*3.3. Immersion Test*

Figure 8 presents the cross-sectional SEM micrographs of corrosion morphology of the Mg-RE alloy ingot sample and the sheet with the TRC sample in PBS solution at 310 K for 10 days. It can be observed from Figure 8a that the corrosion layer covered the Mg-RE ingot sample surface, which extended to the inside of the matrix with crack features. In addition, the maximum length of the corrosion cracks extending to the interior of the matrix was 30 μm, and they were distributed unevenly along the cross section of the Mg-RE ingot sample. The improved corrosion-resistance properties of the Mg-RE alloy with TRC are reflected in Figure 8b, in that the corrosion layer is thinner and more

uniform. The results of the energy spectrum analysis indicate that the content of elements in different location of Mg-RE alloy ingot is dissimilar, which is mainly connected with the corrosion behavior of the alloy in PBS solution. The mechanism of corrosion analysis is as described below.

**Figure 8.** Cross-sectional SEM micrographs of corrosion morphology of (**a**) the Mg-RE alloy ingot sample and (**b**) the sheet with TRC sample in the PBS solution at 310 K for 10 days.

The multiple layers of Mg-RE alloy ingot primarily consisted of C, O, Mg, Al, Ce and other elements during the immersion test in PBS solution. Mg transformed into the stable Mg<sup>2</sup>+ ion in the initial stages. Meanwhile, the cathodic reaction occurred on account of the galvanic corrosion between the matrix and the secondary phase, accompanied by the hydrogen evolution [32]. The cathode reaction formed a heterogeneous thin porous layer which was predominantly magnesium hydrate on the surface of the Mg-RE alloy. This prevented contact of the solution and the substrate, resulting in a decrease in the corrosion rate [33]. Moreover, the chloride ions in the PBS solution were able to easily penetrate the membrane and react with the magnesium hydrate compounds. Therefore, magnesium hydrate compounds were converted into magnesium chloride compounds, which are more likely to dissolve into magnesium ions and chloride ions [34]. The dissolution of the compound leads to a decrease in the magnesium hydrated compounds around the protective layer, leading to further dissolution of the matrix.

Magnesium hydrated films constituted on the surface of magnesium are generally referred to as crystals. Previous studies have found that the composition and structure of magnesium and magnesium alloy surface films can be changed using a rapid solidification process. The conversion of magnesium hydrated films from a crystal form into an amorphous film structure improves the corrosion resistance. Amorphous films have better protection than crystalline films, and films without grain boundaries are better able to resist ion motion than crystalline films [35]. The Mg-RE alloy sheet obtained by the TRC process has a unique structure due to its crystal structure surrounding the amorphous structure, which maybethereasonfortheimprovementofthecorrosionresistanceofthemagnesiumhydratedfilm.

Figure 9 presents the Mg, Al, Ce and La metallic ion concentrations of the solution for (a) the Mg-RE alloy ingot sample and (b) the sheet with the TRC sample at day 10, day 20 and day 30 under immersion testing in PBS solution at a temperature of 310 K. Specifically, both alloys showed a general trend of increasing Mg ion concentration in the PBS solution with increasing immersion time from day 10, day 20 and day 30. In contrast, for Al ion concentration, both alloys showed a decreasing trend. Generally, the Mg-RE alloy ingot sample showed greater average Mg and Al ion concentration in the PBS solution at each time point—day 10, day 20 and day 30. From day 10 to day 20 after immersion, the magnesium ion concentration for the sheet sample was obviously less than the ingot sample after 10, 20 and 30 days. This may be caused by the rapid corrosion of alloys in the initial corrosion stage and then the beginning of passivation to slow down the corrosion. The decrease of aluminum in the corrosive solution may have resulted from the formation of corrosion products on the surface of the sample.

**Figure 9.** Metallic ion concentrations of the PBS solution at 310 K with (**a**) Mg-RE alloy ingot sample and (**b**) sheet with TRC sample at day 10, day 20 and day 30 under immersion testing.

Figure 9a,b also shows that throughout the entire 30 day culture period, the Mg-RE with TRC sample group showed significantly lower release rates of Ce and La ions than the Mg-RE alloy ingot sample, and the group showed a general trend of increasing Ce and La ion concentration with increasing immersion time, but no significant change for the Mg-RE with TRC sample group. Overall, the content of metallic ions concentrations in the corrosion solution of the Mg-RE alloy sheet produced by rapid solidification TRC method less than that of Mg-RE alloy ingot sample within the same amount of immersion time. Which is probably induced by the special microstructure formed under rapid cooling and exhibits an improved corrosion resistance.
