Online analysis of copper, tin, and zinc in white bronze baths by XRF

Single-metal plating is a viable surface finishing solution. However, there is a limit to how much it can improve a surface’s properties. In contrast, co-depositing two or more metals as an alloy coating allows for improvements tailored to specific applications [2].

White bronze is a type of tri-metal alloy, meaning it consists of three different metal elements [3]. Specifically, it is an alloy of Cu, Sn, and Zn (otherwise known as CTZ), carefully engineered to provide superior corrosion resistance and a bright, uniform finish.

The white bronze bath used in electroplating considerably improves both the chemical and physical properties of various metal products. When carefully applied to surfaces, the tri-alloy improves corrosion resistance while providing a visually appealing, silvery-white finish [1]. This procedure is commonly used in the manufacture of jewelry and decorative goods, as it increases the durability and appeal of metal products [1].

One of the primary challenges of maintaining a white bronze bath is ensuring the correct ratio of Sn, Cu, and Zn [1]. An imbalance in the concentration of these elements can lead to non-uniform deposits, which affects the aesthetic and functional properties of the plated layer.

Cyanide compounds are often present in white bronze plating baths, primarily because of their ability to form stable complexes with copper [4]. They ensure efficient metal deposition and a smooth, uniform coating. This cyanide-copper complex helps to control the plating rate and improve the overall quality of the finished layer.

Small fluctuations in metal concentrations can significantly impact the performance of the bath. This leads to issues such as dull deposits, fragile coatings, or poor adhesion. These fluctuations can arise from variations in bath replenishment, consumption rates, or contamination. This makes continuous monitoring essential for stable operation.

Traditional monitoring methods often result in process downtime due to the time required for manual sampling, chemical preparation, and analysis. The labor-intensive nature of these methods also increases the potential for human error, which in turns reduces the reliability of the data collected.

Furthermore, the use of cyanide brings significant handling and safety concerns, as it is highly toxic and requires stringent safety protocols to prevent accidental exposure or environmental contamination [5]. Operators must take great care in storing, handling, and disposing of cyanide solutions, which adds an extra layer of complexity to bath maintenance.

As a result, there is a growing need for online monitoring, which enables real-time adjustments to the bath composition. This prevents potential disruptions and helps maintain optimal plating conditions without the delays associated with offline analytical testing methods.

The 2060 XRF Process Analyzer from Metrohm Process Analytics can be implemented at key points in the plating process to ensure optimal bath composition and coating quality. Its use is beneficial after the pre-treatment and acid activation steps (Figure 1) where the bath composition must be precisely controlled to avoid undesired variations in the plating layer.

Additionally, real-time monitoring during the electroplating process helps maintain the correct Sn, Cu, and Zn ratios. This prevents plating defects such as dull deposits or poor adhesion. By integrating the 2060 XRF Process Analyzer into the workflow, operators can make immediate adjustments to reduce waste and ensure consistent, high-quality plating results.

White bronze bath samples were measured using a tungsten anode source XRF spectrometer. This system ensures high accuracy for Sn, Cu, and Zn detection by using characteristic X-ray excitation. Figure 2 shows the generated spectra with distinct peaks corresponding to Sn, Cu, and Zn in the electroplating bath solution.

While the 2060 XRF Process Analyzer (Figure 3) provides real-time monitoring of the metal concentrations in plating solutions, complementary techniques such as titration can also be incorporated to monitor additional critical bath parameters – like pH and cyanide levels. This combination of methodologies not only enhances process control but also provides a comprehensive solution unique in the market, allowing operators to ensure both plating quality and operator safety with a single, integrated analytical approach. 

AN-PAN-1064 Monitoring complexing agents in galvanic baths inline with Raman spectroscopy

­AN-T-223 Analysis of electroplating baths

­AN-T-024 Metal contents of alkaline plating baths for cadmium, copper, lead or zinc

1. White Bronze, Copper-Tin-Zinc Tri-metal: Expanding Applications and New Developments in a Changing Landscape | Products Finishing. https://www.pfonline.com/articles/white-bronze-copper-tin-zinc-tri-metal-expanding-applications-and-new-developments-in-a-changing-landscape (accessed 2025-02-11).
2. Replacing Nickel with Tri-Metal in Electronics Plating. https://www.pfonline.com/articles/replacing-nickel-with-tri-metal-in-electronics-plating (accessed 2025-02-12).
3. White Bronze Decorative Electroplating Chemistry | Technic Inc. https://www.technic.com/applications/decorative/plating-chemistry/white-bronze-decorative-electroplating-chemistry (accessed 2025-02-12).
4. Zanella, C.; Xing, S.; Deflorian, F. Effect of Electrodeposition Parameters on Chemical and Morphological Characteristics of Cu–Sn Coatings from a Methanesulfonic Acid Electrolyte. Surface and Coatings Technology 2013, 236, 394–399. DOI:10.1016/j.surfcoat.2013.10.020
5. Quality, N. R. C. (US) S. on G. for M. F. D.-W. Guidelines for Cyanide. In Guidelines for Chemical Warfare Agents in Military Field Drinking Water; National Academies Press (US), 1995.