Corrosion and Volta potential evolution of high purity Mg treated in chloride solutions of bi-valent Cu, Mn, Ni, and Zn cations

The corrosion resistance of magnesium-based materials is known to be highly sensitive to even trace amounts of noble impurities. In this study, the surface and electrochemical properties of high-purity magnesium (51 ppm Fe) (HPMg) treated in 1 mM chloride solutions of bivalent Cu, Mn, Ni, or Zn cations were investigated by AFM/SKPFM, H 2 evolution measurements, DC-polarization, SEM, and XPS. The goal was to understand the potential effects of galvanic replacement induced by the respective cations. The cathodic kinetics drastically increased on HPMg samples treated in Cu solutions. Additionally, the Volta potential difference (VPD) of HPMg increased following treatment in Cu and Zn solutions. Furthermore, the VPD dropped after subsequent exposure to NaCl, which was attributed to the loss of galvanic coupling between magneisum and the noble deposits formed in a galvanic replacement reaction. In contrast, no significant VPD changes were observed on HPMg surface for Mn and Ni treatments. XPS analysis confirmed that Cu and Zn metals/oxides and Mn and Ni hydroxides were displaced by the corrosion film formed on the HPMg after exposure to NaCl. The observed changes in surface properties and corrosion kinetics of HPMg were rationalized. • Volta potential difference (VPD) of Mg increased due to deposition of Cu and Zn noble species • Mn and Ni species precipitate as hydroxides on Mg, which showed low VPD • Kinetics of H 2 evolution were substantially increased on Mg with Cu deposits • Mg treated in the Zn solution had lower H 2 kinetics despite its higher VPD • Cu, Mn, Ni, and Zn species were displaced by the corrosion product film on Mg after exposure to NaCl