The thermodynamic state of hydrogen in aluminum during alkaline corrosion was investigated, using a two-compartment hydrogen permeation cell with an Al/Pd bilayer membrane. The open-circuit potential of the Pd layer in a pH 7.0 buffer solution was monitored to sense the hydrogen chemical potential, pH. At pH 12.5-13.5, the measurements established a minimum pH of 0.55 eV relative to the ideal gas reference, equivalent to a H-2 gas pressure of 5.7 GPa. Statistical mechanics calculations show that vacancy-hydrogen defects are stable in Al at this condition. A dissolution mechanism was proposed in which H at very high pH is produced by oxidation of interfacial aluminum hydride. The mechanism explains the observed rapid accumulation of H in the metal by extensive formation of vacancy-hydrogen defects. (C) 2010 Elsevier Ltd. All rights reserved.