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    Home > Coatings News > Paints and Coatings Market > To verify the applicability of various types of zinc-rich coating standards and salt spray resistance

    To verify the applicability of various types of zinc-rich coating standards and salt spray resistance

    • Last Update: 2020-12-10
    • Source: Internet
    • Author: User
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    Zinc-rich coating as a pre-coated and rust-proof primer, with excellent salt spray resistance, with other coatings supporting the formation of heavy anti-corrosion coating system in China's metal anti-corrosion field is widely used. For zinc-rich paint countries have not established a unified standard, but for different applications, various industries including chemical, marine, railway, etc. have established their own industry standards. Various industry standards in the process of use there is a certain specificity, some standards can not be timely revised to supplement, and many existing coatings enterprises do not have professional standard personnel, often the implementation of industry standards are not allowed to grasp, but simply use the existing industry standards or national standards, ignoring the details of the standards and the quality of their own enterprises, resulting in some indicators can not meet the requirements of the standards, can not meet the needs of the project
    China
    . In the coating film process, whether the epoxy zinc-rich coating or inorganic zinc-rich coating is due to the coating contains a large number of zinc powder, the metal substrate has an excellent protective effect, excellent salt spray resistance. For zinc-rich coatings salt spray resistance requirements, the existing relevant coating standards in The requirements are also different.
    1 Zinc-rich coatings using various types of standards
    1.1 GB/T 6747-1986 "Marine workshop primer general technical conditions"
    standard workshop primer as a temporary protection of steel Rust-proof primer, divided into zinc-containing powder and zinc-free two workshop primers, zinc-containing workshop primer paint film thickness of 15 to 20 m, the standard technical indicators are relatively simple, there is no salt spray resistance performance requirements. The commonly used zinc-containing workshop primers mainly include several types of zinc-rich epoxy, inorganic zinc-rich and water-based inorganic zinc-rich and zinc phosphate. This is also the only implementation standard for marine workshop primer inspection by the China Class Society.
    1.2 HG/T 3668-2000 Zinc-rich Primer
    standard specifies two types of epoxy-rich zinc primer and inorganic zinc-rich primer, mainly used as rust-proof steel substrates. In the technical indicators are salt-resistant fog resistance, paint film thickness of 20 to 25 μ m, and requires the test model to draw diagonal permeable lines, wherein the epoxy zinc-rich primer needs to meet 72 h salt-resistant fog, inorganic zinc-rich primer needs to meet 168 h salt-resistant fog. The zinc content in the dry film of epoxy-rich primer is greater than 70%, and the zinc content of inorganic zinc-rich primer dry film is greater than 80%. The coating film within 10mm surrounding the test model and the coating film within 3 mm on each side of the diagonal permeable line on the coating film is not evaluated, and at least 2 of the 3 models do not see bubbles, peeling, rust spots and other phenomena on the coating, it is considered that the coating can withstand salt spray.
    1.3 HG/T 3656-1999 "Steel Structure Bridge Paint"
    standard contains the paint for steel structure bridge paint, intermediate paint and primer general technical conditions, but also applicable to other steel structures in atmospheric environments with finish, intermediate paint and primer. Products are divided into ordinary and long-lasting two categories. The long-lasting primer used in steel structure bridge and other steel structures in China is mainly epoxy-rich zinc-rich primer and inorganic zinc-rich primer, and the thickness of the paint film in the actual application is 75 to 90 μ m. The primer in this standard requires a 240 h salt-resistant water test, with no individual salt-resistant fog resistance, but the matching coating requires a salt-resistant spray resistance of 1,000 h.
    1.4 TB/T 2772-1997 Technical Conditions for the Supply of Rust-proof Primers for Railway Bridges
    This standard applies to rust-proof primers used in the re-painting and maintenance of steel beams in new steel beams and operations, as well as to rust-proof primers used in the coating of other steel structures. The zinc content in the dry film in the zinc-rich primer of epoxy is not less than 80%, and under the condition that the thickness of the paint film is (80 ± 5)μ m, through the salt spray test of 1,000 h, the surface of the model is free of foam and rust, and 24 h is free of red rust at scratches.
