Pipeline cathodic protection
Scope of application: water pipelines, natural gas pipelines, oil pipelines, urban gas pipelines can also be used for cathodic protection of steel structures in fresh water.
In the natural environment, the soil contains moisture, oxygen, acid-base mineral salts and microorganisms. These factors cause corrosion of metal pipelines. The role of pipeline coatings is mainly physical barrier, which separates the metal matrix from the external environment, so as to avoid metal and The effect of the surrounding environment, but there are two reasons for metal corrosion, one is the inevitable defects of the coating itself, the existence of pinholes; the other is the inevitable damage to the coating during construction and operation, making Metal is exposed to corrosive environment. The existence of these defects leads to the phenomenon of large cathode and small anode, which greatly accelerates the corrosion of the damaged coating, and causes corrosion and perforation of the pipeline, resulting in the occurrence of safety accidents. Years of practice at home and abroad have proved that the most effective corrosion The control measure is the combined protection of coating and cathodic protection.
There are two methods of cathodic protection: sacrificial anode cathodic protection and impressed current cathodic protection.
Sacrificial anode cathodic protection >>>
Sacrificial anode cathodic protection is to connect the metal with a more negative potential to the protected metal, and in the same electrolyte, so that the electrons on the metal are transferred to the protected metal, so that the entire protected metal is at a negative and the same potential. . This method is simple and easy to implement, does not require external power supply, and rarely produces corrosion interference. It is widely used to protect small (generally less than 1 ampere) or metal structures in low soil resistivity environments (soil resistivity less than 100 ohm·m). . For example, when designing a sacrificial anode cathodic protection system for urban pipe networks, small storage tanks, etc., in addition to strictly controlling the anode composition, the anode bed location with low soil resistivity must be selected.
Schematic diagram of sacrificial anode cathodic protection
Selection of sacrificial anode types:
Soil resistivity
The type of anode to be selected
<15Ω•m
Zinc alloy sacrificial anode
15～150Ω•m
Magnesium alloy sacrificial anode
Note: 1. When the soil resistivity is greater than 15Ω•m, field tests should be performed to confirm the effectiveness of the zinc alloy sacrificial anode.
2. When the soil resistivity is greater than 150Ω•m, field tests should be performed to confirm the effectiveness of magnesium alloy sacrificial anodes.
3. For high resistivity soil environment and special purpose, the ribbon sacrificial anode can be selected.
Advantages of sacrificial anode cathodic protection:
1. No external power supply required
2. Less corrosion interference to adjacent structures
3. No management is required after commissioning
4. The smaller the project, the more economical
5. The protection current is evenly distributed and the utilization rate is high
Disadvantages of sacrificial anode cathodic protection method:
1. Not suitable for high resistivity environment
2. The protection current is hardly adjustable
3. The quality of the covering layer must be good
4. The initial commissioning work is complicated
5. Consuming non-ferrous metals, the cost of one-time input is high
Impressed current cathodic protection >>>
Impressed current cathodic protection is to add a large amount of electrons to the metal by adding a DC power supply and an auxiliary anode, so that the protected metal is in a state of excess electrons as a whole, so that all points on the metal surface reach the same negative potential, so that the potential of the protected metal structure is lower than surroundings. This method is mainly used to protect large metal structures or in high soil resistivity soils, such as long-distance buried pipelines, large tank groups, etc.
