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. However, 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.

中文翻译：

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高温对混凝土环境中钢筋钝化和去钝化性能的影响

了解各种环境条件下的氯化物阈值 （CTV） 对于易受腐蚀损伤的钢筋混凝土 （RC） 结构的状况评估和使用寿命估计至关重要。本研究通过采用不同的电化学测量和表面表征技术，研究了三种暴露温度（25 °C、35 °C 和 45 °C）对模拟混凝土孔隙溶液中钢筋钝化和去钝化（即腐蚀起始）的影响。结果表明，暴露于高温下会加速 Fe 金属的氧化，与 Fe（II） 氧化物相比，Fe（III） 氧化物的形成优于 Fe（II） 氧化物，更高的 Fe（III）/Fe（II） 比率和更惰性的腐蚀电位证明了这一点。净氧化物含量也较低，缺陷浓度较高，这降低了钝化膜的整体保护性。因此，随着在钝化结束时（即 14 天）逐渐添加氯化物，平均 CTV 从 25 °C 时的 3.5 M 急剧下降到 35 °C 时的 1.9 M 和 45 °C 时的 1.1 M。 温度升高 10 °C 时，CTV 降低了近 50%，而温度升高 20 °C 时，CTV 降低了不到初始值的三分之一，凸显了 CTV 对暴露温度的强烈依赖性。然而，无论暴露温度如何，当极化电阻下降到 500-600 kΩ∙cm² 的范围时，都会触发钢样品的钝化，而固溶电阻接近 5 Ω∙cm²。这些发现强调了重新评估不同气候条件下 RC 结构腐蚀风险的必要性，以避免高估使用寿命。