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

1. The 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. The implementation department of surface pretreatment must have relevant equipment and technical operators, and all surface pretreatments should have special technical supervision and inspection.

To correctly understand surface pretreatment, we should first have a complete understanding of the factors affecting its process. The thick vertical arrow connects the spraying object with the spraying purpose. The arrows connected to the left and right of the thick arrow indicate the factors that act to achieve the expected purpose. The spraying method, abrasive, and carrier for conveying abrasive are selected according to the characteristics, type, and size of the workpiece to be sprayed, as well as the expected purpose after spraying. Because there are many influencing factors involved, it should be very cautious.

In the construction of pipeline anti-corrosion layer, there is a saying that "30% material and 70% construction", which shows the importance of construction. In the construction process, the surface pretreatment of steel pipes (the most basic is "rust removal") is the top priority, and its quality is directly related to the quality and life of the covering layer. There are such statistics in some literature, indicating that surface treatment is the most important factor among many factors affecting the life of the covering layer.

Through the cost analysis of the covering layer, the cost of surface treatment generally accounts for about 50%. The covering layer of the drag reduction inner coating has a thin film layer, a small number of coatings, and a small amount of paint, so the cost of surface treatment is higher, about 70%. Therefore, in the process design and construction of the drag reduction inner coating, special attention should be paid to the quality of surface pretreatment.

2. Main factors affecting the quality of the covering layer

2.1 The 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 the steel pipe. Its main component is a mixture of iron oxides. From the 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. Under the influence of external environmental conditions, such as temperature, humidity, external force, oxygen, and salt, these oxide scales will crack, peel off and loosen. If they are not removed completely, they will have three major destructive effects on the covering layer: first, the electrode potential of the oxide scale is 0.26V more positive than that of the steel, so that the steel surface exposed at the oxide scale peeling and cracks becomes the anode of the galvanic cell and suffers corrosion; second, the cracks in the oxide scale are prone to condensation of water vapor. If SO2 is dissolved in them, ferrous sulfate can be generated, increasing the conductivity of the electrolyte and promoting corrosion; second, the oxide scale that has not been removed but has loosened may completely fall off and bulge when the temperature of the pipeline fluctuates greatly, causing the covering layer to crack and peel off.

2.2 The influence of surface dirt: The dirt mentioned here refers to rust products and dust that have not been completely removed from the surface of the steel pipe. It should also include the residual particles that have not been cleaned up on the surface of the steel pipe after surface treatment and the new rust that has not been coated within the specified time after surface treatment. Due to their existence, it is difficult to obtain a smooth and uniform coating, weakening its adhesion to the substrate, so that the coating cannot directly contact the steel surface, resulting in reduced adhesion of the coating and affecting the service life of the coating.

2.3 The 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. Among them, chloride is the most important soluble salt. Because of its strongest penetration ability, it is stipulated in the Q/SYXQ11 "Supplementary Technical Conditions for Drag Reduction Coating Layer on the Inner Wall of West-East Gas Pipeline" standard, especially for steel pipes shipped by sea and steel pipes stored in coastal areas for some time. This point should be emphasized.

2.4 Effect 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. After the steel pipe surface is treated with abrasive spraying (blasting), the surface roughness increases significantly, and the metal surface area can even increase by 20 times. With the increase of roughness, the surface area increases significantly, and the adhesion between the coating and the steel pipe surface increases accordingly. When the abrasive sprayed (blasted) has edges and corners, the metal surface treated with it not only increases the surface area but also provides a suitable surface geometry for the adhesion of the coating, which is conducive to molecular attraction and mechanical anchoring.

However, unreasonable surface roughness will also hurt the coating. For example, if the roughness is too large, the amount of coating required to fill the "trough" of the anchor pattern will also increase. Too deep troughs are also prone to cause bubbles, which directly affect the quality of the coating. In addition, when the coating is thin, the tip of the crest is easy to expose the surface, destroying the integrity of the coating and causing pitting corrosion.

For the drag-reducing inner coating, the surface roughness of the inner wall of the steel pipe should be required, usually 30-50μm after surface treatment. The surface roughness depends on the process parameters such as the particle size, shape, material, spraying speed, and action time of the abrasive, among which the particle size of the abrasive has the greatest impact on the roughness.

There are many surface treatment methods. The most reasonable one for pipelines is the commonly used spray (shot) method. This is because the violent impact of the abrasive can increase the fatigue strength of the material by about 80%; the surface hardness is also improved to varying degrees; it can also eliminate the internal stress at the weld so that the corrosion resistance of the steel is significantly improved.

3. Basic requirements for surface treatment of steel pipes

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 rust removal of steel surfaces 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 before coating and related products - Visual assessment of surface cleanliness - Part 1: Rust grade and rust removal grade of uncoated steel and steel 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 surfaces before painting" concerning ISO standards. The petroleum industry has also formulated SY/T 0407 "Specifications for surface pretreatment of steel before painting" to be used in conjunction with GB 8923. Here are some excerpts from the key points in the standard.

3.1 GB 8923 "Rust Grade and Rust Removal Grade of Steel Surfaces Before Painting"

GB 8923 "Rust Grade and Rust Removal Grade of Steel Surfaces Before Painting" focuses on the classification of rust grades and rust removal grades, visual assessment, and the use of color photos of standard samples.

(1) Rust grade Before rust removal, the original rust state of the steel surface is divided into four grades, represented by A, B, C, and D. After rust removal, it should be compared with the original rust grade:

A. Steel surface covered with oxide scale and almost no rust; B. Steel surface that has rusted and part of the oxide scale has peeled off; C. Steel surface where the oxide scale has peeled off due to rust, or can be scraped off, and there is a small amount of pitting; D. Steel surface where the oxide scale has been completely peeled off due to rust and pitting has occurred widely.

