Corrosion mechanism of Mg alloys involving elongated long-period stacking ordered phase and intragranular lamellar structure

Abstract

It is a long-term challenge to further improve the corrosion resistance while ensuring the strength of magnesium (Mg) alloys. Revealing the effect of potential fluctuation on the micro-galvanic corrosion and the subsequent film formation is important for understanding the corrosion mechanism of Mg alloys with multiple strengthening phases/structures. Here, we prepared the high-strength Mg-14.4Er-1.44Zn-0.3Zr (wt.%) alloys containing hybrid structures, i.e., elongated long-period stacking ordered (LPSO) blocks + intragranular stacking faults (SFs)/LPSO lamellae. The Mg alloy with elongated LPSO blocks and intragranular LPSO lamellae (EZ-500 alloy) obtains good corrosion resistance (2.2 mm y–1), while the Mg alloy containing elongated LPSO blocks and intragranular SFs (EZ-400 alloy) shows a significantly higher corrosion rate (6.9 mm y–1). The results of scanning Kelvin probe force microscopy (SKPFM) show the elongated LPSO blocks act as cathode phase (87 mV in EZ-400 alloy), and the SFs serve as the weak anode (30 mV in EZ-400 alloy), resulting in high potential fluctuation in EZ-400 alloy. On the contrary, both elongated blocks and intragranular lamellae are cathodic LPSO phase (67–69 mV) in EZ-500 alloy, leading to a lower potential fluctuation. Quasi in-situ atomic force microscope (AFM) observation indicates that high potential fluctuation would cause strong micro-galvanic corrosion, and subsequently leads to the failure in rapid formation of corrosion film, finally forming a loose and porous film, while relatively low potential fluctuation could result in more uniform corrosion mode and facilitate the rapid formation of protective film. Therefore, we propose that it is an effective way to develop high-strength corrosion-resistant Mg alloys by controlling the potential fluctuation to form a “uniform potential” strengthening microstructure.