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In the process of using underground gas pipeline, there are different kinds of corrosion. Electrochemical corrosion is the most serious threat to buried gas pipe. Because the electrochemical corrosion is concentrated and fast, once the corrosion occurs, its speed will not slow down or stop, and it will often cause local perforation.
1¡¢ Causes of electrochemical corrosion
Due to the different physical and chemical properties of the soil, the metallographic structure of each part of the pipeline itself is different (such as defects of character and impurities, deformation of metal due to cold and hot processing and internal stress, different surface roughness of steel pipe, etc.), some metals are easily ionized and positively charged metal ions leave the metal and transfer to the soil. In this part of the tube, the electrons are more and more surplus, and the potential is more and more negative; the other part of the metal is not easy to ionize, and the potential is relatively positive. Therefore, electrons flow along the pipe from the easily ionized part to the less ionized part.
The electron between the two metals has its advantages and disadvantages, which leads to a redox reaction. The metal tube with electron loss becomes anode region, and the metal pipe section with electron becomes cathode area. The corrosion current flows from the cathode to the anode, and then flows away from the anode to the cathode through the soil to form a circuit. In the soil as electrolyte solution, ion migration occurs, the positive cations tend to the cathode, and the negatively charged anions tend to the anode. In the anode area, the positive metal ions interact with the negatively charged anions, causing the metal ions in the anode area to ionize and corrode, resulting in pits on the surface of the steel pipe, resulting in perforation, while the cathode remains intact.
Based on the above principle, the sacrificial anode protection technology can protect the buried sodium pipe from electrochemical corrosion. The metal material with negative electrode potential is used to connect the steel pipe, and the metal with negative electrode potential is connected with positive electrode potential.
The protected steel pipe forms a galvanic cell in the soil as a protective power source, and the metal with negative potential becomes anode and is damaged in the process of output current, so the effect of protecting steel pipe is achieved.
2¡¢ Application of sacrificial anode protection technology
In the past, cast iron pipe was mainly used in Changzhou City gas medium pressure pipe network, and the connection mode was flexible mechanical interface, and there were few projects using steel pipe. However, with the development of gas users and the improvement of pipe network pressure, considering the introduction and transition of natural gas in the future, steel pipes are more and more widely used.
Compared with cast iron pipe, steel pipe has high compressive strength; for long pipe section pre processed, it reduces the difficulty of site construction; welding u has the advantages of high seismic and compressive performance, so we use buried steel pipe in newly laid small pressure pipe network such as Changxi road and Chengzhong North Road. However, in the reconstruction project of Huaide bridge in our city, some steel pipes which have been laid for more than ten years have been excavated out, although there is still residual anti-corrosion insulation layer on the surface of the steel pipes. Due to the lack of sacrificial anode protection technology, there are pits on the surface of steel pipe.
These results show that the main reason of soil corrosion is the damage of the outer wall of buried steel pipe. The damage of insulation layer is inevitable in the process of construction and maintenance. Once the insulation layer is damaged, the corrosion will be carried out in the damaged parts. In order to prolong the service life and obtain good economic benefits, we decided to adopt the method of combining sacrificial anode protection and epoxy coal tar pitch anti-corrosion insulation layer protection to achieve the purpose of anti-corrosion.
3¡¢ Design of sacrificial anode protection
Take Chengzhong North Road medium pressure gas pipe project as an example. According to the test, the pipeline section belongs to medium strength corrosive soil, and the soil resistivity is 450m. We choose 11kg mug-3 magnesium alloy sacrificial anode with the anode size of 700x90mm.
Protection object and scope: A. medium pressure gas buried steel pipe from outer ring road junction to North Ring Road: pipe ¦µ 426. The length is 750m. The total surface area is 1003 m2. b. Main pipe of outer ring road: ¦µ 426, length of 115m, total surface area of 154m2.
The protection period is 25 years.
Within the effective protection period, the protection potential of the protected underground steel pipe should be controlled within -0.85v.
