Effect of Intermetallic Particle Size and Distribution on the Corrosion of an Mg-Al Alloy
Authors: Bland Leslie Gail , Bhattacharyya Jishnu , Agnew Sean , Scully John 
Published: February 2020

The corrosion rates for Mg alloys are dependent on their composition. In particular, the corrosion resistance is usually increased if the alloying elements remain in solid solution and decreased if the alloying elements are above the solid solution solubility limit [1, 2]. Al additions up to 4 wt. % reduce the corrosion rate [2, 3].
During heat treatment, intermetallic particle (IMP) [4, 5] sizes increase while the area fraction remains relatively constant [4]. The size and spacing of these particles impacts the corrosion behaviour and will likely alter the cathodic reaction rate and corrosion rate as observed in other alloys systems [6].
In Mg-Al alloys, IMPs are more noble than typical Mg-Al alloy solid solutions [7, 8]. The IMPs function as cathodes when coupled with the α-Mg phase [9]. These IMPs support cathodic reduction reactions at faster rates than high purity Mg [7]. The extent of attack is dependent on the composition of the IMP. For instance, the cathodic activity of Al8Mn5 is much greater than that of Mg12Al17 or Al3Mg2 [10, 11]. However, little is known about how the size and spacing of these particles alters the cathodic rates and subsequently affects the corrosion rate.
Mg-Al alloy, AZ31B was heat treated at temperatures ranging from 300 to 450 °C for durations ranging from 240 to 604,800 sec to produce samples varying in particle size and spacing. The corrosion behavior was examined in terms of its instantaneous corrosion rate and cathodic kinetics as a function of IMP size and spacing for a fixed area fraction of IMPs. The corrosion resistance was determined utilizing electrochemical impedance spectroscopy (EIS) [3]. The cathodic reaction rate was determined at -1.8 VSCE after 24 hrs immersed at the open circuit potential in 0.6 M NaCl. There was a decrease in both icorr and ic (Figure 1) with increasing particle size and spacing at a fixed area fraction of IMPs.
The fast reactions around IMPs were mapped using Scanning Electrochemical Microscopy [12-15]. A non-aqueous electrolyte, such as ethelyne glycol or methanol mixed with a NaCl, was found to slow the kinetics of Mg corrosion while enabling mapping of active corroding sites [16]. The solution of 50 wt% methanol and 50 wt% H2O was found to give the best results. The mediator, hexaammineruthenium(III), Ru(NH3)6Cl3, has successfully been used to map Mg corrosion and was utilized herein [12]. The reduction of Ru(NH3)6Cl3 occurred at actively corroding sites. The subsequent oxidation of Ru(NH3)6Cl3 at the tip provided an indicator of sites where significant cathodic electron transfer reactions occurred [17]. The evolution of the reactivity of an AZ31B surface with time was mapped in 0.1 M NaCl (Figure 2). The regions of high cathodic reactivity are shown to spread across the surface as a function of exposure time.

Acknowledgements

This work was funded by the Office of Naval Research Grant N000141210967 with Dr. David A. Shifler.

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Figure 1