THE REDUCTION PROCESS OF THE ANODIC PB(II) OXIDE FILM ON LEAD PLUS BISMUTH ALLOY IN SULFURIC-ACID-SOLUTION

The anodic film studied was formed on Pb + 9 at. % Bi at 0.9 V (vs. Ag /Hg2SO4) in 4.5 M H2SO4 solution (25 degrees C). The change of the pha se composition of the anodic film during reduction at -0.91 V (vs. Hg/ Hg2SO4) (slightly more positive than that of the reduction potential o f PbSO4) was determined by X-ray diffraction (XRD). The experimental r esults show that the crystals of PbO.PbSO4 and t-PbO in the anodic fil m can be reduced, but Bi2O3 crystals cannot. However, Bi2O3 crystals c an be reduced when PbSO4 crystals are cathodically reduced. The existe nce of bismuth in alloy increases the corrosion rate of lead. The curr ent-time transient shows that the process of reduction is limited by t hree-dimensional growth of circular cones with progressive nucleation when the potential is stepped from 0.9 to -0.91 V for the anodic film on Pb + 9 at. % Bi, but for a thicker film it is also controlled by io n transport through the film. The PbSO4 film formed on Pb is denser th an that on Pb + 9 at. % Bi. The reduction process of the anodic film o n Pb cannot be described by a simple nucleation and growth mechanism.