Clarifying the Role of Mg₂Si and Si in Localized Corrosion of Aluminum Alloys by Quasi In Situ Transmission Electron Microscopy

The role of magnesium silicide (Mg₂Si) and silicon (Si) particles in the localized corrosion of aluminum (Al) alloys was investigated herein. Sub-micrometer-sized Mg₂Si and Si particles were grown in the Al matrix of Al-Mg-Si and Al-Si alloys, respectively, and characterized by transmission electron microscopy (TEM). A quasi in situ TEM technique was used to study an identical location containing Mg₂Si or Si particle in the Al matrix, prior to and following a period of immersion in 0.1 M NaCl at pH 6, 2, and 12. At pH 6 and 2, Mg₂Si was initially “anodic,” preferentially dealloying via selective dissolution of Mg, resulting in the development of SiO-rich remnants that are electrochemically inert. The SiO-rich remnants at pH 2 physically detached from the Al matrix. Silicon particles were electrochemically inert at pH 6, while “cathodic” at pH 2, dissolving the Al matrix at their periphery. It was observed that copper (Cu) was redeposited on Si particles at pH 2. At pH 12, Mg₂Si and Si were “cathodic” to the Al matrix. This study clarifies, and provides new insights into, the characteristics of Al alloy physical manifestation of corrosion associated with Mg₂Si and Si at the nanoscale.