**5. Conclusions**

A finer eutectoid structure was shown to improve the corrosion resistance of the Zn-4Al alloy, which indicates that the small phase distances between the eutectoid structures may help protect the anode phase. Importantly, this was also accompanied by an increase in the alloy hardness. This issue is important from the point of view of the heat treatment design of machine components exposed to chloride ions.

Although corrosion was initiated in the α-phase, the polarity of the η-phase changed, and its corrosion was observed. This was most likely due to the formation of a corrosion product film on the surface of the α phase that protected it from further corrosion. A scheme of the corrosion mechanism is presented in Figure 15.

**Figure 15.** Schematic illustration of the corrosion mechanism based on as-delivered Zn-4Al alloy.

**Author Contributions:** Conceptualization, M.M.L.; methodology, M.M.L.; validation, R.J.; formal analysis, M.M.L.; investigation, M.M.L.; resources, M.M.L.; data curation, M.M.L.; writing—original draft preparation, M.M.L.; writing—review and editing, M.M.L. and R.J.; visualization, M.M.L.; supervision, M.M.L. and R.J.; project administration, R.J.; funding acquisition, R.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by the Polish Ministry of Science and Higher Education for statutory activities No. S/1/S/KPBMiM/20.

**Conflicts of Interest:** The authors declare no conflict of interest.
