Pattern Formation on Aluminum Electrodes

The mechanisms of the cells forming the almost periodic structure onanodized aluminum electrodes in contact with various acid solutions was investigated. Their size was correlated to the product of the electrode wettability variation in-duced by switching the applied potential from a hydrogen evolution potential to an anodizing potential and the current density during the anodizing process, The mechanism of cell formation is related to solute concentration i homogeneities at the film-solution interface generating local variations in the film-solution interracial tension. Interfacial tension gradi-ents induce spatial and temporal variations in the electrolyte velocity. Particles dragged by electrolyte motion will also have different velocities, so that interparticle contacts will occur. The internal motion of the fluid film accelerates the ag-gregation of particles into preferential geometries which were observed onthe electrode surface by SEM. The size of cells forming the structure, for various acids and for various concentrations of sulfuric acid, were determined experimentally and a successful matching of these results with the model recently developed was obtained. The anodic oxidation of aluminum is a technique which has been known for many years and which has found widespread industrial application (1, 2). Coatings can be formed in various electrolytes under a wide variety o