Surface film effect on the electrochemical behaviour of active metals

Abstract

It has been suggested that a film on the surface of an active metal should be a dielectric with low ionic and electronic conductivity. The film-electrolyte boundary can be regarded either as an interface between two electrolytes or as the surface of an insulating electrode. In this zone a potential difference is set up and redox reactions proceed. These processes, controlled by the concentration and by the transfer of charged particles through the film, determine the anode-cathode behavior of metals under consideration. Corrosion and anodic dissolution of lithium and magnesium have been investigated in dilute solutions of water in propylene carbonate. The potential of the lithium electrode reveals the properties of the equilibrium potential and does not depend on either the content of water in the solution or the lithium corrosion rate. The observed orders of interaction between lithium and water, however, does depend on the water concentration, on the electrolyte concentration, and on the thickness of the initial film. At the beginning of the process the rate of the interaction between lithium and water is described by the equations of topochemical kinetics, but when the maximum is reached, by the Tamman equation. A stationary potential is realized on magnesium; its value depends on the water content in the solution. The potential shifts towards negative values with increased water concentration.