pH-dependent electrochemical behaviour of Al3Mg2 in NaCl solution

Abstract The effects of pH on the electrochemical behaviour of β-phase (Al 3 Mg 2 ) and its role in corrosion of AA5000 series alloys in NaCl solution were investigated. The galvanic corrosion behaviour of synthesized Al 3 Mg 2 intermetallic particle coupled with Al in 0.01 M NaCl solution over a wide pH window of 2–12 was studied using the scanning vibrating electrode technique (SVET). The results show that bulk Al 3 Mg 2 acts as anode at pH 2–10.5, while cathode at pH 12. The corrosion resistance properties of bulk Al 3 Mg 2 were then evaluated via potentiodynamic polarization and electrochemical impedance spectroscopy tests. Surface characterization were carried out by confocal laser scanning microscope (CLSM), scanning electron microscope (SEM) and X-ray photoelectron spectrometry (XPS). These results indicate that Al 3 Mg 2 exhibits active dissolution of both Al and Mg elements at pH 2, while selective dissolution occurs at pH 3.5–12. At pH 3.5–10.5, any deviation from neutral pH causes a significant increase in the selective dissolution rate of Mg element, which results in a rough outer layer and a relatively compact inner layer of corrosive surface. At pH 12, the selective dissolution of Al element becomes dominant and the formation of compact Mg(OH) 2 achieves complete passivity on the surface of Al 3 Mg 2 . This work contributes to further understanding of the electrochemical behaviour of β-phase in AA5000 series alloys.