Elucidating the Role of Intermetallics in Metastable Pitting of 7068 Alloy Using Electrochemical Noise

The role of intermetallic precipitates (IMPs) of Al2MgCu, Al7Cu2Fe, and MgZn2 on metastable pitting of high-zinc 7068 aluminum alloy is studied by using electrochemical noise. Experimental results and microstructural constituents characterized by using scanning electron microscopy analysis are compared. Findings reveal a strong correlation between the MgZn2 IMPs and metastable pitting events. However, the shape of the current transient events depends on the electrode potential and passive film protective ability. Besides, MgZn2 IMPs dominantly enhance the complex electrochemical activities such as microgalvanic coupling and reactivation or repassivation of pits near the breakdown potential. The cathodic current transients depend on the electrode potential, the protective ability of the passive layer, and the presence of nobler Al7Cu2Fe IMPs.