# Time-Dependency in the Corrosion Process of Reinforced Concrete Subjected to a Chloride Solution
## 1 Introduction  
Reinforced concrete (RC) is widely used worldwide owing to its durability and structural integrity, particularly in infrastructures requiring a prolonged service life [1]. The durability of RC can be ensured by improving the quality of the cover concrete with a denser pore structure and with the formation of a passive film on the steel embedded in the concrete through the concrete alkalinity [2; 3]. However, preventing steel corrosion remains the primary challenge in ensuring a long service life of RC. Steel corrosion is commonly influenced by the environment; steel is exposed to extreme environmental conditions that cause corrosion in RC, leading to the breakdown of the passive film formed on the reinforcement surface. This could be attributed to either carbon dioxide or chlorides. Steel corrosion can have serious effects on RC structures, such as a reduced effective thickness of steel, a reduced load-carrying capacity of the structure, and the formation of corrosion byproducts, with volumes reaching three to six times more than that of the original steel. The expanded steel increases the internal stresses around the reinforcement, leading to the formation of cracks or spalling. Consequently, the corroded reinforcing steel reduces the structural performance, leading to a catastrophic collapse. Many countries including South Korea, USA, Canada, Australia, India, European countries, Japan, etc., specify limiting values in the codes to avoid steel corrosion of steelin concrete, but the limiting values suggested in the codes cannot fully cover prevention of RC corrosion. Therefore, evaluation and prevention of RC corrosion are crucial for ensuring safety. Many studies have examined the modelling and mechanisms of steel corrosion through the effects of chloride attack and carbonation [4]. However, the studies examining corrosion detection in RC structures have had significant limitations owing to differences in the corrosion behaviour between laboratory experiments and actual structures, nonhomogeneous concrete materials, and local conditions which can influence the results. Various corrosion detection methods have been developed and applied in this field, including non-electrical and electrical methods. Non-electrical methods typically include the following: (1) the half-cell potential (HP) method, in which the potential of steel is measured with respect to a standard electrode, such as a copper-copper sulphate electrode (CSE), a saturated calomel electrode (SCE), or a silver-silver electrode [5]; (2) the galvanic current method, in which the currents are measured between anodes for steel in corrosive environments and the cathode for a noble metal, such as stainless steel, graphite, or titanium [6]. Electrical methods artificially apply an electrical current or potential to steel to measure its polarisation resistance which can be used to quantify steel corrosion. Polarisation resistance can be obtained using direct current (DC) methods, such as the linear polarisation resistance technique (LPR), the Tafel extrapolation technique, the galvanostatic pulse (GP) technique, and the potentiostatic technique; however, among the alternating current (AC) methods, electrochemical impedance spectroscopy (EIS) typically uses AC in the frequency domain. EIS provides a better understanding of the corrosion process and a more accurate corrosion rate; however, this technique has a high potential sensitivity to the response under various conditions. A previous study demonstrated that a complete description of an electrochemical system can be made for impedance using an equivalent circuit [7]. However, determining the equivalent circuit for steel embedded in concrete remains challenging because the responses regarding the number of phenomena related to the corrosion process are ambiguous [8]. In addition, controlling the steel embedded in concrete after casting is also challenging. Thus, the corrosion process of steel in concrete appears as black box data.