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"Pretreatment of the inner surface of steel pipes" in the construction of pipeline anti-corrosion layer

Views: 7     Author: Site Editor     Publish Time: 2024-10-31      Origin: Site

1. Importance of surface pretreatment: The purpose of surface pretreatment is to make the surface to be coated reach the rust removal quality and roughness required by the selected coating, and ensure good adhesion between the surface to be coated and the covering layer. The method and index of surface pretreatment are determined by the type of covering layer.


2. Main factors affecting the quality of the covering layer

2.1 Influence of oxide scale: Under the high-temperature conditions of rolling and welding, a layer of oxide scale is naturally generated on the surface of steel pipes. Its main component is a mixture of iron oxides. From a structural point of view, it is roughly three layers, the outermost layer is Fe3O4 or Fe2O3, the middle layer is FcO and Fe3O4, and the one close to the steel surface is FeO.

2.2 Influence of surface dirt: The dirt mentioned here refers to the rust products and dust that have not been removed from the surface of the steel pipe, including the residual particles that have not been cleaned on the surface of the steel pipe after surface treatment and the new rust caused by failure to apply the coating within the specified time after surface treatment.

2.3 Influence of soluble salts: When there are soluble salts on the steel surface under the coating, due to the different osmotic pressures inside and outside the coating, the moisture in the air will penetrate the coating to reach the surface of the steel, and combine with the soluble salts to cause corrosion on the steel surface and peel off the coating.

2.4 Influence of roughness: The adhesion between the coating and the steel pipe surface is determined by the mutual attraction between the polar groups in the coating molecules and the metal surface molecules. In addition to physical effects (dispersion force, induction force, and orientation force), it is mainly mechanical.


3. Basic requirements for plug surface treatment

The surface treatment of steel pipes usually follows the requirements of technical standards. Industrially developed countries have successively formulated their quality grade standards for rust removal. The most famous of these is the Swedish industrial standard SIS 055900 "Standard for Surface Rust Removal of Steel Materials Before Painting", which has long been adopted by countries around the world. The International Organization for Standardization has formulated ISO 8501-1 "Pretreatment of Steel Materials Before Painting with Coatings and Related Products - Visual Assessment of Surface Cleanliness - Part 1: Rust Grade and Rust Removal Grade of Unpainted Steel Materials and Steel Materials After Complete Removal of the Original Coating" by the Swedish standard. My country has also formulated GB 8923 "Rust Grade and Rust Removal Grade of Steel Materials Before Painting" concerning ISO standards. The petroleum industry has also formulated SY/T 0407 "Specifications for Surface Pretreatment of Steel Materials Before Painting" to be used in conjunction with GB 8923. Here are some excerpts from the key points in the standard.


4. Pretreatment of the inner surface of steel pipes

4.1 Shot blasting

4.2 Sandblasting (shot)

4.3 Cleaning operation


5. Quality control

There are two main aspects of quality control for the inner surface treatment of steel pipes, namely cleanliness and roughness.

5.1 Cleanliness: According to the requirements of ISO 8501-1 and GB 8923 standards, the inner surface of the steel pipe with drag reduction coating should reach Sa2.5 level after treatment. This level is defined as there should be no visible grease, dirt, scale, rust, or paint on the steel surface, and any remaining traces should only be slight spots or strips. This cleanliness requirement can be visually inspected. In addition, the ISO 8502 standard also provides a method for detecting surface cleanliness.

5.2 Roughness: GB 13288 standard compiled concerning ISO standard makes corresponding provisions for the roughness evaluation after surface treatment. The steps are: remove the dust and debris on the surface, select the appropriate roughness comparison sample ("G" sample and "S" sample) according to the abrasive, and place it close to a certain measuring point on the surface of the steel to be tested for visual comparison. The roughness grade indicated by the sample that is closest to the appearance of the steel surface is the evaluation grade. If a magnifying glass is used for evaluation, the appearance of the sample and the surface of the steel to be tested should be observed in the magnifying glass at the same time. If the visual evaluation is difficult, you can use the thumbnail or the thumb and index finger to hold the wooden stylus and move it on various parts of the surface to be tested and the comparison sample. The roughness grade indicated by the closest touch is the evaluation result.


Hunan Great Steel Pipe Co.,Ltd
Hunan Great Steel Pipe Co.,Ltd is a world-class production and service provider of submerged arc straight seam welded pipe as the first subsidiary of Shinestar Group. Hunan Great Steel Pipe Co.,Ltd pays more attention to in the pipeline engineering research areas as a pioneer of China Petroleum Pipeline & Gas Pipeline Science Research Institute.

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