Corrosion behavior of melt-spun Mg₆₅Ni₂₀Nd₁₅ and Mg₆₅Cu₂₅Y₁₀ metallic glasses

The corrosion characteristics of melt-spun metallic glasses Mg-Ni and Mg-Cu with and without Nd and Y addition in 0.01 M NaCl (pH 12) electrolyte were studied by hydrogen evolution testing and potentiodynamic polarization methods. Both the dissolution rate and corrosion current density of ternary Mg₆₅Ni₂₀Nd₁₅ and Mg₆₅Cu₂₅Y₁₀ were found to be lower than those of binary Mg₈₂Ni₁₈ and Mg₇₉Cu₂₁ alloys. Potentiostatically formed surface layers on Mg₆₅Ni₂₀Nd₁₅ and Mg₆₅Cu₂₅Y₁₀ were characterized with depth profile X-ray photoelectron spectroscopy (DP-XPS). The native and breakdown films were also characterized for comparison. For both Mg₆₅Ni₂₀Nd₁₅ and Mg₆₅Cu₂₅Y₁₀, it revealed that both the surface products of the native and passive films consisted of oxide species of all metallic elements and the depth distributions of oxide species were same: MgO was enriched at the outer region while NiO or CuO was depleted throughout the whole layer; Nd₂O₃ and Y₂O₃ were depleted in the outer region and then enriched in the inner region. The breakdown films consisted of the mixture of oxides and hydroxides, mainly of Mg(OH)₂. The incorporation of NiO and Nd₂O₃ (for Mg₆₅Ni₂₀Nd₁₅) and CuO and Y₂O₃ (for Mg₆₅Cu₂₅Y₁₀), especially the rare earth (RE) oxides, in the passive layers has a beneficial effect to improve the corrosion resistance while their hydroxidation leads to the passivity breakdown.