Chemical and electrochemical characterization of TiO2/Al2O3 atomic layer depositions on AZ-31 magnesium alloy

In this study, innovative TiO2/Al2O3 mono/multilayers were applied by atomic layer depositions (ALD) on ASTM-AZ-31 magnesium/aluminum alloy to enhance its well-known scarce corrosion resistance. Four different configurations of ALD layers were tested: single TiO2 layer, single Al2O3 layer, Al2O3/TiO2 bilayer and Al2O3/TiO2/Al2O3/TiO2 multilayer deposited using Al[(CH3)](3) (trimethylaluminum, TMA), and TiCl4 and H2O precursors. All depositions were performed at 120A degrees C to obtain an amorphous-like structure of both oxide layers. The four coatings were then investigated using different techniques, such as scanning electron microscope (SEM), stylus profilometer, glow discharge optical emission spectrometry (GDOES) and polarization curves in 0.05-M NaCl solution. The thickness of all the coatings was around 100 nm. The layers compositions were successfully investigated by the GDOES technique, although obtained data seem to be affected by substrate roughness and differences in sputtering rates between ceramic oxides and metallic magnesium alloy. Corrosion resistance showed to be strongly enhanced by the nanometric coatings, giving lower corrosion current densities in 0.05-M NaCl media with respect to the uncoated substrate (from 10(-4) to 10(-6) A/cm(2) for the single layers and from 10(-4) to 10(-8) A/cm(2) for the bi- and multilayers). All polarization curves on coated samples also showed a passive region, wider for the bi-layer (from -0.58 to -0.43 V with respect to Ag/AgCl) and multilayer (from -0.53 to -0.38 V with respect to Ag/AgCl) structures.