Abstract
Scratch depassivation of AA2024-T351, AA7075-T6, and 99.999% aluminum in molybdate (MoO4 2-) and chromate (CrO4 2-) containing NaCl solutions was used to examine electrochemical repassivation kinetics over a range of potentials and inhibitor concentrations. Single frequency impedance measurements were taken in the capacitive region of frequency response. Capacitance measurements enabled the observation of the effects of inhibitor additions on oxide growth as protective oxide films formed on the scratched electrode surface. Chromate was found to suppress scratched electrode current transients at high potentials on both AA2024-T351 and AA7075-T6. Capacitance measurements revealed the rapid growth of a thick, protective oxide when concentrations as low 5 mM CrO4 2-E were in solution. Molybdate did not suppress transient current density in AA2024-T351 or AA7075-T6. In some cases the addition of molybdate increased the total charge passed after the scratch. However, capacitance measurements in the presence of molybdate indicate the rapid growth of a passive film.
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