A key observation is that the refractive indexes of Fe 2 O 3 phases differ significantly between the two most common Fe 2 O 3 polymorphs, -Fe 2 O 3 and -Fe 2 O 3 . Both phases were considered in the simulations, represented as orange and blue curves in Fig. . The results show that -Fe 2 O 3 provides a better fit and aligns with the XPS results, supporting the presence of a mixed Cr 2 O 3 /Fe 2 O 3 passive layer.