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because steel pipes are prone to chemical or electrochemical reactions and corrosion due to the influence of the use environment and the conveying medium during use and transportation, the use of anti-corrosion paint for steel pipes is an effective way to prevent steel pipes from being corroded.
1. analysis of the status quo of the steel pipe anti-corrosion coating process. at present, there are mainly the following four kinds of industrial automatic coating processes for seamless steel pipes such as welded pipes and oil casing pipes commonly used at home and abroad:
steel pipe anti-corrosion coating process 1: using the curtain coating method, the front, back, and middle rollers are conveyed in a straight line to form a coating film. then the fork is transferred, the hook-shaped chain is rolled and conveyed, and the steel pipe enters the steam oven horizontally, heated and dried
steel pipe anti-corrosion coating process 2: electrostatic coating method is used. before, after, and in the middle of the coating, it is conveyed by an inclined roller table spirally to form a coating film. then the screw elevator transfers and lifts it to the material rack for cooling and natural drying
steel pipe anti-corrosion coating process three: the steel pipe is conveyed in a straight line, using a uv coating system, vacuum coating method, and air scour to form a coating film. immediately after application, uv radiation cures the coating film. the characteristic is that the formation and curing of the coating film are completed between the two rollers
steel pipe anti-corrosion coating process four: the coating adopts the heating airless spraying method. before coating, it should be conveyed by roller table, and after coating, it should be conveyed by segmented and synchronous "v-shaped tooth" chain to form a coating film. after coating, the steel pipe is transferred by the stepping machine to the horizontal "v-shaped tooth" chain conveyor and enters the steam drying box, where it is heated and dried.
2. comparison of various coating processes
steel pipe anti-corrosion coating process 1: due to the use of the flow coating method, the coating film sags seriously. and due to the unreasonable design of the roller table and chain, there are two longitudinal and multiple circular scratches on the coating film. this craft is being phased out. the only saving grace of this process is the heat drying after application
steel pipe anti-corrosion coating process 2: the coating film has quality defects such as sag, whole body spiral scratches, and whitening. what is more serious is that the thickness of the coating at the spiral scratch is only one-fifth of the specified thickness, and the appearance feels very bad. at the same time, there are potential fire hazards in the process of electrostatic ignition. several fire accidents have occurred in recent years, posing a threat to safe production. the absence of a drying process is also an important defect of the process. due to the many insurmountable and mutually restrictive contradictions in this process, it is becoming increasingly obsolete and can no longer meet the requirements of modern factory automatic coating, and will gradually withdraw from the field of steel pipe coating
steel pipe anti-corrosion coating process three: it is an advanced but not very mature process. the advantages of spraying and curing in an instant between two rollers are self-evident. but some weaknesses are difficult to overcome, such as the pre-treatment requirements of the steel pipe surface are extremely strict, and the adhesion will drop significantly if there is a slight carelessness; the uv coating and equipment are expensive, and the technical management requirements are high; it is easy to partially fall off due to bumps and bumps, and it is difficult to reapply. because there are so many problems, the popularization of this technology is restricted
steel pipe anti-corrosion coating process four: it is a relatively advanced and relatively mature process developed in recent years. it overcomes the disadvantages of serious sagging, scratches, whitening, and fragility of the coating film existing in other processes. the coating film produced by it has strong adhesion, flexibility, good anti-rust effect, little sag, and is beautiful and complete. the process also has the characteristics of simple operation, complete supporting facilities, low technical management requirements, and safety. due to the improvement of technology, it is called "steel pipe heating airless spraying complete set technology"
3. the advanced nature of "steel tube airless heating spraying complete set technology" "process 1" to "process 3" reflects the common coating defects of traditional processes, that is, the coating film is seriously "sagging", scratched, and "whitening" wait. the latest "process four" comprehensively solves this series of problems, and forms a complete "steel tube heating airless spraying complete set of technology". this technology has the following technical advantages.
