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Journal of the Electrochemical Society, Vol.151, No.11, B613-B620, 2004
Surface pretreatments of aluminum alloy AA2024-T3 and formation of chromate conversion coatings - II. Composition and electrochemical behavior of the chromate conversion coating
Aluminum alloy AA2024-T3 samples, subjected to various pretreatments, were conversion coated using Alodine 1200S. The ratio of hexavalent chromium to total chromium in the chromate conversion coating (CCC) was determined by X-ray absorption near-edge spectroscopy. The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pretreatment. The coated surfaces were analyzed by X-ray photoelectron spectroscopy to determine the surface composition of the coatings. Cr(VI) forms the major constituent (46-74% of total chromium) on the surface. Protective passive films of aluminum oxide and aluminum phosphate inhibit the formation of CCCs. Corrosion behavior of the CCCs were studied using open-circuit potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Conversion coated alloys exhibited up to 40-fold higher corrosion resistance when compared to bare pretreated alloys. EIS behavior was modeled using a seven-element equivalent circuit. EIS results indicated industrially used bromate pretreatment to be best suited for CCC formation. Coupling the AA2024-T3 alloy with platinum during bromate pretreatment resulted in lowering the amount of copper intermetallics on the surface. This led to an increase in the corrosion resistance of the subsequently formed CCC by over an order of magnitude as compared to CCCs formed following alternative pretreatments. Therefore, this simple modification of coupling this alloy with platinum is recommended. (C) 2004 The Electrochemical Society.