Studies of the surface reaction mechanisms of Pb-3 wt%Sn-0.5 wt%Ag anode in CrO3 solutions

This work examines the change of surface properties and the reaction process of lead alloy anode in CrO3 solution under conditions of high current density, by means of SEM, EDX, XRD, electrochemical test and Raman spectroscopy, for understanding the corrosion mechanism of lead alloy anode. In the initial oxidation stage of lead alloy anode, the diffraction peaks of monoclinic PbCrO4 and PbO are clearly shown in the XRD spectrum of the lead alloy surface. With increasing reaction time, PbCrO4 and PbO diffraction peaks are weakened and those of lead dioxide strengthened. Cyclic voltammetry reveals that the electrode potential change is among Pb, PbO, PbCrO4, PbO1+x (0 < x < 1) and beta-PbO2. According to impedance tests, charge transfer and a corrosion product film on lead alloy surface happen under the high and medium frequencies, and chromium-oxide ion clusters are absorbed on free sites of the anode surface for the low frequency. The corrosion products have characteristic of complex semiconducting behavior shown in the Mott-Schottky plots. Raman spectra indicates that Cr2O72- and HCrO4- ions are firstly absorbed on the anode surface, and then PbCrO4 is produced between Cr2O72- and Pb, PbO is reacted from HCrO4-. The mechanism of the anode reaction at the anode/solution interface is discussed using the experimental results.