The use of electrochemical techniques to study the corrosion behaviour of organic coatings on steel pretreated with sol-gel zirconia films

The two main features of a protective organic coating are its adhesion and corrosion protection. In order to improve both, chemical pretreatments have been used. The use of chromates was very popular, but recently they have been highly restricted because of their toxicity, so that chromate-free pretreatments have been developed and tested. An interesting alternative seems to be the deposition on the metallic surface of thin layers of zirconia by the sol–gel process. In this study thin films of amorphous zirconia on low carbon steel sheets have been obtained by the dip-coating technique, using two different complexing reagents. Control of the hydrolysis allowed the formation of ZrO2 films suitable as pretreatments, promoting the adhesion of organic coatings. The behaviour of these samples was compared with steel samples pretreated in conventional phosphatation baths. The adhesion of a polyester organic coating was evaluated by the pull-off technique, by measuring the detachment of cross-scratched samples after salt fog chamber testing, or by swelling the organic coating in methyl pyrrolidone. According to the results, the samples pretreated with zirconia layers showed promising performance, in comparison with commercial chemical treatments (tricationic phosphate and iron phosphate). The organic coating adhesion on zirconia films was found to depend strongly on the process parameters, e.g. concentration of the precursors solutions and chelating agents, which determine the thickness and the amount of organic residuals of the amorphous zirconia. Electrochemical impedance spectroscopy (EIS) was also used in sodium sulphate solutions to evaluate adhesion and the corrosion behaviour of these materials. No barrier properties of the zirconia films were observed. Resistance to delamination was studied by using samples where an artificial defect was made by mechanical tools of different diameter. The information obtained by EIS was in good agreement with the data obtained by salt fog chamber tests.

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