Influences of elevated temperature on the passivation and depassivation properties of reinforcing steel in concrete environment

Abstract

Understanding the chloride threshold value (CTV) under various environmental conditions is essential for condition assessment and service life estimation of reinforced concrete (RC) structures susceptible to corrosion damage. This study investigates the influence of three exposure temperatures (25 °C, 35 °C and 45 °C) on the passivation and depassivation (i.e., corrosion initiation) of steel bars in a simulated concrete pore solution by employing different electrochemical measurements and surface characterization techniques. Results demonstrate that exposure to elevated temperatures accelerates the oxidation of the Fe metal, favouring Fe(III) oxide formation over Fe(II) oxides, as evidenced by a higher Fe(III)/Fe(II) ratio and nobler corrosion potentials. The net oxide content is also lower, with a higher concentration of defects, which reduces the overall protectiveness of passive film. Consequently, as chlorides were progressively added at the end of passivation (i.e., 14 days), the average CTV decreased sharply from 3.5 M at 25 °C to 1.9 M at 35 °C and 1.1 M at 45 °C. A temperature increase of 10 °C reduced the CTV by nearly 50 %, while a 20 °C rise reduced it to less than one-third of its initial value, highlighting the strong dependency of CTV on exposure temperature. Notably, irrespective of the exposure temperatures, the depassivation of the steel samples was triggered when polarization resistance dropped to the range of 500–600 kΩ∙cm2, and solution resistance was nearly 5 Ω∙cm2. These findings emphasize the need to reassess the corrosion risk in RC structures under different climate conditions to avoid overestimating service life.