Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings Equilibrium Aspects of Cr[sup VI] Concentration

Abstract

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...

Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys

“…15 For decades, hexavalent chromium (Cr(VI))-based pigments and conversion coatings have been used to protect the high-strength aluminum alloys. 16,17 Chromates are well-known for their corrosion inhibiting efficiency over a wide pH range and their ability to reduce the oxygen reduction reaction. 18,19 Unfortunately, Cr(VI)-based compounds are toxic and known carcinogens, and new regulations and legislations are restricting their current and future use.…”

Utilization of chemical stability diagrams for improved understanding of electrochemical systems: evolution of solution chemistry towards equilibrium

“…49,50 Additionally, corrosion protection of aerospace assets was made possible in large part due to the corrosion inhibition afforded by chromium conversion coatings (CCC). [51][52][53][54] With the growing demands for CCC alternatives, other inhibitor solutions (such as, molybdenum and cerium systems) are being heavily researched. 21,22,[55][56][57][58][59][60] The storage, release, and deposition of these inhibitor species is dependent upon pH, and the critical inhibitor concentration is usually dependent on metal ion concentration in solution.…”

Kinetic Studies of Catalytic Oxidation of Cyclohexene Using Chromium VI Oxide in Acetic Acid Medium

“…An aqueous solution of CrO 3 (3 cm 3 , 5 x 10 -3 M) was transferred into a spectrometer cell and the cell was fitted into the sample hole of the sample compartment of the spectrophotometer (Shimadzu UV/VIS instrument, UV-1600 Series). The instrument was set to 339 nm which is the pH independent λ max for CrO 3 solution (Xia et al, 2000). The initial absorbance of the CrO 3 solution was then measured and recorded.…”