The effect of etching and pickling on the reactivity of intermetallics in cast aluminium alloys towards conversion coating deposition

In Al–Si–Cu–Fe alloys, localized cerium conversion coating (CeCC) deposition on intermetallic (IM) particles is strongly influenced by surface preparation. Here, we investigate how single-step etching (NaOH only) and multi-step etching and pickling (NaOH + HNO3​) preparation times affect CeCC nucleation and growth on Cu-rich θ-Al2Cu and Fe-rich β-Al5FeSi IMs in directionally solidified Al-7Si-2Cu-1Fe and Al-7Si-1Fe alloys. The reactivity of surface was characterized with AFM–SKPFM and surface coverage of CeCC deposits characterized with SEM. A quantitative analysis was performed to discern the effects of different surface preparation procedures on CeCC deposition. Single-step NaOH activates both IMs but leaves substantial Al(OH)3​ smut on θ-containing alloys, which elevates local pH yet introduces variability in coating coverage. In contrast, multi-step etching removes smut, ensuring more consistent CeCC on θ-Al2Cu driven primarily by cathodic activity of the IMs. Fe-rich β-Al5FeSi IMs exhibit nearly 100% coverage even after a short 30 s NaOH etch and 0.5 h CeCC treatment, whereas Cu-rich θ-Al2Cu requires 120 s NaOH etching plus HNO3​ pickling for reliable coverage. Prolonged CeCC times promote lateral deposit growth on both IM types when using single-step NaOH, but such growth is absent for θ-Al2Cu in multi-step samples. These findings reveal that Al(OH)3​ smut, coupled with local Volta potential and alkaline microenvironments, governs IM reactivity during conversion coating formation. Accordingly, single-step NaOH is sufficient for Fe-rich alloys, while multi-step etching is recommended for Cu-rich alloys. This work provides insights into optimizing surface preparation for improved and predictable CeCC deposition on distinct intermetallic phases.