New insights on the corrosion mechanism of a peak-aged Mg–9Al–1Zn alloy in a chloride environment

It has been documented that the corrosion behavior of AZ91 (Mg–9Al–1Zn, wt.%) alloy is strongly influenced by the morphology, distribution, and volume fraction of secondary phases, however, the proposed corrosion mechanisms are questioned due to new experimental findings. The present study focused on providing a more detailed understanding of the corrosion mechanism of a peak-aged AZ91 alloy exposed to a chloride environment by using scanning Kelvin probe force microscopy, immersion testing, optical microscopy, scanning and transmission electron microscopy techniques and electrochemical microcell technique. It was found that corrosion was simultaneously initiated in the interior of α-Mg grains and in the local α-Mg phase within the α + β lamellar precipitate; furthermore, the lamellar precipitate/α-Mg matrix boundary acted as a barrier for corrosion propagation. The proposed corrosion initiation mechanism is opposed to previous investigations reporting the development of micro-galvanic coupling between the β-Mg17Al12 phase and the adjacent α-Mg matrix, which cannot be supported by our experimental findings. The corrosion propagation in the peak-aged alloy was dominated by the preferential anodic dissolution at the interior of the α-Mg matrix due to its faster corrosion reaction kinetics compared to the micro-galvanic corrosion process between the β-phase and the local α-Mg phase occurring inside the lamellar precipitate.