Cr-induced enhancement of pitting resistance and passive film stability in alkaline chloride solutions: From electronic properties to pit morphology

To elucidate the modulation mechanism of Cr on the corrosion evolution trajectory of steel reinforcement in simulated chloride-contaminated concrete pore solutions, this study systematically investigated the cross-scale correlations ranging from microstructure to macroscopic corrosion morphology using HRB400, 0.3Cr, and 4Cr rebars. The results indicate that Cr alloying significantly refined the matrix grains and increased the proportion of low-angle grain boundaries (LAGB) to 26.70%, effectively blocking corrosion active sites and diffusion channels at grain boundaries. In chloride-containing environments, Cr not only significantly increased the pitting potential (with 4Cr exhibiting a positive shift of over 400 mV compared to HRB400) but also induced a fundamental transformation in pitting morphology: corrosion pits shifted from the "deep-narrow" longitudinal penetration observed in HRB400 to the "shallow-wide" lateral expansion in 4Cr, thereby significantly reducing the risk of localized failure. XPS and Mott-Schottky analyses revealed the electronic structural origin of this transition: Cr promoted the enrichment of dense Cr 2 O 3 /Cr(OH) 3 phases within the passive film and induced the film to exhibit unique p-n junction composite semiconductor characteristics. This heterojunction structure established a strong built-in electric field within the film, effectively inhibiting electron transition and anodic dissolution kinetics at the pit bottom. These findings elucidate the corrosion resistance mechanism wherein Cr impedes Cl - penetration through the synergistic optimization of microstructure, rust layer phase composition, and the electrochemical-structural properties of the passive film, providing a theoretical basis for material selection and the application of high-durability reinforcement in severe chloride environments.