Corrosion properties of oxygen plasma immersion ion implantation treated magnesium

Wan G.J., Maitz M.F., Sun H., Li P.P., Huang N.

Surf Coat Techn 201(19-20): 8267-8272 (2007). DOI:10.1016/j.surfcoat.2006.02.088

Oxygen plasma immersion ion implantation (PIII) was conducted in magnesium with oxygen doses in the range of 2.5 × 10¹⁵ ions cm^-2 up to 2.0 × 10¹⁶ ions cm^-2, and the corrosion properties were investigated in neutral phosphate buffer solution (PBS, pH 7.4) and chloride ion enriched PBS (145 mM Cl-, pH 6.4) (PBS(Cl-)). Significantly enhanced corrosion resistance against PBS was achieved on Mg treated with high oxygen doses above 1 × 10¹⁶ ions cm^-2. The corrosion current density Icorr of treated Mg was almost five orders less than that of untreated Mg; no intermediate values are obtained for samples with intermediate doses. However, all samples we prepared in this case could not withstand the more aggressive PBS(Cl^-). No improvement of corrosion resistance in such solution has been observed even for surfaces implanted at 2.0 × 10¹⁶ ions cm^-2. The enhanced corrosion resistance against neutral PBS is ascribed to increased Mg-O bonding states formed on the surface layer of magnesium by the PIII process, as discerned by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements. Further the more homogenous surface morphology, due to the ion bombardment effect, as observed by atomic force microscopy (AFM) may contribute. The Mg-O bondings can be dissolved easily in Cl^- enriched and more acidic ambience, as shown with PBS(Cl^-).