# Advances and Challenges of Corrosion and Topology Detection of Grounding Grid
## 2 Corrosion Detection Method of Substation Grounding Grid
### Electrochemical Method  
The electrochemical method determines the corrosion degree or rate of metals by measuring the electrochemical characteristics of the corrosion system between metals and soil, including potentiometry, electrochemical noise, linear polarization, AC impedance, and electrostatic method. The electrochemical method has successfully been applied to detect corrosion of pipelines, concrete reinforcing bars, and metal components. However, the corrosion detection of groundwork has only begun in recent years. The detection targets of the electric network and surface electromagnetic field methods are the corrosion degree of the grounding grid. The electrochemical method mainly detects the corrosion rate of metals. Therefore, it can be used to evaluate the state of the grounding grid and predict its service life span. The electrochemical method has been widely used in the study of corrosion resistance of grounding conductor materials [3; 36; 37]; by contrast, the corrosion state detection in grounding grid is seldom used. [PERSON] et al. [38] proposed a constant current step method to detect the corrosion of the grounding grid and obtained the polarization resistance value by analyzing the charging curve to reflect the corrosion of the grounding grid at the test location. [PERSON] and [PERSON] [39] used the quasi-steady-state measurement of step voltage excitation to obtain the corrosion rate of the ground conductor.  
The physical and chemical significance of electrochemical measurement method is clear and it reveals the nature of corrosion. From this point of view, it is a research direction with development prospects. However, electrochemical method is not yet mature and the problems with it focus on:  
1. Reliability of corrosion rate detection. This problem has three causes: (1) The influence of AC and DC dispersion. (2) The conductor dispersion is not uniform. The dispersion is affected by the location of the injection point and the distribution of the geoelectric field, and the dispersion of conductors in each segment is extremely uneven. (3) Sensor installation brings influences to the original soil environment. The electrochemical method can only detect the corrosion rate of a test point, and the measurement results are discrete in time and location.
2. Sensor current limiting problem. As a whole, the ground grid is a working electrode. The polarization current is not confined to the measured conductor. The uncertainty of the polarized ground network area makes the corrosion rate impossible to calculate. [PERSON] et al. [38] designed a small hole current-limiting device for corrosion detection of the grounding grid. The basic principle of this device is to use the electric field to guide the focusing and directional flow of the polarization current in soil, so as to achieve current limiting. This method has been applied in concrete corrosion monitoring [40].
3. Strong electromagnetic noise. Strong electromagnetic interference is present in power plants, substations and transmission lines, ground grid dispersion, and stray current effects. There are methods such as wavelet denoising, but the polarization current is on the order of \(\mu\)A. Compared with ground grid and stray current, it is far less than an order of magnitude, and the signal feasibility is not high.
4. Poor accuracy of corrosion assessment. The electrochemical method usually obtains the corrosion rate by measuring the corrosion current and then estimates the corrosion degree by integral. It is an indirect measurement method. The integral calculation produces cumulative error.  
In addition, the electrochemical method for the grounding grid is a microscopic measurement. The measurement can only measure the corrosion rate of the measurement point, but it is impossible to judge the location of the breakpoint and cannot applied in the case of the reinforced concrete ground.