Electrochemical Impedance Study on Dissolution of Cu-Zn Alloy in Fresh Water

The brass (Cu-Zn alloy) is widely used for the water pipe of constructional plant and the joint of the device material. One of the corrosion problems of Cu-Zn alloy is the dezincing, which is the selective dissolution of Zn from Cu-Zn alloy. The fundamental studies of anodic dissolution of Cu-Zn alloy were reported by Pickering et al. (1-3). They indicated that a porous Cu-rich layer was formed on the surface of Cu-Zn alloy due to the preferential dissolution of Zn from Cu-Zn alloy. They proposed the two types of the dissolution mechanisms, which were simultaneous and selective. However, the stability of the Cu-rich layer formed on the Cu-Zn surface has not been clarified yet.

In the present study, the dissolution of Cu-Zn alloy in the fresh water was investigated by an electrochemical impedance spectroscopy (EIS). The measurement of the electrochemical impedance was performed by three-electrode system. The working electrode (0.785 cm²) was Cu-Zn alloy (Cu-35at%Zn) and the counter electrode was the platinum wire. The KCl-saturated silver/silver chloride electrode (SSE) was used as the reference electrode. All the potentials in this study were referred to SSE. In order to investigate the stability of the Cu-rich layer formed on the surface of Cu-Zn alloy, the formation of the Cu-rich layer was conducted as follows. The Cu-Zn alloy (Cu-35at%Zn) was abraded by #1200 emery paper under running water. Then, the galvanostatic polarization of Cu-Zn alloy was measured at 1.0 mA/cm² for 24 h. The electrolyte solution was 0.5M NaCl containing 5mM NaHCO₃. The temperature of the electrolyte solution was keep at 60 ^oC by thermostatic bath.  

The impedance spectra of the Cu-Zn alloy before and after the formation of Cu-rich layer on the surface of the Cu-Zn alloy were measured to examine the stability of the Cu-rich layer. In addition, the results of the rest potential and the anodic polarization curve were also discussed in the present study.  

Reference 

1.               H. W. Pickering and C. Wagner, Journal of the Electrochemical Society, 114, 698 (1967).

2.               H. W. Pickering, Journal of the Electrochemical Society, 115, 143 (1968).

3.               H. W. Pickering and P. J. Byrne, Journal of the Electrochemical Society, 116, 1492 (1969).