Structure and initial corrosion resistance of sputter deposited nanocrystalline Mg-Al-Zr alloys

Nanocrystalline Mg-Al-Zr alloys with a grain size of the order of 30-50 nm have been deposited by magnetron sputtering on glass and quartz substrates. The anodic behaviour of the alloys was studied in 3.5% NaCl and 3.5% NaCl + 50 ppm Cu(II) solutions in order to evaluate anticorrosive resistance. The alloys with 4 and 15 wt.% zirconium exhibited an increased corrosion resistance when compared to their Zr-free counterparts. By contrast, the resistance decreased markedly at higher zirconium concentrations (32 and 50 wt.%). Analogous sequence of corrosion resistance was determined by quartz crystal microgravimetry (QCM). It has been shown by X-ray diffraction (XRD) that the alloys Mg-4Al-4Zr and Mg-3Al-15Zr are solid solutions supersaturated with zirconium. The sample with 50 wt.% Zr was composed of two phases: Mg-Al and nanostructured Zr. Alloy structure refinement by Zr was demonstrated by X-ray diffraction and atomic force microscopy (AFM). The superior corrosion resistance of magnetron-sputtered Mg-Al-Zr films is attributed by enhanced formation of a barrier-type surface film with a more uniform microstructure.