(1) avoid strip or spiral scratches on the coating film in the late 1990s, a company in beijing and relevant experts from the beijing iron and steel design and research institute first analyzed the problem of scratches on steel pipe coating in china. the scratches at that time were mainly manifested as two longitudinal full-length scratches with a width of 40 mm on the surface of the steel pipe. given the steel pipe coating production line imported from abroad at that time adopted the design of a roller conveyor before and after coating, and the roller conveyor is also installed in the middle of the coating box, the two parties proposed to use the transmission method of synchronous chain conveyor in the front of the coating and the rear of the roller conveyor for airless spraying technical solutions. this scheme was realized by a company in beijing at the beginning of the 21st century and put into production and application. it has been proved by the practical application that this process can effectively prevent the longitudinal scratches of steel pipes. after painting, there are only two small tooth marks distributed at an interval of 600mm. in addition, the self-healing property of the paint itself makes the formed coating film macroscopically complete and user-friendly. reflected very well. the further improved process (adopting the segmental conveying process of the front and rear synchronous toothed chains before and after spraying) makes the steel pipe always in partial contact with the "v" shaped teeth of the chain, and the front and rear chains move synchronously, ensuring the distance between the sprayed surface and the fulcrum. the minimum contact between them avoids the disadvantages of strip scratches conveyed by the roller table, and the coating film is beautiful and complete. at present, the technical level is relatively advanced both internationally and domestically. this technology is not only advanced but also mature and has been verified on 13 domestic automatic oiling lines. as for the prevention of spiral ribbon abrasions, it is very simple, just remove the spiral conveying and change to this method
(2) to overcome the problem of "sag" "sag" can be divided into five types: specific heavy type, over-thick type, low-viscosity type, special shape type, and contact type. steel pipe anti-rust coatings generally have a relatively small specific gravity, and the impact on "sagging" can be ignored. the main types of steel pipe "sagging" is an over-thickness and low viscosity, and the secondary types are special shapes and contact types. since the cross-section of the steel pipe is round and close to streamlined, the paint is easier to flow and easy to "sag". this is unavoidable, but it can be overcome. the only way to reduce the flow of paint on the surface of the steel pipe is to overcome the "sag" by spraying paint with a high initial viscosity but a reduced viscosity after heating, and recovering to a higher viscosity as the temperature decreases. the contact type "sagging" is because the support point of the steel pipe contacts the surface of the steel pipe, forming a nearly vertical contact surface with the steel pipe surface, guiding the paint to leave the coating film to form "sagging". this is inevitable. but it can be reduced by film thickness control. fortunately, the number and area are small, and the appearance has little effect. as for the over-thick type "sagging", there are many reasons for the formation, so there are many problems that must be paid attention to. this is the focus of control. the following points must be guaranteed: (1) steel pipes with anti-corrosion coating must ensure a certain operating speed, and it can be adjusted within a certain range. the steel pipe anti-corrosion coating steel pipe must be transported at a uniform speed. the steel pipe anti-corrosion coating adopts the airless heating spraying method. the working pressure of the coating can be adjusted, and the pressure must be kept stable at the same time. the heating temperature of the paint can be adjusted, and the model of the steel pipe anti-corrosion coating nozzle is correctly selected through experiments, including flow rate, spray width, and other parameters.
(2) the airless automatic spray gun for steel pipe anti-corrosion coating is evenly distributed on the circular section of the steel pipe. the angle of the steel pipe surface and the distance to the steel pipe surface can be adjusted. the air volume and wind speed of the layer exhaust air must match the spraying state, and the steel pipe anti-corrosion coating nozzle is in a good working state.
if the above points are carefully adjusted for each specification of steel pipe, the over-thick type "sagging" can be completely overcome. the problem is to establish a complete coating process management system to ensure that when the variety is replaced, the technical adjustment work is carried out in an orderly manner
(3) prevent "whitening" of the coating film the formation mechanism of the "whitening" defect of the coating film is the result of water condensation and mixing into the interior of the coating to produce emulsification and become a translucent white film. there are many reasons for this, such as air humidity in the construction environment, excessive dispersion of paint, temperature drop during dispersion, the local temperature on the surface of the coating film, water in the paint, the low surface temperature of the workpiece, improper selection of solvent boiling point or ratio, water in the atomized air wait. for steel pipe coating, the first three items are the main reasons. these appear as a formation process under certain conditions. as the main reason for electrostatic coating, the "whitening" of the coating film can be described as follows: under the condition of relatively high air humidity and low temperature in the environment, the coating is dispersed after it comes out of the gap-type electrostatic atomizer, and then accumulated on the steel pipe surface. in this process, because the paint is excessively dispersed into a large number of extremely fine paint droplets, its specific surface area is large, and the area in contact with humid air is large, which can attract a large number of water molecules to reach its surface and mix into the paint liquid. the flow enters the interior of the coating, eventually causing the "whitening" defect of the coating film.
it can be seen from the above discussion that the air humidity of the environment is only the condition for production, and the real reason is the excessive dispersion of paint. to prevent the defect of "whitening" of the coating film, it can be realized by controlling the excessive dispersion under the condition of constant ambient air humidity. the use of heated airless spraying is considered the best way to control the above two reasons. the so-called heating airless spraying, as the name suggests, heating is to heat the paint to a certain temperature before spraying. generally, the temperature is selected between 40-80°c. airless spraying means that the paint is atomized without compressed air. the atomization principle is: after the liquid paint is pressurized to a certain pressure with a booster pump, it is sent into the spray gun through the pipeline, and a high-speed liquid film is formed after being sprayed out through the nozzle; the high-speed liquid film is unstable and dispersed into small particles after friction with the air. atomization of droplets. its atomization effect is moderate, and excessive dispersion will not occur, so it is suitable for pipeline coating. airless spraying can reduce the amount of solvent, spray high-viscosity paint, save paint than air spraying, and use a little more paint than electrostatic spraying. but this weakness can be compensated by the recycling unit. due to the temperature and atomization advantages of heated airless spray, it is the best choice for steel pipe coating. the effect of this selection process is obvious. as a result of using multiple manufacturers and multiple coating lines, no "whitening" phenomenon has been found. even the overseas coating film can guarantee a good anti-rust effect. in addition, the effect of heating and drying after airless heating coating to prevent "whitening" defects cannot be ignored.