Selection of Processing Parameters for the Conversion Coatings on High-Strength Aluminum Alloys by Cyclic Voltammetry
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SELECTION OF PROCESSING PARAMETERS FOR THE CONVERSION COATINGS ON HIGH-STRENGTH ALUMINUM ALLOYS BY CYCLIC VOLTAMMETRY L. Kwiatkowski,1,2 M. Grobelny,1, 3 and P. Konarski4 A method for recording copper enrichment on AA2000-series alloys by means of cyclic voltammetry in borate buffer preceded by the immersion of the samples in a 0.6 M NaCl solution is extended to the assessment of various surface treatments like alkaline cleaning, desmutting, phosphating, and also inorganic and organic corrosion inhibitors. Since the height of the peak determined from the voltammograms results from the Cu/Cu + charge transfer taking place on the Al–Cu surface, the choice of ade-
quate substances or technological parameters inhibiting this reaction may find an application in the elaboration or modification of surface technologies. In the present work, the particular technological steps of the complex electrophosphating process of AA2017 were elaborated by using the results obtained from voltammetric transients. It was shown that a partial removal of Cu from the surface without affecting its bulk content was the most efficient desmutting stage, while the inhibitors of Al/Cu corrosion were efficient in the post-treatment stage. Keywords: Al alloys, 2000 series, surface inhomogeneity, corrosion, Cu enrichment, cyclic voltammetry, phosphate coatings, impedance
Pure aluminum is resistant against corrosion due to passivity. The oxide film is adherent, transparent, stable for pH 4–9, self-healing, and also is an electric insulator. However, to increase its strength, the metal is alloyed with several wt.% of Mg, Zn, Cu, and other elements producing a large number of alloys, usually inferior in corrosion resistance as compared with the pure metal. One of the most common representative alloy used in the aircraft industry is AA2024-T3. Even newer and improved versions of precipitation/age-hardened Al alloys which offer combinations of strength, toughness, and weight reduction often exhibit susceptibility to localized corrosion as a result of surface heterogeneity. Numerous constituent particles formed in these alloys can be divided into three major groups based on the composition [1]: (1) Al–Cu–Fe–Mn particles, which are more noble than the Al matrix and act as cathodes; (2) Al–Cu–Mg particles, which are less noble than the Al matrix and act as anodes; (3)
Al 2Cu particles, which are more noble than the Al matrix.
According to [2], corrosion leads to the preferential dissolution of Al and Mg leaving Cu-rich remnants, which act as cathodes. Pitting is often induced by local galvanic cells formed by Cu-intermetallics or replated 1 2 3 4
Institute of Precision Mechanics, Warsaw, Poland.
Corresponding author; e-mail: [email protected]. Motor Transport Institute, Warsaw, Poland.
Tele&Radio Research Institute, Warsaw, Poland.
Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 50, No. 5, pp. 13–22, September–October, 2014. Original article submitted July 7, 2014. 634
1068-820X/15/5005–0634
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