Water uptake and failure of epoxy coatings on LY12 aluminum alloy in NaCl solution

Impedance parameters of LY12 Al alloy epoxy electrodes immersed in NaCl solution for different times were calculated by using proper equivalent circuit (EC). A two-stage sorption for water in epoxy coatings was detected from the time dependence of coating capacitance, CC. The apparent diffusion coefficient of water was obtained from the linear part of InCc-t(0.5) curve at the early immersion stage. The diffusion coefficient of Cl-, calculated from the occurrence time of impedance components of Cl- participating salt film in EIS, is 4.67x10(-12) cm(2) . s(-1). Due to the water uptake the increase of coating capacitance and the decrease of the coating resistance mainly take place in the early immersion time, whilst the open-circuit potentials, E-oc, changes throughout the whole immersion time. The variation of E-oc display four stages: a sharp increase in the early time, a slow increase in the second stage, then dramatic increase again in the intermediate stage and a decrease in the final stage with an activation of alloy base and a failure of coatings. The experimental results show a close relationship between the behavior of Cl- containing salt film (the formation, growth and dissolution etc.) and the change of E-oc, indicating a major role of the salt film on the degradation of LY12 Al alloy/epoxy system.