Effect of mixture ratio on water uptake and corrosion performance of silicone-epoxy hybrid coatings coated 2024 Al-alloy

A silicone-epoxy hybrid coating cured with amino silane was developed to provide corrosion protection on 2024 Al-alloy using air spraying. Water uptake characteristics of the silicone-epoxy hybrid coatings were investigated by electrochemical impedance spectroscopy in a 5 wt% NaCl solution. The effect of mixture ratio of silicone-epoxy and amino-silane on the water uptake (solubility, diffusion coefficient and permeation) was studied by using a single frequency (10 kHz) capacitance method. The glass transition temperature (Tg) was also investigated through differential scanning calorimeter (DSC) before and after immersion in the NaCl solution. Consequently, the excess of silicone-epoxy resin or amino silane improved the solubility of water in the coatings. A low water permeation coefficient was obtained with the mixing ratio 8/2 of silicone-epoxy and amino-silane, in which the Tg value was found to be larger than other three mixing ratios before immersion. After immersion for 750 h, the impedance modulus of EFA 2 coating (mixing ratio 8/2) in the low frequency was still close to 108 Ω cm2 that accounts for the good protective performance.