Corrosion properties of sol–gel silica coatings on phosphated carbon steel in sodium chloride solution

Surface coating is considered to be an efficient method to control the corrosion of carbon steel. Sol–gel silica coatings and hydrophobic silane–silica coatings were prepared on the substrates of polished and phosphated carbon steel and were characterized with scanning electron microscopy, energy-dispersive spectrometry, X-ray photoelectron spectroscopy and contact angle analyzer. Corrosion behaviors of these coated samples were investigated by potentiodynamic polarization tests in 3.5 wt% sodium chloride solution at 25 °C. Uniform, flat and dense silica coatings were formed on the phosphated carbon steel. The contact angle of water increased from about 70° on silica coating to 127° on hydrophobic silane–silica coating, while there was no significant change on contact angle by phosphating pretreatment. Both of the corrosion potential and polarization resistance of the coatings increased and the corrosion rate decreased with phosphating pretreatment. However, the addition of silane could reduce the corrosion resistance of the coatings to a certain degree, and most coatings had defects such as cracks, which were adverse to corrosion prevention.