Shielding and improvement of cathodic protection of buried steel tube by casing

Shielding and improvement of cathodic protection of buried steel tube by casing

1 introduction

Underground steel oil and gas pipeline through ditches, roads, Bridges and other special areas, in order to protect the pipeline and facilitate future maintenance, often in the main outside the addition of a layer of casing, casing is usually a section of steel pipe slightly larger than the main diameter. According to the standard requirements, the annular gap between the two ends of the casing and the main pipe should be filled with oil hemp wire and strictly sealed with asphalt to prevent the infiltration of groundwater. In practice, complete sealing is difficult to achieve and maintain for a long time. Groundwater that seeps in from cracks during rainy season, but cannot flow out during dry season, remains in the casing and forms a corrosive environment. Due to the shielding effect, the main pipe inside the casing could not be effectively protected by the sacrificial anode, resulting in serious corrosion of the main pipe.

2 Cathodic protection test

In order to explore the protective effect of sacrificial anode on the buried casing main pipe, we designed the following tests to test the shielding condition of the casing to cathodic protection current and the distribution of cathodic protection current when the anticorrosive layer of the casing main pipe has a damaged point.

2.1 Test Conditions

FIG. 1 Schematic diagram of the test device

The main length of the test is 2.5m, the specification is ф 38X3.5, the outer anticorrosive layer is used with anti-rust primer and two layers of polyethylene tape. Both ends of the main pipe are plugged, one end of which is connected with a 4kg magnesium alloy sacrifice anode through a wire, and the distance between the anode block and the main pipe is 5m. Solder joints should be treated with anti-corrosion, and a microammeter is connected in series between the connection of the sacrific anode and the main. The length of casing is 2m, the specification is ф 57X3.5, and there is anticorrosion layer outside. When the casing test is carried out, the casing should be evenly arranged outside the main pipe, and the annulus at both ends should be wrapped with anti-corrosion tape. Use water as the medium around the pipe. The test device is shown in Figure 1.

2.2 Test contents and methods

(1) The main anticorrosive layer is intact, without the casing, the two ends of the casing is not sealed, the two ends of the casing is sealed three cases, the main cathodic protection total current change. Test method: the main pipe is put into water without casing, read the current reading: add the casing to read the current reading, the two ends of the casing are sealed to read the current reading.

(2) The cathodic protection potential distribution of the chief is measured every 0.20m under the condition that the main anticorrosive layer is intact without the casing and the two ends of the casing are not sealed. Test method: the negative end of the multimeter and the supervisor is connected and wrapped to prevent leakage, the positive electrode is connected to a reference electrode (Cu/CuSO4), from one end of the supervisor in the supervisor every 0.20m to measure a protection potential value; Then add the casing, put the reference electrode between the main tube and the casing, and measure a protection potential value every 0.20m on the main tube from one end of the main tube.

(3) The main anticorrosive layer is damaged, no casing, the two ends of the casing are not sealed, the two ends of the casing are sealed, the total current change of the main cathodic protection. Test method: In the middle part of the main, artificially manufactured a 10mm2 anticorrosion layer damage. The main pipe is put into the water without the casing, and the current reading is read: then the casing is added to the current reading: the current reading is read after the two ends of the casing are sealed.

(4) When the main anticorrosive coating is damaged, there is no casing and the two ends of the casing are not sealed, the potential distribution of the main cathode protection (a cathodic protection potential value is measured every 0.20m). Test method: the negative electrode of the multimeter is connected with one end of the supervisor and wrapped to prevent leakage, the positive electrode is connected with a reference electrode (Cu/CuSO4), from one end of the supervisor in the supervisor every 0.20m to measure a protection potential value: Then add the casing, put the reference electrode between the main tube and the casing, and measure a protection potential value every 0.20m on the main tube from one end of the main tube.

2.3 Test Results

(1) The total current test results of cathodic protection are shown in Table L.

Table 1 Record of total current test results of cathodic protection

(2) Cathodic protection potential test results (see Figure 2)

FIG. 2 Potential change curves of cathodic protection at different positions of the casing

The curve in the figure is:

(a)△ When there is no casing, the corrosion coating is intact and the protection potential is in charge

(b)● When there is a casing, the corrosion coating is intact and the protection potential is in charge

(c) when there is no casing at the mouth, there is a point of damage in the anticorrosive coating.

(d)■ When there is a casing corrosion coating has a point of damage, the main protection potential.

3. Analysis of experimental results

3.1 Analysis of test data

(1) When the corrosion coating of the main pipe is intact, the total current of cathodic protection with or without the casing is very small, and the cathodic protection potential of the main pipe is negative, indicating that the cathodic protection is playing a role. It is proved that the casing has no effect on the cathodic protection current under test conditions.

(2) When the main anticorrosive layer has a damaged point (located in the middle of the casing), with or without the casing, the cathodic protection current has significantly increased and increased value, indicating that the increased cathodic protection current is the main anticorrosive layer at the damaged point, the cathodic protection current plays a protective role in the main anticorrosive layer at the damaged point. It is indicated that whether there is casing outside the supervisor under the test condition has no effect on the total current increment of cathodic protection.

(3) According to the potential measurement curve of cathodic protection of the main pipe (FIG. 1), whether there is a sleeve outside the main pipe has no significant effect on the potential distribution of cathodic protection. The potential distribution is basically the same whether there is casing protection outside or not when the corrosion protection layer in the main tube is damaged.

(4) when there is no conductive medium between the seal at both ends of the casing and the main pipe, the anticorrosive layer of the main pipe has or does not have a damaged point, and the total current of cathodic protection does not change, indicating that the damaged point is not protected by the cathodic protection current -- that is, the casing has a shielding effect on the cathodic protection current.

3.2 Theoretical Analysis

The above test results show that the main pipe in the casing can get cathodic protection because there is a conductive medium (water) between the main pipe and the casing, and the damage point and the power supply of cathodic protection can form a closed conductive loop. When the air medium is in the middle of the seal between the casing and the main pipe, because the air is not conductive or the conductivity is very low, the cathodic protection current is blocked by the air layer at the damaged point, and the oxygen in the air causes corrosion of the main pipe, then the casing has a shielding effect. The necessary conditions for effective cathodic protection of the main pipe inside the casing are that there is conductive medium between the main pipe and the casing. When the anticorrosive layer of the main pipe is damaged, the damaged point can form a closed loop with the cathodic protection power supply.

4 conclusion

(1) casing parts of buried pipeline, as long as the head of the surrounding between casing and there is the conductive medium (water or soil) with cathodic protection power supply form a closed electric circuit, current cathodic protection can protect of casing head, the casing will not produce shielding effect, sleeve length has no impact on the effect of cathodic protection.

(2) there are pipe casing part of cathodic protection, between casing and director is air or implement insulation sealing on both ends, when the casing head, break point on the coating due to breakage points with the protection of cathodic protection power supply not form a closed loop current cannot flow into the damaged point, cathodic protection will not be able to play a role -- casing has shielding effect.

(3) The buried pipe has a casing part. According to the situation at both ends of the casing construction, it is not necessary to seal and waterproof, as long as the soft material is filled between the main pipe and the casing to prevent the sand and stone from entering the mechanical damage to the anticorrosive layer. When the water enters the casing and can fill the gap, the cathodic protection can play an effective role.

(4) The buried pipe has a casing position. When the main pipe is buried in a shallow depth, it is easy to form a hollow-out inside the casing, which has a shielding effect on the cathodic protection current. In this case, both ends of the casing need to be sealed.