Abstract
Abstract 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.