Amorphous Zirconia Films as Adhesion Promoter for Organic Coating

The two main features of a protective coating are the adhesion and the anticorrosive protection. In order to improve both, chemical pretreatments have been used. The use of chromate was very popular, but recently they have been highly restricted because of their toxicity, so that chromate-free pre-treatments have been developed and tested. An interesting alternative seems to be the deposition on the metallic surface of thin layers of zirconia by sol-gel process. In this study thin films of amorphous zirconia on low carbon steel sheets have been obtained by the dip-coating technique. The behaviour of the zirconia films as promoters of adhesion was evaluated measuring detachment of cross-scratched paint coated samples after Salt Fog Chamber Test. In addition, the determination of adhesion of organic coatings was performed by swelling in N-methyl pyrrolidone. According to the results, the samples pretreated by zirconia layers showed a good performance, in comparison with commercial chemical treatments (tricationic phosphate and iron phosphate). The protective properties of the zirconia films were found strongly depending on the process parameters, e.g. concentration of the precursors solutions and chelating agents, which rule the thickness and the organic residuals amount of the amorphous zirconia. In order to assess the behaviour of these sol-gel systems, preliminary tests using Electrochemical Impedance Spectroscopy were carried out too.