Bath purity, deposit quality: the role of copper control in ruthenium plating processes

Galvanotecnica Year XXXVI - March – April 2026 - ISNN 1121 - 855X

Ruthenium is a precious metal widely used in the electroplating industry as an electrolyte. Despite its rarity, it is generally cheaper than other commonly used precious metals: approximately 42 €/g compared to 138 €/g for gold and 311 €/g for rhodium (February 2026 data). Although the cost remains significant, ruthenium is a valid alternative to other noble metals, allowing for a reduction in costs while maintaining the same functional performance. Its properties allow it to be used in high-tech sectors, such as microelectronics and fuel cells production, while also being widely used in high-end decorative electroplating.

From an aesthetic point of view, ruthenium naturally offers a bright silver-white finish; however, the introduction of specific additives allows the deposit to be modulated towards dark gray or deep black tones.

Historically, the spread of ruthenium in the decorative sector has been hampered by the cost of the metal and the complexity of managing the baths, with simpler alternatives such as black gold or black nickel often being preferred. The turning point came with the introduction of European regulations limiting the release of nickel for objects in prolonged contact with the skin (nickel-free). In this scenario, black ruthenium has established itself as the solution of choice: a technically superior response which, when integrated with appropriate post-process treatments, guarantees finishes of the highest quality, stable over time and compliant with the most rigorous safety standards.

While technical applications require thicknesses of up to 1 µm to exploit its exceptional hardness (greater than 1000 HV), decorative applications usually use “flash” thicknesses of 0.1–0.2 µm. In this context, ruthenium is used as a barrier layer to limit the diffusion of the underlying metals, ensuring high-performance finishes.

The management of ruthenium baths is known to be one of the most challenging aspects of decorative electroplating. The operating temperature necessarily fluctuates between 50 and 70°C, as a drop below 50°C causes a collapse in deposition efficiency, while exceeding 75°C can lead to rapid degradation of organic additives, especially in black ruthenium baths. Temperature is not the only factor affecting good deposition; precise pH control is also necessary. Values usually remain below 1.5, often close to 1.0, and require almost daily monitoring, as too high a pH can cause ruthenium salts to precipitate as hydroxides, rendering the bath unusable. The process also requires a current density of between 0.5 and 2.0 A/dm²: currents that are too high cause “burns” (dusty or opaque black deposits that do not adhere) and excessive hydrogen development.

Simplifying the management of ruthenium is difficult: the process must contend with minimal cathodic efficiency and the metal's natural tendency to generate internal stresses (micro-cracks). Vulnerability to cross-contamination (cross-metals) is another critical obstacle to ensuring consistent, defect-free deposition.

Unlike common galvanic processes, ruthenium is characterized by extremely low cathodic efficiency (10–20%), which results in massive hydrogen gas development. This dynamic represents a significant technical challenge: the accumulation of hydrogen alters the pH locally on the piece and generates pitting, in which trapped micro-bubbles create craters and structural discontinuities.

At the same time, the ruthenium crystal lattice develops high internal stresses: even at very low thicknesses (0.2–0.5 µm), these stresses exceed the cohesive strength of the metal, triggering micro-cracking. The problem can be solved with the use of ‘anti-stress’ additives, but their management is complex due to rapid degradation and the resulting chemical instability of the bath over time.

Added to this is the extreme vulnerability to external metal contamination (cross-metals). The acidity required by the bath indirectly catalyzes pollution: the electrolyte attacks any exposed non-noble surface, such as unprotected areas of zamak parts or worn frames, instantly dissolving impurities of copper, zinc, and iron. This contamination causes two types of damage: it compromises the immediate appearance, preventing the correct color point from being achieved, and triggers latent degradation processes that lead to mechanical failure of the coating.

Copper contamination remains one of the most persistent problems in ruthenium bath management. The main sources of pollution are linked to three common causes: operational accidents (pieces falling into the bottom of the tank), equipment wear (compromised frame insulation), and insufficient coverage in hollow areas, which expose the base metal to direct attack by the electrolyte. Although copper contamination is a common problem in all precious metal baths, in the case of ruthenium it becomes an intolerable limitation even at infinitesimal concentrations, between 5 and 10 ppm. A presence of copper that in other contexts would be considered barely detectable, in ruthenium already compromises the integrity of the color point and the brilliance of the finish.

This contamination remains “hidden” during the deposition process and can clearly manifest itself with effects on the finished piece. First and foremost, the anthracite gray or gunmetal color is one of the most appreciated characteristics of ruthenium. The presence of copper shifts the color towards brownish tones or creates opacifying effects, depriving the piece of its typical metallic brilliance.

