Influence of Solution Viscosity on Protectiveness of SiO₂ Films Formed by Sol-Gel Methods

Abstract

Silica films were formed on Ni substrate from sol-solutions of various viscosities. The viscosity was varied by changing the aging time of the prepared solution at 308K from 3.6 to 900ks, with resulting viscosities 2 to 10mPa·s, respectively. The most suitable curing temperature was found to be 673K. The protectiveness was related to the structural defects of the film and was assessed by anodic polarization properties in 0.5kmol·m^-3 H₂SO₄ aqueous solution. The dissolution of the substrate metal was significantly suppressed by the silica coating. However, if the film were associated with defects, local corrosion takes place. The sol-solutions of viscosities around 2.8mPa·s results in the films of very good protectiveness, while those having viscosities larger than 3.7mPa·s lead to exfoliation of the films.