The most effective and practical steel pipe surface treatment method

With the continuous development of China's economy, the country vigorously develops the energy industry, and long-distance oil and gas pipelines are an important means of energy security. During the construction of anti-corrosion pipelines for oil (gas) pipelines, surface treatment of steel pipes is a key factor in determining the service life of pipelines. First, it is the premise that the anticorrosion layer and the steel pipe can be firmly combined. After verification by the research institute, the life of the anti-corrosion coating depends on the coating type, coating quality and construction environment. The effect of surface treatment of the steel pipe on the life of the anti-corrosion coating accounts for approximately 50%. Therefore, the anticorrosion layer should be strictly adhered to. The requirements of the surface of the steel pipe are continuously explored and summarized, and the steel pipe surface treatment method is continuously improved.

1. Cleaning Uses solvents and emulsions to clean the surface of steel to remove oil, grease, dust, lubricants and similar organics. However, it cannot remove rust, oxide scales, and welding flux on the surface of steel. Therefore, it is only used as an antiseptic product. Aids.

2. Tool rust removal The main use of wire brushes and other tools to sand the steel surface, can remove loose or tilt of the scale, rust, welding slag and so on. The manual tool derusting can reach Sa2 level, and the derusting of power tools can reach Sa3 level. If the steel plate surface is attached with a solid iron oxide scale, the rust removal effect of the tool is not satisfactory, and the depth of the anchoring lines required for anti-corrosion construction cannot be achieved.

3, pickling generally use chemical and electrolytic methods to do pickling treatment, pipeline anti-corrosion chemical pickling only, can remove oxide scale, rust, old coating, and sometimes can be used as a blasting rust after reprocessing. Although chemical cleaning can make the surface reach a certain degree of cleanliness and roughness, its anchorage is light and it is easy to cause pollution to the environment.

4, spray (throw) shot rust spray (throw) shot rust is driven by high-power motor spray (throw) shot blade rotation at high speed, so that steel sand, steel shot, wire segment, minerals and other abrasives under the action of centrifugal force The sprayed (throw) shot treatment on the surface of the steel pipe can not only completely remove the rust, oxides and dirt, but also can achieve the required uniform roughness under the action of the abrasive impact and friction force of the abrasive.

After spraying (throwing) shot rust removal, not only can the physical adsorption of the pipe surface be enlarged, but also the mechanical adhesion of the anti-corrosion layer to the pipe surface can be enhanced. Therefore, spraying (throwing) shot rust removal is an ideal rust removal method for pipeline anti-corrosion. In general, shot peening (sand) rust removal is mainly used for inner surface treatment of tubes. Shot peening (sand) rust removal is mainly used for outer surface treatment of pipes. There are several issues that need to be taken into account when using spray (throw) shot rust removal.

4.1 Rust-removing grades For the construction process of anti-corrosion coatings such as epoxy, vinyl, and phenolic, which are commonly used for steel pipes, the surface of the steel pipe is generally required to reach a level of nearly white (Sa2.5). Practice has proved that this kind of derusting grade can remove almost all scales, rust, and other dirt. The depth of anchoring lines can reach 40-100 μm, fully satisfying the adhesion requirements of the anticorrosion layer and the steel pipe, and spraying (throwing) the injection. The rust process can achieve near-white (Sa2.5) technical conditions with lower operating costs and stable and reliable quality.

4.2 Spray (throw) shot abrasive In order to achieve the desired rust removal effect, the abrasive should be selected based on the hardness of the surface of the steel pipe, the original degree of rust, the required surface roughness, the type of coating, etc., for a single layer of epoxy, two or three layers. Polyethylene coating, the use of mixed abrasive steel grit and steel pellets more easily achieve the desired effect of rust. Steel shots have the effect of strengthening the steel surface, while steel sand has the effect of etching the steel surface. Mixed abrasives of steel grit and steel shots (usually steel shots with a hardness of 40-50 HRC, steel grit with a hardness of 50-60 HRC can be used on various steel surfaces, even if used on grades C and D rusted steel surfaces, except The rust effect is also very good.

4.3 Abrasive particle size and ratio In order to obtain a better uniform cleanliness and roughness distribution, the abrasive particle size and proportioning design is very important. If the roughness is too large, the anti-corrosion coating may become thinner at the peak of the anchorage pattern. At the same time, due to the deeper anchorage pattern, the anti-corrosion coating may easily form bubbles during the anti-corrosion process, which will seriously affect the performance of the anti-corrosion coating.

If the roughness is too small, the adhesion of the coating and the impact strength will be reduced. For severe internal pitting, high-strength impact cannot only be achieved by large-grained abrasives, but also by small particles to remove corrosion products to achieve the cleaning effect. At the same time, a reasonable proportioning design can not only reduce abrasive wear on pipes and nozzles (blades). , And the utilization of abrasives can also be greatly improved. In general, the particle diameter of the steel shot is 0.8 to 1.3 mm, and the particle size of the steel grit is 0.4 to 1.0 mm, of which 0.5 to 1.0 mm is a main component. Sand pills are generally 5-8.

It should be noted that in practice, the ideal ratio of steel grit and steel shots in abrasives is difficult to achieve because hard and brittle grit has a higher breakage rate than steel shots. For this reason, the mixed abrasive should be continuously sampled during operation. According to the particle size distribution, new abrasives should be added to the derusting machine, and the amount of steel sand should be the major one among the new abrasives.

4.4 Rust removal rate The derusting speed of the steel pipe depends on the type of abrasive and the displacement of the abrasive, ie the total kinetic energy E applied by the abrasive to the steel pipe per unit of time and the kinetic energy E1 of the single-particle abrasive.

In the formula: m——spray (throw) quantity of abrasive;

V——abrasive running speed;

M1—mass of single particle abrasive.

The size of m is related to the abrasive crushing rate. The crushing rate directly affects the cost of surface treatment operations and the cost of rust removal equipment. When the equipment is fixed, m is a constant, y is a constant, so E is also a constant, but due to abrasive crushing, m1 changes, therefore, generally should choose the abrasive with a lower loss rate, which will help improve the cleaning speed and long The life of the blade.

4.5 Cleaning and preheating Before spraying (throwing) shot processing, cleaning method is used to remove the grease and scale on the surface of the steel pipe. The furnace is preheated to 40-60°C with a heating furnace to keep the surface of the steel pipe dry. In spray (throw) shot processing, since the surface of the steel tube does not contain grease and other dirt, the effect of rust removal can be enhanced, and the dried steel tube surface is also conducive to the separation of steel shot, steel sand and rust and scale, so that after the rust removal The steel surface is cleaner.