Role of hexavalent chromium in the inhibition of corrosion of aluminum alloys

Kendig, M.; Jeanjaquet, S.; Addison, R. C.; Waldrop, J. R.

doi:10.1016/s0257-8972(01)01099-4

Cited by 219 publications

(122 citation statements)

References 12 publications

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“…Due to rapid adsorption and passivation at the intermetallic sites, the main role of the inhibition process by Cr 6+ is to limit the oxygen reduction on intermetallics, which are the main initiation sites for localized corrosion of aluminum alloys. 2,3 Further, the susceptibility to pit initiation is reduced, because the Cr 6+ compounds are adsorbed to aluminum oxide, minimizing the susceptibility to adsorption of chloride. Despite these excellent characteristics, the use of CCCs is restricted because of their hazards for the environment and humans.…”

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confidence: 99%

“…New alternatives include a broad spectrum of possibilities -from various cerium, molybdenum and organic inhibitors, barrier coatings including plasma deposited, sol-gel and electrodeposited coatings, to conducting polymers. [3][4][5][6] An alternative yielding highly effective and long lasting protection that is fully comparable to CCCs is still to be found.…”

mentioning

confidence: 99%

“…1 Corrosion performance was tested under simulated aircraft conditions in air using electrochemical and immersion measurements complemented by analysis of coating morphology and adhesion. 3 , ≥ 98% Sigma) and zirconium tetrapropoxide (Zr(OPr) 4 , 70 wt%, in 1-propanol, Aldrich) as a non-silicon precursor were used as initial alkoxide reagents for synthesizing the hybrid sol. Other chemicals were methacrylic acid (H 2 C=C(CH 3 )COOH, 99.0% Aldrich) as chelating reagent, deionized water Milli-Q Direct water (resistivity of 18.2 M cm at 25…”

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Corrosion Properties of UV Cured Hybrid Sol-Gel Coatings on AA7075-T6 Determined under Simulated Aircraft Conditions

Rodič

Milošev

2014

J. Electrochem. Soc.

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Novel hybrid sol-gel coatings have been developed for aluminum alloys as an alternative to the restricted use of chromate conversion coatings. They were prepared from two individual sols: zirconium tetrapropoxide (ZTP) and methacrylic acid (MAA) were added to siloxane sol prepared by mixing tetraethyl orthosilicate (TEOS) and organically modified silica 3-methacryloxypropyl trimethoxysilane (MAPTMS). The mixtures were deposited on alloy AA7075-T6 substrate and cured in air under daylight at room temperature or at 100 • C without the addition of photo-initiator. Different molar ratios of ZTP and MAA were investigated, as well as the effects of curing temperature and aging time. The corrosion behavior of the coated alloy was examined by electrochemical potentiodynamic measurements and six-month immersion tests in Harrison's solution, which simulates aircraft conditions. The coatings provided excellent corrosion protection in the atmospheric region and at high anodic potentials up to 7 V, as well as effective protection during long-term immersion. Curing under daylight improved the corrosion properties compared to curing in the dark. The corrosion behavior of synthesized coatings was compared to standard chromate coatings, including primer and topcoat. The morphology of the coatings was analyzed by scanning electron microscopy, and their adhesion to aluminum alloy surface measured using the X-cross cut test. Chromate conversion coatings (CCCs) have been used successfully for corrosion protection of aluminum alloys.1-3 The formation of CCCs involves the reduction of the oxy chromium species and consequent oxidation of aluminum. Stable, hydrophobic Cr(III) oxide/hydroxide film thus formed contains residual, releasable hexavalent chromium which maintains the active inhibition of the film. Due to rapid adsorption and passivation at the intermetallic sites, the main role of the inhibition process by Cr 6+ is to limit the oxygen reduction on intermetallics, which are the main initiation sites for localized corrosion of aluminum alloys.2,3 Further, the susceptibility to pit initiation is reduced, because the Cr 6+ compounds are adsorbed to aluminum oxide, minimizing the susceptibility to adsorption of chloride. Despite these excellent characteristics, the use of CCCs is restricted because of their hazards for the environment and humans. New alternatives include a broad spectrum of possibilities -from various cerium, molybdenum and organic inhibitors, barrier coatings including plasma deposited, sol-gel and electrodeposited coatings, to conducting polymers.3-6 An alternative yielding highly effective and long lasting protection that is fully comparable to CCCs is still to be found.Over the last decade, inorganic-organic (hybrid) sol-gel coatings have been studied as important alternatives to CCCs.7-13 They combine the properties of inorganic (scratch resistance, durability, adhesion, chemical and thermal stability) and organic (density, flexibility and functional capability) components. Of the one-component hybrid coatings...

