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It is generally accepted that Cr VI in chromate conversion coatings (CCCs) and in SrCrO 4 containing primers c is the critical component in corrosion protective coating systems used on aluminum aircraft alloys such as AA-2024-T3. The ability of a CCC to release Cr VI as soluble chromate species is likely to be important to "self-healing" exhibited by CCCs, in which CrO 4 Ϫ2 or related species can migrate to defects or corrosion sites and inhibit further damage. 1-8 Several investigators have reported that CCCs contain both Cr III and Cr VI 9-11 and a Cr III-Cr VI mixed oxide has been identified as a major CCC component. 12 Furthermore, release of CrO 4 Ϫ2 from a CCC has been demonstrated, as has protection of an initially untreated alloy surface by dilute CrO 4 Ϫ2 in a chloride solution. 7 The mechanism of corrosion protection by chromate is currently being debated, but storage and release of Cr VI by a CCC appear to be essential for its long term protection property. In addition, release of chromate from sparingly soluble SrCrO 4 in primers may also provide a source of dilute chromate for self-healing. The current investigation addresses the storage and release of Cr VI in more detail. The release of Cr VI from a CCC and SrCrO 4 into water and salt solution was monitored quantitatively with ultraviolet-visible (UV-vis) spectroscopy, in order to examine solution concentrations, saturation (if any), release rate, and possibly storage mechanism. By considering a variety of conditions, a quantitative model for Cr VI storage and release was formulated, and its implications for corrosion protection were considered. The kinetics and rate-controlling factors during release will be addressed in a separate communication. Experimental All the chemicals used were analytical grade. Solutions were prepared with "deionized" water (Barnstead, Nanopure 18 M⍀-cm). The absorbance vs. concentration behavior of chromate solutions is complicated by the equilibria between HCrO 4 Ϫ , CrO 4 Ϫ2 , and Cr 2 O 7 Ϫ2 , which depend on both concentration and pH. 13-17 The combined concentration of these species in solution is indicated herein as [Cr VI ]. In order to determine the relationship between UV-vis absorbance and [Cr VI ], solutions of K 2 Cr 2 O 7 were prepared at various concentrations in the range of 1.0 ϫ 10 Ϫ5 M to 4 ϫ 10 Ϫ4 M of Cr VI , and adjusted to different pH values with HClO 4 or NaOH and a pH meter. UV-vis absorption spectra for solutions in 1 cm quartz cuvettes were collected using either a Perkin Elmer Lambda 20 spectrometer or a custom system based on an ISA Triax monochromator. Spectra for one Cr VI concentration are shown in Fig. 1 for a pH range from 2.01-9.47. Since the absorption at 339 nm (A 339) was independent of pH, this wavelength was used to construct the pH-independent calibration curve shown in Fig. 2. The absorbance at 339 nm was preferable to that at 295 nm for quantitative analysis of [Cr VI ] due to the smaller interference from nitrate ion. Over the range of total [Cr VI ] from 1 ϫ 10 Ϫ5 to 4 ϫ 10 Ϫ...

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