4¡¢ Construction of sacrificial anode protection technology
1. Detection of electrical connection between anode and cable
After the arrival of the anode, the electrical connection between the anode and the electrode must be detected with a multimeter one by one. If the anode with poor electrical connection or broken wire is found, it can not be used in the construction. In addition, it is strictly forbidden to lift the cable during the construction to prevent the cable joint from breaking.
2. Anode surface treatment
In order to accelerate the activation of anode surface, the grease and oxide on the surface of anode should be removed before assembling anode. The anode is polished with sandpaper or manual grinding wheel, and then cleaned with acetone or anhydrous ethanol.
3. Assembly of anode packing package
The anode packing package is filled with bentonite, calcium sulfate and magnesium sulfate in the proportion of 50%, 25% and 25%. Each special white cloth bag is filled with 50kg filling material. A 11kg grade magnesium alloy sacrificial anode with surface treatment is installed. The anode is placed in the center of the packing package. The anode must be tightly packed with filler and obvious eccentricity is strictly prohibited.
4. Water the anode bed
After the anode packing package is put into the anode pit, it must be watered in the pit, the water level in the pit must completely submerge the anode packing package, and the water accumulation time in the pit must exceed 10 hours, so as to completely immerse the packing package.
5. Anode performance test
After the packing of anode packing is soaked with water, the open circuit potential of anode must be tested by testing personnel before connecting with pipeline. After the anode is connected with the pipeline, corrosion protection can be carried out at the weld joint only after the anode working potential test is carried out by the tester.
6. Connection form of anode and pipeline
The anode and the protected pipeline are connected by electric welding, that is, the square steel sheet with wide copper nose at the end of anode conducting cable is directly welded on the protected steel pipe. There are two small welds, the total length is 60mm, and the same anti-corrosion coating as the pipeline is timely applied at the welding point, and the anti-corrosion grade is the same as that of the pipeline, and the quality acceptance of the anti-corrosion coating shall be carried out according to the same specification requirements.
7. Anode bed backfilling
When the anode bed is backfilled, it is strictly forbidden to backfill sand and gravel, cement block, plastic and other sundries into the anode pit. The fine soil with low resistivity should be backfilled, and the cable should be prevented from breaking in the subsequent construction.
5¡¢ Acceptance of sacrificial anode protection
After the anode filler edge is soaked with water, the anode parts must be tested. The test contents and indicators are as follows:
Anode open circuit potential: the open circuit potential of magnesium alloy sacrificial anode must be negative to - 1.5V. This data is an important standard to measure the quality of anode. After the anode is connected with the pipeline, the working potential of anode is measured and the data is the standard for evaluating the implementation quality of sacrificial anode protection project.
Natural potential of pipeline: it belongs to the natural property of pipeline. When the pipeline is corroded, the potential will tend to be positive.
6¡¢ Investment measurement and economic analysis
1. Investment estimation
A total of 865mm dn400 spiral welded medium pressure steel pipes were buried in Chengzhong North Road project, and 18 magnesium alloy anodes with the model of mug-3 were set up.
Note: 0 ¡Á 376, 0 ¡Á 612 and 0 ¡Á 706.4. Since the square steel sheet of the welding point is attached by dirt, the natural potential is negative.
Anode material cost: 25000 yuan / piece, x18 = 45000 yuan
Anode installation cost: RMB 036 / piece x 18 = RMB 648
Subtotal: 514800 yuan
Total investment cost of pipeline: 1.66 million yuan / km x 0.865 = 1.4359 million yuan
Anode protection cost accounts for 3.58% of the total investment
2. Economic analysis
Assuming that there is no anode protection on the medium pressure steel pipe of Chengzhong North Road, its service life is 15 years, if the residual value is not included, the annual depreciation cost is about 100000 yuan. If anode protection is used, its service life can be up to 25 years. If the residual value is not considered, the annual depreciation cost is about 60000 yuan. Compared with LL, the annual depreciation cost can be saved by 40000 yuan and the economic benefit is very significant.
Shandong Linyi Bosi conductive materials Co., Ltd. is a high-tech enterprise specializing in the production of MMO flexible anode, conductive polymer flexible anode and titanium coated anode, mainly providing cathodic protection technology, engineering design, installation and technical services.