Establishing the passive film stability formed at different depths from the surface of machined type 304 L SS

Abstract Using a novel approach, passive film stability was established at increasing depths from surface for machined and heat-treated (675 °C/1 h) type 304 L SS. Severely plastically deformed (SPD) top surface had nano/ultrafine grains till ∼8 µm depth. Double loop-electrochemical potentiodynamic reactivation test and potentiostatic test showed passive film over SPD layer was unstable. Film stability improved till ∼100 µm depth, beyond which it matched with that of mid-thickness (protective). Defect density (Mott-Schottky analysis) in film over SPD surface was 1.7-1.8 times higher than that over mid-thickness. Point defect model was used to explain the role of machining on passive film stability.