Corrosion Protection of Aluminium Alloys Using Thin ZrO2 Coatings Deposited with Sol-Gel Technique

Thin ZrO₂ pre-treatments applied with the sol-gel technique are a possible replacement of chromium conversion coatings on aluminium alloys. ZrO₂ pre-treatments should provide good barrier properties, promote coating adhesion and possibly introduce self-healing ability.
ZrO₂ pre-treatments were deposited on aluminium alloys AA2024 and AA3105 by means of dip-coating technique in a sol of film precursor. Two types of precursor were employed: metal-organic precursor (zirconium butoxide) in an organic solvent and inorganic salt precursor (zyrconil nitrate) in a water-based solution. The sol-gel technique enables deposition of films with thickness in the range of 100-200 nm. Application of the pre-treatments by means of spraying technique was also investigated. Corrosion inhibitors (2-mercaptobenzothiazole and cerium nitrate) were introduced in the ZrO₂ pre-treatments in order to improve their corrosion behaviour. Moreover, substrates pre-treated with ZrO₂ films were painted with an epoxy water-based primer and top-coat non-inhibited.
Morphology of ZrO₂ pre-treatments was investigated by means of SEM-EDXS. Film thickness and composition were studied by means of glow discharge optical emission spectroscopy (GDOES). Electrochemical behaviour of ZrO₂ pre-treatments and coated systems was characterized by means of potentiodynamic polarization and electrochemical impedance spectroscopy. Adhesion properties of coated systems were also evaluated by means of electrochemical measurements.