The deconvoluted Cr 2p spectrum exhibits several distinct features that reflect the complex electronic structure of the material (Figure ). The peak at 586.3 eV is assigned to the Cr 2p 1/2 main emission of Cr(III)) in Cr 2 O 3 , confirming that the predominant oxidation state of chromium in the film is +3 . A pronounced signal observed at 583.2 eV is interpreted as a shake-up (satellite) feature associated with the Cr 2p 1/2 emission; such satellites arise from charge transfer processes or multiplet splitting effects characteristic of Cr(III) in an oxidic environment . The main Cr 2p 3/2 peak at 576.5 eV further substantiates the formation of Cr 2 O 3 , while a lower binding energy feature at 573.7 eV is attributed either to a metallic chromium (Cr 0 ) component or to a reduced chromium species.In summury, XPS analysis indicates Cr 2 O 3 and Fe 2 O 3 in the passive film, matching the typical bi-layered structure of stainless steel passive films (inner compact Cr 2 O 3 , outer less compact Fe 2 O 3 ). Given the known chemical composition, the passive layer thickness can be estimated from two independent measurements recorded during Period 1 (Figure ): the electrical charge (via Faraday's law) and the optical reflectivity change (via Fresnel's law). However, we needed to assume a specific ratio for the thicknesses of Cr 2 O 3 and Fe 2 O 3 . For simplicity, a 1:1 ratio was chosen. While different ratios would alter the absolute thickness estimates, the trend in thickness distribution across the 5-100 mM NaCl concentrations would remain unchanged, which is siffucuent to rationalize optical signatures. Details of these calculations can be found in the SI-4.