A NEW POTENTIAL-PH DIAGRAM FOR AN ANODIC FILM ON PB IN H2SO4

To study the composition, structure, and electrochemical behavior of the anodic film on lead electrode, a potential-pH diagram for the Pb-H2O-H2SO4 SYStem has been recalculated and reconstructed according to the activities of the SO42- ion in an actual anodic film. The pH values, the activities of the other ions, and the potentials On the interfaces of the three-phases have been found in the equilibrium condition. There mainly exist the H+ and SO42- ions in the PbSO4 layer and the Pb2+ and OH- ions in the PbO layer at potential < 0.9 V. The distribution of the ionic activities in the basic lead sulfate layer is complex. A maximum and minimum of the activity of the Pb2+ ion exist simultaneously in the layer. The transport number of the ion and the diffusion potential in the anodic film vary with different polarization conditions. By using the diffusion potential and the diagram, the formation potentials of tetragonal PbO and basic lead sulfates have been calculated and are in agreement with the experimental results. The composition and electrochemical properties of the anodic film have also been discussed.