Long-distance oil and gas pipelines are an important means of energy security. During the anti-corrosion construction process of oil (gas) pipelines, the surface treatment of steel pipes is one of the key factors that determine the anti-corrosion service life of the pipelines. It is the prerequisite for whether the anti-corrosion layer and the steel pipe can be firmly combined. . It has been verified by research institutions that, in addition to factors such as coating type, coating quality, and construction environment, the surface treatment of steel pipes accounts for about 50% of the impact on the life of the anti-corrosion layer. Therefore, the anti-corrosion layer specifications should be strictly followed. The requirements for the surface of steel pipes are constantly explored and summarized, and the surface treatment methods of steel pipes are constantly improved. the

1. Cleaning

Solvents and emulsions are used to clean the steel surface to remove oil, grease, dust, lubricants and similar organic matter. However, it cannot remove rust, oxide scale, welding flux, etc. on the steel surface, so it is only used as an auxiliary means in anti-corrosion production. the

2. Tool derusting

Mainly use tools such as wire brushes to polish the steel surface to remove loose or lifted oxide scale, rust, welding slag, etc. The rust removal of hand tools can reach Sa2 level, and the rust removal of power tools can reach Sa3 level. If the iron oxide scale is firmly attached to the surface of the steel, the rust removal effect of the tool will not be ideal and the anchor pattern depth required for anti-corrosion construction will not be achieved. ​

3. Pickling

Generally, chemical and electrolytic methods are used for pickling treatment. Only chemical pickling is used for pipeline anti-corrosion, which can remove scale, rust, and old coatings. Sometimes it can be used as a reprocessing after sandblasting and rust removal. Although chemical cleaning can achieve a certain degree of cleanliness and roughness on the surface, its anchor lines are shallow and it can easily cause environmental pollution. the

4. Spray (throw) to remove rust

Spraying (throwing) rust removal uses a high-power motor to drive the spraying (throwing) blades to rotate at high speed, so that steel sand, steel shots, wire segments, minerals and other abrasives are sprayed (throwing) on the surface of the steel pipe under the action of centrifugal force. , not only can rust, oxides and dirt be completely removed, but the steel pipe can also achieve the required uniform roughness under the action of violent impact and friction of abrasives. After spraying (throwing) rust removal, it can not only expand the physical adsorption on the pipe surface, but also enhance the mechanical adhesion between the anti-corrosion layer and the pipe surface. Therefore, spraying (throwing) rust removal is an ideal rust removal method for pipeline anti-corrosion.

4.1 Derusting level

For the construction technology of epoxy, vinyl, phenolic and other anti-corrosion coatings commonly used for steel pipes, the surface of the steel pipe is generally required to reach near white level (Sa2.5). Practice has proven that using this level of rust removal can remove almost all oxide scale, rust and other dirt. The depth of the anchor pattern can reach 40-100µm, which fully meets the adhesion requirements between the anti-corrosion layer and the steel pipe. However, spraying (throwing) can remove The rust process can achieve near-white level (Sa2.5) technical conditions with lower operating costs and stable and reliable quality.

4.2 Spraying (throwing) abrasives

In order to achieve the ideal rust removal effect, the abrasive should be selected according to the hardness of the steel pipe surface, the original rust degree, the required surface roughness, the coating type, etc. For single-layer epoxy, two-layer or three-layer polyethylene coatings, use The mixed abrasive of steel sand and steel shot is easier to achieve the ideal rust removal effect. Steel shot has the function of strengthening the steel surface, while steel grit has the function of etching the steel surface. Mixed abrasives of steel grit and steel shot (usually the hardness of steel shot is 40 to 50 HRC, and the hardness of steel grit is 50 to 60 HRC) can be used on various steel surfaces, even on grade C and D rusted steel surfaces. , the rust removal effect is also very good.

4.3 Abrasive particle size and ratio

In order to obtain better uniform cleanliness and roughness distribution, the particle size and proportion design of the abrasive are very important. Too much roughness will easily cause the anti-corrosion layer to become thinner at the peaks of the anchor lines; at the same time, because the anchor lines are too deep, bubbles will easily form in the anti-corrosion layer during the anti-corrosion process, seriously affecting the performance of the anti-corrosion layer. If the roughness is too small, the adhesion and impact strength of the anti-corrosion layer will decrease. For severe internal pitting corrosion, we cannot rely solely on high-intensity impact with large-grain abrasives. We must also rely on small particles to grind away the corrosion products to achieve the cleaning effect. At the same time, reasonable ratio design can not only slow down the wear of the abrasives on the pipes and nozzles (blade) , and the utilization rate of abrasive can be greatly improved. Usually, the particle size of steel shot is 0.8~1.3 mm, and the particle size of steel sand is 0.4~1.0 mm, of which 0.5~1.0 mm is the main component. The ratio of sand to shot is generally 5-8.

