How is seamless carbon steel pipe sprayed and derusted?

How is seamless carbon steel pipe sprayed and derusted?

For seamless steel pipes, the surface treatment of steel pipes is one of the key factors that determine the anticorrosion service life of pipelines, and it is the premise of whether the anticorrosion layer can be firmly combined with steel pipes. 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. 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. the

2. Tool derusting

Mainly use tools such as wire brushes to polish the steel surface, which can remove loose or raised scale, rust, welding slag, etc. Hand tool derusting can reach Sa2 level, and power tool derusting can reach Sa3 level. If the steel surface is attached with a firm oxide scale, the derusting 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. the

4. Spray (throw) shot 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 rust removal by spraying (throwing), 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 derusting is an ideal derusting method for pipeline corrosion. Generally speaking, shot blasting (sand) derusting is mainly used for the inner surface treatment of the pipe, and shot blasting (sand) derusting is mainly used for the outer surface of the pipe. Several issues should be paid attention to.

4.1 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 near-white level (Sa2.5). 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 40~100µm, which fully meets the adhesion requirements between the anti-corrosion layer and the steel pipe. The rust process can achieve near-white (Sa2.5) technical conditions with low operating costs and stable and reliable quality.
4.2 Spraying (throwing) abrasives
In order to achieve the ideal derusting 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, use 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
In order 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, it is easy to 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, 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 and impact strength of the anti-corrosion layer will decrease. For severe internal pitting, it is not only possible to rely on the high-strength impact of large-grain abrasives, but also to grind away the corrosion products with small particles to achieve the cleaning effect. At the same time, a reasonable proportion design can not only slow down the abrasion of abrasives on pipes and nozzles (blades). , and the utilization rate of abrasive 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 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 derusting 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 Rust removal speed

The derusting 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 time and the kinetic energy E1 of a single abrasive particle. the
In the formula: m——abrasive injection (throwing) amount; V——abrasive running speed; m1——mass of single abrasive.
The size of M is related to the breakage rate of the abrasive, which directly affects the cost of surface treatment operations and the cost of derusting equipment. When the equipment is fixed, m is a constant, y is a constant, so E is also a constant, but due to the abrasive breaking, m1 changes, so generally, the abrasive with a lower loss rate should be selected, which is conducive to improving the cleaning speed and the long-term leaf 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 the spraying (throwing) treatment, since the surface of the steel pipe does not contain grease and other dirt, the effect of rust removal can be enhanced. The surface of the steel pipe is cleaner.

5.Conclusion 

Pay attention to the importance of surface treatment in production, and strictly control the process parameters during derusting. In actual construction, the peel strength value of the steel pipe anti-corrosion layer greatly exceeds the standard requirements, ensuring the quality of the anti-corrosion layer. On the basis of the same equipment , greatly improving the level of technology and reducing production costs.