Effect of Na-Montmorillonite sonication on the protective properties of hybrid silica coatings

This work deals with the effect of the sonication treatment applied to an aqueous suspension of sodium montmorillonite nanoparticles used to prepare hybrid clay-organosilane derived sol-gel films. The sol-gel films were obtained from a mixture of 3-glycidoxypropyltrimethoxysilane (GPTMS), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) adding 2 wt% of sodium montmorillonite suspension after sonication. The sol-gel layers were applied onto hot dip galvanized (HDG) steel plates. The influence of the duration of the sonication treatment (i.e. 0, 1, 3 and 12 hours) on both the structure as well as on the electrochemical properties of the sol-gel films was analyzed in details. The effect of different sonication times on the structure of the sol-gel film was investigated by means of solid state Nuclear Magnetic Resonance (NMR), X-Ray diffraction (XRD) and Infra-Red (FT-IR) spectroscopy. The corrosion protection properties of the different sol-gel layers were investigated mainly by means of electrochemical techniques. The structural characterization (NMR, XRD, FT-IR) of the coating proved that interactions between the montmorillonite plates and the silica-based sol-gel network take place, with changing the local environment of Al sites in the clay. The potentiodynamic measurements as well as the EIS confirmed the beneficial effects of the presence of the Na-MMT and of the sonication treatment.