Steel-framed bridge heavy anti-corrosion paint coating railway and road steel bridge coating

With the increase of bridge span, the application of steel bridge is more and more. The corrosion problem is an important problem affecting the safety of long-term use of steel bridge, the system introduces the main anti-corrosion measures of steel structure, the rust removal process and rust removal grade and roughness requirements commonly used on steel surface before coating, and compares the similarity and differences of the recommended anti-corrosion coating system of railway steel bridge and highway steel bridge for reference in the design of anti-corrosion coating of steel bridge.

Key Words" Steel Bridge Rust Removal Roughness Anti-corrosion CoatingIn order to further expand domestic demand and promote steady and rapid economic growth, China has set off a new round of road network construction climax . In a large number of engineering construction, due to the high strength of steel, weight, with good plasticity and toughness, across the river, deep mountain canyons of the large span bridges are using steel bridges, such as the 1385m cross-diameter Jiangyin Yangtze River Bridge, the main span of 550 meters of Lupu Bridge. However, steel bridges have the disadvantage of being prone to rust. According to the survey, the economic losses caused by metal corrosion in the United States in 1975 were about $70 billion, while the gross domestic product of the United States was $1677 billion, and the economic losses caused by metal corrosion in a year accounted for about 4.2% of GNP that year. To date, the United States still estimates the annual economic losses from metal corrosion at 4.2% of GNP. Surveys have also been carried out in other countries, which show that the economic losses caused by metal corrosion amount to about 4 per cent of the country's gross national product. In order to ensure the long-term safety of steel bridge in the design period, it is very important to do a good job of corrosion protection of steel bridge.
coating method: coating method anti-corrosion method is generally not as good as long-lasting anti-corrosion method. It is low cost at one time, but relatively expensive to maintain. The first step in coating construction is rust removal. High-quality coatings rely on thorough rust removal, requiring a high coating that generally uses blasting pellets to remove rust, revealing the luster of the metal and removing all rust and oil. Coatings for on-site construction can be removed by hand. The choice of coating should take into account the surrounding environment, different coatings have different resistance to different corrosion conditions. The coating is generally divided into primers, intermediate paints and finishes. The primer contains more powder, less base material, rough film, strong adhesion with steel, the role of the middle paint is to increase the thickness of the coating to improve the shielding performance of the entire coating system, the middle paint for primer and finish should have a good adhesion. The paint is more base material, the film has a luster, can protect the primer from atmospheric corrosion, and can resist weathering. Pay attention to the compatibleness of different coatings when using them. The construction of the coating should have an appropriate temperature (between 5 and 38 degrees C) and humidity (relative humidity is not greater than 85%). The construction environment of the coating should have less dust, and the surface of the component should not be exposed. Do not get caught in the rain for 4 hours after coating. The coating is generally made of 4 to 5 layers, and the total thickness of the dry paint film can be thickened to 200 to 300 m.
coating protection is widely used because of its convenient construction and good anti-corrosion effect. The anti-corrosion of steel bridges in China generally adopts the coating protection method, and forms the recommended standards of the railway and highway industries.
2. Steel surface treatment method before coating
Steel bridge components in the process of processing, transportation, storage, etc. , the surface will often be with oxidized skin, rust mold residue sand, solder slag, dust and oil and other dirt. In order for the coating to be firmly attached to the surface of the steel bridge component, the surface must be cleaned before coating, otherwise, not only affect the binding force of the coating and the substation metal, but also make the substation metal can continue to corrode even under the protection of the coating, so that the coating peels off, affecting the service life of the steel bridge component. In order to provide a good work surface, the treatment of the surface of the steel bridge component before coating has the following points: (1) no oil and moisture, (2) no rust and oxides, (3) no adhesive impurities, (4) no acid and alkali residues, (5) surface has a certain roughness.
commonly used surface treatment methods are :
1. Hand-processed. The symbol "St". With manual and power tools, such as a blade, manual or power wire brush, power sandpaper tray or grinding wheel and other tools to remove rust, by hand can remove the steel bridge component surface rust and oxidation skin, but manual treatment of labor intensity, low production efficiency, poor quality, cleaning is not thorough.
