  https://www.columbiachemical.com/wp-content/uploads/Ask-the-Expert-by-Angela-Heintel.pdf