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Derusting of Steel Pipes

Posted: 12/26/2022 19:55:28 Hits: 7

Derusting levels
The surface of steel pipes is generally required to reach the near-white level (Sa2.5) for epoxy, vinyl, phenolic and other anti-corrosion coatings commonly used for steel pipes. 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 to 100um, which fully meets the requirement for adhesion between the anti-corrosion layer and the steel pipe. Shot blasting can achieve technical conditions of near-white (Sa2.5) with low operating costs and stable and reliable quality.

To achieve the ideal rust removal effect, abrasives should be selected according to the hardness of the steel pipe surface, the degree of original rust, the required surface roughness, and the type of coating. For single-layer epoxy, two-layer or three-layer polyethylene coatings, it is easier to achieve the ideal rust removal effect by using the mixed abrasive of steel grits and steel shots. Steel shot has the effect of strengthening the steel surface, while steel grit has the effect of etching the steel surface. A mixed abrasive of steel grits and steel shots can be used on various steel surfaces, even on C-level and D-level corroded steel surfaces. Usually, the hardness of steel shots is 40 to 50 HRC, and that of steel grits is 50 to 60 HRC. The rust removal effect is also very good.

The ratio of the particle size and abrasives
To obtain better uniform cleanliness and distribution of roughness, the ratio of the particle size and abrasive is very important. If the roughness is too great, it will easily cause the anti-corrosion layer to become thinner at the peak of the anchor pattern; at the same time, the anti-corrosion layer is easy to form bubbles in the anti-corrosion process due to too deep anchor patterns, 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, not only the high-strength impact of large-grained abrasives should be relied on, but also the corrosion products with small particles should be polished to achieve the cleaning effect. At the same time, a reasonable ratio design can not only slow down the abrasion of abrasives on pipes and nozzles, but also greatly improve the utilization rate of abrasives. Usually, the particle diameter of steel shot is 0.8 to 1.3 mm, and the particle diameter of steel grit is 0.4 to 1.0 mm. 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 distribution of the particle size. The amount of steel grit should be the main one in the new abrasives added.

Rust removal speeds
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 and the kinetic energy E1 of a single abrasive applied by the abrasive to the steel pipe per unit of time. In the formula: m is the abrasive amount.

V is the abrasive running speed. mi is the mass of the single-grain abrasive. The size of the abrasive is related to the breakage rate of the abrasive, which directly affects the cost of surface treatment operations and derusting equipment. When the equipment is fixed, both m and y are constant. E is also constant. Because the abrasive is broken, m1 changes. Generally, the abrasive with a lower loss rate should be selected, which is conducive to improving the cleaning speed and service life of blades.

Cleaning and preheating before shot blasting treatment, use the method of cleaning to remove the grease and scale on the surface of the steel pipe. The heating furnace preheats the pipe body to 40 to 60°C to keep the surface of the steel pipe dry. When shot blasting, the surface does not contain grease and other dirt due to steel pipe, which can enhance the effect of rust removal, and the dry steel pipe surface is also conducive to the separation of steel shot, rust and oxidation, making the surface of the steel pipe after derusting cleaner.

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