How should the surface of high-frequency welded steel pipes be treated

With the continuous development of my country's economy, the country has vigorously developed the energy industry. Long-distance API oil and gas pipelines are an important way to ensure energy security. In the process of anti-corrosion construction of oil (gas) pipelines, the surface treatment of high-frequency welded steel pipes is one of the key factors that determine the service life of pipeline anti-corrosion. It is the premise for the firm combination of the anti-corrosion layer and the high-frequency welded steel pipe. According to research institutions, the life of the anti-corrosion layer depends on factors such as the type of coating, coating quality, and construction environment. The influence of the surface treatment of the steel pipe on the life of the anti-corrosion layer accounts for about 50%. Therefore, the requirements of the anti-corrosion layer specification for the surface of thick-walled steel pipes should be strictly followed, and the surface treatment methods of thick-walled steel pipes should be continuously explored and summarized to continuously improve.

1. Cleaning
Use solvents and emulsions to clean the surface of the steel to remove oil, grease, dust, lubricants, and similar organic matter, but it cannot remove rust, scale, welding flux, etc. on the surface of the steel. Therefore, it is only used as an auxiliary means in anti-corrosion production.

2. Tool rust removal
Mainly use tools such as wire brushes to polish the steel surface, which can remove loose or raised oxide scales, rust, welding slag, etc. Manual tool rust removal can reach the Sa2 level, and power tool rust removal can reach the Sa3 level. If the steel surface is firmly attached to the oxide scale, the tool rust removal effect is not ideal, and the anchor pattern depth required by the anti-corrosion construction cannot be achieved.

3. Pickling
Generally, chemical and electrolytic methods are used for pickling treatment. Pipeline anti-corrosion only uses chemical pickling, which can remove oxide scale, rust, and old coating. Sometimes it can be used as a re-treatment after sandblasting and rust removal. Although chemical cleaning can make the surface reach a certain degree of cleanliness and roughness, its anchor pattern is shallow and it is easy to pollute the environment.

4. Blasting (blasting) rust removal
Blasting (blasting) rust removal is to use a high-power motor to drive the blasting (blasting) blade to rotate at high speed, so that steel sand, steel shot, wire segments, minerals, and other abrasives are blasted (blasted) on the surface of the steel pipe under the action of centrifugal force. It can not only completely remove rust, oxides, and dirt, but also the steel pipe can achieve the required uniform roughness under the violent impact and friction of the abrasive. After blasting (blasting) rust removal, can not only expand the physical adsorption effect of the pipe surface but also enhance the mechanical adhesion between the anti-corrosion layer and the pipe surface. Therefore, blasting (blasting) rust removal is an ideal rust removal method for pipeline corrosion protection. Generally speaking, shot blasting (sand) rust removal is mainly used for the inner surface treatment of the pipe, and shot blasting (sand) rust removal is mainly used for the outer surface treatment of the pipe. Several issues should be paid attention to when using blasting (blasting) rust removal.

4.1 Rust removal level
For the construction process of epoxy, ethylene, phenolic, and other anti-corrosion coatings commonly used on steel pipes, the surface of the steel pipe is generally required to reach the near-white level (Sa2.5). The practice has proved that this rust removal level can almost remove all scales, rust, and other dirt, and the anchor pattern depth reaches 40~100μm, which fully meets the adhesion requirements of the anti-corrosion layer and the steel pipe, and the spray (blasting) rust removal process can achieve the near-white level (Sa2.5) technical conditions with lower operating costs and stable and reliable quality.

4.2 Spray (blasting) abrasive
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, the mixed abrasive of steel sand and steel shot is easier to achieve the ideal rust removal effect. Steel shot has the effect of strengthening the steel surface, while steel sand has the effect of etching the steel surface. The mixed abrasive of steel grit and steel shot (usually the hardness of steel shot is 40-50HRC, and the hardness of steel grit is 50-60HRC) can be used for various steel surfaces, and even on steel surfaces with C and D rust, the rust removal effect is also very good.

4.3 Particle size and ratio of abrasive
To obtain better uniform cleanliness and roughness distribution, the particle size and ratio design of abrasive is very important. Too much roughness can easily cause the anti-corrosion layer to become thinner at the peak of the anchor pattern; at the same time, because the anchor pattern is too deep, bubbles are easily formed in the anti-corrosion layer during the anti-corrosion process, which seriously affects the performance of the anti-corrosion layer. Too little roughness will cause the adhesion and impact resistance of the anti-corrosion layer to decrease. For severe internal pitting, it is not only impossible to rely on the high-intensity impact of large-particle abrasives but also to rely on small particles to grind off the corrosion products to achieve the cleaning effect. At the same time, a reasonable ratio design can not only slow down the abrasive on the pipe and nozzle (blade) Wear but the utilization rate of abrasives can also be greatly improved. Usually, the particle size of steel shot is 0.8-1.3mm, and the particle size of steel grit is 0.4-1.0mm, of which 0.5-1.0mm is the main component. The sand-shot ratio 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 crushing rate than steel shot. For this reason, the mixed abrasive should be sampled and tested continuously during operation, and new abrasive should be added to the rust remover according to the particle size distribution, and the amount of steel grit should account for the majority of the new abrasive.

4.4 Rust removal speed
The rust removal speed of steel pipes depends on the type of abrasive and the displacement of the abrasive. Generally, abrasives with lower loss rates should be selected, which is conducive to improving the cleaning speed and prolonging the life of the blades.

4.5 Cleaning and preheating
Before spraying (blasting) treatment, the grease and scale on the surface of the steel pipe are removed by cleaning, and the pipe body is preheated to 40-60℃ in a heating furnace to keep the surface of the steel pipe dry. During spraying (blasting) 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 surface of the steel pipe after rust removal cleaner.

In production, the importance of surface treatment is emphasized, and the process parameters during rust removal are strictly controlled. In actual construction, the peeling strength value of the steel pipe anti-corrosion layer greatly exceeds the standard requirements, ensuring the quality of the anti-corrosion layer. Based on the same equipment, the process level is greatly improved and the production cost is reduced.