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Corrosion Properties of UV Cured Hybrid Sol-Gel Coatings on AA7075-T6 Determined under Simulated Aircraft Conditions

Rodič

Milošev

2014

J. Electrochem. Soc.

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“…Therefore a coating that maintains a low current density at high overpotential is desirable. The coating properties such as corrosion current densities (I corr ) and potential (E corr ) were estimated by the Tafel method [36], while the polarisation resistance (R p ) was calculated using Stern-Geary equation [37] (Eq. (1)).…”

Section: Pdsmentioning

confidence: 99%

Effect of organic chelates on the performance of hybrid sol–gel coated AA 2024-T3 aluminium alloys

Varma

Colreavy

Cassidy³

et al. 2009

Progress in Organic Coatings

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a b s t r a c tSol-gels are organic-inorganic polymers formed by hydrolysis/condensation reactions of alkoxide precursors, primarily silanes, which have found applications as electronic, optical and protective coatings. These coatings possess important characteristics such as chemical stability, physical strength and scratch resistance. Further performance improvement is achieved through the incorporation of zirconium and titanium based nanoparticles, also formed through the sol-gel process. However due to the inherent difference in the reactivity of the precursors, the hydrolysis of each precursor must be carried out separately before being combined for final condensation. Zirconium precursors are commonly chelated using acetic acids, prior to hydrolysis, to lower the hydrolysis rate.In this body of work various ligands such as organic acids, acetyl acetone (AcAc) and 2,2 -bipyridine (Bipy) were used to control the zirconium hydrolysis reaction and form nanoparticles within the silane sol matrix.Nanoparticle modified coatings formed from the silane sol on AA 2024-T3 aluminium were characterised spectroscopically, electrochemically and calorimetrically to evaluate the potential effect of the different chelates on the final film properties while neutral salt spray tests were performed to study their anti-corrosion performance. Results indicate that the acid ligand modified coatings provided the best performance followed by AcAc, while Bipy was the poorest. In all cases the zirconium nanoparticle improved the protective properties of the sol-gel coating.

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“…The corrosion parameters such as corrosion current densities (I corr ) and corrosion potential (E corr ) were estimated by the Tafel method within the range OCP r 1 mV [32]. The polarisation resistance is calculated using Stern-Geary…”

Section: Potentiodynamic Scanningmentioning

confidence: 99%

Influence of magnesium nitrate on the corrosion performance of sol–gel coated AA2024-T3 aluminium alloy

Varma¹,

Duffy

Cassidy³

2009

Surface and Coatings Technology

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Traditional anti-corrosion technology has relied heavily on using reducible metal species, predominantly hexavalent chromium (Cr(VI)), for protecting reactive metal alloys such as aluminium which is extensively used in the aerospace sector. However, the impending changes in the use of Cr(VI) in Europe and the United States have forced aerospace manufacturers to examine alternative materials for protecting aluminium. One of the most promising alternatives being investigated are organosilane based sol-gels containing anticorrosion additives. In this work the anti-corrosion properties of magnesium (II) nitrate (Mg(NO 3 ) 2 ) as a inhibitor was investigated at different concentrations (0.1% -1.0 wt %) in a methyltriethoxysilane (MTEOS) sol-gel on the aluminium alloy AA 2024-T3 and compared to Alodine TM 1200 (the established Cr(VI) pre-treatment). Electrochemical evaluation of the coating system by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) measurements correlated strongly with results obtained from Neutral Salt Spray (NSS) exposure data. The surface morphology of the coating was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results indicated the optimum performance was achieved using a Mg (NO 3 ) 2 . level of 0.7% w/w. It is proposed that the superior anticorrosion properties of the Mg 2+ rich sol-gel is due to the pore blocking mechanism of insoluble Mg(OH) 2 formed during the hydrolysis process.

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Role of hexavalent chromium in the inhibition of corrosion of aluminum alloys

Cited by 219 publications

References 12 publications

Corrosion Properties of UV Cured Hybrid Sol-Gel Coatings on AA7075-T6 Determined under Simulated Aircraft Conditions

Corrosion Properties of UV Cured Hybrid Sol-Gel Coatings on AA7075-T6 Determined under Simulated Aircraft Conditions

Effect of organic chelates on the performance of hybrid sol–gel coated AA 2024-T3 aluminium alloys

Influence of magnesium nitrate on the corrosion performance of sol–gel coated AA2024-T3 aluminium alloy

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