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How should the surface of high frequency welded steel pipe be treated?

Date:2019-07-17    keywords:welded steel pipe,surface

With the continuous development of China's economy, the country vigorously develops the energy industry. Pipeline long-distance oil and gas pipelines are an important way of energy security. In the process of anti-corrosion construction of oil (gas) pipelines, the surface treatment of steel pipes is the key factor determining the service life of pipelines. One of them is the premise that the anti-corrosion layer and the steel pipe can be firmly combined. According to the research institute's verification, the life of the anti-corrosion layer depends on the coating type, coating quality and construction environment. The surface treatment of the steel pipe affects the life of the anti-corrosion layer by about 50%. Therefore, it should be strictly in accordance with the anti-corrosion layer specification. The requirements of the surface of the steel pipe are continuously explored and summarized, and the surface treatment method of the steel pipe is continuously improved.

1. Cleaning

  Use solvent and emulsion to clean the surface of steel to remove oil, grease, dust, lubricant and similar organic matter, but it can not remove rust, scale, flux, etc. On the surface of steel pipe, so it is only used as an auxiliary means in anti-corrosion production of steel pipe. .

2. Tool rust removal
  The surface of the steel pipe is mainly polished by using a wire brush or the like to remove loosened or lifted scale, rust, welding slag, and the like. The rust removal of the hand tool can reach the Sa2 level, and the rust removal of the power tool can reach the Sa3 level. If the surface of the steel material is adhered to the iron oxide scale, the rust removal effect of the tool is not ideal, and the anchor depth required for the anti-corrosion construction cannot be achieved.

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

4. Spray (throw) shot derusting
    Spraying (polishing) rust removal is a high-speed motor that drives the spray (bounce) to rotate at high speed, so that the abrasive grains such as steel grit, steel shot, wire segment and mineral are sprayed (throwing) on the surface of the steel tube under the action of centrifugal force. Not only can rust, oxides and dirt be completely removed, but the steel tube can also achieve the required uniform roughness under the action of abrasive impact and friction.

After spraying (polishing) and removing rust, not only can the physical adsorption of the surface of the tube be enlarged, but also the mechanical adhesion of the coating layer to the surface of the tube can be enhanced. Therefore, spray (polishing) rust removal is an ideal rust removal method for steel pipe pipeline corrosion protection. In general, shot blasting (sand) derusting is mainly used for the inner surface treatment of steel pipes, and shot blasting (sand) derusting is mainly used for the outer surface treatment of steel pipes. There are several problems to be aware of when using blasting to remove rust.

a. Derusting grade For the construction process of epoxy, vinyl, phenolic and other anticorrosive coatings commonly used in steel pipes, it is generally required that the surface of the steel pipe reaches near-white level (Sa2.5). Practice has proved that the use of this descaling level can almost remove all scale, rust and other dirt, anchor depth of 40 ~ 100μm. It meets the adhesion requirements of the anti-corrosion layer and the steel pipe, and the spray (polishing) derusting process can achieve the near-white (Sa2.5) technical condition with lower operating cost and stable and reliable quality.

b. Spraying (polishing) abrasives In order to achieve the desired descaling effect, the abrasive should be selected according to the hardness of the surface of the steel pipe, the degree of original corrosion, the required surface roughness, the type of coating, etc., for single layer epoxy, second layer or Three-layer polyethylene coating, using a mixture of steel sand and steel shots to achieve the desired rust removal effect. Steel shots 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 shot (usually the hardness of steel shot is 40 ~ 50HRC, the hardness of steel sand is 50 ~ 60HRC can be used on various steel surfaces, even on the surface of steel grade C and D rust, except The rust effect is also very good.

c. Abrasive particle size and ratio To obtain better uniform cleanliness and roughness distribution, the particle size and proportion design of the abrasive is very important. The roughness is too large, which may cause the anti-corrosion layer to become thinner at the anchor peak; at the same time, because the anchor pattern is too deep, the anti-corrosion layer is easy to form bubbles during the anti-corrosion process, which seriously affects the performance of the anti-corrosion layer.

If the roughness is too small, the adhesion of the coating and the impact strength will be lowered. For severe internal pitting, it is not only possible to rely on large-grain abrasives for high-strength impact, but also to remove the corrosion products by small particles to achieve the cleaning effect, while the reasonable proportioning design can not only slow the wear of the pipes and nozzles (blades). And the utilization rate of abrasives can be greatly improved. Usually, the steel pellet has a particle diameter of 0.8 to 1.3 mm, and the steel grit has a particle diameter of 0.4 to 1.0 mm, of which 0.5 to 1.0 mm is a main component. The sand pill ratio is generally 5-8.

It should be noted that in actual operation, the ideal ratio of steel sand and steel shot in the abrasive is difficult to achieve because the hard and brittle steel sand has a higher breaking rate than the steel shot. For this reason, in the operation, the mixed abrasive should be continuously sampled and tested. According to the particle size distribution, a new abrasive is added to the descaling machine, and the amount of the steel sand is dominant in the new abrasive.

d. Derusting speed The descaling speed of the steel pipe depends on the type of abrasive and the displacement of the abrasive. Generally, the abrasive with lower loss rate should be selected, which is beneficial to improve the cleaning speed and the life of the long blade.

e. Cleaning and preheating of the seamless pipe network in China: Before the spraying (polishing) treatment, the grease and scale on the surface of the steel pipe are removed by washing, and the pipe body is preheated to 40-60 ° C by using a heating furnace. Keep the surface of the steel pipe dry. In the spraying (polishing) treatment, since the surface of the steel pipe does not contain dirt such as grease, the effect of rust removal can be enhanced, and the surface of the dried steel pipe is also advantageous for the separation of the steel shot, the steel sand, the rust and the scale, and the rust removal. The surface of the steel pipe is cleaner.

In the production, the importance of surface treatment is emphasized, and the process parameters during rust removal are strictly controlled. In actual construction, the peel strength 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, greatly improve the process level and reduce production costs.

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