How to treat the surface of high-frequency welded steel pipes

With the continuous development of our country's economy, the country is vigorously developing the energy industry. Long-distance API oil and gas pipelines are an important way to ensure energy security. During the anti-corrosion construction process of oil (gas) pipelines, the surface treatment of high-frequency welded steel pipes is the key to determining the anti-corrosion service life of the pipelines. The key factor is the premise of whether the anti-corrosion layer can be firmly combined with the high-frequency welded steel pipe. 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 thick-walled steel pipes are continuously explored and summarized, and the surface treatment methods of thick-walled steel pipes are continuously improved.

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.

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

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 pattern is shallow and it is easy to pollute the environment.

4. Spray (throw) rust removal
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) to remove rust, it can not only expand the physical adsorption on the surface of the pipe but also enhance the mechanical adhesion between the anti-corrosion layer and the surface of the pipe. Therefore, spraying (throwing) shot de-rusting is an ideal de-rusting method for pipeline corrosion. Generally speaking, shot blasting (sand) de-rusting is mainly used for pipe inner surface treatment, and shot blasting (sand) de-rusting is mainly used for pipe outer surface treatment. Several issues should be paid attention to when spraying (throwing) to remove rust.

4.1 Rust removal level
For the construction process of epoxy, vinyl, phenolic, and other anti-corrosion coatings commonly used for steel pipes, the surface of steel pipes is generally required to reach the near-white level (Sa2.5). The 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. The rusting process can achieve near-white (Sa2.5) technical conditions with low operating costs and stable and reliable quality.

4.2 Spray (throw) 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 grit and steel shot makes it easier to achieve the desired rust removal effect. Steel shot has the effect of strengthening the steel surface, while steel grit has the effect of etching the steel surface. Mixed abrasives of steel grit and steel shot (usually the hardness of steel shot is 40~50HRC, the hardness of steel grit is 50~60HRC) can be used on various steel surfaces, even on C-grade and D-grade rusted steel surfaces, except The rust effect is also very good.

4.3 Abrasive particle size and ratio
To obtain a better uniform cleanliness and roughness distribution, the particle size and proportion design of abrasives 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, a reasonable ratio design can not only slow down the wear of the abrasives on the pipes and nozzles (blade) but the utilization rate of abrasives can be greatly improved. Usually, the particle size of steel shot is 0.8-1.3 mm, and the particle size of steel grit 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. Moreover, among the new abrasives added, steel grit should account for the majority.

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 will help increase 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.
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. Based on the same equipment, greatly improves the level of technology and reduces production costs.