Film Formation and Hydrogen Evolution on the Alkaline Zinc Electrode

The duplex structure of anodic films formed in concentrated solutions with convection nearly absent—a hydrodynamic condition present near a battery plate in proximity to a separator—has been confirmed on high‐purity polycrystalline zinc and to a more limited extent on some polycrystalline zinc alloys as well. Previously, the use of a very slow potential scan with simultaneous microscopic observation of single‐crystal zinc electrodes showed that a precipitated coating of particles (type I film) overlies a more coherent film also of (type II). The experimental conditions required for following stages in the formation of the precipitated film, the one most difficult to observe, are defined more explicitly in this paper. Evidence is presented that the type II film serves as a catalyst for hydrogen evolution at potentials anodic to the zinc/zinc oxide equilibrium value. Hydrogen bubbles, whose formation is so catalyzed, can mechanically dislodge overlying passivating film to cause reactivation of the electrode. The presence of complex tin or lead anions at concentrations about 10 −3 M (molar) in solutions gives rise to very smooth, almost imperceptible, films of metallic tin or lead over the zinc electrode surface. Such films are made visible, though, during zinc dissolution as they rumple and eventually gather into a wad after being undercut. These foreign metal films inhibit the dissolution of zinc to an extent that depends on the particular complex anion, its concentration, and exposure time to the zinc surface. Lead films can be made sufficiently inhibitive that zinc dissolution is reduced to such a rate that very little precipitation of occurs.