Avalanches during micro-galvanic corrosion of Mg-Cu-Al alloys in NaCl solution

Alloying metals improves their mechanical properties. In the case of Mg, alloying atoms, such as Cu and Al, act as microcathodes and accelerate corrosion. We establish the relationship between the corrosion mechanisms and avalanche dynamics as observed by acoustic emission (AE) spectroscopy. The micro-galvanic corrosion of Mg-Cu-Al alloys is simultaneously investigated under in situ conditions by AE and free-corrosion potential measurements, and optical observations. Three corrosion mechanisms are found, namely Mg matrix dissolution, crack propagation, and bubble burst. Mg matrix dissolution events show predominantly correlated avalanches with power law distributed AE signals. Their energy exponents decrease when initial sparse, localized corrosion events develop into large-scale corrosion. The crack propagation involves avalanches which accelerate micro-galvanic corrosion. To better understand the role of alloying of Mg we compare our results of a Mg-Cu-Al alloy with those of pure Mg. The incubation time in Mg alloys is much shorter (41 s versus 1236 s) than in pure Mg, and the initial attack regime is shortened (1236 s versus 1903 s). The power law exponents are generally reduced, and the power law regimes are wider in the Mg alloys than in pure Mg. The bubble bursts in the two materials are virtually identical. The statistical analysis demonstrates in situ monitoring of corrosion based on these techniques is possible.