ANODIC BEHAVIOR AND PASSIVATION OF A LEAD ELECTRODE IN SODIUM-CARBONATE SOLUTIONS

Cyclic voltammograms of a lead electrode were obtained in Na2CO3 solut ion as a function of the starting potential, electrolyte concentration and voltage scanning rate. The shape of the voltammograms was found t o depend on the starting potential as well as the sweep number. This i s probably due to changes in the activation state of the electrode sur face. The first anodic portion of the voltammograms is characterized b y a shoulder and two peaks corresponding to the formation of PbCO3, Pb O and PbO2, respectively. The cathodic portion shows the occurrence of two peaks corresponding to the reduction of PbO2 to PbO and PbO to Pb , successively, followed by the formation of PbH2. An increase in conc entration of CO32-ions leads to a negative shift in the values of the peak potentials, E(p), accompanying the formation of PbO and PbO2. In addition, the current density for both the anodic oxidation peaks show ed marked dependence on the concentration of the electrolyte. An incre ase in the scanning rate was observed to lead an increase in the size of the voltammograms. The current density of both the anodic peaks and the anodic passivation region were proportional to nu1/2. Such behavi our is expected in a diffusion-controlled processes. In addition, the anodic peaks are shifted towards more positive values of potential, wh ereas the cathodic peaks are shifted in the negative direction, indica ting irreversible formation of the passive film on the electrode surfa ce.