It should be noted that in actual operation, the ideal ratio of steel grit and steel shot in the abrasive is difficult to achieve because the hard and brittle steel grit has a higher breakage rate than the steel shot. For this reason, the mixed abrasives should be continuously sampled and tested during operation, and new abrasives should be added to the rust remover according to the particle size distribution. Among the new abrasives added, steel grit should account for the majority.

4.4 Derusting speed

The rust removal speed of the steel pipe depends on the type of abrasive and the displacement of the abrasive, that is, the total kinetic energy E applied to the steel pipe by the abrasive per unit time and the kinetic energy E1 of the single-grain abrasive. the

Generally, abrasives with lower loss rates should be selected, which will help improve the cleaning speed and extend the life of the blades.

4.5 Cleaning and Preheating

Before spraying (throwing) treatment, use cleaning methods to remove grease and scale on the surface of the steel pipe, and use a heating furnace to preheat the pipe body to 40-60°C to keep the surface of the steel pipe dry. During spraying (throwing) treatment, since the surface of the steel pipe does not contain grease and other dirt, the rust removal effect can be enhanced. The dry steel pipe surface is also conducive to the separation of steel shot, steel sand, rust and oxide scale, making the rust removed The steel pipe surface is cleaner.

5.Conclusion

Pay attention to the importance of surface treatment in production and strictly control the process parameters during rust removal. In actual construction, the peel strength value of the anti-corrosion layer of the steel pipe greatly exceeded the standard requirements, ensuring the quality of the anti-corrosion layer. On the basis of the same equipment, , greatly improving the process level and reducing production costs.

Anti-corrosion steel pipe is a new type of steel pipe, after anti-corrosion treatment, it can effectively prevent or slow down the corrosion of chemical or electrochemical reactions during transportation and use; however, it is an excellent anti-corrosion steel pipe, please pay attention when using it; keep it warm , especially in the cold winter,

In fact, there are many insulation methods for anti-corrosion steel pipes, including coating insulation coatings, wrapping anti-corrosion materials around anti-corrosion steel pipes, and filling and insulating anti-corrosion steel pipes. Specifically, they are:

1. Anti-corrosion steel pipes are insulated by coating heat-insulating coatings, that is: using expanded perlite, expanded frog stone, asbestos powder, asbestos fibers, diatomite clinker and other amorphous heat-insulating materials, and then adding cement, water glass , refractory binder (such as clay) or coagulant (such as sodium fluorosilicate), then add water in a certain proportion and mix evenly to form a slurry, or use these insulating materials on bare hands or apply them on anti-corrosion steel pipes with tools , This insulation method for anti-corrosion steel pipes is also called coating insulation.

2. Anti-corrosion steel pipes are insulated by wrapping insulating materials, that is, directly wrapped with insulating materials such as slag felt, glass wool felt, straw rope, asbestos rope or cotton tape, so that there is no need to worry about the anti-corrosion steel pipe being frozen and cracked. Does not affect the use of anti-corrosion steel pipe.

3. The anti-corrosion steel pipe is filled with thermal insulation material, that is, when the thermal insulation material is a block material, it can also be filled with thermal insulation; however, during the construction process, the support ring made of round steel is fixed on the pipe wall, and its thickness and insulation The same layer, then wrap the support ring with iron, aluminum or barbed wire, and then insulate it with heat insulating material; material filling; filling method can also use prefabricated rigid arc-shaped blocks made of porous porous material as the supporting structure, with a spacing of about 900mm , according to the shape and size of the pipeline insulation layer, the flat woven barbed wire mesh is cut, and the winding machine is processed into a circle, so that the slag wool covers the support ring, and then the metal protective shell is used to fill the insulation structure.

4. In addition, we can also use prefabricated thermal insulation treatment of anti-corrosion steel pipes to maintain thermal insulation. The main materials of prefabricated thermal insulation products are foam concrete, asbestos, diatomaceous earth, slag wool, glass wool, rock wool, expanded perlite, expanded vermiculite , calcium silicate, etc.; prefabricated pipe insulation structures, usually with a diameter of DN ≤ 80 mm, using a semicircular shell, such as DN ≥ 100 mm, using fan-shaped tiles (curved tiles) or trapezoidal tiles.

Epoxy powder coating is applied by electrostatic spraying process, forming a film at one time. This epoxy powder coating is a kind of thermosetting coating, which is processed by mixing and destroying solid epoxy resin, curing agent and various additives. The steel pipe is preheated by shot blasting and intermediate frequency before painting, and then the epoxy powder coating is sprayed on the surface of the heated steel pipe by electrostatic spraying method, melted and bonded to the surface of the steel pipe, and solidified to form a coating.

FBE coating is generally a film-forming structure epoxy powder coating. The production raw materials include: solid epoxy resin with an epoxy content in the middle and a narrow molecular weight distribution, which has no reaction activity under natural temperature conditions and responds quickly at high temperatures. Curing agent, catalyst and co-catalyst, in addition to leveling agent, pigment and filler.

