Insight into the Effect of Mg(OH)₂ Films vs. Noble Element Enrichment on the Global and Local Cathodic Activation of Corroding Mg

The anodically-enhanced cathodic activation of high-purity Mg was studied by global and local electrochemical measurements in unbuffered 0.6 M NaCl, 0.6 M NaCl saturated in Mg(OH)₂, 0.1 M MgCl₂, 0.1 M Na₂SO₄, and 0.1 M tris(hydroxymethyl)aminomethane hydrochloride (TRIS). The selection of diverse aqueous environments was deliberate in order to explore the effect of dissolution product upon subsequent cathodic activation. Cathodic activation was found to be strong in all of the chloride-containing environments where a porous hydroxide forms; weak in Na₂SO₄ where a denser, more compact hydroxide forms; and negligible in TRIS where no hydroxide forms. Elemental enrichment, mapped using microparticle-induced x-ray emission, revealed the enrichment of Fe only in Cl⁻-containing environments, suggesting that noble metal enrichment provides a stronger influence on cathodic activation than that of Mg(OH)₂ films.