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. The implementation department of surface pretreatment must have relevant equipment and technical operators, and all surface pretreatments should have special technical supervision and inspection.
In order to correctly understand surface pretreatment, we should first have a complete understanding of the factors affecting its process. Figure 5-1 lists the system diagram of various factors affecting the quality of spraying rust removal. 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.
Through the cost analysis of the covering layer, the general surface treatment cost accounts for about 50%. The coating of the drag-reducing inner coating is thin, the number of coatings is small, and the amount of coating is small. Therefore, the cost of surface treatment is higher, about 70%. Therefore, in the process design and construction of the drag-reducing inner coating, special attention should be paid to the quality of surface pretreatment.
2. Main factors affecting the quality of the coating
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 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 (as shown in Figure 5-2) [3]. 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 the rust products and dust that have not been removed from the surface of the steel pipe. It should also include the residual particles that have not been cleaned 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 clearly 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 a period of 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), the main effect is mechanical. After the steel pipe surface is treated with abrasives, the surface roughness increases significantly, and the metal surface area can even increase by 20 times [3]. As the roughness increases, the surface area increases significantly, and the adhesion between the coating and the steel pipe surface increases accordingly. When the abrasives sprayed (polished) have 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 can also have a negative impact on 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 can easily cause bubbles, which directly affects the quality of the coating. In addition, when the coating is thin, the tip of the wave 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 them, the particle size of the abrasive has the greatest impact on the roughness. Table 5-2 gives the recommended values of the corresponding relationship between abrasive and roughness by the American Steel Structure Coating Association (SSPC).
There are many methods for surface treatment. The most reasonable one for pipelines is the commonly used spraying (blasting) 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.
