A key defense against corrosion in passive metals, such as stainless steels (SSs), is the formation of a nanometer-thick oxide/hydroxide layer. Modern structure-sensitive techniques-scanning tunneling microscopy (STM), atomic force microscopy (AFM), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) among anothers-have revealed that this layer forms almost instantaneously upon metal oxidation, reaches 0.1 to 5 nm in thickness, and exhibits a bi-layered structure. The inner layer is compact and Cr-rich, while the outer layer is Fe-rich. Growth kinetics models explain the dissolution of the underlying SS as a process of cation transfer through this protective layer. This cation transport accelerates markedly at defective regions, including oxide and metal grain boundaries, inclusions, second phase particles, and atomic defects in crystalline oxide structures. The scientific consensus indicates that Cr plays the most critical role in corrosion protection of SSs. When Cr becomes locally depleted due to defective regions, particularly in solutions containing Cl -or other halides, film breakdown occurs, leading to accelerated localized dissolution, commonly known as pitting. Therefore, effective corrosion monitoring requires in-situ screening of passive layer properties to identify pitting-susceptible zones (PSZs) or Cr depletion-a challenge currently limited to laboratory settings with high-precision tools (STM, ToF-SIMS, AFM etc.) unsuitable for real-world monitroing.