Stress generated on aluminum during anodization as a function of current density and pH

Anodic oxidation is accompanied by stress generation during the growth of the oxide. The present study focuses on the stress-generation mechanism on aluminum as a function of the applied current density in acidic solutions of sulfuric acid of different strengths giving variable pH. A beam-deflection technique was utilized for the determination of the magnitude and direction of stresses generated during the anodic oxidation process. Generally, thickness of the oxide determines whether the stress is compressive or tensile in nature. The results have been interpreted in terms of the formation and annihilation of anion (O2-) and cation (Al3+) vacancies. A reduction in the aluminum-ion vacancy concentration (VAl3-) results in a compressive stress deflection, whereas tensile-stress deflection is introduced by an increase in the oxygen vacancy (Vo(2+)) concentration. The mechanism is further elucidated by current-density jump/drop experiments. The results show that stress in this case is affected by the dissolution of the oxide at the oxide-electrolyte interface.