Abstract: Experiments have shown that the dielectric spectrum of rebar corrosion in mortar has obvious characteristics due to the presence of a passivation film on the surface of rebar in alkaline medium. Therefore, the capacitance characteristics of the passivation film can be obtained from its Cole-cole plot. Keywords: rebar corrosion; dielectric spectrum; Cole-cole plot; corrosion capacitance Since John[1] proposed using AC impedance spectroscopy to measure rebar corrosion in concrete and mortar, this method has been widely accepted. Shi Meilun et al.[2] used admittance to represent the experimental results, which can more intuitively measure the polarization resistance R[sub]p[/sub] and study the properties of the passivation film on the surface of rebar. Since the passivation film on the surface of the steel bar is a typical dielectric, the characteristics of the steel bar corrosion process should be reflected in the dielectric spectrum. The characteristics and measurement of the dielectric spectrum are briefly described in reference [3]. The physical meaning of the dielectric spectrum of the steel bar corrosion process is: the corrosion system can be regarded as a leakage capacitor. The greater the corrosion rate of the system, the more serious the leakage of the capacitor. 1 Experimental section 1.1 Raw materials Cement: 32.5R ordinary cement produced by Ningguo Cement Plant; Sand: fineness modulus 2.5; Steel bar: size 7mmX100mm; Mortar specimen: size 7.07cm×7.07cm×7.07cm, with built-in steel bar, the exposed area of ​​the steel bar in the mortar is 11cm2. 1.2 Test instruments and methods The test instruments are: Model 273A potentiostat/galvanometer and Model 5210 lock-in amplifier, both produced by EG&G Princeton Applied Research Co. During the measurement, the steel bar was used as the working electrode, stainless steel as the counter electrode, and a calomel electrode as the reference electrode. The electrolytic cell used saturated Ca(OH)₂ as the electrolyte. Dielectric spectroscopy was performed using M398 software, with a measurement frequency ranging from 100 kHz to 100 mHz, and five sampling points at each order of magnitude. The sinusoidal AC voltage amplitude was 5 mV. 2. Results and Discussion The impedance spectra of mortar and steel bar corrosion are shown in Figure 1. As can be seen from Figure 1, the impedance spectrum of steel bar corrosion lacks obvious characteristics, and its Nyquist curve does not converge, making it difficult to directly obtain the polarization resistance R from the figure. Figure 2 shows the corresponding admittance diagram. As can be seen from Figure 2, the admittance diagram of the steel bar corrosion system is a slightly flattened semicircle, a characteristic not found in the admittance diagram of a typical mortar system. A disadvantage of the admittance diagram is its inaccuracy in measuring the thickness and capacitance of the passivation film on the steel bar surface. The dielectric spectrum is represented by a Cole-cole plot with the real part of the complex capacitance as the abscissa and the imaginary part as the ordinate (see Figure 3). As can be seen from the figure, the dielectric spectrum of the general mortar system is hyperbolic (only the first quadrant is shown), while the dielectric spectrum of the steel corrosion system is characterized by a vertical line passing through the origin in the high-frequency region and a turning point in the mid-frequency region. Since there is a corresponding relationship between the dielectric spectrum and the admittance spectrum, the results obtained from the admittance spectrum can be directly transferred to the dielectric spectrum [2]. The curve in Figure 3(b) can be divided into three segments based on the slope. The length of segment ab is related to the thickness of the passivation film; the thicker the passivation film, the longer segment ab. The length of segment bc is related to the capacitance of the passivation film; the capacitance value of the passivation film can be directly obtained from the length of segment bc. The projection of segment cd onto the vertical axis is called the corrosion capacitance C[sub]corr[/sub] of the steel reinforcement. In the past, corrosion processes were treated as electrochemical processes, and the corrosion system was considered as an electrochemical cell. In reality, the passivation film formed on the surface of the steel reinforcement is a good insulator. To gain a deeper understanding of the insulating properties of the passivation film and its inhibitory effect on steel reinforcement corrosion, it may be more appropriate to study it from the perspective of dielectric physics, especially for steel reinforcement in a concrete environment. From the perspective of dielectric physics, the steel reinforcement corrosion process can be regarded as a dielectric loss behavior, and the degree of steel reinforcement corrosion can be represented by the dissipation tangent tgδ. The dissipation tangent is obviously larger because the more severe the leakage current of the capacitor, the larger the dissipation tangent. tgδ is also a function of frequency f. The process of the dissipation tangent of the steel corrosion process changing with frequency is shown in Figure 4. The results of the dielectric spectrum can also be expressed using the complex permittivity. For a parallel plate capacitor, its real part of capacitance is given by the formula, where ε is the real part of the complex permittivity, S is the area of ​​the plate, and d is the distance between the plates. Conventionally, ε can be taken relative to the vacuum state, i.e., in a vacuum, ε_o = 1. Therefore, the physical meaning of ε is the capacitance per unit area and unit distance of the capacitor. In some literature, the relative permittivity is also called dielectric permittivity. 3 Conclusion For steel reinforcement in concrete and mortar media, in addition to the traditional electrochemical viewpoint of corrosion, the corrosion problem can also be addressed from the perspective of dielectric physics. The dielectric spectrum of the steel corrosion process has a unique shape. From its curve, we can understand the thickness and capacitance of the passivation film, discuss the corrosion capacitance and dissipation tangent, and the complex permittivity. Since the complex permittivity is directly related to the material structure, further work can be done to understand its corrosion properties from the molecular composition of the material. References: [1] JOHN DG, SEARSON PC, DAWSON JL. Use of AC impedance technique in studies on steel in concrete in immersed conditions[J]. Br Corros J, 1981, 16(2): 102-106. [2] Shi Meilun, Shi Tao. AC admittance method for measuring steel corrosion in concrete[J]. Corrosion and Protection, 1997, 18(3): 23-28. [3] Ni Erhu. Dielectric spectroscopy technique in materials science[M]. Beijing: Science Press, 1999. [4] FROEHLICH H. Theory of dielectrics, dielectric constant and dielectric loss (2nd edition)[M]. Oxfordl[sn], 1956. [5] COELHO R. Physics of dielectrics for the engineer[M]. Amsterdam: Elsevier Scientific Publishing Company. 1979.