A localized study on the influence of surface preparation on the reactivity of cast Al-7Si-1Fe and Al-7Si-2Cu-1Fe alloys and their effect on cerium conversion coating deposition

The spontaneous triggering of cerium-based conversion coating deposition occurs due to active micro-galvanic couple induced local rise in pH at cathodic sites. Since surface preparations prior to conversion coating treatment modify the surface reactivity or the extent of micro-galvanic coupling between the anodic and cathodic phases, they are of crucial importance. While many past works have studied the effect of various preparations including alkaline etching and/or acid pickling and their parameters on the resulting conversion coating, very few studies focus on the local surface reactivity changes from the surface preparation. Moreover, most of the studies use high Cu containing AA2024, and related work on other alloys are scarce. In this study, two model cast alloys Al-7Si-1Fe and Al-7Si-2Cu-1Fe have been created to obtain a relatively homogeneous microstructure containing two different cathodic activity intermetallics Fe-rich IM β-Al5FeSi and Cu-rich IM θ-Al2Cu. Changes in the surface state of each alloy are monitored with AFM-SKPFM after subjecting them to four specific surface preparations: (1) Mechanical polishing (2) NaOH etching (3) NaOH etching + HNO3 Pickling (4) NaOH etching + H2SO4 pickling and correlations have been made with localized conversion coating deposit observations.