**4. Discussion**

#### *4.1. SEM vs. Mass Balance after NSST*

Both weight loss and weight gain were observed for a significant portion of the as-cast samples due to several related properties of the ZnAlMg alloy system. During directional cooling of these alloys, the η(Zn) phase forms the primary dendrites, while the interdendritic spaces are formed by a fine mixture of various phases including α(Al) solid solution, MgZn2, Mg2Zn11, and Mg2Sn intermetallic phases. There is an inherent difference in the open circuit potential (OCP) of these phases mainly compared to the η(Zn) phase (Figure 13). Consequently, the interdedritic phases seem to offer the galvanic

protection to the η(Zn) phase dendrites. Due to this phenomenon, the interdendritic spaces corrode prior to the η(Zn) phase. The still intact η(Zn) phase dendrites act as anchors holding these corrosion products in place. These corrosion products cannot be removed by the environment during the NSST, nor by chemical cleaning done in the preparation for the sample weighing after the test. Naturally, the total weight of such corrosion products is greater as the weight of the original metallic phases. This phenomenon will cause weight gain for several samples even after the corrosion products were removed as much as possible before weighing. This increase in weight is also followed by an increase in volume. Following the BSEM images, it is clear that the η(Zn) phase dendrites are cracking as seen in Figure 8. This could be attributed to volume expansion-induced cracking (Figures 3a and 8a).

**Figure 13.** Overview of OCP values for phases present in the investigated system [34,40–44].
