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 the β-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 Al3Mg2 is 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 Al3Mg2 while 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 Al3Mg2 indicating its self-dissolution while the surrounding Al matrix remained relatively inert. However, at pH 13, a reversal of polarity occurs and Al3Mg2 becomes cathodic relative to Al. The severity of the galvanic coupling of Al3Mg2 to Al is in the order pH 13 > pH 2 > pH 6 > pH 10. ToF-SIMS analysis reveals that the dealloying of Al3Mg2 in 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.