Martin W. Kendig

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings II. Composition and Electrochemical Behavior of the Chromate Conversion Coating

Rockwell Scientific Company, Surface and Electrochemical Processes Department, Thousand Oaks,California 91360, USAAluminum alloy AA2024-T3 samples, subjected to various pretreatments, were conversion coated using Alodine 1200S. The ratioof hexavalent chromium to total chromium in the chromate conversion coating ~CCC! was determined by X-ray absorptionnear-edge spectroscopy. The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pre-treatment. The coated surfaces were analyzed by X-ray photoelectron spectroscopy to determine the surface composition of thecoatings. Cr~VI! forms the major constituent ~46-74% of total chromium! on the surface. Protective passive films of aluminumoxide and aluminum phosphate inhibit the formation of CCCs. Corrosion behavior of the CCCs were studied using open-circuitpotential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy ~EIS!. Conversion coatedalloys exhibited up to 40-fold higher corrosion resistance when compared to bare pretreated alloys. EIS behavior was modeledusing a seven-element equivalent circuit. EIS results indicated industrially used bromate pretreatment to be best suited for CCCformation. Coupling the AA2024-T3 alloy with platinum during bromate pretreatment resulted in lowering the amount of copperintermetallics on the surface. This led to an increase in the corrosion resistance of the subsequently formed CCC by over an orderof magnitude as compared to CCCs formed following alternative pretreatments. Therefore, this simple modification of couplingthis alloy with platinum is recommended.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1806393# All rights reserved.Manuscript submitted December 1, 2003; revised manuscript received May 11, 2004. Available electronically October 27, 2004.

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings I. Composition and Electrochemical Behavior of the Oxide Film

Aluminum alloy AA2024-T3 has been subjected to various pretreatments including those routinely carried out in the industry prior to conversion coating of the alloy. The surface chemistry of the samples after pretreatment was analyzed using variable angle X-ray photoelectron spectroscopy (VAXPS). Electrochemical techniques were used to evaluate the corrosion behavior of the samples in quiescent 0.05 M NaCI solution. The results were compared with those obtained from an untreated sample used in as-received condition that acted as a control. The surface of the alloy was observed to be enriched in copper and magnesium after certain pretreatments. Composition of the surface oxide film as well as the electrochemical behavior was found to vary significantly depending on the pretreatment. In all cases, the overlayer formed on the alloy was less than the sampling depth of the XPS (∼90 A). Pretreatment of the alloy using Alconox® was found to result in the formation of a highly protective aluminum oxide (Al 2 O 3 ) surface film. The corrosion current of the samples was observed to be directly proportional to the aluminum oxyhydroxide content of the film, while the polarization resistance of the sample was inversely proportional to the surface copper content.