The role of microstructure and cathodic intermetallics in localised deposition mechanism of conversion compounds on Al (Si, Fe, Cu) alloy

Cerium-based conversion coating formation is triggered by a local rise in pH at cathodic sites produced by the oxygen reduction reaction. Therefore, size, morphology distribution and electrochemical potential of those sites play a crucial role. While the deposition reaction is sensitive to both immersion bath concentrations and underlying substrate microstructure, only the former has been widely investigated. This research attempts to fill the gap by studying the effect of controlled microstructure variables like the cathodic intermetallics' geometry and spatial distribution on the conversion compound deposit initiation. A controlled cast Al alloy was synthesised for this study and consisted of distinct cathodic phases: Cu-rich intermetallics, Fe-rich intermetallics and Si particles. The localised deposition preferentially formed only on strong cathodic Cu-rich intermetallics. Size (surface area) of the Cu-rich intermetallic correlated linearly with the deposited area over it in terms of lateral and z-direction spread. The pH gradient occurring from the oxygen reduction near an IM is very local and does not affect pH gradients of a neighbouring IM. When immersion time is increased from 0.5 h to 1 h, the percentage of Cu-rich IM covered with conversion coating increases. Big Cu-rich IM activate faster for deposition reaction than small Cu-rich IM.