Preferential anodic oxidation of second-phase constituents during anodising of AA2024-T3 and AA7075-T6 alloys

Model analogues of the metallurgical phases found in 2xxx and 7xxx series aluminium alloys were produced by magnetron sputtering and employed to investigate the local and general anodising behaviour of the alloys. Electrochemical tests, allied with scanning and transmission electron microscopy, enabled insight into the local anodising behaviour of the constituents and related effects on the overall porous oxide morphology. Under potentiodynamic conditions, the observed anodic current peaks of the commercial alloys were related with the anodic oxidation of specific second-phase particles. At 0 V, magnesium-containing particles, including S-phase, were preferentially removed from the alloy surface; at 5-6 V-SCE, the copper- and/or iron-containing particles, such as theta phase and Al7Cu2Fe particles were anodically oxidised. The initial voltage transient revealed for the commercial alloys during galvanostatic anodising was related to the previous findings and reproduced by the use of coupled alloy analogues. Transmission electron microscopy revealed that the voltage transient associated with oxidation of second-phase material influence the morphology of the anodic film formed on the aluminium matrix. Copyright (C) 2010 John Wiley & Sons, Ltd.