Sn microalloying-enhanced corrosion resistance of passive films on high-Cr steel in chloride-alkaline solution

Through systematic electrochemical tests, surface analyses, and thermodynamic/kinetic calculations, this work elucidates the mechanism by which Sn micro-alloying enhances the passivity of high-Cr steel in chloride-containing alkaline environments. Results indicate that trace Sn addition significantly reduces the passivation current density and enhances the impedance and thickness of the passive film. XPS and ToF-SIMS analyses reveal the coexistence of Sn^{2 +} and Sn⁴⁺ species within the film, with the anionic Sn(OH)₃^- species generated via the triple hydrolysis of Sn²⁺ playing a decisive role. Sn(OH)₃^- not only serves as heterogeneous nucleation sites for Fe and Cr hydroxides to improve film compactness but also reduces cationic defects and interstices, effectively hindering chloride ion penetration to the metal surface. Mott–Schottky analysis further confirms that Sn addition decreases carrier density and enhances the energy barrier of the passive film. Thus, Sn micro-alloying significantly improves corrosion resistance and durability of high-Cr steel passive films through a combined mechanism of interfacial charge modulation and structural optimization.