Exponential pH dependence of chloride threshold for rebar in simulated concrete pore solutions

This study investigates the pH dependence of chloride threshold value (CTV) for Q235 carbon steel in simulated concrete pore solutions (pH 11.5–13.2). Using multiple electrochemical techniques and Mott-Schottky analysis, the results show that as pH increases, CTV rises from 0.02 to 0.25 mol/L. Over the studied pH range, this relationship can be approximated by an exponential function (R 2 = 0.96). Higher pH promotes a thicker and less defective passive film, reducing oxygen vacancy density by 65%. A unified kinetic model, in which CTV increases with OH − concentration and film thickness but decreases with defect density, accurately predicts CTV (R 2 = 0.96). SEM confirms higher pH mitigates both depassivation and pit propagation. These findings provide quantitative data that can support durability design of concrete structures in chloride environments.