Influence of mixing ratio on the permeability of water and the corrosion performance of epoxy/amine coated un-pretreated Al-2024 evaluated by impedance spectroscopy

Water uptake by epoxy coatings was measured in 0.5 M NaCl solution using a single frequency (1 kHz) capacitance method. The influence of substrates as well as the epoxy/amine mixing ratio on the water permeation coefficient was studied. Corrosion reactions at the coating/substrate interface affected the results of water uptake when the coating is applied on Al-2024 alloy. Consequently, the permeation coefficient of supported coating can be determined accurately only if the coating is applied on an inert substrate. Excess of epoxy or amine improves the solubility of water in the coating. A low water permeation coefficient was obtained with an epoxy/amine mixing ratio 1.05/1. But this formulation has a relatively poor corrosion performance when applied on an un-pretreated Al-2024 alloy. Under-film corrosion which is the characteristic of defective coatings was observed using electrochemical impedance spectroscopy.