Organic-inorganic hybrid coatings have been prepared by a sol-gel route from 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) with triethylenetetramine (TETA) added in the sol formulation. The main goal of this research was to evaluate how the chemical structure of the GPTMS/TEOS/TETA hybrid coatings influences the corrosion of mild steel. The presence of TETA as an epoxy opening agent offers the possibility to form interconnected epoxy-amine and silica networks. It was found by means of 29Si HRMAS NMR spectroscopy that TETA acted as an efficient basic catalyst for the condensation of GPTMS and TEOS. This accelerates the gelation and gives limited shelf life to GPTMS/TEOS/TETA sols. Structural characterization of the cured hybrid coatings was performed using 13C and 29Si Solid State NMR spectroscopies. The results show that the epoxy amine reaction is the principal epoxide ring opening reaction. The slow formation of diol units with sols prepared with an excess of epoxy groups has also been observed. It was found that the polymerization of the organic network hindered the polymerization of the siloxane network. As a result, the film based on an epoxy rich off-stoichiometric formulation had a higher Persoz hardness value compared to the stoichiometric film.
The corrosion behavior of the coatings in neutral chloride solution was studied through electrochemical impedance spectroscopy (EIS). The results revealed that barrier properties have been improved by the addition of TETA to sol-gel films. The films based on epoxy rich off-stoichiometric formulation and those base on stoichiometric formulation resulted in similar barrier properties but a higher resistance to interfacial delamination was observed for the epoxy rich off-stoichiometric films. The relationship between the film structure and its corrosion behavior was discussed.
Corrosion, Sol-gel, 29Si and 13C NMR, Epoxies, EIS