Corrosion of grounding grids and anti-corrosion measures

Due to being buried in the soil for extended periods, operating in harsh conditions, and being constantly subjected to chemical and electrochemical corrosion, the grounding grid sees its thermal stability and electrical continuity significantly affected. Using conductive anti-corrosion coatings is a relatively economical and practical solution. For preventing corrosion in ordinary soil, it clearly has significant advantages over galvanizing; its service life can exceed 30 years. If two or more of the anti-corrosion methods mentioned above are combined, that is, by adopting what is known as a multi-layer protection approach, then the anti-corrosion problem of grounding grids can be completely resolved. Perhaps some may ask: Is it possible to achieve corrosion resistance while also reducing resistance? The answer is yes, it is possible. That is, the goal can be achieved by using conductive anti-corrosion coatings in combination with low-resistivity auxiliary materials to replace the soil surrounding the grounding grid. Here, we recommend using physical resistance reducers as an auxiliary material to replace the soil around the grounding grid, thereby achieving both corrosion protection and resistance reduction. After 1997, a new type of physical resistivity reducer was introduced, thereby clearly dividing resistivity reducers into two main categories: chemical resistivity reducers and physical resistivity reducers. When selecting resistivity reducers, it is necessary to distinguish between these two types of reducers with different mechanisms of action. Regardless of whether chemical resistivity reducers are in liquid, solid form, or are organic or inorganic in nature, their common feature is that electrolytes serve as the conductive medium; their mechanism of conduction is similar to that of soil. It is only when water is involved that electrolytes can ionize and thus become the conductive medium. The higher the electrolyte concentration, the greater its electrical conductivity (i.e., the lower its resistivity), and the more pronounced the effect of reducing resistance. However, this electrical conductivity inevitably leads to corrosion of the metal electrodes. Since its electrolyte content typically accounts for 25% to 64% of the total composition, this is more than 10 times higher than the soluble salt content of ordinary soils, which is 2% to 5%. This undoubtedly constitutes the main cause of corrosion in grounding systems. Since chemical resistivity reducers rely on water’s participation to reduce resistivity, this leads to a **weakening of their resistivity-reducing effect when used in arid and water-scarce areas. In rainy areas, due to years of rainfall erosion and fluctuations in groundwater levels, it will also gradually lose its ability to reduce resistance as a result of the loss of electrolytes. This is an inherent weakness of chemical drag reducers that is difficult to overcome. Physical resistivity reducers use solid powder of non-electrolytes as the conductive material, along with strong bases and weak acids as gelling agents, which significantly reduces corrosion of metal electrodes. At the same time, it is not affected by seasonal rainfall (water only plays a cementing role during construction); therefore, its electrical conductivity is not constrained by acids, bases, salts, high or low temperatures, or humidity, and it relies solely on its own conductive powder to reduce resistance. This makes it easier to meet the requirements for grounding resistance stability in arid, low-rainfall, and cold regions. The resistivity of chemical resistivity reducers increases after water loss, decreases after bubbling, and rises sharply after freezing, which fully illustrates the characteristics of ionic conduction. Physical resistivity reducers, being composed of conductive powders, have a resistivity lower than that of the condensation film; their resistivity even decreases slightly after losing moisture during storage. Moreover, their resistivity remains below 1 Ω•m under both high and low temperature conditions. Additionally, the semi-chemical and physical resistivity reducers were wrapped around the metal blocks respectively, and these blocks, along with those without resistivity reducers, were buried in acidic soil with a pH of 6 for comparative testing. A check after 60 days revealed that the metal blocks without a corrosion inhibitor suffered severe rusting, while those coated with a chemical corrosion inhibitor showed obvious corrosion spots in certain areas; the metal blocks coated with a physical corrosion inhibitor had almost no such spots. This shows that physical corrosion inhibitors have minimal corrosive effect on metals, **thereby extending the service life of grounding systems. It can be seen that physical resistivity reducers are significantly superior to chemical resistivity reducers. Therefore, in recent years, there has been a trend to gradually replace chemical resistivity reducers in the reduction of resistance in grounding systems. Here, we fully took advantage of the significant feature of physical resistance reducers – their nearly non-corrosive nature – to form another layer with a certain thickness of anti-corrosion coating around the grounding device. This allows the grounding device to meet the required grounding resistance values while also extending the service life of the steel structure coated with the conductive anti-corrosion paint, thereby achieving dual protection. Some may ask whether using physical resistance reducers directly cannot achieve both anti-corrosion and resistance-reduction purposes. This is a one-sided view. By directly using physical corrosion inhibitors, although the steel structure is covered and it may seem that the steel will not corrode, this is not actually the case. This is because it is impossible for the steel body phase to be completely isolated from external corrosive agents through the use of a resistance-reducing layer. Studies have shown that the average corrosion rate of steel is 70 mg/cm²•year. And only 11 mg/cm²•year of water and 30 mg/cm²•year of oxygen are required to cause this type of corrosion. The typical outer protective layer has a water permeability of 190–1122 mg/cm²•year and an oxygen permeability of 4–53 mg/cm²•year; therefore, over time, the steel itself cannot prevent the penetration of water and oxygen, leading to corrosion. Moreover, the surface of steel already contains about 2 g/m² of adsorbed water in dry air, and it absorbs even more water in humid air near the dew point, around 20 g/m². Therefore, after the steel surface is coated with a conductive anti-corrosion coating, it can resist corrosion caused by the penetration of water and oxygen from the surrounding environment. This surrounding layer actually prevents the penetration of water and oxygen, thereby extending the service life of the coating on the steel surface and consequently prolonging the lifespan of the grounding grid. This is the basic principle of the double layer, as well as a new approach to comprehensive anti-corrosion measures that takes into account resistance reduction. For areas with more severe corrosion or important facilities, composite materials (such as galvanized ones), conductive anti-corrosion coatings, and physical resistance reducers are used to provide triple protection for the grounding system, which is sufficient to ensure that its service life reaches 50 years. Of course, in areas where resistance reduction is not necessary, or in regions with good soil conditions, as well as in some low-voltage distribution networks, we can use conductive anti-corrosion coatings alone to prevent corrosion of the grounding grid. For most substations of 110 kV and above, as well as 110 kV and above transmission lines and other power facilities, it is recommended to use a so-called dual-protection method that combines conductive anti-corrosion coatings with physical protection measures of mg/cm²•year. It is estimated that this dual protection approach of combining the two methods increases costs by about 10%, making it an economical, practical, and easy-to-implement solution. Regarding the corrosion prevention of grounding devices, many new technologies from home and abroad have been introduced. It is possible to envision a fundamental solution to the corrosion problem of grounding grids; by carrying out the work only once, permanent benefits can be expected. By adopting two or more comprehensive protection methods, although the initial investment increases by over 10%, from a long-term perspective, as the power grid system undergoes reforms and the requirements for the reliability of power generation and supply grow ever higher, the additional investments mentioned will yield significant social and economic benefits. Why not go for it then? It is believed that with the widespread adoption of comprehensive anti-corrosion measures for grounding systems, this will not only lead to a qualitative change in people’s attitudes toward investment but, more importantly, will enhance the reliability of power grid operation, ultimately bringing substantial benefits to the various operating entities and users.