    1.5 TB/T 1527-2004 Railway Steel Bridge Protection Coatings
    This standard applies to the initial coating of steel bridges and the re-painting and maintenance of steel bridges after deteriorating, as well as to the initial coating, re-painting and maintenance of other ancillary steel structures of bridges. Appendix C" Railway Steel Bridge water-based inorganic zinc-rich rust-proof primer technical conditions" provides that the zinc content of the dry film of the zinc-rich primer is not less than 90%, in the paint film thickness of (80 ± 5) μ m conditions, through the 1,000 h salt spray test, the surface of the model is free of rust, scratches 124 h no red rust. Appendix E "Technical Conditions for Inorganic Zinc-Rich Anti-Slip Coatings for Railway Steel Bridges" stipulates that the zinc-rich primer needs to pass the 500 h salt spray test under conditions of 80±5) μ m of paint film thickness, and the surface of the model is free of foam and red rust.
    1.6 DOD-P-24648 Zinc-containing primer (U.S. army standard)
    The zinc-rich coating in the military standard is suitable for coating steel surfaces under ocean, high humidity, coastal environmental conditions, including organic and inorganic zinc-rich coatings. Among them, the thickness requirement of the salt spray test is 3 to 5 mils (equivalent to 76 to 127 μ m), according to assTM B 117 method, through 30 d (720 h) salt spray test, the surface of the model is free of foaming, wrinkles and rust.2 Differences between standards and scientificity
    Zinc-rich coatings as a heavy anti-corrosion coating to prevent corrosion of metal substrates, salt spray resistance is an indispensable and important technical indicator to measure their corrosion resistance. According to the construction experience and reference to a variety of application cases, zinc-rich paint film thickness can be determined according to specific construction requirements, in order to apply uniform, generally need to be at least 2 coating, paint film thickness is not less than 45 m, but also can not be too high, generally maintained at 120 m below, otherwise, paint film is too thick, not only affect the drying time, but also will cause the flow, cracking and other ills, the entire coating system has adverse effects.
    TB/T2772-1997 and TB/T 1527-2004 give a reasonable evaluation method for the salt spray resistance of zinc-rich coatings applied to rust-proof primers, and the thickness of the paint film selected is close to the specific requirements of the construction, so that the measured data have a more realistic and convincing. HG/T 3656-1999 only for long-lasting supporting coating salt spray test, with rust-proof primer only 144 h or 240 h salt water test, no primer salt spray test, primer rust resistance can not be reflected, especially for metal bridge disposable anti-corrosion input, post-maintenance and maintenance difficult conditions, increase the salt spray test of primer is necessary. GB/T 6747-1986 as a general technical conditions, primers mainly play a temporary protective role, zinc-containing workshop primer in the ocean climate to ensure that 6 months do not rust, the test cycle is longer, if you can use accelerated testing methods, the appropriate amount of time to increase the salt spray test, can also ensure 6 months of weather resistance.
    H G / T3668-2000 provides for zinc-rich primer salt spray test, from the text of the standard and the scope of application, the standard should be used as the general standard of zinc-rich primer, but the general performance and salt spray test paint film thickness index was carried out Strict regulations, generally controlled at 15 to 25 μ m, from this point of view, this standard does not apply to all rust-proof zinc-rich primers, but should be similar to G B / T6747-1986 zinc-rich workshop primers or pre-coated primers. Because of the misdirection of the text, many paint manufacturers take this standard as the quality basis of zinc-rich coating products, in the actual testing work, resulting in salt spray testing can not meet the requirements set out in the standard, resulting in misjudgment. The U.S. military standard DOD-P-24648 not only carried out the salt spray test, but also gave the highest and lowest limit to the thickness of the model paint film for the salt spray test.
    compared with the above standards of various industries at home and abroad, it can be seen that the salt spray test with zinc-rich coating for metal structure is a very important technical indicator, and the thickness of the paint film of the model plays a vital role in the salt spray test.3 Effect of paint film thickness on salt spray test
    Through the salt spray test of inorganic zinc-rich and epoxy-rich zinc-rich coatings used in a key engineering steel structure, it is found that the thickness of paint film has a great influence on salt spray test. Paint suppliers include a number of domestic production enterprises and foreign well-known paint companies in the domestic joint venture or wholly-owned enterprises, some suppliers use their own enterprise standards, some based on H G / T3668-2000, in order to standardize the inspection and evaluation of the comprehensive performance of coatings, by the construction department, supervision and suppliers of coordinated approval, unified in accordance with HG/T 3668-2000 standards, the use of zinc-rich coating testing. Since the HG/T 3668-2000 zinc-rich primer standard stipulates that the thickness of the model paint film for salt mist testing is 20 to 25 m, the epoxy zinc-rich coating needs to have 72 h salt spray resistance. Two sets of epoxy zinc-rich coatings A and B were selected in advance for salt spray resistance tests, the results of which are shown in Table 1.
