What are the electrical protection methods for buried steel pipelines?

For shallowly buried anodes, steel pipelines buried underground should generally be provided with cathodic protection immediately after burial. For pipelines whose construction takes a long time and for which permanent cathodic protection cannot be implemented in a short period, temporary cathodic protection measures should also be considered for those sections located in highly corrosive environments. Ensuring the cathodic protection current is generally a necessary condition for pipeline cathodic protection, and the longitudinal resistance of the pipeline has a significant impact on the protection range. Therefore, the electrical continuity design of the pipeline is essential. To prevent cathodic protection current from flowing to unprotected structures that are electrically connected to the ground, the cathodic protection pipeline system should be electrically insulated. Electrical insulation can serve the following purposes: preventing current leakage, reducing galvanic corrosion, minimizing interference from stray currents, avoiding unnecessary disturbances, and controlling the direction of current flow. Due to its very low resistivity, it can help increase the size of the anode, thereby reducing the grounding resistance of the ground bed. Most of the current is transferred from the anode to the packing through direct contact; therefore, most of the material consumption should occur outside and at the edges of the packing column. Since a positive potential ie is applied across the entire anode bed structure, it is absolutely necessary to ensure that the cable insulation and connectors remain intact and are sealed to prevent moisture ingress. In shallow-buried anode beds for steel pipelines, prefabricated anodes are typically used. These anodes can be either high-silicon cast iron anodes or precious metal oxide anodes. Prefabricated anode beds eliminate the need for on-site fabrication; the coke built into the anodes is sufficient to improve the soil conditions around them, and generally no additional coke needs to be added to the anode bed. Shallow-buried anodes are typically used in areas with a relatively favorable soil environment, where electrons can reach the surface of the pipelines to be protected through the upper layers of soil, thereby reaching the farthest pipelines within the design range. The installation of shallowly buried anodes is simpler compared to that of deep-well anodes; no specialized drilling team is required – it is sufficient to dig pits for the anode bed near the area where the anodes are to be placed. Typically, the depth of excavation for the anode bed is two meters. Deep well anode: The deep well anode system is a type of forced current cathodic protection installation method. In this system, the anode is placed in specialized or existing boreholes that extend vertically downward from the surface, and is located entirely or partially at an electrically remote position. Deep-well anodes can provide effective cathodic protection for various facilities such as underground pipelines, storage tanks, refineries, power plants, sewage treatment plants, steel pile structures, and oil well casings. It is suitable for areas where the resistivity of surface soil is very high, or in situations where the resistivity is relatively low but the soil layer is shallow with high-resistivity soil beneath it. However, the underlying strata must have a sufficiently low resistivity to be suitable for installing deep-well anodes. When a suitable voltage is applied, a deep-well anode can generate a certain amount of current. Compared to the surface soil, installing the anode in deep soil layers with low resistivity enables a very uniform current distribution along the structure. Even if the surface soil is perfectly suitable for designing shallowly buried anode beds, deep-well anodes are more appropriate in areas with dense structures. Since shallowly buried anodes are difficult to install, they can only be placed in locations far away from the pipelines to be protected as well as other structures. Installing multiple anodes in a carbonaceous packing column allows for the even distribution of the maximum anode current. The highly dense carbon filler in the deep-well anode bed facilitates the discharge of the maximum current from the anode surface, thereby extending the anode’s lifespan. Since the possibility of encountering groundwater is high, it is possible to achieve the lowest loop resistance. A properly installed ventilation system can minimize the risk of gas buildup and facilitate wetting the anode bed with drinking water. The low loop resistance of this type of anode system allows electricity consumption costs to be minimized. In areas with external underground structures, it is recommended that the maximum output current for deep-well anodes be 30A. Sacrificial anodes: The two commonly used materials for sacrificial anodes in buried pipelines are magnesium and zinc. Aluminum has a higher theoretical capacitance than magnesium, but to date there is no evidence indicating that it is suitable for use buried in soil. This is because there is a problem in soil regarding how to maintain the electrochemical activity and high efficiency of aluminum. Aluminum anodes are mainly used in marine environments. For pipeline cathodic protection, sacrificial anodes are typically used in situations where a low level of protection current is required (usually less than 1 A), as well as in areas with low soil resistivity (usually less than 10,000 ohm·cm), where an appropriate amount of anode can provide the necessary protection current. If there are still doubts about which protection method to adopt, then an economic analysis should be conducted. A small number of well-coated pipelines will require a moderate level of protective current. In areas with suitable soil resistivity, the use of sacrificial anodes can meet the requirements for such protective current. On well-coated pipelines that are protected by forced current cathodic protection, there may still be some isolated points where a small amount of protective current is required. A sacrificial anode can be used to meet this requirement. Typical application scenarios include buried valve installations with poor coating or no coating at all, short-circuited sleeves that cannot be eliminated, isolated pipe sections with severely damaged coatings, and areas where current shielding may affect the effective current distribution from distant forced-current systems. Following general practice, zinc anodes are best used in soils with low resistivity (less than 1500 ohms). centimeters), while magnesium anodes are best suited for soils with higher resistivity (1500–10,000 ohms). centimeters). This principle is not universally applicable; it depends on the context of use. Compared to the usual method of installing forced-current anode beds, the installation of a sacrificial anode system is relatively simple. The simplest way to install a sacrificial anode is to bury a single, packaged anode at the leak repair point of the pipeline, or to use distributed single anodes buried along the pipeline. Of course, packaged zinc anodes can also be used in soils with low resistivity. When conditions permit, heavier packaged magnesium anodes can also be used to achieve a longer service life. Typically, to protect a well-coated pipeline where multiple magnesium or zinc anodes need to be installed at one location, a bus cable can be used to connect these anodes together. Leading the collection cable to the test pile allows for the detection and regular measurement of the output current, so as to calculate the service life of the anode.

1. Cathodic protection ； 2. Sacrificial anode protection.