AVS 50th International Symposium
    Applied Surface Science Wednesday Sessions
       Session AS-WeA

Paper AS-WeA8
The Mechanism of Protection of Mechanical Damages to Chromate Conversion Coatings Formed on Aluminum Alloys

Wednesday, November 5, 2003, 4:20 pm, Room 324/325

Session: Fuel Cell & Battery Materials/Corrosion
Presenter: D. Chidambaram, State University of New York at Stony Brook
Authors: D. Chidambaram, State University of New York at Stony Brook
C.R. Clayton, State University of New York at Stony Brook
G.P. Halada, State University of New York at Stony Brook
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Inhibitive action of dichromate based solutions have been known for nearly a century. Hexavalent chromium present in these coatings is toxic and a well-known mutagenic agent. Aluminum alloys used widely in the aerospace industry are given a treatment to form chromate conversion coatings (CCCs). The development of alternative coatings requires the precise knowledge of the underlying mechanisms of protection offered by hexavalent chromium, which are unclear. Defects or mechanical damages in CCCs are protected without further treatment by "self-healing" properties. "Self-healing" has been associated with migration of chromates to actively corroding sites. The formation of a protective Al(III)-Cr(VI) complex at damaged sites was first suggested by Abd Rabbo et al. Electrochemical techniques have been used in this study to provide a direct observation of the repassivation of a scratch. XPS analysis of pure aluminum exposed to chromate solution indicates the presence of high amount of hexavalent chromium at higher depth of analysis. This result is in agreement with theories proposing the formation of stable Al(III)-Cr(VI) compounds. Atomic force microscopy (AFM) confirmed the physical presence of these compounds. Synchrotron infrared micro spectroscopy (SIRMS) clearly showed the formation of Al(III)-Cr(VI) complex in these regions. Al(III)-Cr(VI) complex was found to form at active regions and protect the surface irrespective of the nature of the chromate source (adsorbed chromate film or CCCs). A novel study involving scratching of AA2024-T3 prior to the formation of CCC was performed. The surface morphology studied using scanning laser confocal microscopy showed the physical presence of some compounds in the scratches. The increase in the hexavalent chromium content with the number of scratches as observed using XANES explained the mechanism of protection of mechanical damages to the CCCs formed on aluminum alloys.