Storage and release of soluble hexavalent chromium from chromate conversion coatings. Equilibrium aspects of Cr^VI concentration

The release of soluble Cr^VI species by a chromate conversion coating (CCC) was monitored quantitatively by ultraviolet-visible spectroscopy. By careful selection of measurement wavelength (339 nm), the Cr^VI concentration could be determined without regard to solution pH or Cr^VI speciation. The Cr^VI concentration in solution over a CCC reached an equilibrium value that depended on pH, ionic strength, and the ratio of the CCC surface area to the solution volume (A/V). In separate experiments, the adsorption of Cr^VI by synthetic Cr^III hydroxide to form a Cr^III-Cr^VI mixed oxide was observed, and also led to an equilibrium concentration of Cr^VI in solution. The equilibrium Cr^VI concentration was determined for a variety of A/V values on both AA1100 and AA2024-T3 aluminum alloys. The results are inconsistent with release mechanisms based on the solubility of a Cr^VI salt in the solution or depletion of Cr^VI from the CCC. However, the observed concentrations are consistent with a mechanism similar to a Langmuirian adsorption-desorption equilibrium of Cr^VI on a porous, insoluble Cr^III hydroxide matrix. The Cr^III hydroxide matrix has a finite number of Cr^VI binding sites and exhibits a nonlinear relationship between solution and solid Cr^VI concentrations governed by an equation similar to a Langmuir adsorption isotherm. The proposed model incorporates reversible adsorption and desorption of Cr^VI, with adsorption favored at low pH during formation of the CCC, and desorption favored in field conditions. The model quantitatively predicts the observed concentrations after determining the binding constant from fits to the data. The model explains the capacity of a CCC to release active Cr^VI corrosion inhibitor and provides strong evidence that Cr^VI storage in a CCC involves reversible formation of a Cr^VI-O-Cr^III mixed oxide.