Localized corrosion mechanism of 2×××-series Al alloy containing S(Al2CuMg) and θ′(Al2Cu) precipitates in 4.0% NaCl solution at pH 6.1

To study localized corrosion mechanism of 2×××-series Al alloys containing θ′ and S precipitates, the electrochemical behaviors of a three-electrode coupling system of α(Al) substituting for alloy base, simulated bulk θ′ and S in 4.0% NaCl solution at pH 6.1 have been investigated. The potential of θ′ is positive to that of α(Al) and S, indicating that θ′ acts as cathodic zone in the alloy. S is main anodic phase, and the anodic dissolution of S will cause its pitting corrosion. However, during the corrosion of S, Mg is preferentially dissolved, resulting in noble Cu-rich remnants, which makes its potential move to positive direction with immersion time. As a result, corroded S becomes cathodic to α(Al), leading to anodic dissolution of α(Al) and pitting corrosion of the alloy base at adjacent periphery of S precipitate.