Enhancing water repellency and anticorrosion properties of a hybrid silica coating on mild steel

In this study, a sol-gel route has been designed for the synthesis of a hydrophobic organic-inorganic polymer network used to protect mild steel substrate against corrosion in 3.5 wt% NaCl solution. The underlined protective hybrid coating is prepared using tetraethoxysilane (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) with dimethoxymethyl octadecylsilane (DMOMS), bearing a C18 alkyl group, added at room temperature in order to provide a water repelling property to the coating. The superior protective property of this coating matrix has been found to be related to high water contact angle (θw) values, even after modification with surface active zinc-based corrosion inhibitor pigments. Insight into the protection mechanisms of these hybrid coatings (modified and unmodified) on the mild steel electrodes has been explained using electrochemical impedance spectroscopy. Results from electrochemical and SEM surface analyses reveal prolonged corrosion protection with better performance obtained for the modified coating matrices due to their reticulated make-up and effective barrier properties against the diffusion of aggressive species. The presence of corrosion inhibitors in this rigid anticorrosive silica network offers superior surface and bulk properties to the modified sol-gel coatings, and hence, fosters greater impedance against ionic current of corrosive species. The reduced protection on prolonged immersion must have been due to higher coating permeability via conductive pathways to the metal surface, and this is associated with the bulk pores on the sol-gel coating. Nuclear magnetic resonance (NMR) and Fourier transform infra-red (FTIR) spectroscopies have been employed to probe the extent of hydrolysis/condensation, and to ascertain the presence of the chemical group offering the needed hydrophobicity to the coating matrix.