Forms of electrochemical corrosion in ground grids 1. Pitting corrosion: The carbon steel that makes up ground grids can be regarded as numerous tiny galvanic cells composed of countless carbon particles and iron particles. In humid soil environments, carbon steel in grounding grids suffers from micro-cellular corrosion; this electrochemical corrosion is the main cause of pitting corrosion in grounding grids. 2 Macroscopic corrosion: In low-resistivity soil environments, a potential difference exists between the entire grounding grid and the soil, resulting in the formation of a corrosion cell; the grounding grid, having a lower potential than the soil, is corroded. This type of macro-cellular corrosion is particularly severe in saline-alkali areas or where resistivity reducers are used. 3 Local corrosion: When there are differences in the electrolyte concentration (i.e., varying soil resistivity) or oxygen concentration at various parts of the grounding grid, a potential difference is generated across those parts, resulting in the formation of corrosion cells. Local galvanic corrosion in the ground grid is the main cause of ground grid failure. 4 Electrolytic corrosion: Electric trains, urban trams, and high-voltage direct current transmission systems all use the ground as a circuit; the nearby AC grounding grid suffers from severe electrolytic corrosion under the influence of direct current. 5 Weak alternating current corrosion: When alternating current flows through the grounding grid, it causes mild electrolytic corrosion in the grid; the lower the frequency, the more severe this electrolytic corrosion becomes, with ultra-low frequency harmonic currents accelerating the corrosion of the grounding grid. Analysis of anti-corrosion measures for grounding grids in China 1: The effects of hot-dip galvanizing and increasing the cross-sectional area are not satisfactory. The Code for Construction and Acceptance of Grounding Installations (GB50169 —92) stipulates that grounding installations in large and medium-sized power plants, substations of 110 kV and above, or locations with high corrosion levels should be constructed using hot-dip galvanized steel or with an appropriately increased cross-sectional area. Since the standard reduction electrode potential of zinc (–0.7630 V) is lower than that of iron (–0.0360 V), in a corrosion galvanic cell composed of zinc and iron, zinc acts as a sacrificial anode to protect the grounding grid. Due to the extremely limited thickness of the galvanized layer, the service life of the ground grid is also limited. As mentioned earlier, carbon steel corrodes at a high rate in soils with low resistivity; even if the cross-sectional area of the flat steel is doubled, the lifespan of the grounding grid cannot be doubled as well. 2 Conductive cement cannot protect the ground grid. There are reports on using conductive cement as a “permanent ground grid,” but this lacks theoretical support. It is well known that cement is not waterproof, which means electrochemical corrosion of the ground grid still occurs. The corrosion protection of underground reinforced concrete poses a challenge for building engineers, and it should not be used as a method for corrosion protection by electrical engineers. Once the ground grid is subjected to the high current surge caused by lightning strikes or short circuits, the flat steel bars fixed in place with cement will be broken due to the intense thermal stress. 3. It is difficult to carry out online monitoring of the sacrificial anode protection ground grid. Based on the power generated by the sacrificial anodes and the optimal protection current density for carbon steel in specific soil corrosion environments, it is possible to easily calculate the lifespan of the protected ground grid. This process is widely used in heavy anti-corrosion projects such as oil pipelines, steel-pile docks, and dam gates. Its drawback is the difficulty in online monitoring of current density. The protection current density is too low, so corrosion of the ground grid still occurs ； The protection current density is too high (i.e., so-called over-protection), and the ground grid will suffer from electrolytic corrosion under the effect of excessive current! Other cathodic protection methods have the same problems. 4 Corrosion-resistant conductive coatings are easy to apply, but high performance is required of them. These coatings have been used for the corrosion protection of ground grids for over a decade; they feature simple design and easy installation. Coatings used in China for the anti-corrosion of ground grids, classified by conductive additives, include three types: nickel powder type, graphite type, and nano-carbon type. However, anti-corrosive conductive coatings require not only good electrical conductivity and anti-corrosion properties; the 5JR nano-carbon anti-corrosive conductive coating exhibits excellent preparation quality, performance, and application effects