Tannic acid reinforced sol-gel silica coatings for corrosion protection of zinc substrates

Silica (SiO2) coatings are a widely discussed inorganic coating alternative, offering a feasible and simple method of passive corrosion protection, with the option of enhancement by incorporating different inhibitors into the matrix. The novelty of the present study lies in the reinforcement of the sol-gel SiO2 coating system, by the addition of different tannic acid (TA) amounts in the sol phase and producing nano scale coatings by dip-coating technique, on Zn. Tannic acid is a non-toxic substance, which acts as anodic corrosion inhibitor. Long term corrosion measurements were carried out to check the durability and corrosion resistance of the thin coatings. They were characterized by Electrochemical Impedance Spectroscopy and Potentiodynamic polarization curves, as well as by physical methods (SEM, contact angle and adhesion measurements). The pseudo-porosity of the coatings was also evaluated. It has been concluded that SiO2 coatings containing TA in an optimal concentration show lower corrosion current densities than simple SiO2 coatings, good adhesion to the zinc substrate, and lower porosity. The interactions occurring between the SiO2 and TA put in evidence by FT-IR, Raman spectroscopy offer an explanation of the better corrosion behavior of the SiO2+TA coating compared to the pure SiO2.