2. Chemical treatment. The symbol "Be". Mainly by using acidic or alkaline solution and steel bridge component surface oxides and oil pollution chemical reaction, so that it dissolves in acidic or alkaline solution, in order to remove the surface rust of the work piece oxidation skin and oil pollution purposes. Chemical treatment is suitable for the cleaning of sheet parts, but the disadvantage is that if the time is not controlled properly, even with corrosion inhibitors, steel can produce erosion phenomenon. For more complex structural parts and porous parts, after acidic solution acid wash, immersed in the gap or hole of residual acid is difficult to completely remove, improper treatment, will become a hidden danger of corrosion after the work parts, and chemicals volatile, high cost, after treatment of chemical fluid discharge work difficult, improper treatment, will cause serious pollution to the environment. With the improvement of people's environmental awareness, this treatment method is being replaced by mechanical treatment.
3. Mechanical treatment. The symbol "Sa" mainly includes the blast method and the blast method. Blast cleaning is a method of using centrifugal force to accelerate the projectiles and projectiles to the work piece for rust removal. However, the projectile flexibility is poor, limited by the site, cleaning the work piece is somewhat blind, in the workware surface is prone to clean up the dead end. Equipment structure is complex, many fragile parts, especially blades and other parts wear fast, maintenance work time, high cost, a large one-time investment. The spray is divided into blasting and blasting. Surface treatment with the spray, the impact is large, the cleaning effect is obvious, but the treatment of the sheet work piece of the spray, easy to deform the components. Steel pellets hit the surface (whether blasting or spraying) to deform the metal substrate, because Fe3O4 and Fe2O3 have no plasticity, after fragmentation, and the oil film with the substrate deformation, so with oil-stained work parts, blasting, blasting can not completely remove oil pollution. In the existing work piece surface treatment method, the best cleaning effect of the number of blast cleaning. Blasting is suitable for cleaning of work surfaces that require high demands. China's current general sand blasting equipment is mostly composed of articulated dragons, scrapers, bucket lifters and other original bulky sand transport machinery. With
households need to build a deep pit and do waterproof layer to install machinery, high construction costs, maintenance work, sand blasting process produced a large amount of dust can not be removed, affecting the health of operators and pollution of the environment. The blasting method and the blast method require the ambient humidity to be less than 85%.
4. Flame rust removal. The symbol "FI" is a method of rust removal using oxyacetylene flame and nozzle. The shape and size of the nozzle should be suitable for the steel surface condition to be rusted. Flame rust removal is suitable for steels with a thickness of more than 5mm, without coating or with the old coating removed completely. When rusting steel with a thickness of nearly 5mm, pay attention to the temperature of the flame.
3. Rust removal grade and roughness of steel surface before coating
China's national standard "pre-coating steel surface rust grade and rust removal grade" GB 8923-88, the steel surface rust removal grade is clearly defined.
The mechanical treatment method (injection or ejection rust removal) is divided into four grades: (1) Sa1 mild injection or ejection rust removal, steel surface should be free of visible grease and dirt, and there is no attachment such as non-secure oxide skin, rust and paint coating; Steel surface should be free of visible grease and dirt, and oxide skin, rust and paint coating and other attachments have been basically removed, its residue should be firmly attached; (4) Sa3 makes the steel apparent clean injection or ejection rust removal, steel surface should be free of visible grease, dirt, oxide skin, rust and paint coating and other attachments, the surface should show a uniform metallic color.
