Figure provides experimental correlations of two independent electrochemical and optical measurements, along with fitted curves of the bi-layered passive layer under the assumptions considered above. We considered two polymorph Fe 2 O 3 phases for fitting: α-Fe 2 O 3 (blue curve) and γ-Fe 2 O 3 (orange curve). Both curves exhibit the same trend: a linear decrease in normalized intensity and a linear increase in charge as NaCl concentration increases, with the blue curve more closely matching the experimental observations.