Effects of sol–gel process parameters on the anticorrosive performance of phosphosilicate hybrid coatings for carbon steel: structural and electrochemical studies
The effects of several sol–gel process parameters such as acid catalyst addition and the heat treatment procedure on porosity and anticorrosive properties were investigated for phosphosilicate sol–gel hybrid coatings prepared from 3-[(methacryloyloxy)propyl] trimethoxysilane (MEMO) and bis-[2-(methacryloyloxy)ethyl] phosphate (BMEP). Three coatings, denoted SG/air (sol–gel coating without catalyst addition and dried under air), SG/HNO3/air (coating with HNO3 catalyst addition and dried under air) and SG/HNO3/vacuum–air (coating with HNO3 catalyst addition and dried using a two-step process vacuum/air), were prepared and analysed using different methods such as Fourier Transform Infrared Spectroscopy, ATR mode (ATR-FTIR), Scanning Electron Microscopy (SEM), Open Circuit Potential (OCP) as a function of time and Electrochemical Impedance Spectroscopy (EIS). The results obtained by ATR-FTIR during MEMO sol preparation indicated that MEMO slowly hydrolysed at neutral pH, but rapidly in the presence of acidic catalysts by forming silanol Si–OH groups after a few minutes. ATR-FTIR spectra and SEM images also showed a less porous microstructure and a better cross-linked network with a low amount of non-condensed Si–OH groups in the case of SG/HNO3/vacuum–air coating in comparison with the two other coatings dried under air. EIS results in 3 wt% NaCl demonstrated that HNO3 addition and the drying process under vacuum pressure allow enhancement of the protection properties of hybrid coating by reducing its porosity.