On the Composition of the Passivating Films Formed on Aluminum in Chromate Solutions

Abstract

The chemical constitution of the passive oxide films formed on aluminum in chromate and chromate/chloride solutions under simple immersion (at open-circuit potential) and under anodic polarization has been investigated by means of x-ray photoelectron spectroscopy. It has been shown that these films, whose thickness is comparable with the mean escape depth of photoelectrons, are composed of a mixture of hydrated chromium (lII) oxide, aluminum (III) oxide, and chromium (VI) specms. A considerable amount of chloride species has been detected in the films formed in chromate/chloride solutions, independently whether chlorides were initially present in the solution or were added only after 18 hr immersion of the sample in chromates. The extent of chloride incorporation within the oxide film from CRO42-/C1 -solution was not modified by the anodic polarization of aluminum from open-circuit potential up to critical pitting potential. This situation is quite different from that observed in chloride solutions, in the absence of chromates, where the amount of chlorides in ~he oxide film increases appreciably between Est and Ecr. The nature of the inhibiting effect of chromate ions on the pitting corrosion of aluminum is discussed on the basis of these results.Although numerous sttidies have been devoted to clarifying the mechanism by which chromate ions inhibit the corrosion of aluminum, the exact nature of this phenomenon is still under discussion. Edeleanu and Evans (1) first attributed the inhibiting effect of chromate solutions to a redox reaction occurring between CrO42-ions and aluminum metal to form aluminum oxide and Cr ~+ ions. Their suggestion was further examined by Pryor (2), and by Heine and Pryor (3), who found that 7-A12Oz films formed on aluminum by passivating chromate solutions exhibited lower ionic resistance and higher electronic resistance than amorphous films of comparable thickness produced by thermal oxidation. According to these authors, the inhibiting effect of chromate ions on the pitting corrosion of aluminum in chloride solutions can be explained by strong specific adsorption of divalent CrO42-ions on the film-solution interface, preventing simultaneous adsorption of monovalent C1-ions and their entry into the 7-A1~O3 lattice. This latter theory has since received indirect support in the investigation of BShni and Uhlig (4), who showed the existence of a linear relationship between the logarithm of the activity of C1-ions, and the logarithm of the activity of CrO4 uions required to shift the critical pitting potential of aluminum to an arbitrary noble value of 0.8V vs. NHE. Recently, Abd Rabbo et aI. (5) have studied the changes in the composition of anodically formed aluminum oxide films after immersion in chromate/dichromate solutions, using secondary ion mass spectrometry (SIMS). A layer of uniformly distributed Cr + ions 1 was detected in the outer regions of the films on immersed specimens, followed by localized Cr+-rich areas down to the metal-oxide interface. On the basis of these results ...

Chromate Conversion Coatings on 2024 Al Alloy

“…For example, using XPS, Treverton and Davies have observed levels between 0%3 and 15%.2 Much higher levels of Cr6`have also been reported. Koudelkova et al detected D20% Cr6`, 6 Hagans and Haas D 60%,7 Davenport and Isaacs detected D50% Cr6`using XANES33 and Lytle et al8 detected only 23% Cr6à lso using XANES. The XANES studies by Hawkins et al10 on oxide Ðlms of di †erent thicknesses which had been immersed in chromate/chloride solution demonstrated qualitatively that the amount of Cr6`incorporated into the protective coating depended on the Ñaw density, i.e.…”

Mechanisms of corrosion inhibition of AA2024-T3 by vanadates

“…The susceptibility of aluminium alloys to localized corrosion strongly depends on the distribution and electrochemical properties of intermetallic compounds (IMCs) and second phase particles [3][4][5][6][7][8][9]. Aluminium alloys for aerospace applications are protected from the aggressive environment using multi-layered coating systems based on the chemistry of chromates [10][11][12][13][14][15][16][17]. A suitable candidate for chromate replacement has not yet been developed for the most demanding applications [10].…”

A study of the mechanisms of corrosion inhibition of AA2024-T3 by vanadates using the split cell technique

“…Due to the susceptibility to localized corrosion, aluminum alloys for aerospace applications are protected from the aggressive environment using multi-layered coating systems based on the chemistry of chromates [18][19][20][21][22][23][24][25]. It has been shown that the formation of a Cr 3+ monolayer inhibits oxygen and further Cr 6+ reduction [18,[27][28][29][30][31][32].…”