PASSIVITY OF ALPHA-BRASS (CU-ZN/67-33) AND ITS BREAKDOWN IN NEUTRAL AND ALKALINE-SOLUTIONS CONTAINING HALIDE-IONS

The passivation behaviour of alpha-brass (Cu: Zn = 67:33) in alkaline solutions was studied using cyclic voltammetry and potentiostatic current transient measurements. The recorded cyclic voltammograms exhibited the main features usually observed for pure copper and zinc, and one additional anodic peak on the reverse potential scan. The height, sharpness and location of the different peaks depended greatly on the alkali concentration and the scan rate. The results show that the formation of Cu2O and Cu(OH)2 films proceed under ohmic resistance control following a dissolution-precipitation mechanism. The effect of F-, Cl- and Br- ions on the passivity was also studied. The pitting potential was found to decrease with algarithm of halide ion concentration. The current transients in the absence and presence of halide ions were analysed. In the absence of pitting the current, after a few seconds, was found to increase linearly with the reciprocal of the square root of time while in the presence of pitting it was found to fit the Engell-Stolica equation.