Accelerated Corrosion Mechanism of a Bainitic U75V Rail in a Neutral Salt Spray Environment: Pseudopassivation from an Active Dissolution Perspective
摘要
In this study, U75V rails were subjected to isothermal quenching to obtain a lower bainite microstructure. The corrosion behavior of lower bainite samples under laboratory-simulated marine atmospheric conditions and its mechanism with respect to the microstructure were evaluated. The samples were subjected to an accelerated corrosion test for 0-240 h using a neutral salt spray test (NSS, 5% NaCl, 35 °C). The open-circuit potential (OCP) and potentiodynamic polarization (PDP) curves were collected at regular intervals using an electrochemical workstation. The evolution of the corrosion product morphology, composition, and phase assemblage was tracked by SEM/EDS and XRD. The experiments demonstrate that the corrosion resistance of the samples evolves through a pseudopassivation sequence comprising transient surface passivation, interfacial degradation beneath a macroscopically dense rust layer, and the eventual structural collapse of the corrosion product barrier. Throughout the NSS test, the open-circuit potential (OCP) shifted positively (from − 0.65 to − 0.45 V), while the corrosion current density (Icorr) increased steadily from 4.98 × 10−5 A cm−2 (24 h) to 25.06 × 10−5 A cm−2 (240 h), with a brief plateau (17.17-16.11 × 10−5 A cm−2) at 144-192 h. The corrosion rate increased from 0.960 to 1.153 mm/a, the rust protectiveness (α/γ*) decreased from 0.072 to 0.043, and the pit area fraction increased from 2.92 to 23.79%. This paradox (a narrow OCP shift alongside deteriorating protective metrics and accelerating localized dissolution) defines “pseudopassivation”: Macroscopic electrochemical behavior mimics passivation, but microstructural and kinetic data confirm progressive active breakdown. This trend is attributed to the enrichment of Cl− through the porous structure of γ-FeOOH toward the rust layer/matrix interface and the induction of pitting corrosion, as well as the numerous microgalvanic effects created by the refined lath belts of bainite. Therefore, efforts to protect bainitic rails from corrosion in marine atmospheric environments should focus on blocking the penetration of chloride ions.