By combining optical and electrochemical estimations, we validate the evolution of surface film thickness during passivation. The experimental data shown in Figure (main text) demonstrate a clear correlation between charge transfer and normalized optical intensity across different NaCl concentrations. Notably, the model assuming a bilayer structure of Cr 2 O 3 /𝛼-Fe 2 O 3 exhibits better agreement with experimental reflectivity changes compared to the Cr 2 O 3 /𝛾-Fe 2 O 3 model. This suggests that the passive film formed under these conditions is more accurately described by a Cr 2 O 3 /𝛼-Fe 2 O 3 configuration, reinforcing the validity of our approach in characterizing the oxidation process.