Epoxy powder coating: The dense structure of the coating determines its strong anti-corrosion performance. The polar structure of the epoxy molecule determines its strong adhesion. It is a coating with good anti-corrosion effect. However, the coating is thin and brittle, and there is a high possibility of mechanical damage during hoisting, transportation, and stacking, and the epoxy structure has poor anti-ultraviolet ability, so it is not suitable for coating the outer wall and outer surface of the pipeline.

Although both polyethylene and epoxy have excellent corrosion resistance, polyethylene is a thermoplastic material with good flexibility and bump resistance. Because it is a non-polar molecule and the durability of adhesion to steel pipes is poor, epoxy resin is a polar molecule. Under high temperature, it is easy to react with the steel pipe, and the adhesion is very strong, but because it is a thermosetting material, it is not resistant to bumps.

Therefore, the combination of the two materials belongs to the current anti-corrosion industry collocation. The plastic-coated steel pipe industry has developed from the earliest inner and outer polyethylene, due to adhesion problems, to inner and outer epoxy, but the outer epoxy layer is not resistant to bumps, and later developed to the first inner epoxy outer polyethylene, but the single-layer polyethylene is directly bonded to the steel pipe In combination, there is still an adhesion problem, and it is upgraded to 3PE external anti-corrosion fusion-bonded epoxy powder internal anti-corrosion pipeline.

The quality requirements of galvanized square tubes are very high. However, due to the inevitable corresponding quality defects in each process of steel pipe production, and some steel pipes are in service under some special environmental conditions, in addition to the overall performance of the steel pipe, the accuracy of the outer diameter and wall thickness, and the flat surface In addition to the requirements for straightness, special requirements are also put forward for its surface, end face, anti-corrosion, etc.

In order to meet the above requirements, the steel pipe needs to be straightened and defect repaired after cooling; the pipe end must be processed; the steel pipe after passing the performance inspection (test) of the steel pipe is checked, and then the length measurement, weighing, Logo, packager library. In short, the finishing process of steel pipes is an important process that is indispensable for removing defects of steel pipes, further improving the quality of steel pipes, meeting the needs of special uses of products, and clarifying the “identity” of products. Steel pipe finishing mainly includes: steel pipe straightening, end cutting (chamfering, sizing)

Inspection and inspection (including surface quality inspection, geometric dimension inspection, non-destructive inspection and hydraulic test, etc.), grinding, length measurement, weighing, painting, spray printing and packaging and other processes. Some special-purpose steel pipes also require surface shot blasting, machining, and anti-corrosion treatment.

In the various processes of steel pipe finishing, the requirements of steel pipe inspection and inspection procedures have been introduced in the first chapter. The length measurement, weighing, painting, spray printing and packaging of the steel pipe will generally not change the shape, size and performance of the steel pipe except for minor defects such as bruises and scratches on the steel pipe body. Therefore, this chapter aims to focus on the quality defects and preventive measures of steel pipes in the three processes involving steel pipe deformation or processing, such as straightening, grinding, and surface treatment.

The standard specifies the “surface finish” requirements for steel pipes. However, there are as many as 10 surface defects of steel pipes caused by various reasons in production (see appendix “Examples of typical galvanized steel pipe defects”). These defects mainly include: surface cracks (cracks), hairlines, inner folds, outer folds, crushing, inner straight roads, outer straight roads, separation layers, scars, pits, convex hulls, hemp pits (pockmarked surfaces), Abrasion (scratch), internal spiral, external spiral, blue line, concave correction, roll printing, etc. Among the surface defects of the above-mentioned steel pipes, some defects are very harmful to the performance of the steel pipe, which are called dangerous defects, such as steel pipe cracks (cracks), inner folds, outer folds, crushing, delamination, knotting, pulling, etc. Concave, convex, etc.; some defects have relatively little impact on the performance of steel pipes, which are called general defects, such as steel pits (surfaces), blue lines, scratches (scratches, bumps), slight inner straights and outer straights , Slight internal spiral and external spiral, concave correction, roll printing, etc.

Although some general surface defects that are very slight and have little impact on the use of steel pipes can remain on steel pipes, the standard still has very strict restrictions on the depth and length (size) of the defects. For dangerous steel pipe surface defects, it must be completely removed by cutting or grinding. When grinding those steel pipe surface defects that allow grinding, the depth of the specified grinding point and the shape of the grinding point must meet the requirements specified in the standard. In order to improve the surface quality of steel pipes, sometimes the inner and outer surfaces of steel pipes are shot blasted (sanded), sanded or machined and turned.

There are two main reasons for the surface defects of galvanized steel pipes. On the one hand, it is caused by surface defects or internal defects of the pipe. On the other hand, it is produced in the production process, that is, if the design of the rolling process parameters is incorrect, the surface of the tool (mold) is not smooth, the lubrication conditions are not good, the pass design and adjustment are unreasonable, etc., may cause the steel pipe to appear. Surface quality problems; or during the heating, rolling, heat treatment and straightening process of the tube blank (steel tube), if the heating temperature is improperly controlled, the deformation is uneven, the heating and cooling speed is unreasonable, or the straightening deformation is too large. Excessive residual stress is also likely to cause surface cracks in steel pipes.