Silica-based hybrid coatings for corrosion protection of carbon steel. Part I: Effect of pretreatment with phosphoric acid

This work studies the synthesis and characterization of hybrid organic–inorganic coatings based on silica to improve the corrosion resistance of carbon steel. Hybrid organic–inorganic silica sol–gel coatings were obtained by dipping in an organically modified silica sol synthesized through hydrolysis and condensation of 3-glicidoxipropyl-trimetoxisilano (GPTMS) and tetraethoxysilane (TEOS) in acidic catalysis. The coatings were doped with a cerium salt (Ce(NO3)3·6H2O) and loaded with silica nanoparticles in order to improve both barrier effect and the anticorrosive behavior of the coatings by the inhibitory effect of cerium. Prior to the application of the coating, some samples were treated with a phosphoric acid 2% v/v in order to improve coating adherence. A two layered coating was applied onto AISI 1010 carbon steel, the outer containing with a cerium salt and the inner one with silica nanoparticles without the cerium salt, producing homogeneous and cracks-free films. Raman spectroscopy was used to characterize the compounds present in the surface of steel after pretreatment with phosphoric acid. Microstructural characterization of coatings was performed by means of scanning electron microscopy (SEM). The evolution with time of the protective properties of films was studied by electrochemical impedance spectroscopy (EIS), providing quantitative information of the role of the pre-treatment. Electrochemical behavior in each stage of the corrosion processes was modeled by equivalent circuits. Additionally, films adhesion was evaluated by nano-scratch demonstrating that the phosphate treatment improves adhesion of the hybrid coating