Anodic Crystallization on Pure and Antimonial Lead in Sulfuric Acid

Electrochemical cycling, x-ray diffraction, and electron microscopy were used to study anodic crystallization on pure and antimonial Pb in H2SO4. On pure Pb a maximum electrochemical capacity developed that did not increase with further cycling. The anodic coating was comprised of small needlelike crystals that grew with cycling. The amount and crystallinity of beta PbO2 gradually increased in a soft porous outer layer. Attached to the metal was a layer of alpha PbO2. On the Sb-Pb alloy the capacity continually increased with cycling. A compact eutectoidal coating of small crystals of alpha and beta PbO2 was formed. It was concluded that on pure Pb beta PbO2 does not bond to alpha PbO2 and that Sb in the Sb-Pb alloy acts as a nucleating catalyst for beta PbO2 in the corrosion product attached to the metal surface. Antimony also promotes intercrystal bonding between the two polymorphs of PbO2. The morphologies of the PbSO4 crystals were also studied. The crystals formed on soaking in the electrolyte, and during discharge of PbO2 coatings, developed by electrochemical cycling, were examined. Well-developed prisms, dendrites, and hopper crystals were observed. The discharge of the antimonial coatings appeared to be limited by the growth rate of the PbSO4 crystals. The fundamental aspects of electrocrystallization are discussed, and a crystal chemical mechanism is proposed for the action of Sb in the PbO2 electrode. A broad program for future investigation is outlined.