The manual treatment method (hand and power tool rust removal) is divided into two grades: (1) St2 thorough manual and power tool rust removal, steel surface should be free of visible grease and dirt, and no adhered to the non-secure oxidizing skin, iron Adhesives such as rust and paint coatings; (2) St3 very thorough manual and power tools to remove rust, steel surface should be free of visible grease and dirt, and no attachments such as oxides, rust and paint coatings. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic sheen.
in order to improve the adhesion performance of coating, there are certain surface roughness requirements for the steel surface before coating. Surface roughness refers to the micro-geometry characteristics of small spacing and peaks and valleys on the processing surface, which is a kind of micro-geometry error, also known as micro-unevenness. Attention should be paid to the difference between surface roughness and shape error (macro geometry error) and surface wave. Usually, the wave distance is less than 1mm belongs to the surface roughness, the wave distance is 1 to l0mm belongs to the surface wave degree, the wave distance is greater than l0mm belongs to the shape error. China's national standard "surface roughness parameters and their values" GB/T 1031-1995 stipulates that the main evaluation parameters of surface roughness can be used the following three kinds of: (1) contour arithmetic average deviation Ra;
4. Comparison of the recommended standards for railway and road steel bridge coating
The recommended standards for the anti-corrosion coating industry in China's existing railway (rail bridge protection coating TB/T 1527-2004) and highway (road bridge steel structure anti-corrosion coating technical conditions JT/T 722-2008) are recommended for coating protection. Overall, road standards are introduced later than railway standards, more detailed and clear than railway standards, the basic content of the two standards is consistent, that is, the initial coating (the initial coating of the new bridge steel structure includes two years of defect liability period of coating, repair coating (bridges in the whole process of its operation of the coating maintenance), re-painting (Complete removal of the old coating, re-surface treatment, in accordance with the complete coating specifications of the coating), are given before coating steel surface must be mechanical treatment and manual treatment method rust removal, rust removal grade and roughness of the specific requirements, are recommended coating matching system, coating quality requirements and the corresponding various test methods .The main differences are the following:
1. The highway standard clearly gives the classification of the protection years of the coating system, that is, the general type (10 to 15 years), the long-lasting type (15 to 25 years), and the railway standard does not give the protection years of the coating system.
2. The highway standard clearly gives the classification of coating parts, which are divided into seven categories: outer surface, non-closed environment inner surface, closed environment inner surface, steel bridge surface, wet and dry alternating area and underwater area, anti-slip friction surface, ancillary steel components, including anti-collision guardrails, armrest guardrails and base, lamp mounts, water pipes, steel rim stone, etc. The classification of coating parts by railway standards is implicit in the provisions and coating supporting system, and the classification is not as detailed as the highway standards.
3. Surface roughness requirements, highway standards slightly improved: highway standards recommend hot zinc spray (aluminum), steel surface roughness of Rz60 m to 100 m, railway Rz50 m to 1 Road standards recommend the spraying of other protective coatings for inorganic zinc-rich primers, with steel surface roughness of Rz30 m to 75 m and railroads from Rz25 m to 60 m. Highway standards separately put forward the spray inorganic zinc-rich primer steel surface roughness of Rz50 m to 80 m requirements.
4. The highway standard clearly states that the coating supporting system is designed according to the corrosion environment, working conditions and anti-corrosion years, and the coating supporting system is more easy to operate with the international classification of the corrosion environment (ISO12944-2), and the railway standard directly describes the classification of the corrosive environment in the applicable parts column of the coating supporting system.
. Under the same corrosion environment, the total dry film thickness of the general coating system of highway standard is about 20 m to 60 m thicker than that of railway standard, and the thickness of the long-lasting type of total dry film is thicker. Railway standards offer only one coating system, with no common and long-lasting coating systems.
, conclusion and corrosion
affect the long-term use of steel bridge safety and full performance of important issues. With the development of China's transportation infrastructure construction, the construction of large-span steel structure bridge has increased rapidly, and the problem of anti-corrosion of bridge steel structure has been paid more and more attention. This paper systematically introduces the relevant contents of anti-corrosion of the steel structure of the bridge, and makes a comparative analysis of the industry standards for the coating of railway and highway steel bridges for reference in the design of anti-corrosion of steel bridges. At present, a new type of coating and dehumidification system integrated anti-corrosion system is gradually being applied in large bridges. The anti-corrosion principle is that if the humidity of the air drops below 40%, the corrosion of steel structure is very difficult to occur, the anti-corrosion effect is good, but the daily operating costs are higher, dehumidification system is only applicable to the closed system.