Early stages of Mg–Nb alloy corrosion in a balanced salt solution – STEM-HAADF and EIS study

Department of Electrochemical Materials Science, Centre for Physical Sciences and Technology, 3 Saulėtekio Avenue, Vilnius, Lithuania Thin Mg–Nb alloy films having a thickness below 1 μm were deposited by magnetron sputtering on glass substrates with Nb concentrations of 4, 26 and 32 at.%. Distribution of the elements within the deposits was identified by a scanning transmission electron microscope (STEM) equipped with a high angle annular dark field (HAADF) detector, which showed on a nanometer scale fine Nb-enriched structures. XRD identified nano-crystallinity of the deposits with grain sizes of the order of tens of nanometers. The corrosion behaviour of the films has been studied in a balanced salt solution, whose pH and ion concentrations were close to those of the human blood plasma. The electrochemical impedance spectroscopy implied a localized corrosion with diffusion limitations within a layer of corrosion products. The corrosion rates were determined in situ by Tafel extrapolation and EIS-derived polarization resistance; both methods provided reliable results for corrosion resistant alloy with ca. one third of Nb. Alloying of Mg with Nb makes it possible to adjust the corrosion activity of the alloy according to biomedical requirements for in vivo applications.