Impedance evaluation of permeability and corrosion of Al-2024 aluminum alloy coated with a chromate free primer

The corrosion of AA-2024 aluminum alloy protected with a chromate free primer is investigated after immersion in a 0.5 M NaCl aqueous solution. The water uptake by the coating increases continuously when the film, applied on an aluminum AA-2024 substrate, is placed in the 0.5 M NaCl solution. This increase is attributed to corrosion reactions taking place at the alloy/coating interface when water molecules reach the interface. The maximum water volume fraction absorbed by a similar coating applied on platinum substrate is 3.5 vol% and the permeability is 7.6 × 10−12 m2 s−1. After 72 h immersion in the 0.5 M NaCl solution, the Nyquist representation of impedance data shows transmission line behavior that can be assigned to percolation pathway along the filler particles after water uptake. Charge transfer and diffusion of corrosion reactants and products occur, but no delamination was observed for immersion longer than 172 h. Furthermore, the coating resistance is still close to 108 Ω cm−2 after this immersion time. This accounts for the good protective performance of the coating.