Sealing effect of TiO2 films deposited by atomic layer deposition on AA6xxx alloys

In this work, the sealing effect of nanometric coatings of titanium dioxide (TiO2), deposited by means of atomic layer deposition (ALD) technique, was evaluated for an anodized Al 6XXX alloy. ALD coatings with controlled thickness (varying the number of cycles and the deposition temperature) were deposited on anodized substrates. In particular, three different thicknesses of the anodic oxide were considered (7, 14 and 26 µm). The corrosion resistance of the ALD coated specimens was compared to that of the current industrial state of the art, i.e. anodized Al specimens sealed by immersion in boiling water. The structure of the ALD deposits was characterized by field emission scanning electron microscope (FE-SEM) and their chemical composition was investigated by glow-discharge optical emission spectroscopy (GDOES). Corrosion resistance was assessed by means of potentiodynamic polarization measurements in a 3.5% NaCl solution. The quality of the ALD coatings is dependent on the temperature at which the ALD process is performed. A deposition process at 130°C leads to crack-free TiO2films for all investigated substrates. In contrast, deposition at higher temperature (200°C) leads to the formation of defective films. In particular, crack-free ALD films were obtained for anodized Al specimens with thickness of the oxide film of 7 µm, while sealing of thicker anodized layers was more difficult to achieve. A complete sealing of the anodized layer was obtained with 280 ALD deposition cycles, with thickness of the ALD film in the order of 30 nm. Potentiodynamic polarization curves show that TiO2 films obtained with 280 deposition cycles significantly decrease the corrosion current density as compared to specimens sealed in boiling water.