Microstructure and corrosion behaviors of AZ31 alloy with an amorphous-crystallin nano-composite film

Magnesium (Mg) alloy has drawn considerable attention for lightweight structural and functional materials, whereas its corrosion resistance still requires to be enhanced. A new strategy for corrosion resistance has been proposed as making an amorphous-crystalline nano-composite film on Mg alloys. The film as the composition as Al₂O₃/GaN with a thickness of 20 nm was prepared on AZ31 Mg alloy by atomic layer deposition. Grazing incidence x-ray diffraction, scanning electron microscopy equipped with energy-dispersive spectroscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, and nano indentation tester have been used to characterize the film in details. It is verified the sample has an amorphous/crystalline/Mg interface structure, and a surface with homogeneous elemental distribution and higher hardness. Neutral salt spray test shows the film changes the corroded mode from pitting corrosion to uniform corrosion. Furthermore, electrochemical measurements indicate that the film would raise E_corr (ΔE_corr = +0.295 V), drop i_corr (about 1/10 times), and make electrical equivalent circuits change from R_s (CPE R_ct (R_L L)) to R_s (CR_f) (CPE R_ct (R_L L)). All evaluations show that better corrosion resistance has been by inducing the amorphous-crystalline nano film. The amorphous layer in the film would make a more homogeneous Cl⁻ distribution in the surface and act as a barrier to block the penetration of corrosion medium in the early stage. During corrosion, the interface between the layers in the film could retard the corrosion crack propagating further. The film would be favorate to form a denser corrosion product layer finally. A more uniform and lower corrosion occurs for AZ31 Mg alloy with this nano-composite film. (paper)