CORROSION AND PASSIVATION OF PB IN CO32- SOLUTIONS

The electrochemical behaviour of lead in carbonate solutions has been investigated using impedance, cyclic voltammetry and open-circuit pote ntial measurements. Under open-circuit conditions, the lead electrode has been passivated by the formation of PbCO3 film. The rate of passiv ation is found to be dependent on the carbonate ion concentration. The reciprocal capacitance, the polarization resistance and the equilibri um electrode potential are found to increase with time following a sim ple logarithmic law. Cyclic voltammograms of Pb in 0.15 M Na2CO3 displ ay three anodic current peaks. The peaks correspond to the oxidation o f Pb into PbCO3 the transformation of PbCO3 into PbO, and then the fur ther oxidation of PbO into PbO2. On reversing the scan, two reduction peaks could be observed. The first is due to the reduction of PbO2 int o PbO and the second corresponds to the transformation of PbO into PbC O3. The peak currents are found to be dependent on both the sweep rate and electrolyte concentration. The non-linear dependence of the peak current on the sweep rate indicates that the passivation process is ir reversible and controlled by several factors other than diffusion of P b2+ ions.