Electrochemical behaviour of ZrO2 sol-gel pre-treatments on AA6060 aluminium alloys

ZrO2 pre-treatments applied with the sol–gel technique are a possible replacement of chromium based pre-treatments on aluminium alloys. The thickness and homogeneity of the films deposited on AA6060 alloy are strongly related to the process parameters like preparation of the surface, number of dips and thermal treatment of the film. ZrO2 films were prepared using the dip-coating technique in sol obtained from metal-organic precursors in an organic solvent (0.1MZr(OBun)4 in anhydrous n-butanol with addition of acetic acid as complexing agent). Different layers were applied on AA6060 changing number of dips and thermal treatment (150 ◦C for 1 h or 250 ◦C for 4 min). The typical thickness of the deposited layers was in the range 70–180 nm depending on process parameters. The electrochemical behaviour of the pre-treated alloy in diluted Harrison solution (0.05 wt% NaCl + 0.35 wt% (NH4)2SO4) was investigated by means of potentiodynamic polarization, open circuit potential measurements, and electrochemical impedance spectroscopy. In addition, the electrochemical behaviour of ZrO2 sol–gel films was compared with that of chromatized AA6060 and fluotitanated/fluozirconated AA6060. In order to evaluate the adhesion properties of the films, ZrO2 pre-treated AA6060, chromatized AA6060 and fluotitanated/fluozirconated AA6060 were painted with a polyester resin and subjected to thermal cycles in 0.05 wt% NaCl. Each thermal cycle consisted of heating the samples at 90 ◦C, permanence at 90 ◦C for 6 h, cooling at room temperature and permanence at room temperature for 18 h. Impedance measurements were performed at the end of each cycle. Potentiodynamic polarization curves and impedance spectra indicate that ZrO2 pre-treatments have similar barrier properties to those of chromatized AA6060. However, no self-healing ability is observed for ZrO2 films. The barrier properties of ZrO2 films are strongly dependent on process parameters. In particular, the number of dips determines the amount of defects in the film and its homogeneity. The electrochemical behaviour strongly improves increasing the number of dips in the deposition bath. Thermal aging cycles evidence good adhesion properties for ZrO2 pre-treatments.