    The results of the salt spray test compared to table 1 of
    show that the salt spray resistance of epoxy-rich zinc-rich coatings A and B is poor, and for the analysis of the reasons, the results are basically the same as 1 for repeated tests of coatings A and B according to the same test conditions. From this point of view, the poor salt spray resistance of the coating is not caused by the sample process, may be related to the structure of the paint itself and the thickness of the paint film, throughout the relevant standards, respectively, the use of a variety of coatings for different paint film thickness salt spray test, the results are shown in Table 2.
    the results of different paint film thickness salt mist test results
    in Table 2, A, B, C, E, G, H, I for different epoxy zinc-rich coatings, D for water-based inorganic zinc-rich coatings, F for zinc silicate pre-coated primers. From the test results, it can be seen that when the thickness of epoxy zinc-rich paint film is less than 50 μ m or thinner, the salt-resistant fog is poor, when the thickness of the paint film is higher than 50 m or thicker, the salt-resistant mist is significantly better, while the water-inorganic inorganic zinc-rich coating and zinc silicate pre-coated primer regardless of the thickness of the paint film, salt-resistant fog resistance is very good. This is due to the epoxy zinc-rich anti-corrosion coating mainly composed of high molecular weight epoxy resin, polyamide and zinc powder, generally selected 325 zinc powder, zinc powder particle size distribution is uneven, and the roughness of the blasting model is close to 10 to 30 μ m, when preparing a low paint film thickness, often painted 1 coating, easy to cause leakage phenomenon, pinholes, in the process of forming a coating film, high molecular weight resin and zinc powder formed uneven distribution of the film, after high humidity salt spray test, it is easy to foam and rust phenomenon.
    When the thickness of the paint film is increased, the general coating 2 to 3 coatings, thick coating not only comprehensively covers the blasting roughness of the steel plate, but also avoid leakage, multi-coating forms the uniform distribution of the coating internal resin and zinc powder, thus playing a good protective role on the metal structure, resulting in excellent salt spray resistance. Water-based inorganic zinc-rich coatings and zinc silicate pre-coated primers are generally composed of a high volume of matter than silicate or modified ethyl silicate and zinc powder. These two types of coatings have low viscosity, easy to prepare thin coating film, silicate and zinc powder in the coating and steel surface of the active iron reaction, the formation of zinc-siliconic acid-iron compounds, while the coating film also contains a large number of active zinc powder, zinc powder oxidation of the compound will easily close the pinhole on the surface of the coating film, forming a dense protective film, a large amount of zinc powder also play an excellent cathode protection, which is also the main reason for such film scratching.
    Through the test results and the reason analysis, it can be seen that the thickness of zinc-rich paint film has a great influence on the salt spray test, and the thickness of the paint film can be reasonably selected in order to make a correct evaluation of the salt spray test performance of zinc-rich coating.4 Conclusion
    Many zinc-rich coatings adopted standards due to the different industries applicable, there are different advantages and disadvantages, can not form an all-encompassing general standard, but as anti-corrosion coatings, its salt spray resistance test should be its most important technical indicators, through test verification shows that the salt spray test and paint film thickness has a close relationship. HG/T 3668-2000 Zinc-rich Primer standards are only suitable for workshop pre-coated primers, can not meet the requirements of zinc-rich anti-corrosion coatings. In the existing standards, zinc-rich anti-corrosion coating salt spray testing should be carried out according to the specific requirements of the user, or epoxy-rich zinc-rich anti-corrosion paint by reference to "steel structure bridge paint" or "rail bridge with rust-proof primer supply technical conditions", water-based inorganic zinc-rich anti-corrosion coating in accordance with the provisions of the "rail bridge protection coating" requirements, so as to make a correct and reasonable judgment of the corrosion-resistant paint.
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