A Duplex Mechanism-Based Model for the Interaction Between Chromate Ions and the Hydrated Oxide Film on Aluminum Alloys

Abstract

In order to understand more clearly the nature of the anodic film formed on aluminum and its alloys, and the mechanism of inhibition of corrosion by chromates, a novel investigation has been made on the anodic film formed on AA1060 using X-ray photoelectron spectroscopy. The goal has been to elucidate whether the interaction of CrO 4 2Ϫ on Al alloy surfaces is in certain aspects analogous to that of MoO 4 2Ϫ on stainless steel surfaces and investigate whether a bipolar mechanism is applicable to aluminum surfaces. The aluminum hydroxide film was freshly formed on the metal surface and exposed to chromate and chloride solutions. Both chromate and chloride ions were observed to lead to a structural change in the hydrated aluminum anodic film consistent with deprotonation. Although both lead to deprotonation, exposure to chromate resulted in a structural transformation to an oxide layer associated with higher corrosion resistance and exposure to chloride lead to the formation of an oxyhydroxide film that is associated with a lower corrosion resistance. When the alloy was exposed to chromate prior to chloride exposure, the adsorbed chromate acted as a fixed negative charge substantially inhibiting chloride ingress. A reaction scheme, involving the formation of aluminum hydroxychlorides, has been proposed for deprotonation due to chloride. Deprotonation by chromate is explained for the first time in terms of bipolarity. Evidence suggests that two models may be operative on the surface of roughened AA106.Sakashita and Sato 1,2 have presented a molecular mechanism for metal passivation, in which a bipolar film consists of an anion selective hydrated oxide film on the metal with a cation selective layer in contact with the solution. This mechanism was shown to be active on thick films formed on iron. 1,2 It is well known that deprotonation within a film may occur at an applied anodic passivating potential thereby leading to the formation of a crystalline oxide inner layer and a hydrated gel-like outer layer. Deprotonation is a field-driven process and hence will be supported by a bipolar film that acts like an ionic rectifier, with its anion selective inner layer and cation selective outer layer. Hydrogen ions from bound water or hydroxide will be expelled from the film and the oxygen ion will migrate toward the metal, thickening the oxide barrier layer. The effectiveness of MeO 4 2Ϫ ions as cation selective agents may be explained using the pioneering work by Rozenfeld and Larkin, wherein the authors attempt to explain the electron distribution on the electron acceptor molecules using quantum mechanical calculations and experimental data to support it. 3-5 According to these calculations, the additional electrons reside on the oxygen ligands, thus increasing their negative fixed charge and it is this fixed negative charge that accounts for the cation-selective outer film. This is contrary to the expectation that the high positive charge residing on the core metal atom will attract electrons, thereby resulting in a fixed...

Interactions of the Components of Chromate Conversion Coating with the Constituents of Aluminum Alloy AA2024-T3

“…We have earlier shown that aluminum hydroxide films, which impart lesser corrosion resistance than aluminum oxide passive films, are susceptible to chloride ingress, while aluminum oxide films formed by deprotonation of hydrated films by chromate are protected. 57 Hence, we consider this hydration to be the reason for the observed activation.…”

The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

“…With regards to the progression of Cr(III) adsorption onto nanoboehmite surfaces, they [203] concluded that initial Cr(III) adsorption onto nanoboehmite under alkaline conditions occur by hydroxyl ligand exchange which leads to inner-sphere binding of Cr(OH) 4 −monomers, but at Cr(III) concentration in solution >20 ppm these monomers polymerize into clusters. Chromate ions are reported by several authors [204][205][206][207] to inhibit both hydration and deprotonation of anodic oxide films on aluminum.…”

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“…All XPS Spectra were taken at 90 • take off angle with respect to the sample surface and all spectra were corrected for charging by using the C 1 s line of adventitious carbon at 284.6 eV as a reference. All curve fitting followed the methods outlined by Savitsky and Golay [11], and Sherwood [12], as well as procedures developed over the years by Halada and Clayton [13][14][15] using CASA software version 1.001. XPS was performed on select samples to determine the surface (depth analysis 5-10 nm) chemical changes taking place on the spool material and the resulting printed 3D structures.…”