Enhancement mechanisms of Cr and RE on the corrosion resistance of HRB400 rebar in chloride-containing concrete pore solution

This study deciphers the chloride-induced early-stage corrosion of Cr/RE-microalloyed HRB400 rebars in saturated Ca(OH)₂ pore solutions (0.5–3.5 wt% Cl⁻). Rare-earth addition transforms original MnS and Al₂O₃–MnO–CaO inclusions into RE–Al–O–S particles, many encapsulated by thin MnS shells. Preferential MnS dissolution creates occluded cells whose acidification simultaneously attacks the exposed RE–Al–O core and adjacent steel, yet the RE phase markedly retards this sequence. Across all chloride levels, HRB400-Cr-RE exhibits the highest pitting potential, lowest passive current density, largest charge-transfer resistance and thinnest passive-film donor density; after 7 d immersion its corrosion rate is one-third that of HRB400 and confocal microscopy confirms the shallowest pits with the lowest aspect ratios. Elevated Cl⁻ progressively lowers pitting potentials, raises passive currents, shrinks Nyquist arcs and increases donor densities in all steels, evidencing accelerated dissolution of inclusions and surrounding matrix under high-chloride conditions.