After obtaining the data of cathodic polarization tests on the pure metals, the ohmic potential drop (IR Drop) was removed, and the corresponding cathodic polarization curves were shown in Fig. 1. Since the corrosion potential of Mg alloys is about −1.7 V_SCE, the cathodic current density for different metals at −1.7 V_SCE was plotted and compared, as shown in Fig. 2. Generally, Fe, W, Cr, Co and Cu were regarded as impurities that can significantly accelerate the corrosion of Mg alloys. Fe, W, Cr, Co and Cu can support very large cathodic hydrogen reduction current density, as revealed in Fig. 2. For Fe, W, Cr, Co and Cu, the hydrogen reduction ability of Cu is the weakest, which is about 1/10 of other 3 elements. Ag is an interesting element. Although the corrosion potential of Ag was the highest in the studied metals, Ag only showed the medium ability of supporting hydrogen reduction. The cathodic current density of Ag at −1.7 V_SCE was about 3.4 mA/cm². Ti and Zr showed very weak cathodic hydrogen reduction ability, with only about 0.25 mA/cm² cathodic current density at −1.7 V_SCE. Ti has very low solid solubility in Mg and it does not form second phase particles with Mg. Ti can be used as a grain refiner for some Mg alloys. The weak hydrogen reduction ability of Ti and its low solid solubility in Mg indicate the galvanic corrosion effect of Ti in Mg almost need not be considered. Zr is the most common grain refiner for Al-free Mg alloy, whose content is generally below 0.6 wt% [18]. This indicates a small amount of Zr, as long as it does not combine with other elements to form the second phase, should not be a problem in terms of galvanic corrosion with Mg. Zn, Sn and Al are common alloying elements for Mg alloy. They are generally added as the main alloying elements to obtain good mechanical properties [19,20]. Their content in Mg is generally between 1 wt% to 10 wt%. The hydrogen reduction ability of Zn, Sn and Al is relatively low. The cathodic current density for Zn, Sn and Al at −1.7 V_SCE were about 0.65 mA/cm², 0.45 mA/cm² and 0.3 mA/cm², respectively. In summarize, Ti, Al, Sn, and Zr have lower cathodic current density, while Fe, Cr, Co and W behave inversely. As a consequence, it is recognized that Ti, Al, Sn, and Zr have weak hydrogen evolution ability and can be alloyed with magnesium in terms of corrosion property. Oppositely, Fe, Cr, Co and W are strong in hydrogen evolution and can accelerate the galvanic corrosion of magnesium alloys significantly, which should be avoided even as impurities.

Anodic polarization was performed on pure Zn, Al, Ag, and Sn (Fig. 3) as they have relatively large solid solubility and are often used as main alloying elements in Mg alloy. Performing this test is to indirectly check if they can provide protective effect on the corrosion of Mg alloy when they are oxidized to corrosion products. Zn does not show passive behavior, as the current density increases stably with increasing potential. But Ag, Sn, and Al show passive effect and the breakdown potential can be seen on the anodic polarization curves. The potential ranges of passive region for Al, Sn, and Ag are 430 mV, 250 mV, and 110 mV, respectively. This indicates the protective effect of the oxides of alloying elements might be in the following order Al > Sn > Ag. The enhanced passive effect due to alloying with Al, Sn and Ag were also observed in Mg alloys [21–24]. This is consistent with our study. Some work also reported that Zn alloying can improve the stability of corrosion film of Mg alloy slightly [25,26], which is not fully in agreement with our study on pure Zn. However, by considering the relatively strong cathodic role of Zn and Zn-containing phases and the weak or no passive effect of Zn-containing film, it is reasonable to infer that alloying with Zn could not improve corrosion resistance of Mg.

The curves of potential and current density variation as a function of time during galvanic coupling between Mg and other metals are presented in Fig. 4. The mixed potential of all the galvanic couples only varies slightly, around −1.68 V_SCE. Thus, the potential is considered to be similar and its effect on current is ignored. The initial current density for all the galvanic couples was similar to the value obtained by the potentiodynamic polarization test. However, with increasing time, the current density varied differently for different metals, which indicates the initial cathodic polarization behavior could not completely reveal the long term galvanic corrosion behavior between Mg and other metals. The variation of current density on pure Sn was relatively small, which decreased initially and then increased, and finally kept relatively stable at about 0.7 mA/cm². It was also been studied by Cain et al. [20] that Mg-Sn alloys have lower corrosion rates when compared to high purity Mg, especially for Sn addition of 10%. This indicates Sn does not generate strong cathodic hydrogen evolution effect, which agrees with our work. For Zr, Cu, and Ag, the current density increased initially and then reached a relatively stable value after about 2.5 hours. The increased percentages for Zr, Cu, and Ag are about 50%, 100%, and 200%, respectively. For Ti, the current density increased very fast in the first 20 minutes and then increased slowly but linearly, which increased by 300% and was still increasing after 20 hours. Therefore, the enhanced cathodic activity should be considered when alloying with Ag and Ti during long term corrosion. The accelerating corrosion behavior is also reported in Ag-containing alloys [23,27].

For Zn and Al, the current density decreased with increased time and then kept stable. However, the decreased degree has a large difference. The stable current density for Zn was about half of the current density initially. Yim [28] studied the as-extruded TZ alloys and found that both the corrosion potential measured by potentiodynamic test and the current density at cathodic region increased with increase of Zn content. It means Zn content should be controlled strictly in magnesium alloys though the current density of Mg-Zn galvanic couple is low. But the stable current density for Al was only about 1/50 of the initial value. This indicates Al is not only a weak cathode but also has the potential to significantly reduce its cathodic role when alloying in Mg, which is quite interesting. However, the reported corrosion behavior of Mg-Al alloys is very complexed. Bland [29] et al. studied the Mg-Al alloys with ≥ 3 wt% Al content and found that deeper local corrosion occurs. The current density of the alloys is high in cathodic polarization test. This is not consistent with our study. According to our result of Mg-Al galvanic corrosion test, such low current density equals to pretty good corrosion resistance. Therefore, we deduce that impurity elements, such as Fe, may be the cause for the high corrosion rate of the reported Mg-Al alloys [27]. Effects of Fe in magnesium alloys have been studied and it is confirmed that Fe can significantly increase corrosion rate by forming Fe-rich particles in magnesium alloys [30–32]. Generally, less than 9 wt% Al is alloyed in Mg in commercial Mg alloys [27,33]. Based on our study, higher amount of Al is suggested to be alloyed for new alloys development in terms of long term corrosion resistance.