Brasses are widely used in watch manufacturing and jewelry as based materials, thanks to their cost, good mechanical properties and corrosion resistance. These industrial sectors face major changes in substrate composition in particular due to environmental regulations (changes in the grade of lead-free brass substrates) leading to investigate other brass grades.

Smooth and bright surfaces are usually obtained by mechanical polishing requiring a succession of manual operations. Electropolishing is an anodic dissolution process currently used in industries to replace mechanical polishing entirely or as a finishing treatment for complex shapes. The first patent for copper polishing in an aqueous orthophosphoric acid solution at high current density, was deposited by Jacquet in the 30s (1). The electropolishing quality depends on the material/electrolyte pair and the operating conditions (potential, time process, temperature…) (2,3).

The aim of this work is to observe the impact of alloying composition on polishing quality. Two grades of brass were used CuZn₄₂ which has a diphasic composition, and CuZn₃₇ which has a monophasic composition. They were electropolished in a phosphoric acid solution using a rotating disk electrode. At first, an electrochemical study was performed to investigate the electropolishing mechanism. A kinetic dependence on diffusion following the Levich law, is verified for the two grades of brass. This electrochemical study also allows to determine the best treatment conditions which correspond to the middle of electropolishing current density plateau.

The quality of polishing was evaluated by surface characterization which consists of a set of methods (Stylus Profiler, Secondary Electron Microscopy, Atomic Force Microscopy, X-Ray Diffraction). SEM and AFM images highlight a discrepancy between the two grades of brass (Figure 1). Indeed, CuZn₃₇ sample (monophasic composition) shows a smooth surface and grain boundaries are almost not visible, while CuZn₄₂ sample still presents a biphasic composition with a slight level difference between α and β phases. The roughness parameter Ra confirmed this behavior difference. Because CuZn₃₇ sample presents a roughness equal to 42 nm, while CuZn₄₂ is about 10 times rougher. All these characterization results indicate that there is a preferential attack of β phase (richer in Zn) for biphasic materials which induces some difficulties to apply standard conditions for the electropolishing process.

1. Jacquet PA. Electrolytic Method for obtaining Bright Copper Surfaces. Nature. 1935;135:1076.

2. Lin C-C, Hu C-C, Tai-Chou Lee. Electropolishing of 304 stainless steel interactive effects of glycerol content bath temperature and current density on surface roughness and morphology. Surf. Coat. Tech. 2009;448‑54.

3. Gabe DR. Electropolishing of copper and copper-based alloys in ortho-phosphoric acid. Corros Sci. 1972;12:113‑20.