The Role of the Chelating Agent on the Structure and Anticorrosion Performances of an Organosilane—Zirconium Sol-Gel Coatings

This work investigates the role of the structure of a zirconium complex (ZC) on the condensation and anticorrosion properties of an organosilane sol-gel coating. The structure and reactivity of the ZC were modified by varying the content of methacrylic acid employed as a chelating agent. The structures of the developed materials were characterised by Dynamic Light Scattering, Fourier Transform Infrared Spectroscopy, Differential Scanning Calorimetry and Scanning Electron Microscopy. The passive anticorrosion properties were evaluated by Electrochemical Impedance Spectroscopy of the sol-gel coatings deposited on AA2024-T3 substrates. It was highlighted, that a competition in the condensation process of the silicate (Si-O-Si) and silicon-ZC oxides (Si-O-Zr) species can be tailored by the structure of the ZC, with the least chelated ZC exhibiting the highest content of Si-O-Zr bonds. At the same time, it was also found that the coatings containing the highest content of Si-O-Zr groups exhibited the best anticorrosion barrier performances amongst all sol-gel coatings investigated here, therefore presenting the highest condensation degree. This suggested that Si-O-Zr bonds were the essential chemical species responsible for the formation of condensed coatings. A direct correlation between the structure of the coatings and their anticorrosion performances is proposed.