Effects of sol-gel process parameters on the anticorrosive performance of phosphosilicate hybrid coatings for carbon steel: structural and electrochemical studies

摘要

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.