An Insight into the Critical Role of Microstructure and Surface Preparation on Localized Conversion Coating Deposition on Cast Al Alloys

Sammanfattning: The spontaneous cerium conversion coating formation over aluminium alloys is affected by the chemical process conditions, the surface preparation prior to conversion treatment and the microstructure of the underlying alloy. Most research performed until now focus only on the former whereas the influence of the latter two is poorly understood. The microstructure of aluminium alloys contains cathodic intermetallic particles that have a crucial role in conversion coating formation as they are responsible for the increase in pH to a critical level where chemical precipitation reaction of conversion coating compounds becomes possible. There are many different types of intermetallics in the aluminium alloys’ microstructure whose cathodic potential depends on their chemical composition, but no systematic study exists on the influence of their geometric properties on the reactivity of intermetallics. The surface preparation also critically affects reactivity of intermetallics, and a consensus exists regarding which preparation procedure makes an alloy surface most conducive for conversion coating deposition, but little is known about the topographical and volta potential changes occurring around the intermetallics from the surface preparations. An insight into both the role of microstructure and surface preparation is thus crucial as they can help design better treatments.To address this knowledge gap, model microstructures of hypoeutectic cast Al-7Si alloys have been created in the study with the desired intermetallic composition and geometry. By addition of Cu and Fe to the hypoeutectic cast alloy, two types of intermetallics, namely θ-Al2Cu and β-Al5FeSi form and are the main objects of investigation in the study. The geometrical dimensions of the intermetallics were modified by directionally solidifying the cast alloys at different rates. Different surface preparation procedures, namely mechanical polishing, NaOH, NaOH-HNO3 and NaOH-H2SO4 have been tried in the study. Furthermore, parameters such as etching time have been varied to understand their influence. The microstructural features, particularly intermetallic geometry and cathodic potential have been systematically compared with localized deposition on them through conversion coating treatment. Experimentally derived data-based analyses have been conducted to come to conclusions in the study.Firstly, the study found differences in the initiation of localized deposits on θ-Al2Cu and β-Al5FeSi attributed this to the conductivity variation due to compositional difference. Among the three differently sized θ-IM investigated in the study, namely fine, coarse and bulky θ, increasing the size from fine to coarse led to increase in reactivity of the IM for localized deposition. But increasing the size even further from coarse to bulky θ decreased the reactivity. Such a decrease in reactivity was found to be due to a combination of factors such as volta potential difference relative to the matrix and geometry.Surface preparation affected localized deposition and the choice of procedure applied critically depends on the intermetallics present in the microstructure. The β-Al5FeSi is most reactive for conversion coating deposition when prepared with NaOH etching solution but becomes passive when further pickled with HNO3 solution. The θ-Al2Cu, on the other hand, becomes most active when subjected to multi-step NaOH- HNO3 procedure. Such observations make the choice of a surface preparation procedure difficult when the alloy microstructure contains both θ-Al2Cu and β-Al5FeSi intermetallics. The localized deposition of cerium conversion compounds after surface preparation are triggered due to a combination of surface factors like including volta potential and presence of Al(OH)3 smut, with varying degrees of dominance depending on the type of IM. Al(OH)3 smut content was found sensitive to the NaOH etching time. A further HNO3 pickling step cleans the surface. A “cleaner” surface was found to be associated with more consistent coverage numbers, while the presence of Al(OH)3 smut, on the one hand provides additional source of alkalinity, does not consistently result in good coverage.