Correctly Treat the Surface of Steel Pipe to Prevent Corrosion
The surface treatment of steel pipes is one of the key factors that determine the anti-corrosion service life of pipelines. It is a prerequisite for whether the anti-corrosion layer and the steel pipe can be firmly combined. The research institute has verified 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 the steel pipe has an impact on the life of the anti-corrosion layer. Therefore, the anti-corrosion layer specifications should be strictly followed. The requirements of the surface of steel pipes are constantly explored and summarized, and the surface treatment methods of 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, but it cannot remove the rust, oxide scale, welding flux, etc. on the steel surface, so it is only used as an auxiliary means in the production of anti-corrosion.
2. Tools to remove rust
Mainly use tools such as wire brushes to polish the steel surface, which can remove loose or warped oxide scale, rust, welding slag, etc. The rust removal of hand tools can reach Sa2 level, and the rust removal of power tools can reach Sa3 level. If the steel surface adheres to a firm oxide scale, the tool rust removal effect is not ideal, and the anchor pattern depth required for anti-corrosion construction cannot be reached.
3. Pickling
Generally, two methods of chemical and electrolysis are used for pickling. Pipeline anticorrosion only uses chemical pickling, which can remove oxide scale, rust, and old coatings. Sometimes it can be used as reprocessing after sandblasting and rust removal. Although chemical cleaning can make the surface reach a certain degree of cleanliness and roughness, its anchor pattern is shallow and easily pollutes the environment.
4. Spray (throw) to remove rust
Spraying (throwing) rust removal is to drive the spray (throwing) blades to rotate at a high speed through a high-power motor, so that steel grit, steel shot, iron wire segments, minerals, and other abrasives are sprayed (thrown) 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 the violent impact and friction of the abrasive.
After spraying (throwing) to remove rust, it can not only expand the physical adsorption of the pipe surface but also enhance the mechanical adhesion of the anti-corrosion layer and the pipe surface. Therefore, spray (throwing) rust removal is an ideal way to remove rust from pipelines. Generally speaking, shot blasting (sand) rust removal is mainly used for pipe inner surface treatment and shot blasting (sand) rust removal is mainly used for pipe outer surface treatment. Several problems should be paid attention to when using spray (throwing) to remove rust.
Rust removal grade
For the construction process of epoxy, ethylene, phenolic, and other anticorrosive coatings commonly used in steel pipes, the surface of the steel pipe is generally required to reach the nearly white level (Sa2.5). The practice has proved that the use of this rust removal level can remove almost all oxide scale, rust, and other dirt, and the depth of the anchor pattern can reach 40-100μm, which fully meets the adhesion requirements of 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.
Spray (throw) abrasive
In order to achieve the ideal rust removal effect, the abrasive should be selected according to the hardness of the steel pipe surface, the degree of original corrosion, 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 ideal 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 40~50HRC, and the hardness of steel grit is 50~60HRC. It can be used on various steel surfaces, even if it is used on C-grade and D-grade rusted steel surfaces, except The rust effect is also very good.
Abrasive particle size and ratio
In order to obtain better uniform cleanliness and roughness distribution, the design of abrasive particle size and ratio is very important. Too much roughness can easily 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 corrosion, the high-strength impact of large abrasives cannot be used alone, but small particles must also be used to polish away the corrosion products to achieve the cleaning effect. At the same time, the reasonable ratio design can not only slow down the abrasive wear on the pipeline and nozzle (vane).
And the utilization rate of abrasives can be greatly improved. Generally, the grain size of steel shot is 0.8~1.3mm, and the grain size of steel grit is 0.4~1.0mm, of which 0.5~1.0mm is the main component. The sand shot ratio is generally 5-8.
It should be noted that in actual operation, the ideal ratio of steel grit to steel shot in the abrasive is difficult to achieve because the hard and fragile steel grit has a higher crushing rate than steel shot. For this reason, the mixed abrasives should be sampled and tested continuously in the operation. According to the particle size distribution, new abrasives should be added to the rust remover, and the amount of steel grit should be the main one.
Rust removal speed
The rust removal speed of the steel pipe depends on the type of abrasive and the displacement of the abrasive. Generally, an abrasive with a lower loss rate should be selected, which will help improve the cleaning speed and extend the life of the blade.
Cleaning and preheating
Before spraying (throwing) treatment, a cleaning method is used to remove grease and scale on the surface of the steel pipe, and a heating furnace is used to preheat the pipe body to 40-60℃ to keep the surface of the steel pipe dry. During the spray (projection) treatment, since the surface of the steel pipe does not contain grease and other dirt, the effect of rust removal can be enhanced. The dry steel pipe surface is also conducive to the separation of steel shot, steel grit, rust, and oxide scale so that The surface of the steel pipe 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 steel pipe anti-corrosion layer greatly exceeds the standard requirements, ensuring the quality of the anti-corrosion layer, based on the same equipment, Greatly improving the technological level and reducing production costs.