*3.2. EDS Analysis*

Figures 3 and 4 show the collective results of metallographic studies for selected alloys using the EDS system. The presented results indicate the presence of a loose oxide layer (blue), which is adjacent to the passive layer. The passive layer is an area also marked in blue, where the area has an increased silicon content (intense yellow bands on the maps—see Figure 3e, Figure 4e, Figure 5e). The increased Si content in the passive layer results from the diffusion of iron atoms from this layer to the loose oxide layer, which makes the area richer in Si. Penetrating oxygen reaches the area enriched with silicon and forms compounds with it (e.g., SiO2) creating a tight barrier to the propagation of corrosive phenomena. Of course, a SiO2 compound can react with Fe, O, and FeO, forming a fayalite Fe2SiO4 [8,17].

**Figure 3.** Microstructure of SiMo cast iron, 0.01% Mo (**a**). Elements decomposition maps. Map for (**b**) Fe, (**c**) C, (**d**) O, and (**e**) Si.

The characteristic phenomenon of these alloys is their ability to perform so-called "self-healing". Removing the passive layer creates a new one in its place. The average thickness of the loose oxide layer was 10 μm for the tested samples, while the thickness of the passive layer was also around 10 μm. Of course, the thickness of this layer depends on the local conditions on the surface of the sample exposed to the corrosive agents.

**Figure 4.** Microstructure of SiMo 3 cast iron, 1.09% Mo (**a**). Map for (**b**) Fe, (**c**) C, (**d**) O, and (**e**) Si. D E F

**Figure 5.** View of the carburized layer. SiMo sample 5. 2.51% Mo (**a**). Map for (**b**) Fe, (**c**) O, (**d**) C, (**e**) Si, and (**f**) Mo.

Figure 5 shows the analysis of the sample area (for the SiMo melt 5), where a significant degree of carburation of the casting material layer was observed. The layer is placed under the passive layer. This can be explained by the high concentration of carbon in the corrosive atmosphere of the furnace, and the penetration of carbon atoms through the passive layer towards the center of the casting.
