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The release of chromate ions from chromate conversion coatings ͑CCCs͒ on Al alloys was studied, and the effect of aging of CCCs on the chromate release kinetics was investigated. Chromate release from CCCs into aqueous solutions was monitored by measuring the change in the chromate concentration in solution using UV-visible spectroscopy. Heat-treatment of the CCC greatly reduced the chromate release rate. The chromate release rate also decreased with increasing aging time at room temperature. A diffusion-control model was proposed based on the notion that the CCC in an aqueous solution is a porous, two-phase structure consisting of a solid phase with adsorbed Cr͑VI͒ species that is in local Langmuir-type equilibrium with an interpenetrating solution phase. This model results in a concentration gradient of soluble Cr͑VI͒ in the solution phase of the CCC as chromate is released. The concentration and diffusion coefficients of soluble Cr͑VI͒ in CCC were estimated. The estimated diffusion coefficient tended to decrease with aging time, suggesting that the CCC is modified with aging time. For aircraft coating technology, understanding the mechanism of aluminum corrosion and the effect of chromates on corrosion inhibition is important. Chromate conversion coatings ͑CCCs͒ are commonly used on aluminum alloy surfaces to improve the corrosion resistance and to establish an adhesive base for organic coatings. Chromate ions have increased the pitting potential of aluminum alloys in chloride media and reduced the rates of pit nucleation and growth if present in a high enough concentration.1,2 One of the important aspects of CCCs is the dynamic repair of newly created breaks on coated metals, which is called self-healing. For example, a CCC-treated Al alloy sample with a scribe mark through the coating to the metal exhibits minimal corrosion in salt-spray testing.Zhao et al. demonstrated the migration of chromates from a CCC to an uncoated metal surface by detecting Cr on the surface of an initially untreated AA2024-T3 sample immersed in chloride solution in close proximity to a CCC-treated sample. 3 The release of Cr͑VI͒ from CCCs was further studied using UV-Visible spectroscopic analysis of solutions exposed to samples with CCCs. 4 The observations were consistent with a release mechanism similar to Langmuir adsorption and desorption of Cr͑VI͒ from a porous, insoluble Cr͑III͒ matrix. To fully understand the inhibition behavior of CCCs, one must fully understand the chromate release phenomenon. In the present study, the release of Cr͑VI͒ from CCCs is examined for samples under a range of conditions, and a model describing the release kinetics is developed. ExperimentalCCCs were prepared on aluminum alloys AA1100 and AA2024-T3 by immersion in Alodine 1200S ͑Henkel Surface Technologies͒ solutions at room temperature. The Alodine bath consisted of 9.0 g L Ϫ1 of a mixture reported to be 5 50-60% CrO 3 , 20-30% KBF 4 , 10-15% K 3 Fe͑CN͒ 6 , 5-10% K 2 ZrF 6 , and 5-10% NaF by weight, which is approximately 0.065 M in Cr. The pH ...

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