Abstract
Anodic coatings grown on aluminum in certain organic electrolytes have been studied by transmission electron microscopy examination of thin cross sections. The electrolytes have low water content and contain ethylene glycol as a solvent component and a dicarboxylate as solute. A cellular porous coating grows although aluminum is insoluble and oxide deposits with about 95% faradaic efficiency. Inner and outer layers in the oxide are easily distinguished, and it is found that the field strength of the outer layer is up to 50% higher than for conventional amorphous alumina due to a high concentration of incorporated organic species. The transition from barrier to porous growth occurs when cusps initiate at concavities that evolve from the initial metal surface texture. A cusp develops into a stable pore because the high field at this site causes Al ion ejection and the unusually small cation transport number in this oxide/organic composite is insufficient for oxide growth at the coating/electrolyte interface. © 2000 The Electrochemical Society. All rights reserved.
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