Corrosion of rebar in concrete. Part IV. On the theoretical basis of the chloride threshold

Highlights: • The chloride threshold for rebar in concrete pore solution has been calculated using the Point Defect Model and Mixed Potential Model. • Expressions have been derived for interconverting different definitions of CT. • CT are scattered over wide ranges, arising from inherent error in the breakdown voltage and the ECP. • The accuracy of CT is limited by fundamental theory- and determined by the standard deviation of cation vacancy diffusivity in the population of available breakdown sites. • The PDM/MPM theory yields CT values that are in good agreement with those reported experimentally. A theory is developed for calculating the chloride threshold (CT) from the Point Defect Model (PDM). CT is defined as the ratio [Cl⁻]/[OH⁻] at which the passivity breakdown voltage (E_b) equals the corrosion potential, ECP. CT is sensitive to the T, ECP, pH, and E_b, because all four appear in the exponent X in CT = 10^X. Small uncertainties in these parameters translate into a large uncertainty in CT, emphasizing the need for accurate T, ECP, pH, and E_b data. We demonstrate the theory by successfully calculating CT using data for selected systems.