(2) Rust removal level GB 8923 "Rust level and rust removal level of steel surfaces before painting" distinguishes rust removal levels according to different rust removal methods, and then gives different levels according to different methods. "Sa", "St" and "Fl" represent spray (blast) rust removal, manual and power tool rust removal, and flame rust removal respectively. The Arabic numerals after the letters indicate the degree of rust removal.

① Spray or blast rust removal is represented by "Sa" and is divided into four levels, which are described as follows.

Sa1: Mild spray or blast rust removal: There should be no visible grease and dirt on the steel surface, and no loosely attached oxide scale, rust, and coating.

Sa2: Thorough spray or blast rust removal: There should be no visible grease and dirt on the steel surface, and the oxide scale, rust, and coating have been basically removed, and the residue should be firmly attached.

Sa2.5: Very thorough blasting and blasting rust removal: There should be no visible grease, dirt, oxide scale, rust, or coating on the steel surface, and any remaining traces should be only slight spots or stripes.

Sa3: Blasting or blasting rust removal to make the steel surface clean: There should be no visible grease, dirt, oxide scale, rust, or coating on the steel surface, and the surface should show a uniform metallic color.

② Manual and power tool rust removal Indicated by "St", GB 8923 gives two levels, namely:

St2: Thorough manual and power tool rust removal: There should be no visible grease and dirt on the steel surface, and no loose oxide scale, rust, and coating.

St3: Thorough manual and power tool rust removal: There should be no visible grease and dirt on the steel surface, and no loose oxide scale, rust, and coating. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster.

③ Flame rust removal Indicated by "F1", flame rust removal should include the use of a powered wire brush to remove the products attached to the steel surface after the flame heating operation. The standard only gives one grade:

F1 Flame rust removal: The steel surface should be free of oxide scale, rust, coating, and other attachments, and any remaining traces should only be surface discoloration (shadows of different colors).

(3) Evaluation of rust grade and rust removal grade The evaluation method and requirements of visual evaluation and standard photos are given in GB 8923. When evaluating the rust grade, the rust grade indicated in the photo of the corresponding more serious rust grade is used as the evaluation result; when evaluating the rust removal grade, the rust removal grade indicated in the photo that is closest to the steel surface appearance is used as the evaluation result. Many factors affect the visual assessment of the rust removal grade of steel surfaces, including the following:

① Abrasives used for rust removal by spraying or blasting, and tools used for rust removal by hand and power tools;

② Rust conditions on the steel surface that do not belong to the standard rust grade;

③ The color of the steel itself;

④ Differences in the roughness of various parts due to different degrees of corrosion;

⑤ Surface unevenness, such as depressions;

⑥ Tool scratches;

⑦ Uneven lighting;

⑧ Shadows caused by different angles of abrasive impacting the surface during rust removal by spraying or blasting;

⑨ Abrasives embedded in the surface.

3.2 SY/T 0407 "Specification for Surface Treatment of Steel Materials Before Painting": This specification requires use in conjunction with GB 8923, and most of its content is written about the American Steel Structure Coating Committee SSPC standard. Combined with the relevant content in the pipeline drag reduction process requirements, a brief introduction is given as follows:

(1) Surface treatment before and after spraying (blasting) rust removal: Before spraying (blasting) rust removal, remove visible oil, grease, and scale on the steel surface. After rust removal and before painting, the surface rust and dust on the workpiece should be removed by blowing dry, sleeveless air, vacuuming, or brushing. The steel surface after blasting (blasting) rust removal should be painted before it is contaminated. If the steel surface is contaminated before painting, it should be cleaned again.

(2) Selection of abrasives: According to the results of the spray test, zircon sand and wire grains are the best abrasives, corundum is the worst, and cast iron particles and two types of fused corundum are in between. The surface rust removal effect of corundum is very slow and poor, and it produces very intense dust flying. Wire grains are particularly suitable for rust removal of delicate cross-sections. Sand also has a good rust removal effect, but both produce dust. For fused corundum, the abrasive delivery volume is almost only half of that of zircon sand, cast iron particles, and wire grains. For the same rust removal work, the amount of iron abrasive required is 2 to 3 times less in volume than that of mineral materials, which means that heavy particles have a better rust removal effect than light particles. The spraying time required for a certain rust removal effect is related to the selected abrasive. The rust removal effect per unit time decreases in the following order: sand, zircon sand, cast iron crushed particles, 0.65 wire particles, 0.97 wire particles, 0.72 electric fused corundum, 0.75 electric fused corundum, and corundum. In actual operation, 0.65 wire particles are faster than 0.97 wire particles in rust removal.

Abrasives should be selected according to the steel grade, type, original rust degree, type of coating used, rust removal method, and surface roughness required for coating. Metal abrasives such as cast steel shot, cast iron shot, cast steel sand, cast iron sand, and steel wire segments can be used for rust removal by spraying (blasting).

It can be based on the requirements of the coating system for the anchor pattern depth of the steel surface. Note that the hardness of the steel shot in the table is HRC 40~50, and the hardness of the steel sand is HRC55~60. The typical anchor pattern depth in the table is the maximum and average surface roughness expected to be achieved under good spraying (blasting) conditions (impeller or nozzle). The appendix of the standard gives the performance requirements of the steel wire segment, such as specifications, composition, hardness, etc. In surface treatment, adding a certain amount of steel wire segments to the abrasive can produce sharp roughness "peaks and valleys", which is very beneficial to increase the mechanical adhesion between the coating film and the steel surface.

The consumption of abrasives is determined by the abrasive life, which is a difficult concept to define. It is usually based on the fragmentation of the abrasive. In engineering, the "usable times" are used to express its life, which determines the relative cost.