An investigation into the role of the acid catalyst on the structure and anticorrosion properties of hybrid sol-gel coatings

This paper investigates the role of the acid catalyst on the structure and anticorrosion barrier performances of a dual hybrid sol-gel coating composed 3-trimethoxysilylpropylmethacrylate and a zirconium complex and deposited on AA2024-T3 alloy. Along with the variation of the acid catalyst in the range 0.3 to 2, the zirconium complex concentration in the silicate matrix is altered from 10 to 30%. Mol. The structure of the materials is characterised using dynamic light scattering, Fourier-transform infrared spectroscopy, and 29Silicon nuclear magnetic resonance spectroscopy, while the thermal behaviour is identified by differential scanning calorimetry and the coating morphology analysed using scanning electron microscopy/ energy dispersive x-ray spectroscopy. The anticorrosion barrier properties are characterised by electrochemical impedance spectroscopy and neutral salt spray. It is shown that the acid catalyst concentration has a direct impact on the kinetics of the hydrolysis and condensation reactions and that those effects can be controlled by the content of zirconium. These individual or combined alterations are found to directly impact the formation of Si-O-Zr bonds, thus the interconnectivity between the silicate matrix and the zirconium complex modifier, resulting in hybrid structures with altered densities. For the materials composed of 10 and 20 % of zirconium complex and when the pH is lowered, the relative concentrations of siloxane bonds is increased at the expenses of the Si-O-Zr bonds. When the concentration of zirconium complex is increased to 30 % and as the pH is increased, a higher competition in the formation of Si-O-Zr bonds takes place and are preferentially formed over the higher condensed Si-O-Si bonds. The corrosion testing show that the coatings with the highest barrier properties are found when the interconnectivity between the silicate and zirconium complex is maximum, thus for the materials exhibiting the highest condensation degrees.