To establish a deterministic link between the properties of the nanothick passive layer and the locations of its subsequent rupture across the entire substrate surface during pit formation, it is necessary to use surface characterization techniques sensitive to nanolayers, in conjunction with operando wide-field observations such as proposed RM approach (Fig. ). In this work, we used a mirror-polished SS 316L sample with a surface area of 50 mm 2 under strong anodic polarization of +1.6 V vs. OCP to induce both in a singl experiment, passive layer formation and subsequent pit inition. A 1 mm 2 area of this sample was dynamically monitored optically, with a spatial resolution of 0.48 µm and a minimum detectable intensity change of 0.4 %. This field of view was large enough to observe the simultaneous formation of dozens of pits, and the sensitivity to intensity changes was sufficient to detect monolayer passive film growth. This section focuses on the correlation of chronoamerometry curve and raw optical images at a 50 mM NaCl concentration to describe the overall observed trends in pit formation. The following sections delve into a detailed analysis of pit locations in relation to passive film properties, employing unsupervised ML on the entire set of optical images from experiments with NaCl concentrations of 5, 10 (repeated twice), 30 (repeated twice), 50, and 100 mM. Fig. presents the chronoamperometry results, 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 Fig. ) 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, preciding chronoamerometry at OCP (Fig. ). The normalized image at = 0 s is uniform, showing no detectable changes with a consistent value of 100 %. Compared to the raw optical image at = 0.8 s, the normalized image at the same time in Fig. shows clearly 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 Fig. at = 0.8 s, stand in contrast to the whiter regions.