Corrosion protection of stainless steel by polysiloxane hybrid coatings prepared using the sol–gel process

Polysiloxane hybrid films were deposited on 316 L stainless steel substrates by dip-coating in a sol prepared by acid-catalyzed hydrolytic co-polycondensation of tetraethoxysilane (TEOS) and 3-methacryloxy propyltrimethoxysilane (MPTS), followed by radical polymerization of methacrylic moieties. Structural features of the polysiloxane hybrids were studied using 13C and 29Si nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA), as a function of the TEOS/MPTS ratio, which ranged between 0 and 2. The efficiency of corrosion protection of hybrid-coated stainless steel was investigated by X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) after immersion of the material in acidic and neutral saline aqueous solutions. The NMR and TGA results indicated a high degree of polymerization and polycondensation (85%) for the hybrid film with a TEOS/MPTS ratio of 2, as well as elevated thermal stability of 410 °C and excellent adhesion. The XPS analysis confirmed the hybridized structure of the polysiloxane network, and showed that no corrosion-induced changes had occurred on the coated steel surface after 3 weeks of immersion in 3.5% NaCl solution. Potentiodynamic polarization curves showed that the hybrid coating prepared using a TEOS/MPTS ratio of 2 yielded the best anti-corrosion performance. It acts as an efficient physical barrier, increasing the total impedance and reducing the current densities by 2 orders of magnitude, compared to the bare electrode. The impedance results obtained for the two electrolytes were discussed based on the electrical equivalent circuits used to fit the experimental data.