THE BEHAVIOR OF LEAD ELECTRODE IN CARBONATE MEDIA UNDER ALTERNATE ANODIC AND CATHODIC POLARIZATION

The electrochemical behaviour of lead has been studied in sodium carbonate solutions of different concentrations using the galvanostic (cyclic anodic and cathodic polarization) method. In solutions with concentration varying from 5 x 10(-2) to 3 x 10(-1) mol L-1 Na2CO3, three arrests were observed corresponding to the oxidation of Pb to PbCO3 which transforms to PbO and later to PbO2, before the evolution of oxygen. In relatively higher concentrated solutions (ca > 3 x 10(-1) mol L-1 Na2CO3), only the first and last arrests are recognized. The second arrest accompanying the formation of PbO in those solution disappears presumably due to increased solubility of PbO in relatively concentrated alkaline media. In the solutions under investigation, a gradual decrease has been observed in the potential accompanying the formation of PbO2 with an increase in the number of cycles. Such behaviour is indicative of a decrease in the nucleation energy necessary to form PbO2 from PbO with cyclization. Passivation of lead in the Na2CO3 solution covering a range from 5 x 10(-2) to 5 x 10(-1) mol L-1 can be satisfactorily represented by the relation. log t = A - n log i where i is the current density and t the time elapsed before the evolution of oxygen. The above equation is an indication that the diffusion of the Pb2+ ion controls the passivation process.