Synthesis and performance evaluation of environmentally compliant epoxysilane coatings for aluminum alloy

Organosilane-based waterborne coatings for corrosion protection of aluminum alloy have been developed by sol–gel process. The coating solutions were prepared by hydrolysis and condensation of methyltrimethoxysilane (MTMS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) in the presence of acid catalyst and amino functional organosilanes namely 3-aminopropyltrimethoxysialne (1N), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (2N) as hardeners. Coating materials were characterized using FT-IR, atomic force microscopy (AFM), UV–visible spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The optimum concentration of GPTMS to hardeners (1N and 2N) and the effect of MTMS:GPTMS molar ratios on corrosion resistance of coatings were studied. Corrosion resistance and porosity in coatings were measured by potentiodynamic polarization studies and electrochemical impedance analysis. Furthermore, hydrophobicity of the coatings was evaluated using contact angle measurements. Bend test, impact resistance test and pencil hardness test were carried out to evaluate the hardness and adhesion behavior of coating.Results show that the coatings are continuous, defect free and well adherent to the substrate. Structural studies confirm the nanometric level of interaction among the coating components. None of the coating materials show the glass transition temperature in the temperature range of 30–700 °C. Coating materials are thermally stable up to 390 °C. EIS and potentiodynamic polarization studies suggest that the optimum concentration of GPTMS:NH are 1:1 and 3:2 (molar ratio) for 1N and 2N harden coatings, respectively. Improvement in corrosion resistance is observed with increasing GPTMS concentration in coatings. The average porosity of coatings is found to be 7.18% and 10.98% for 1N and 2N harden coatings, respectively. Water contact angle of these coatings are found in the range of 59–78 °C.