*3.1. SEM Analysis*

Figure 2 shows photos of SiMo cast iron samples after the chemical corrosion resistance test cycle. The microstructure of the cast iron consisted of a ferritic matrix, graphite nodules, molybdenum carbide (Mo2C), a carburized zone (Figure 2e), and the passive layer and the loose oxide layer on the top surface of the samples. Microstructure components are highlighted in Figure 2.

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**Figure 2.** Microstructure of SiMo cast iron after the corrosion test cycle. SiMo cast iron with addition of: (**a**) 0.01% Mo, (**b**) 0.47% Mo, (**c**) 1.09% Mo, (**d**) 1.92% Mo, (**e**) 2.51% Mo. SEM.

In all cases (SiMo samples 1–5), the thickness of passive layers is around 10 μm. For elevated molybdenum content (Figure 2e, SiMo melt 5, 2.51% Mo) in the near-surface layer, the carburized zone in the form of black inclusions distributed in the vicinity of Mo2C carbide precipitates is clearly visible. All the cases considered are characterized by a cohesive passive layer tightly adhering to the sample. No cracks nor defects in the passive layer were observed, even after the process of preparing metallographic sections (cutting, grinding, and polishing).
