Figure presents the CA results for the experiment with 50mM NaCl solution, identifying two distinct periods. Period 1 begins with the application of potential and persists for approximately 0.8 s, marked by a decrease in current density from 2.4 to 1.1 µA/cm 2 . A comparison of the whole raw optical images at the beginning (t = 0 s) and end (t = 0.8 s) of this period shows no visible changes. However, magnified inset images (marked as Pit 1 in Figure ) already show the formation of pits, visible as the appearance of dark, contrast-enhanced spots. To enhance the sensitivity of optical detection, we normalized the light reflectivity of each image pixel-wise by by the light reflectivity of the first image, preceding CA at OCP (Figure ). The normalized image at t = 0 s is uniform, showing no detectable changes with a consistent value of 100%. Compared to the raw optical image at t = 0.8 s, the normalized image at the same time in Figure shows clearly a decrease in reflectivity of about 1% on average across whole image. The decline in current density and light reflectivity points to the formation of a passive layer on the SS 316L surface, which restricts electron transfer and absorbs light. Magnified optical images already reveal the formation of pits, even as the current continues to decline. Note also that the passive layer is highly heterogeneous, as indicated by the normalized image: areas of greater light absorption, which appear more blue in Figure at t = 0.8 s, stand in contrast to the whiter regions.