Corrosion of 7A04 aluminum alloy under defected epoxy coating studied by localized electrochemical impedance spectroscopy

In this work, corrosion of the coated 7A04 aluminum alloy with and without ultraviolet (UV) aging was studied by salt spray test, surface observation and localized electrochemical impedance spectroscopy measurements. Results demonstrated that, in the absence of UV aging, the impedance at coating defect increases with time, indicating the formation and accumulation of corrosion product, while the impedance in the adjacent coating decreases, which is attributed to the permeation of corrosive solution from defect and the resultant disbondment of coating. Upon UV aging, the impedance values at both defected area and the coating decrease. The coating contains a significant number of wrinkles. The wrinkles provide micro-pathways where the corrosive species, such as chloride ions, could permeate and reach the aluminum alloy surface for corrosion reaction. Different from the coating without UV aging where corrosion is limited to the defected area, a layer of corrosion product would not be formed to cover the defected area when coating experiences UV aging. Therefore, corrosion occurs continuously with time.