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A passive film is a thin, adherent, generally oxide-based layer that forms spontaneously on a metal or alloy surface under anodic conditions and that lowers the dissolution rate by several orders of magnitude relative to the bare metal ^[1]. On stainless steels the film is about 1–3 nm thick and consists mainly of chromium oxide or oxyhydroxide together with iron and nickel species, and its growth is usually described by a high-field mechanism in which ion migration through the film is rate-limiting ^[2]. The Point Defect Model expresses the same behaviour in terms of cation vacancies, anion vacancies, and cation interstitials that are generated and annihilated at the metal/film and film/solution interfaces, and it accounts for both steady-state growth and film breakdown ^[1][3]. Composition and structure are commonly probed by X-ray photoelectron spectroscopy, while the defect structure at atomic and nanometric scales is examined by scanning tunnelling and atomic force microscopy under electrochemical control ^[4]. Because the film behaves as a semiconductor, the donor or acceptor density obtained from Mott–Schottky analysis is often correlated with corrosion resistance ^[5]. Passivity is nevertheless lost in chloride-containing environments once the breakdown potential is exceeded, and the local chemistry that triggers pit initiation, together with the relative roles of film thinning, vacancy condensation, and chloride incorporation, remains debated ^[3][5].