SVET and ToF-SIMS Studies on the Galvanic Corrosion of β-phase/Aluminum Couple in Aqueous Solutions as a Function of pH

The galvanic corrosion of theβ-phase (Al₃Mg₂)/Al couple has been studied using electrochemical methods, scanning vibrating electrode technique (SVET) and Time of flight-secondary ion spectroscopy (ToF-SIMS). Polarization tests reveal that Al₃Mg₂is anodic relative to pure Al at pH ≤ 10. Visualization of the current density distribution over the Al₃Mg₂/Al galvanic couple by SVET reveals that at pH 2, an intense and stochastic anodic current density is evolved from Al₃Mg₂while a heavy cathodic current flux is sustained on the surrounding Al matrix. At pH 6 and 10, localized both cathodic and anodic regions are seen on Al₃Mg₂indicating its self-dissolution while the surrounding Al matrix remained relatively inert. However, at pH 13, a reversal of polarity occurs and Al₃Mg₂becomes cathodic relative to Al. The severity of the galvanic coupling of Al₃Mg₂to Al is in the order pH 13 > pH 2 > pH 6 > pH 10. ToF-SIMS analysis reveals that the dealloying of Al₃Mg₂in the galvanic couple is by selective dissolution of Mg leading to the deposition of corrosion products dominated by Mg compounds in an acidic and neutral environment. In an alkaline environment, a relatively thin hydroxide rich film is formed.