Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy Part I: Nucleation and growth

The microstructure and morphology of the surface of aluminium alloys strong ly affect the nucleation, growth, and protective properties of chromate con version coatings. Samples of AA 2024-T3 cladded with AA1230 were pretreated with two different procedures before they underwent a chromating process, which consisted of dipping in a commercial Alodine 1200 solution. In one pr ocedure, an extra step, consisting of immersion in nitric-hydrofluoric acid solution for 30 s (desmutting bath), was carried out after immersion in su lfuric -phosphoric acid solution (acid pickling bath). Scanning Electron Mi croscopy (SEM), Atomic Force Microscopy (AFM) and electrochemical investiga tions highlighted the fact that this extra step in the pretreatment route g ave rise to a more etched, but less reactive surface, due to the removal of a large amount of cathodic sites, mainly related to Al-Fe-Si precipitates present in the clad layer. A similar effect, but on a smaller scale, was ca used also by the increase of the time of immersion in sulfuric -phosphoric acid solution. The decrease of galvanic coupling gives rise to a more homog eneous nucleation of the chromate conversion coating on the surface of the Alclad 2024-T3. Moreover, the chromate film tends to be denser and with few er defects. (C) 2001 Elsevier Science Ltd. All rights reserved.