Application of Cathodic Protection Process in Buried Steel Pipelines

Cathodic protection is one of the electrochemical protection methods in the metal anticorrosion process. It applies an electric current to the protected metal body, so that the electrode potential is negatively shifted, so that the metal weakens the tendency of spontaneously changing from the atomic state to the ionic state, thus fundamentally inhibiting the occurrence of corrosion. Since this process must be carried out in electrolyte, buried steel pipes are very suitable for cathodic protection. If the cathodic protection and the anti-corrosion layer of the pipeline itself complement each other to achieve a perfect combination in terms of safety and economy, it is currently recognized as the best anti-corrosion solution.
1 Ningbo Urban Water Supply Project
　　Ningbo Urban Water Supply Project is to lay a 1600mm water delivery steel pipe from Xiaozhen, Fenghua City to Meixu, Jiangdong District, Ningbo City. The pipeline is 38 km long. 1.1 Impressed current cathodic protection The 17km pipeline from Xiaozhen to Beidu and the first 4.2km pipeline from Beidu to Jiangdong Meixu, there are basically no underground metal structures, and impressed current cathodic protection is adopted. Xiaozhen Cathodic Protection Station: Set up 2 potentiostats (1 used and 1 prepared), and 30 YJBSiCr 50mm×1200mm double-ended chromium-containing high-silicon cast iron anodes are buried horizontally and horizontally at a distance of 100m from the pipeline and along the vertical direction of the pipeline. . Beidu Cathodic Protection Station: 3 sets of potentiostats (2 used and 1 prepared) are provided to supply power to both sides. Two sets of anodes are buried (the specifications and quantity are the same as above), and each instrument corresponds to one set of anodes. The confluence point is set on the outer side of the inlet and outlet insulation flanges. There are 18 potential test piles (1/km), 2 current test piles and 4 insulation flange test piles. Insulating flanges are set at the entry and exit of the pipeline for effective electrical insulation (insulation resistance ≥ 5 MΩ). Since there is a certain potential difference on both sides of the insulating flange, it will cause cathodic interference and accelerate corrosion to the pipeline in the pump station that is not energized on the other side. Therefore, sacrificial anode protection is implemented for the pipeline in the station, that is, two pump stations are buried in each of the two pump stations. Group of 22 kg/piece magnesium anodes. In addition, in order to prevent the loss of protection current, insulation treatment must be done at the same time. For example, the contact between the steel pipe at the overhead pipe bridge and the buttress should be insulated with rubber gaskets, and all metal parts connected to the pipeline are not allowed to be grounded. 1.2 The implementation effect of impressed current cathodic protection The actual measured protection parameters are shown in Table 1.

Table 1 Measured values ​​of protection parameters

Protection parameters
Xiaozhen Pumping Station
Beidu Pumping Station (inlet side)
Beidu Pumping Station (outlet side)

Soil resistivity (Ω·m)
51.33
17.08
17.9

Anode ground resistance (Ω)
0.8
0.42
0.34

Output voltage (V)
6.22
7.06
3.40

Output current (A)
7.21
7.50
3.96

Control point potential (V)
-1.251
-1.125
-1.053

Protection potential (V)
-0.852～-1.166
-0.970～-1.092
-0.960～-1.062 The
　　results show that: ①The grounding resistance of the anode ground bed is less than 1 Ω, which meets the requirements of the specification. ②The potential data in each test pile meets the protection standard of ≤-0.85V. ③When the highest control potential of the confluence point is -1.251V, it indicates that the entire pipeline has reached the protection potential; the output current and voltage of the instrument are not large, indicating that the anti-corrosion layer of the pipeline is well done, and the insulating flange has good insulation effect; the anode ground bed is grounded The resistance is small, so that the daily maintenance cost of cathodic protection is low. 1.3 Sacrificial anode protection of the pipeline from Beidu to Jiangdong Meixu, except for the 4.2km imposing current, the remaining 17km has many underground metal structures and is parallel and intersecting with other pipelines for many times. Therefore, sacrificial anode protection is adopted. Reduce mutual interference with other metal structures. The sacrificial anode is made of magnesium anodes. There are 4 groups of 1km, each group consists of 4 pieces of 14kg magnesium anodes, of which 3 groups are directly welded to the pipeline, and 1 group is connected by test piles for electrical parameter measurement and understanding of anode service life. 1.4 The implementation effect of sacrificial anode protection The implementation effect of sacrificial anode protection is shown in Table 2.

Table 2 Implementation effect of sacrificial anode protection

Protection parameter
value

阳极开路电位(V)
-1.598～-1.697

管道保护电位(V)
-1.03～-1.05

管道自然电位(V)
-0.573～-0.741

埋点土壤电阻率(Ω·m)
10.36～15.26

阳极输出电流(4支)(A)
0.11～0.15
结果表明：①阳极开路电位符合要求。②管道保护电位≤-0.85V，且比较均匀。③按阳极出电流计算阳极寿命＞20a。
2　陕气进津管道工程
陕甘宁气田—北京输气管道天津支线工程(简称陕气进津管道工程)是天津市的重点工程，对于改善天津市的能源结构具有重大意义。管线西起河北省永清县，东至天津市第一煤气厂，全长64km，选用426mm螺旋缝埋弧焊接钢管。钢管外防腐采用阴极保护与聚乙烯三层结构的联合保护方式。2.1外加电流阴极保护　　从永清至天津西青区子牙河的54km管线主要在农田中敷设，沿线的金属构筑物少，相互干扰较小，采用外加电流的保护方式。为了避免保护电流的流失，在外加电流保护管段的两端加装了高电阻值的绝缘接头(因输气管内壁不做防腐，为防止管内有积水时绝缘接头两端的管段在电解液中导电而失去绝缘作用，故将其埋设在高点处)。在王庆坨设置阴极保护站，沿永清方向保护36km、天津方向保护18km。站内设恒电位仪两台(PS—1型)，可以互相切换，并由自动控制台控制，以保证阴极保护系统连续运行。阳极地床位于管道垂直方向250m处，由40支100mm×1500mm的高硅铸铁阳极组成，阳极为立式埋设，埋深3m，周围填充150mm厚的焦炭(粒径≤10mm，含碳量≥85%)。2.2外加电流阴极保护的实施效果　　外加电流阴极保护的实施效果见表3。

表3　外加电流阴极保护的实施效果

Protection parameter
value

Natural potential along the pipeline (V)
-0.468～-0.693

Insulation joint resistance (before buried installation)
(MΩ)100

Soil resistivity (at anode ground bed) (Ω·m)
25.13

Anode ground resistance (Ω)
0.16

Output voltage (V)
1.7

Output current (A)
1.2

Control point potential (V)
-1.250

Protection potential along the pipeline (V)
-1.0