SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2002; 33: 796–806 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.1456 Spatially resolved microchemical analysis of chromate-conversion-coated aluminum alloy AA2024-T3 M. Jaime Vasquez, J. R. Kearns, G. P. Halada and C. R. Clayton * Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, USA Received 29 April 2002; Revised 1 August 2002; Accepted 2 August 2002 The corrosion of aerospace aluminum alloys has been linked to the electrochemical behavior and chemical composition of constituent intermetallic compounds (IMCs). In this study, secondary ion mass spectrometry (SIMS), x-ray photoelectron spectroscopy (XPS), synchrotron infrared microspectrometry (SIRMS) and x- ray absorption near-edge structure spectroscopy (XANES) were used to characterize chromate conversion coatings (CCCs) formed on AA2024-T3 sheet and cast IMC analogs. The nascent surface film formed on IMC and matrix surfaces was characterized to reveal the influence of microstructure on CCC structure and composition. Copper-rich regions were found on the outermost CCC surface. Intermetallic compounds rich in Cu were found to be surface depleted of Cr and enriched in CN from the ferricyanide present in the CCC as an accelerator compound. By distinguishing particle morphology and chemistry, SIMS data demonstrated that heterogeneities in CCC composition were associated with IMCs. Copyright  2002 John Wiley & Sons, Ltd. KEYWORDS: chromate conversion coatings; surface analysis; XPS; SIMS; SIRMS; AA2024-T3 INTRODUCTION The relatively poor corrosion resistance of AA2024-T3 has generally been attributed to galvanic currents generated between copper-rich intermetallic compounds (IMCs) and the aluminum alloy matrix when exposed to a conducting environment. There has been growing evidence indicating that the situation is more complex than generally believed, e.g. the longstanding criticism made by Galveles 1 of inter- preting aluminum alloy pitting solely in galvanic terms. The compositions of IMCs have generally been deduced from bulk x-ray diffraction measurements: Laue analysis of AA2024-T3 reveals Al 2 Cu, Al 2 CuMg, Al 2 Cu 2 Fe, Al 7 Cu 2 Fe, Al 12 Si⊲FeMn⊳ 3 , Al 20 Cu 2 ⊲MnFe⊳ 3 and Al 20 Cu 3 Mn 3 . 2 However, the composition of individual particles has been shown to be not necessarily stoichiometric, and clusters of parti- cles tend to be mixed depending on the thermomechanical processing and trace element content of a product form. 3,4 Buchheit’s list 5 of ‘corrosion potential’ data for aluminum alloys presents several inconsistencies when electrochemical behavior is viewed solely as a function of bulk composition. Certain particles within a given alloy would be expected to be anodic and others cathodic to the matrix. However, cer- tain compositions/morphologies have shown inconsistent L Correspondence to: C. R. Clayton, Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, USA. E-mail: cclayton@notes.cc.sunysb.edu Contract/grant sponsor: US Air Force Office of Scientific Research; Contract/grant number: F49620-96-1-0479. dissolution behavior: 3 some thought to be cathodic relative to the matrix appeared to suffer anodic dissolution. 6 A ‘cop- per plume’ has been associated with pitting of IMC sites. 7–9 The plume is noteworthy on two counts: it indicates anodic dissolution of what were assumed to be cathodic particles, 9 and Cu redeposition on the matrix would change the relative galvanic areas of the alloy surface. To the extent that galvanic reactions do contribute to pitting, 10 the magnitudes of the currents generated by particle–matrix interactions depend not only on compositional differences but also on particle size distribution. For example, a few large particles are more reactive than many small particles for a constant volume fraction of particles in a matrix. 11 Chromate conversion coatings (CCCs) are used to inhibit this corrosion behavior. Hexavalent chromium, in the form of the oxyanions of chromate or dichromate, displays a seemingly unique ability to inhibit corrosion and to produce corrosion protective coatings. Thus, chromate ions are one of the most effective aqueous corrosion inhibitors for a range of commercial metals and alloys, including aluminum. In addition, chromate in conversion coatings is the most efficient pretreatment before painting to stop the pitting corrosion and filiform corrosion of aluminum alloys. However, chromate ⊲Cr 6C ⊳ is extremely toxic and carcinogenic. As a result, the Environmental Protection Agency has set considerably low limits for their use and several government agencies are working toward the replacement of CCCs. It is necessary to study the chemistry and structure to be able to understand the Copyright  2002 John Wiley & Sons, Ltd.