With the sustainable development of China’s economy and the country’s vigorous development of energy industry, long-distance oil and gas pipeline is an important way of energy security. In the process of oil (gas) pipeline anti-corrosion construction, the surface treatment of spiral steel pipe is one of the key factors to determine the anti-corrosion service life of the pipeline, which is the premise of whether the anti-corrosion layer can be firmly combined with the steel pipe. As verified by the research institution, the service life of the anti-corrosion layer is determined by the type of coating, the quality of coating and the construction environment, etc. Besides, the impact of surface treatment on the anti-corrosion layer is about 50%. Therefore, the surface treatment of the steel pipe should be explored and summarized in strict accordance with the requirements of the anti-corrosion layer, and the surface treatment methods of the steel pipe should be improved continuously.
Solvent and emulsion are used to clean the steel surface to remove oil, grease, dust, lubricant and similar organics. However, it cannot remove rust, oxide scale and solder on the steel surface, so it is only used as an auxiliary means in anti-corrosion production.
2. Cleaning tools
The surface of steel is polished by steel wire brush and other tools, which can remove loose or warped scale, rust and welding slag.Manual tool rust removal can reach Sa2 level, while power tool rust removal can reach Sa3 level. If steel surface is adhered to strong oxidized iron sheet, tool rust removal effect is not ideal, which cannot reach the anchor grain depth required for anti-corrosion construction.
Generally, chemical and electrolytic methods are used for pickling treatment. Chemical pickling is only used for pipeline corrosion protection. Oxide scale, rust and old coating can be removed. Sometimes it can be used as a retreatment after derusting.Although chemical cleaning can make the surface achieve a certain degree of cleanliness and roughness, it is easy to cause pollution to the environment.
4. Spraying (throwing) to eliminate rust
Spraying (throwing) derusting is to make the abrasive materials such as steel sand, steel balls, iron wire segments, minerals and so on spray (throwing) the surface of the steel pipe under the action of centrifugal force through high power motor driving the spraying (throwing) jet blade to rotate at high speed. Not only can thoroughly remove rust, oxide and dirt, but also the steel tube can achieve the required uniform roughness under the impact of abrasives and friction forces. After spraying (throwing) the rust removal, not only can expand the physical adsorption on the surface of the pipe, but also can enhance the mechanical adhesion between the corrosion protection layer and the surface of the pipe. Therefore, spraying (throwing) blasting derusting is an ideal method of pipeline corrosion removal. Generally speaking, sand blasting is mainly used for internal surface treatment, and shot blasting is mainly used for external surface treatment.Several problems should be paid attention to when using spray (throwing) to remove rust.
1.1 Descaling grade
For the construction process of epoxy, ethylene, phenolic and other anticorrosive coatings commonly used in steel pipe, the surface of steel pipe is generally required to reach nearly white grade (Sa2.5). t has been proved by practice that this derusting grade can remove almost all oxide, rust and other contaminants, and the anchor grain depth reaches 40~100 mm, which fully meets the requirements of adhesion of anti-corrosion layer and steel pipe. The spraying (throwing) derusting process can achieve the technical conditions of near white grade (Sa2.5) with lower operation cost and stable and reliable quality.
1.2 Abrasive materials
In order to achieve the ideal effect of rust removal, abrasive materials should be selected according to the surface hardness of spiral steel tube, original degree of rust corrosion, required surface roughness, coating type, etc. For single-layer epoxy, two-layer or three-layer polyethylene coating, it is easier to achieve ideal descaling effect by using steel sand and steel pellets. Steel pellets have the effect of strengthening the steel surface, while steel sand has the effect of etching the steel surface. Mixed abrasive of steel sand and steel pellets (generally, the hardness of steel pellets is 40 ~ 50 HRC, the hardness of steel sand is 50 ~ 60 HRC can be used on various steel surfaces, even if it is used on the surface of C grade and D grade rusting, the effect of derusting is very good.
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