The use of ruthenium for high-end metal finishes is justified by its exceptional chemical inertness. If the electrodeposition process is properly monitored, the coating provides lasting protection against corrosion and high resistance to mechanical wear. However, the co-deposition of copper radically compromises this performance: once trapped in the lattice, copper migrates and oxidizes, triggering premature tarnishing (dark spots or iridescent halos).

In addition to aesthetic damage, contamination alters the structural stability of the coating. The copper atom acts as an intruder element that distorts the ruthenium crystal lattice beyond its physical limits. The resulting accumulation of elastic energy drastically increases the fragility of the layer, promoting the appearance of cracks and, in severe cases, total detachment of the finish (peeling).

Visual and scanning electron microscope (SEM) analysis of the samples reveals how the presence of metallic impurities, particularly copper, compromises the integrity of the ruthenium deposit.

Figure 1 is a direct visual representation of a typical localized “staining” defect: the area has a bluish and greenish iridescence, which is direct evidence of copper co-deposition. When exposed to moisture or the atmosphere, copper tends to oxidize quickly, creating thin layers of oxides that ruin the appearance of the piece.

Figures 2, 3, and 4 below show SEM enlargements that reveal the micro-cracks that form on the coating; the embrittlement of the surface leads to mechanical failure of the surface.

Figure 2 shows a section of the surface where the coating has peeled off, exposing the underlying substrate. In this case, the copper inserts itself into the interface between the galvanic layers, preventing the correct metallurgical bond between the ruthenium and the bonding layer. Figure 3 confirms that the insertion of copper alters the crystalline growth of ruthenium. The result is a fragile deposit that fractures under stress. The uneven growth resulting from contamination is visible in Figure 4: the surface is extremely irregular and cratered, thus compromising the final shine.

Copper pollution remains a crucial issue that requires rigorous and timely analytical monitoring. Traditionally, the quantification of trace metal contaminants relies on two official laboratory methods: ICP (Inductively Coupled Plasma) and AAS (Atomic Absorption Spectroscopy). ICP is considered the most reliable method in terms of accuracy but is not widely accessible due to high operating and maintenance costs, as well as the need for highly specialized personnel to calibrate and use the equipment. The main limitation shared by ICP and AAS remains the management of extremely complex equipment that does not allow for constant monitoring of the process, relegating analysis to a generally weekly basis. To resolve this critical issue, a rapid analysis system such as CDR GalvanLab® can be adopted: the approach with optimized methods allows for a radical change in the management of the ruthenium bath. The system allows a test to be performed in a few minutes directly at the production site, avoiding the need to wait several days for results from an external laboratory. The speed of execution allows quality control to be transformed from weekly to daily, immediately intercepting peaks related to copper pollution.

The advantage lies not only in the speed of execution but also in its ease of use: no specialized technical personnel are required as all procedures are guided, and the reagents are pre-bottled and pre-calibrated, reducing the risks of sample preparation and handling. The method guarantees high sensitivity and repeatability even at extremely low concentrations, down to 3 ppm.

Real-time copper quantification on the production line allows for the introduction of an internal quality control protocol that enables immediate assessment of selective purification, avoiding the risk of producing entire batches of waste between one analysis and the next.

In a market where surface finish quality is a non-negotiable requirement, implementing an internal real-time monitoring protocol is no longer an option, but a strategic necessity.

As analyzed, the vulnerability of the ruthenium bath to copper contamination can compromise the entire production chain in a matter of hours. Moving quality control from the external laboratory directly to the tank edge—thanks to rapid and intuitive systems such as CDR GalvanLab®—allows you to transform what is often “blind” management into a data-driven process.

Dedicating a few minutes to a simplified analysis, accessible to all production staff, generates an immediate economic return:

• It prevents the formation of aesthetic and structural defects on batches in production.
• It avoids the enormous cost of having to rework or discard parts that are already finished and ready for delivery.
• It ensures operational continuity, protecting the company's reputation with its luxury and technology partners.

Ultimately, constant analytical prevention is the best insurance policy for modern electroplating: a small daily gesture that safeguards the value of the finished piece and the overall efficiency of the plant.

We would like to express our sincere thanks to Valmet Plating for its ongoing support in the development of innovative and practical analytical methods dedicated to quality control in electroplating processes. Further thanks also go to Valmet Plating's testing and quality control laboratory for its valuable scientific contribution and for providing the photographic and SEM images that supported the technical documentation of this work.

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