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Corrosion Protection Methods of Gas Pipelines

Posted: 07/12/2021 15:28:34 Hits: 16

Corrosion is a spontaneous process of materials. In nature, substances will spontaneously change from a high-energy state to a low-energy state in order to maintain energy conservation. Iron mostly exists in the form of iron ore in nature, and its state is relatively stable. When iron ore is given the energy to restore to metallic iron, iron has higher free energy than its original state. Therefore, metallic iron will release energy to return to a more thermodynamically stable natural state, that is, metal oxides, sulfides, and other compounds. This changing process is corrosion.

Causes of corrosion of steel gas pipelines
The causes of corrosion of steel pipelines are generally divided into the following four types:
(1) Chemical corrosion: it is caused by a chemical reaction and refers to the reaction between metal and the surrounding media such as oxygen, hydrogen sulfide, sulfur dioxide, etc.
(2) Electrochemical corrosion: it is generated by an electrochemical reaction due to the contact between metal and electrolyte solution, forming the principle of the galvanic cell.
(3) Corrosion caused by the stray current: the stray current is formed in the soil due to the leakage and grounding of various external electrical equipment. When the current flows through the gas pipeline, an electrolytic battery is formed.
(4) Bacteria: underground microorganisms participate in the corrosion process, for example, sulfate reducing bacteria, which transform soluble sulfate into hydrogen sulfide, thereby accelerating the corrosion of the pipeline.

Anti-corrosion methods of gas pipelines
(1) Overhead pipelines
For overhead pipelines, painting the outer wall of the pipeline with anti-corrosion materials can reduce the corrosion of the outer wall of the pipeline to a certain extent, and the inner wall is usually not treated.
Anti-corrosion methods: surface removal → anti-rust painting → applying mixed paint

(2) Anti-corrosion of the outer wall of the buried pipeline
The anti-corrosion of buried pipelines is mainly divided into two categories; one is the anti-corrosion of the outer wall of the pipeline, and the other is the anti-corrosion of the pipeline when it is put into use. For the early anti-corrosion method of the outer wall of the pipeline, petroleum asphalt was used for anti-corrosion of the outer wall. However, it pollutes and has low area resistance; it is eliminated. With the development of technology, epoxy powder (FBE) use for anti-corrosion has been widely promoted and applied due to its excellent performance, long lives and high area resistance. Polyethylene has the same advantages as epoxy powder, and it also has a cathodic shielding effect. The actual situation of the pipeline and the difficulty of subsequent pipeline monitoring should be considered when Polyethylene is used. Three-layer PE anti-corrosion is the mainstream anti-corrosion technology. The three-layer structure of the extruded polyethylene anti-corrosion layer is also called the composite coating, which is a protective layer that bonds epoxy powder and polyethylene through the copolymer glue. This kind of protective layer has good adhesion, good corrosion resistance and a long service life. The 3PE anticorrosive pipeline has good mechanical properties, damage resistance and insulation resistance, which makes it good against mutual interference.

Figure 1 The schematic diagram of the three-layer PE anticorrosion

The thickness of the 3PE anti-corrosion layer of the pipe body shall meet the requirements of Table 2, and the thickness of the anti-corrosion layer of the weld part shall be over 90% of the specified value.

Table 2 The thickness of the 3PE anticorrosion coating for the pipe body

Nominal sizes (mm)		DN being less than and equal to100	DN being between 100 and 250	DN being between 250 and 500	DN being between 500 and 800	DN being greater than or equal to 800
Epoxy powder (Micron)		DN being greater than or equal to 120
Glue layers (Micron)		DN being greater than or equal to 170
Anti-corrosion layers (Millimeters)	Normal grades	1.8	2.0	2.2	2.5	3.0
Anti-corrosion layers (Millimeters)	Enhanced grades	2.5	2.7	2.9	3.2	3.7

(3) Electrical protection-anodic sacrifice protection
A corrosion battery must have four parts: anode, cathode, electrolyte solution and circuit (the flow path of the current). Electrochemical corrosion needs to go through the following processes:
The anode process: the metal releases electrons and enters the solution in the form of ions.

The cathodic process: the electrons flowing from the anode are accepted by the oxide in the electrolyte solution.

Only when the oxide in the cathode continuously accepts electrons and undergoes a reduction reaction, will the anode process continue to occur, causing the metal to be corroded. The flow process of the current: In metals, electrons flow from the anode to the cathode, while in the electrolyte solution, ions move from the anode to the cathode. Electrons and ions flow between the cathode and anode to form an electric current, forming a circuit path in the entire battery in the system. Therefore, the corrosion damage of metals is concentrated in the anode area, and the cathode area is only responsible for the transfer of electrons. The use of highly reducing metals as anodes and pipes as cathodes can avoid corrosion. In the soil environment, magnesium rods are generally used as anodes.

Figure 3 The schematic diagram of the cathodic protection system of anodic sacrifice protection for buried pipelines

Detection methods of defects of gas pipelines and detection methods of anti-corrosion coatings of metal pipelines
The anti-corrosion layer of the pipe should be inspected regularly according to the grade of pressure, and the inspection period is generally as follows:
(1) Low-pressure gas pipelines should be inspected at least once every 8 years, and pipelines that have been in operation for 10 years or more should be inspected at least once every 5 years.
(2) Medium-pressure gas pipelines shall be inspected at least once every 5 years, and pipelines that have been in operation for 10 years or more shall be inspected at least once every 3 years.
(3) Sub-high pressure and above gas pipelines shall be inspected at least once every 3 years, and pipelines which have been used for 10 years and above shall be inspected at least once every 2 years.

According to the provisions of CJJ-95, the evaluation method and classification of anti-corrosion coating defects are shown in Table 4. The ground inspection or a combination of excavation pit inspection can be adopted for the inspection of the anti-corrosion layer. The ground inspection items include the location of the damaged point and the average insulation resistance; the excavation pit inspection items include inspection of appearance, electric spark, and thickness of the anti-corrosion layer.

Table 4 The Evaluation classification of defects of anti-corrosion coatings

Detection methods		Alternating current potential gradient methods (ACVG)	Direct current potential gradient methods (DCVG)	Alternating current attenuation methods	Close interval potential methods (CIPS)	Evaluation results	Handling opinions
Grades	1	Low voltage drop	Small potential gradient, small IR%, CP is in the cathode state when the power is on or off.	The attenuation per unit length is small.	The potential is slightly negative than the cathodic protection potential criterion when the power is one or off.	Have the possibility of passivation or lower corrosion activity.	No excavation detection is required.
	2	Medium voltage drop	Medium potential gradient IR%, CP is in a neutral state when the power is on or off.	Attenuation per unit length is medium.	When the power is on or off, potential is moderately deviated and positive to cathodic protection potential criterion.	There is general corrosion activity.	Excavation inspection should be required.
	3	High voltage drop	Great potential gradient IR%, CP is in the anode state when the power is on or off.	The attenuation per unit length is great.	When the power is on or off, the potential greatly deviates from the cathodic protection potential criterion.	It has a high possibility of corrosion activity.	Immediately excavating for inspection is required.

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