What is the role of pipeline anticorrosi

The anticorrosion effect of pipeline is to prevent chemical or electrochemical reaction of pipeline from corrosion.

Corrosion prevention of pipes refers to measures to mitigate or prevent pipelines from being eroded and metamorphosed by the chemical, electrochemical, or microbial metabolism of the internal and external media.

Anticorrosion method

Coating anticorrosion

Coating evenly and densely on the surface of the corroded metal pipe and isolating it from various corrosive media is one of the most basic methods for pipeline corrosion protection. Since 70s, laying pipes in the harsh environment of the polar, ocean and so on, as well as the heating and transporting of oil to the higher temperature of the pipeline, have put forward more requirements for the performance of the coating. Therefore, more and more composite coatings or composite structures are used for pipeline anticorrosive coatings.

1. The outer wall anticorrosive coating: the type and the use condition of the coating material on the outer wall of the pipeline.

2. Inner wall anticorrosive coating: thin film applied to the inner wall of pipes to prevent corrosion, reduce friction resistance and increase throughput. Commonly used coatings are amine cured epoxy resin and polyamide epoxy resin, the coating thickness is 0.038 to 0.2 millimeters. To ensure the bonding between the coating and the tube wall is firm, the inner surface of the tube must be treated. Since 70s, the same material has been chosen for coating inside and outside the tube, so that the coating inside and outside the tube can be carried out at the same time.

(3) anticorrosion and insulation coating: in the medium and small caliber heat transfer crude oil or fuel oil pipeline, in order to reduce the pipe to the soil heat dissipation, add insulation and anticorrosion composite layer outside the pipeline. Insulation material is commonly used in rigid polyurethane foam, suitable for 185 temperature to 95 DEG C. This material is soft and soft. In order to improve its strength, a layer of high density polyethylene layer is added outside the insulation layer to form a composite structure to prevent groundwater from infiltrating into the insulation layer.

Electrical protection

The method of changing the electrode potential of metal relative to the surrounding medium to protect metals from corrosion. The electrical protection of long distance pipelines only refers to cathodic protection and electric erosion prevention.

Cathodic protection: the way to polarize protected metals into cathodes to prevent metal corrosion. This method has been used for 150 years in marine anticorrosion. The first time used in pipeline in 1928 is to apply the principle of corrosion in metal corrosion battery without corrosion and anode corrosion in metal corrosion protection technology. The corrosion will not happen if the external current is applied to force all the cathodic polarization of the protected metal surface in the electrolyte. There are two indicators to determine whether the pipeline reaches cathodic protection. The first is the minimum protective potential, which is the potential of the cathodic polarization of the metal in the electrolyte to the stop of the corrosion process; its value is related to the environment and other factors, and the commonly used values are 850 MV (relative to copper - copper sulfate reference electrode, the same below). The two is the maximum protection potential, that is, the maximum potential value permitted by the protected metal surface. When the cathode polarization is too strong, hydrogen is precipitated between the surface of the pipe and the coating, and the coating produces cathodic stripping. Therefore, the confluence point potential must be controlled within the allowable range to prevent the coating from being destroyed. This value is related to the properties of the coating, generally between 1.20 and 2 volts. There are two ways to realize cathodic protection of underground pipelines: impressed current method and sacrificial anode method.

The impressed current method is a direct current power supply, the negative electrode is connected to the protected pipeline, and the positive electrode is connected to the anode bed. After the circuit is connected, the pipe is polarized by the cathode. Cathodic protection is achieved when the pipe ground potential reaches the minimum protective potential. The connection is as shown in Figure 3. Commonly used DC power supply can be used, especially in the majority of rectifier. DC output is generally below 60 VOLTS and 30. The new DC power supply has thermoelectric generator, solar cell and so on, and it is mostly used in power shortage areas. The anode ground bed is a conductive body, which is connected with the positive pole of the DC power supply, which is in good contact with the earth, or the anode grounding device. The common materials are carbon steel, high silicon iron, graphite, magnetic iron oxide, etc. The anode bed is located in the place where the soil resistivity is low, the protective current is easy to distribute, and does not interfere with adjacent underground structures. The anode is corresponding to the buried position of the pipeline, and there are two kinds of shallow buried long distance anode and deep anode. In order to determine the cathodic protection parameters and identify the cathodic protection effect of pipelines, check points and inspection sheets should be installed along pipelines. The detection instrument supporting the use of high resistance voltmeter, ammeter, copper sulfate electrode. Since 70s, the cathodic protection parameter telemetry system, which is combined with the pipeline aerial patrol line, has been used, and the data are processed with an electronic computer.  The application of impressed current cathodic protection for single station protection can reach dozens of kilometers.

Sacrificial anode is connected to the protected metal by a more negative metal potential than the protected metal electrode. The negative metals (such as magnesium, zinc, aluminum and their alloys) become anodes, which are gradually lost during the output current, and the protected pipe metal becomes the cathode so as not to be corroded, so the metal with negative potential is sacrificed as a sacrificial anode. The connection is as shown in Figure 4. Sacrificial anode protection is used for underground pipelines, and the decisive factors are anode current, anode quantity and protection length. When the anode type is determined, the influence of the above parameters is the anode grounding resistance and the leakage resistance of the anode protection section. The former depends on the resistivity of the soil, and the latter depends on the coating resistance of the pipe and the quality of the coating. The service life of sacrificial anode is related to weight and can be used for several to several decades as needed. Sacrificial anode has the advantages of low investment, simple management, no need of external power supply, and good effect of preventing interference and corrosion.