Corrosion protection mechanism of Ce(4+)/organic inhibitor for AA2024 in 3.5% NaCl

Cerium is a rare earth element that has been widely proposed for the corrosion protection of aluminium alloys (AA). Both cerium salts, Ce(3+) and Ce(4+), have been used in combination with other compounds to offer synergistic inhibition, however, the inhibitive corrosion mechanism when using Ce(4+) with organic compounds is still not clear. In this study, the synergistic inhibition effect of Ce(4+) and melamine (M) on the corrosion of aluminium alloy 2024 (AA2024) in 3.5% NaCl solution was investigated. Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) techniques were used to study the synergistic effect of different Ce(4+)/M ratios on the corrosion behaviour of AA2024. The PDP study showed that a combination of 50% Ce(4+) and 50% M leads to the lowest corrosion rates, both acting as cathodic inhibitors. Both PDP and EIS results indicated that M or Ce(4+) in isolation did not offer effective corrosion protection, while the combination of M and Ce(4+) significantly enhances the corrosion protection with a synergism parameter equal to 3.5. SEM and EDX observations confirm the findings from the electrochemical techniques. XPS was used to investigate the mechanism of protection, revealing that the reduction of Ce(4+) to Ce(3+) occurs during protection of AA2024. A new mechanism of corrosion synergistic inhibition by Ce(4+) and organic compounds is postulated where the role of the organic compounds is to enhance the reduction of Ce(4+).