Stress induced by incorporation of sulfate ions into aluminum oxide films

The geometry of self-ordered porous anodic aluminum oxide films depends strongly on the type of acid anion in the anodizing solution. Recent studies suggest that electric field-induced anion incorporation in the oxide may generate compressive surface stress that drives oxide flow responsible for formation of self-ordered arrays of pores. This communication reports direct evidence for sulfate ion incorporation-induced compressive stress in native aluminum oxide films from stress measurements during open-circuit Al dissolution in 0.4 M H2SO4, coupled with analysis of the film composition by X-ray photoelectron spectroscopy. The measured force per width (integrated in-plane stress) was compared to the calculated elastic force assuming that all detected sulfate ions are incorporated in the oxide, rather than adsorbed on the oxide surface. Quantitative agreement was demonstrated between the calculated and experimental force, showing that sulfate incorporation dominates over adsorption and induces greater than 1 GPa levels of compressive stress in the oxide. The present work and an earlier study of phosphate incorporation show that anions incorporate at small open-circuit exposure times, when the outer portion of the oxide can support a high electric field because it is not yet completely hydrated. Therefore the electric field in the oxide is the likely driving force for incorporation.