SVET and SIET Study of Galvanic Corrosion of Al/MgZn2 in Aqueous Solutions at Different pH

Abstract

The galvanic corrosion behavior of synthesized MgZn2 intermetallic particle coupled with Al in 0.1 M NaCl solutions across pH 2 to12 has been studied using the scanning vibrating electrode technique (SVET), the scanning ion-selective electrode technique (SIET) and X-ray photoelectron spectroscopy (XPS). Results indicate the galvanic dissolution of MgZn2 depends on pH as well as exposure time. SVET current density maps reveal that MgZn2 is anodic relative to Al at pH 2 but cathodic at pH 12, while self-dissolution of MgZn2 prevails at pH 4 and 6. However, the galvanic coupling effect is more pronounced at pH 2 and 12. SIET maps of H+ and Cl− ion distribution reveal evidence of local alkalization on MgZn2 and migration of Cl− ions away from MgZn2 surface at pH 4 and 6 due to significant accumulation of OH− ions. Preferential dissolution of Mg occurs leading to segregation of Mg compounds in the outer layers of the film and Zn enrichment on the matrix/solution interface. In slightly acidic and near neutral solutions MgO/Mg(OH)2 dominates the outer layers and MgO/Mg(OH)2/ZnO dominates the inner layers, however, at pH 12 a stable MgO/Mg(OH)2 passive film is formed.