Brass (Cu-Zn alloy) is widely used in the plumbing systems because of its good mechanical properties and low cost. The dezincification corrosion of brass is major problem in the plumbing system because a porous copper rich layer is formed on the brass surface due to the selective dissolution of zinc [1, 2]. Thus, it is difficult to evaluate the corrosion conditions of brass. Hoshi et al. [3] applied an electrochemical impedance spectroscopy for the investigation of the dezincification corrosion of brass. They [3] reported that the distorted impedance is observed in the high frequency range on a Nyquist plane, which is associated with the current distribution on the porous copper rich layer formed on the brass surface. In the present study, a transmission line model (TLM) [4] involving a constant phase element (CPE) was applied to the analysis of the distorted impedance of brass in the high frequency range. The impedance of the brass with the different thickness of copper-rich layer on the brass surface was examined in order to develop the evaluation method for the dezincification corrosion of brass.

A susceptibility test of dezincification corrosion of brass was carried out for 24, 36, 48, and 72 hour in order to form the copper-rich layer on the brass surface. The electrochemical impedance measurement of brass with different thickness of copper-rich layer was performed by a three-electrode system. The working electrode and the counter electrode were the brass with different thickness of copper-rich layer and a platinum wire, respectively. A KCl-saturated silver/silver chloride electrode (SSE) was used as a reference electrode. The electrolyte solution was 0.5 M NaCl containing 5 mM NaHCO₃. The impedance spectrum was measured in the frequency range of 0.1 Hz – 10 kHz at five frequencies per decade with an AC amplitude of 10 mV. The applied DC potential was -0.10 V vs. SSE. After the measurement, the surface morphology of brass was investigated by a field emission scanning electron microscope (FE-SEM).

The impedance spectra of brass with different thickness of copper-rich layer on the brass surface described the locus with small slope in the high frequency range. In this case, the locus shows the straight line whose slope is lower than 45◦ against real axis in the high frequency range. These results indicate that the value of the slope is related to the structures of copper-rich layer. We [3] derived the theoretical equation to determine the dezincification corrosion area of brass from the impedance at transition frequency. Although the distributed constant region and lumped constant region are observed in the impedance spectra, it is difficult to determine the impedance at transition frequency precisely from the plots of impedance in the present study. In order to determine the impedance at transition frequency, the impedance in the distributed constant region was analyzed by the TLM involving CPE and discussed by using CPE constant p. The differential values of impedance in the high frequency range and arbitrary value of p are discussed, and the impedance at transition frequency was determined from the impedance spectra of brass with different thickness of copper-rich layer.

References

[1] H.W. Pickering, P.J. Byrne, J. Electrochem. Soc, 116 (1969) 1492-1496.

[2] Y. Hoshi, M. Itagaki, K. Tabei, I. Shitanda, J Solid State Electrochem, 19 (2015) 3551-3557.

[3] Y. Hoshi, Y. Nishihara, I. Shitanda and M. Itagaki, Journal of Japan Institute of Copper, 56, (2017) 178-183.

[4] R. de. Levie, Electrochim. Acta, 8 (1963) 751.