Steel pipe surface treatment method

With the continuous development of my country's economy, the country vigorously develops the energy industry. Long-distance oil and gas pipelines are an important way of energy protection. 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 service life of pipeline anti-corrosion First, it is the premise of whether the anti-corrosion layer can be firmly combined with the steel pipe. It has been verified by research institutions that the life of the anti-corrosion layer depends on factors such as the type of coating, coating quality, and construction environment, and the surface treatment of steel pipes has an impact on the life of the anti-corrosion layer. The surface requirements of steel pipes are constantly explored and summarized, and the surface treatment methods of steel pipes are continuously improved.

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

2. Tool de-rusting
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 anticorrosion, which can remove scale, rust, and old coatings. Sometimes it can be used as retreatment 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) to remove rust
Spraying (throwing) rust removal is driven by a high-power motor to rotate the spraying (throwing) blades at high speed, so that steel sand, steel shots, iron 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 completely remove rust, oxides, and dirt but also the steel pipe can 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. Derusting 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 proved that almost all scale, rust, and other dirt can be removed by using this level of rust removal, and the depth of the anchor pattern can reach 40-00 μm, which fully meets the adhesion requirements between the anti-corrosion layer and the steel pipe. The rusting process can reach near-white (Sa2.5) technical conditions with low operating costs and stable and reliable quality.

4.2 Spraying (throwing) abrasives
To achieve the ideal de-rusting effect, the abrasive should be selected according to the hardness of the steel pipe surface, the degree of original rust, the required surface roughness, the type of coating, etc. For single-layer epoxy, two-layer, or three-layer polyethylene coating, using The mixed abrasive of steel grit and steel shot is 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. The mixed abrasive of steel grit and steel shot (usually the hardness of steel shot is 40-50 HRC, the hardness of steel grit is 50-60 HRC can be used on various steel surfaces, even on C-level and D-level corroded steel surfaces, the rust removal 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. If the roughness is too large, the anti-corrosion layer will become thinner at the peak of the anchor pattern; at the same time, because the anchor pattern is too deep, the anti-corrosion layer will easily form bubbles during the anti-corrosion process, which will seriously affect the performance of the anti-corrosion layer.
 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 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 de-rusting machine according to the particle size distribution, and the amount of steel grit should be the main one in the new abrasives added.

4.4 Derusting speed
The de-rusting 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 by the abrasive to the steel pipe per unit of time and the kinetic energy E of a single abrasive.
In the formula: m—the spraying (throwing) amount of abrasive; V—the running speed of abrasive; m—the quality of a single abrasive.
m. The size of the abrasive is related to the breakage rate of the abrasive, which directly affects the cost of surface treatment operations and the cost of de-rusting equipment. When the equipment is fixed, m is a constant and y is a constant, so E is also a constant. However, due to the breaking of the abrasive, m changes. Therefore, generally, the abrasive with a lower loss rate should be selected, which is conducive to improving the cleaning speed and long life.

4.5 Cleaning and preheating
Before the spraying (throwing) 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°C in a heating furnace to keep the surface of the steel pipe dry. During spraying (throwing) treatment, since the surface of the steel pipe does not contain dirt such as grease, the effect of rust removal can be enhanced. The steel pipe surface is cleaner.