An electrochemical cell was made using transparent acrylic plates. An electrochemical measurement was performed by a three-electrode system. The brass plate and the platinum wire were used as the working electrode and the counter electrode, respectively. A KCl-saturated silver/silver chloride electrode was used as the reference electrode. The electrolyte solution was 0.5 M NaCl containing 5 mM NaHCO₃. The temperature of electrolyte solution was kept at 60 ^oC. The brass plate was fixed at wall surface of the electrochemical cell to record the video of the brass cross-section.

The video of the brass cross-section was taken by an optical microscope combined with a charge-coupled device camera under the galvanostatic polarization for 24 h. The brass cross-section under the galvanostatic polarization was successfully observed from the video recording, indicating that the dissolution of brass progressed inhomogeneously in the depth direction. We estimated the erosion depth from the cross-sectional surface images at each measurement time. The results demonstrated that the erosion depth was increased with time under galvanostatic polarization. The dissolution process involving preferential dissolution of zinc or the simultaneous dissolution of zinc and copper was discussed based on the video images and the potential changes of brass during the measurement.

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5) Y. Hoshi et al., J. Electrochem. Soc., 165, C243 (2018).