Formation of Chromate Conversion Coatings on Al-Cu-Mg Intermetallic Compounds and Alloys

Abstract

"Accelerated" chromate conversion coating (CCC) formulations are distinguished among all CCC types by the fact that they utilize a special chemical additive to increase coating weight, 1 or increase the rate of the film forming Cr VI to Cr III reduction reaction. 2 Ostensibly, accelerated CCCs are used on corrosion-prone Al-Cu-Mg and AlZn-Mg-Cu alloys to ensure maximum protection. 3 Ferricyanide [Fe(CN) 6 3Ϫ ] has been used as an accelerant in commercial CCC formulations since the 1960s. 1 Despite the long history, its role in coating formation has not been precisely established. Both Fe(CN) 6 3Ϫ and Fe(CN) 6 4Ϫ are readily detected in CCCs by various surface-sensitive techniques, supporting the notion that it contributes to coating weight by becoming part of the CCC film. 1 For example, Treverton and Davies used X-ray photoelectron spectroscopy (XPS) and ion-beam etching to study accelerated CCCs formed on 99.8% pure Al substrates. 4 Results indicated the presence of Fe(CN) 6 4Ϫ/3Ϫ concentrated in the near-surface regions. It was suggested that the compound was probably present as a CrFe(CN) 6 salt. However, the findings were insufficient to establish a clear role for Fe(CN) 6 3Ϫ in coating formation. In subsequent studies aimed at clarifying their earlier work, these workers prepared accelerated CCCs on 99.8% Al from a bath formulation based on commercial chemistries. 5 XPS results indicated that Fe(CN) 6 3Ϫ/4Ϫ was present throughout the coating but was concentrated in the near-surface region. Mixed metal cyanides identified in the earlier study were now not present. Additionally, evidence of ferrocyanide [Fe(CN) 6 4Ϫ ] was found. The authors equivocated on the significance of ferrocyanide determination since ferri-to ferrocyanide reduction in X-ray beams was known to occur. Despite these complications, it was proposed that the primary film growth reaction, chromate reduction, was stifled by adsorption of ferricyanide on the nascent CCC, thereby increasing the availability of free chromate for reaction with uncoated metal.Hagans and Haas specifically considered film formation on Cu and Fe intermetallic compounds (IMCs) in studies of accelerated CCC formation on 2024-T3. 6 Auger electron spectroscopy (AES), XPS, and ion-beam depth profiling were used to show that for coating formation times up to 3 min, the film formation rate on various phases in 2024-T3 decreased in the order: matrix phase > Cu IMC > Fe IMC. After 5 min of immersion time, coating thickness appeared to be similar on the different phases. These workers proposed that ferrocyanide interacted with Cu-rich IMCs to form Cu 4 Fe(CN) 6 or Cu 2 Fe(CN) 6 and that these compounds promoted corrosion resistance by eliminating the galvanic couples that would otherwise form between the noble particles and the matrix phase.Lytle et al. used a variety of surface analytical techniques, including X-ray absorption spectroscopy (XAS), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), and Fourier transform infrared s...

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