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Suggestions on the peeling method of 3PE anti-corrosion coating

1.Improvement of mechanical peeling method of 3PE anti-corrosion coating
① Find or develop better heating equipment to replace the gas cutting torch. The heating equipment should be able to ensure that the spray flame area is large enough to heat the entire coating part to be peeled off at one time, and at the same time ensure that the flame temperature is higher than 200°C.
② Find or make a better stripping tool instead of a flat shovel or a hand hammer. The peeling tool should be able to achieve good cooperation with the outer surface of the pipeline, try to scrape the heated anti-corrosion coating on the outer surface of the pipeline at one time, and ensure that the anti-corrosion coating bonded to the peeling tool is easy to clean.

2.Electrochemical peeling of 3PE anti-corrosion coating
Engineering design and construction personnel can analyze the causes of external corrosion of gas buried pipelines and the defects of 3PE anti-corrosion coating, and find new ways to destroy and peel off the anti-corrosion coating.
(1)Causes of external corrosion of pipelines and analysis of 3PE anti-corrosion coating defects
① Stray current corrosion of buried pipelines
Stray current is a current generated by the influence of external conditions, and its potential is generally measured by the polarization probe method [1]. Stray current has a large corrosion intensity and hazard, a wide range and strong randomness, especially the existence of alternating current can cause the depolarization of the electrode surface and aggravate pipeline corrosion. AC interference can accelerate the aging of the anti-corrosion layer, cause the anti-corrosion layer to peel off, interfere with the normal operation of the cathodic protection system, reduce the current efficiency of the sacrificial anode, and cause the pipeline to not get effective anti-corrosion protection.
② Soil environment corrosion of buried pipelines

The main influences of the surrounding soil on the corrosion of buried gas pipelines are: a. The influence of primary batteries. Galvanic cells formed by electrochemical inhomogeneity of metals and media are an important cause of corrosion in buried pipelines. b. Influence of water content. The water content has a great influence on the corrosion of gas pipelines, and the water in the soil is a necessary condition for the ionization and dissolution of the soil electrolyte. c. The effect of resistivity. The smaller the soil resistivity, the stronger the corrosiveness to metal pipes. d. The effect of acidity. Pipes are easily corroded in acidic soils. When the soil contains a lot of organic acids, even the pH value is close to neutral, it is very corrosive. e. The effect of salt. The salt in the soil not only plays a role in the conductive process of soil corrosion, but also participates in chemical reactions. The salt concentration difference battery formed by the contact between the gas pipeline and the soil with different salt concentration causes corrosion of the pipeline in the position with high salt concentration and aggravates the local corrosion. f. Effect of porosity. Larger soil porosity is conducive to the infiltration of oxygen and the preservation of water in the soil, and promotes the occurrence of corrosion.

③ Defect analysis of 3PE anti-corrosion coating adhesion [5]
An important factor affecting the adhesion between the 3PE anti-corrosion coating and the steel pipe is the surface treatment quality and surface contamination of the steel pipe. a. The surface is wet. The surface of the steel pipe after derusting is contaminated with water and dust, which is prone to floating rust, which will affect the adhesion between the sintered epoxy powder and the surface of the steel pipe. b. Dust contamination. The dry dust in the air directly falls on the surface of the rust-removed steel pipe, or falls on the conveying equipment and then indirectly contaminates the surface of the steel pipe, which can also cause a decrease in adhesion. c. Pores and bubbles. The pores caused by moisture exist widely on the surface and inside of the HDPE layer, and the size and distribution are relatively uniform, which affects the adhesion.
(2)Recommendations for electrochemical stripping of 3PE anti-corrosion coatings
Through the analysis of the causes of external corrosion of gas buried pipelines and the adhesion defects of 3PE anti-corrosion coatings, the development of a device based on electrochemical methods is a good way to quickly solve the current problem, and there is no such device on the market at present.
On the basis of fully considering the physical properties of the 3PE anti-corrosion coating, by studying the corrosion mechanism of the soil and through experiments, a corrosion method with a corrosion rate far greater than that of the soil is developed. Use a moderate chemical reaction to create certain external conditions, so that the 3PE anti-corrosion coating reacts with chemical reagents electrochemically, thereby destroying its adhesion with the pipeline or directly dissolving the anti-corrosion coating.

3.Miniaturization of current large-scale strippers

PetroChina West-East Gas Pipeline Company has developed an important mechanical equipment for emergency repair of oil and natural gas long-distance pipelines – large-diameter pipeline external anti-corrosion layer stripping machine. The equipment solves the problem that the anti-corrosion layer is difficult to peel off in the emergency repair of large-diameter oil and gas pipelines, which affects the efficiency of emergency repair. The crawler-type large-diameter pipeline external anti-corrosion layer stripping machine uses a motor as the stripping power to drive the roller brush to rotate to remove the anti-corrosion layer wrapped on the outer wall, and move along the circumference on the surface of the pipeline’s anti-corrosion layer to complete the pipeline anti-corrosion layer peeling. Welding operations provide favorable conditions. If this large-scale equipment is miniaturized, suitable for outdoor small-diameter pipelines and popularized, it will have better economic and social benefits for urban gas emergency repair construction. How to miniaturize the crawler-type large-diameter pipeline outer anti-corrosion layer stripper is a good research direction.

Post time: Oct-14-2022