We have applied radiotracer, electrochemical and ultrahigh vacuum techniques to study sulfate accumulation in passive films on pure aluminum, and Al 2024 alloy in 0.1 M NaClO4 containing 0.1 mM Na2SO4. We have found that the sulfate coverage is pH and electrode potential dependent and that sulfate is bonded to the surface in two distinctively different ways. While the breakdown of the passive film results in sulfate removal, the subsequent repassivation reintroduces the sulfate anion into the passive film. There is a strong tendency of sulfate to remain in the passive film which explains the inhibitive properties of sulfate in aluminum corrosion. Our data reveal that the anomalous sulfate accumulation during the negative-going polarization can be attributed to the copper content of the alloy surface. The formation of copper-containing nodules determines the electrode potential threshold below which sulfate anions desorb.