Effect of nanosilica type on protective properties of composite ceramic coatings deposited on steel 316L by sol–gel technique

Trilayer SiO2 coatings were obtained from sol modified with nanosilica differing in its specific surface and hydrophilic and hydrophobic properties. Each of the successively deposited coating layers was sintered at ever lower temperatures: 300, 250 and 200 °C. Examinations under a JSM 5800LV Joel scanning electron microscope showed that each SiO2 coating includes a smooth, uncracked, thin layer covering the entire steel 316L basis and an outer layer made up of grains forming clusters of different size and density depending on the nanosilica used. The corrosion resistance of the coatings in Ringer’s solution was evaluated on the basis of polarization studies. A comparison of the passive region widths, the passive region current intensities, the cathode current densities at potential ESCE = −750 mV, the anode region potentials corresponding to a current density of 2 μA/cm2, the through-coating porosity and the polarization resistance values shows the coatings obtained from sol modified with hydrophilic nanopowder to be superior. As regards the tested hydrophobic powders, additive ®R972 (SBET = 110 m2/g) ensures good protective properties. If hydrophobic silica with larger specific surface (®R974, ®R812, SBET with respectively 170 and 260 m2/g) is used, the deposited coatings only slightly improve the protecting properties of steel 316L. The deposited coatings are uncracked. Even at their highest compactness, the surface grains and grain clusters, do not form a tight outer shell as evidenced by, for example, the poor protective properties of the coatings produced from sol modified by hydrophobic silica ®R812. The latter coatings’ protective properties are the poorest at the highest grain compactness. If hydrophobic nanosilica with small specific surface (®130 and ®150) is used the resulting layers have low grain compactness and good protective properties (probably smooth thicker layers directly coat the steel basis).