Vacancy defects in aluminum formed during aqueous dissolution

Aqueous dissolution of aluminum is accompanied by extensive absorption of hydrogen, along with formation of hydride and voids. We used in situ stress measurements to discriminate between absorption mechanisms leading to either interstitial or vacancy defects, and to relate defect formation to surface chemistry. Large tensile shifts of the stress–thickness product, approaching 35 N/m, were found during the initial exposure of Al thin films to aqueous NaOH solutions at pH 12–13. The time dependence of the stress–thickness product correlated with mass of metal dissolved, as determined with the quartz crystal microbalance. The observed relationship between stress and mass change was consistent with a significant fraction of dissolved Al atoms forming vacancies or vacancy-hydrogen defects. Electrochemical potential transients indicated that the onset of the tensile stress change corresponds to the presence of aluminum hydride at the metal surface. We propose mechanisms in which vacancy-hydrogen defects form either due to hydride, or because of the elevated hydrogen chemical potential at the Al surface.