Passivity and corrosion of Cu-xZn (x=10-40 wt%) alloys in borate buffer containing chloride ions

The electrochemical behaviour of Cu-xZn alloys and of Cu and Zn metals was studied by cyclic voltammetry and chronopotentiometry in borate buffer, pH = 9.2, with and without the addition of chloride ions in the range from 0.01 M to 1 M. In general, the shape of voltammograms of four Cu-xZn alloys with 10 - 40 wt.% of zinc resembles that of copper more than that of zinc. With increasing zinc content several characteristics of zinc are observed. In borate buffer containing chloride anions, Cu-xZn alloys are susceptible to pitting corrosion. The breakdown potential, E-b, at which the current density in the passive region starts to increase abruptly, becomes more negative with increasing zinc content in the alloy. The general relationship E-b = a + b log c(NaCl) held in all cases, with constants a and b, however, being dependent on the zinc content of the alloy and on the chloride concentration. The corrosion resistance of Cu-xZn alloys was less than that of copper metal but significantly greater than that of zinc.