Anti-corrosion Measures for Straight Seam Steel Pipeline
Anti-corrosive coatings on the outer wall of pipelines: materials and operating conditions of coatings on the outer wall of pipelines.
Inner wall anticorrosive coating: To avoid corrosion in the pipe, reduce friction resistance, improve the flow rate and coat the film on the inner wall of the pipe. The common coatings are amine cured epoxy resin and polyamide epoxy resin. The thickness of coatings is 0.038-0.2mm. In order to ensure the bonding between the coating and the pipe wall, it is necessary to stop the external disposal of the pipe wall. Since the 1970s, it has tended to choose the opposite material for inner and outer wall coatings, so that both inner and outer wall coatings can be stopped at the same time.
Anti-corrosion and thermal insulation coating: In order to increase the heat dissipation of pipeline to soil, a compound layer of flatness and anti-corrosion is added to the pipeline on medium and small diameter pipelines for heat transportation of crude oil or fuel oil. The commonly used heat preservation material is rigid polyurethane foam, and the applicable temperature is - 185 - 95 C. In order to improve its strength, a layer of high density polyethylene (HDPE) is added to the thermal insulation layer to form a composite data structure to avoid groundwater infiltrating into the thermal insulation layer.
Electrical maintenance
Change the electrode potential of the metal absolutely in the surrounding medium, so that the metal is not corroded. Electrical maintenance of long distance pipeline only refers to cathode maintenance and corrosion avoidance.
Cathodic Maintenance: Polarization of the maintained metal into a cathode to avoid metal corrosion. This method has been used for anti-corrosion of ships for more than 150 years. It was first used in pipelines in 1928. The principle that the cathode of metal corroded batteries is not corroded while the anode is corroded was applied to metal anti-corrosion technology. If the applied current is applied to force the cathodic polarization of the surface of the maintained metal in the electrolyte, the corrosion will not occur. There are two criteria to determine whether the pipeline can reach the goal of cathode maintenance. The first is the minimum maintenance potential, which is the potential of metal from cathodic polarization in electrolyte to the end of corrosion process. Its value is related to environment and other factors. The commonly used value is - 850 mV (absolutely determined by copper-copper sulfate reference electrode, the same below). The second is the maximum maintenance potential, which is the maximum potential allowed to be reached by the surface of the maintained metal. When the cathodic polarization is too strong, hydrogen will precipitate between the surface of the pipeline and the coating, which will lead to the cathodic stripping of the coating. Therefore, it is necessary to control the potential of the confluence point within the allowable range in order to prevent the coating from being destroyed. This value is related to the properties of the coating. It is usually taken between -1.20 and -2.0 volts. There are two methods for cathode maintenance of underground pipelines, i. e. external current method and sacrificial anode method.
The sacrificial anode method is to connect the metal with the maintained metal, which is more negative than the potential of the maintained metal electrode. Both of them constitute the primary battery in the electrolyte. Metals with negative potential (such as magnesium, zinc, aluminium and their alloys) become anodes, which are gradually depleted in the process of input current, and the maintained pipeline metals become cathodes to avoid corrosion, so the metal with negative potential is called sacrificial anode. Its wiring is shown in Figure 4. Underground pipelines are maintained with sacrificial anodes, whose key factors are the current of anode onset, the number of anodes and the length of maintenance. When the type of anode is determined, the above parameters are affected by the grounding resistance of the anode and the leakage resistance of the maintenance section of the anode. The former depends on the soil resistivity, and the latter depends on the resistance of pipeline coating and the construction quality of coating. The service life of sacrificial anode is related to its component, and it can be used for years to decades depending on the demand. Sacrifice anode has the advantages of low investment, simple management, no need for external power supply, good effect of avoiding disturbance and corrosion, so it is widely used in underground metal pipeline corrosion protection.
Corrosion avoidance method: first, take measures to reduce leakage current to the minimum limit on stray current source related equipment; second, avoid stray current area as far as possible when laying pipelines, or improve the quality of insulating anticorrosive layer of disturbed pipeline section, adopt shielding, installing insulating flange and other measures; third, make drainage maintenance for disturbed pipelines. The stray current will be discharged from the disturbed pipeline back to the leakage current grid to eliminate the corrosion of the stray current to the pipeline. According to the scope of use and the non-uniform performance of drainage equipment, there are three types: indirect drainage, polar drainage and forced drainage. For the protection of AC disturbance voltage, many countries have formulated technical rules. Secondly, safe interval and pipeline discharge are used to protect pipelines from damage.
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