Effect of the chemical modification of the precursor of ZrO2 films on the adhesion of organic coatings

ZrO2 films were deposited on low carbon steel by a sol-gel process as a chemical pretreatment before the application of a polyester paint. The films were obtained by a dip-coating technique using solutions of Zr(OBun)4 containing complexing agents (acetylacetone or acetic acid). These additives modified the alkoxide at the molecular level, so a new precursor was formed in solution. This new compound shows slower rates of hydrolysis and condensation than Zr(OBun)4, which allows the stability of solutions and the morphology of the gel to be controlled. Moreover, the length and the temperature of the thermal treatment influence the structure of the gel by reducing the amount of organic residues. The behavior of the films in promoting the adhesion of organic coatings was evaluated by measuring the detachment of the paint from a cross-scratch of samples which had been exposed for different times in a salt fog chamber. Tests using electrochemical impedance spectroscopy were also carried out. According to the results, most of the samples pretreated with zirconia layers showed a good performance, even better than commercial chemical treatments, when acetic acid was used as a modifier of the alkoxide precursors.