Über zweiphasige deckschichten auf metallelektroden—I. Thermodynamische stabilitäts- und bildungsbedingungen

Abstract

A group of systems consisting of a metal covered with oxide layers in aqueous solutions is examined, in which the metal forms at least two oxides of different oxidation numbers. The aim is to find necessary conditions of stability and of growth of single-phase and of two-phase layers. These layer systems are characterized by different relations between the free enthalpies of formation of the oxides and of water. The corresponding reversible electrode potentials are U HA, and U HB for the anodic formation of one-phase oxide layers A or B and U HAB of the oxidation of the ‘inner’ oxide layer A producing the ‘outer’ oxide layer B, which contains more oxygen. These three potentials obey the relation of Luther. The potentials of layer formation are compared with the values U HO 2 and U HH 2 of the reversible oxygen and hydrogen electrodes. The metal covered with a two-phase oxide layer can only be stable if U HA < U HB < U HAB. Moreover, a certain anodic overpotential with respect to U HAB is necessary for formation of the two-phase layer. The case of miscibility of the two oxides is also discussed. The results can be used for a possible explanation of the passivity phenomenon of iron. The steps of the above mentioned half-cell reactions forming anodic oxide layers and the corresponding overpotentials on the metal/layer interface, on the layer/layer interface and on the layer/solution interface, as well as the drop of electric potential within the layers are discussed.