Example 1: A buried stainless steel pipe in a factory originally used flange connection. Due to leakage at the joint, it was decided to change to a fully welded joint. During the construction process, heavy rain suddenly fell at night, and the ditch accumulated water, submerging the completed pipe section in the water. The construction continued the next day, and as a result, thousands of holes were drilled / drilled in the stainless steel pipes in the water, and the construction had to be restarted. Analysis: The culprit here is the stray current produced by the welding machine. Direct current leakage into the soil and water can cause serious corrosion damage to underground and underwater steel equipment. Electric current enters the steel pipe from somewhere and then leaves from another place. In this case, it is a stainless steel tube. The water contains chloride ions. The stray current flowing out of the area causes the passivation film on the surface of the stainless steel tube to break down, forming corrosion holes. The corrosion is concentrated in the corrosion holes, and the stainless steel tube is quickly eroded. In order to prevent stray current from leaking into the soil and water, on the one hand, the DC system must form a complete closed loop, and it is strictly prohibited to use soil and water as part of the loop; on the other hand, the conductive lines must be well insulated from the soil and water to prevent current Leak in.
Example 2: A factory selects stainless steel pipes for underground oil pipelines. About 1a after installation, it is ready to be put into use. The oil is input from one end, but does not appear on the other end. The inspection revealed that many small holes were formed in the pipeline due to stray current corrosion, which caused the oil to leak out. The reason is that the nearby carbon steel pipeline is cathodic protected, and it is considered that stainless steel is a corrosion-resistant material that does not require protection, so the stainless steel pipe is not connected to the cathodic protection system. Analysis: For buried pipelines and other facilities, the best protection technology is coating plus cathodic protection. Cathodic protection is to pass cathodic current to the protected device (that is, the device is connected to the negative pole of the DC power supply), electrons flow into the device from the power supply, and the potential of the device is negatively shifted, and the metal anode reaction speed is reduced. Steel pipelines, storage tanks, ships and other facilities in soil and seawater are suitable corrosion protection systems for cathodic protection. This is because a small protective current density can provide good protection. But for a large area of ​​equipment, if the surface is not coated (bare metal), the current required is still very large. On the other hand, the current distribution on the surface of the equipment to be protected is not uniform (the uniformity of current distribution is called dispersion capacity). For long pipelines, the length of the pipeline protected by an anode station is relatively short, making the design of the anode station very complicated. The use of coatings can solve these two problems.
The coating reduces the current requirement for cathodic protection and improves the current dispersion capability. For coatings, cathodic protection prevents corrosion of the exposed metal in the pores and damage of the coating. There is such an example, an underground steel pipe is coated with cathodic protection, and 15a has not failed. Later, another same pipeline was installed. The contractor thought that double protection was unnecessary, and only the coating was used instead of cathodic protection. As a result, it leaked within 1a. This is because it is difficult for the coating to completely avoid pores and damage caused during transportation and installation. The exposed metal at the defect and damage becomes a very small anode.
Example 3: A carbon steel gasoline pipe with a diameter of 30 mm leaked, and gasoline leaked into the sewage system, and eventually exploded in a densely populated area, causing serious consequences. This pipeline uses a coating (coal tar pitch) plus cathodic protection. A few years after the pipeline was built, another 113 cm diameter water pipeline (galvanized pipe) was laid on it. Before the accident, the protection potential was reported to be abnormal, but no measures were taken.
Water leaking from the water pipe scours the surface of the gasoline pipe, which strengthens the environmental corrosion conditions and increases the protection current required. Under the condition that the power supply capacity is fixed, the need to protect the current cannot be met, the cathodic protection system is ineffective, and the gasoline pipe can only rely on the surface coating, and the coating is partially damaged. The gasoline pipe lost its protection and corroded the perforation quickly. In the design of buried steel pipes for cathodic protection, attention must be paid to the problem of stray current corrosion that may be caused to other underground pipes or facilities nearby. The cathodic protection current flows from the buried anode through the soil to the protected pipeline. When there are other pipelines nearby, current may flow into these pipelines from one location and flow out from another location (as shown in Figure 3). The location where the current flows becomes the anode area, causing corrosion of the pipeline at that location. Even stainless steel pipes can suffer from stray current corrosion.
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