Sol–gel silica film preparation from aqueous solutions for corrosion protection

SiO2 coatings were deposited on aluminum (98% Al) surfaces by dip-coating in silicic acid aqueous solutions containing urea as dry chemical control agent (DCCA). These films are of special interest for protection of metals against oxidation and acid corrosion at elevated temperature. In this work, we investigate the effect of two different treatments of aluminum surfaces on the adhesivity of the silica films. The optimum urea/silanol ratio in the precursor gel solution and the number of dips were also investigated. The corrosion protection afforded by the silica film was determined by anodic polarization of the coated surfaces in 0.5 mol/l sodium chloride aqueous solution exposed to air at room temperature. The best performance was obtained for surfaces anodized prior to dip coating, and for urea/silanol ratio and number of dips equal to, respectively, 7 and 5.