What are the technical requirements for anti-corrosion and fireproof coating of steel structures?

Steel structures are widely used in various construction fields such as industrial plants, high-rise buildings, bridge engineering, public venues, etc. due to their advantages of high strength, light weight, convenient construction, and large span. However, steel structures themselves have shortcomings such as easy corrosion and poor high-temperature resistance. Under high temperature environments, the strength of steel will rapidly decrease, and it will lose its load-bearing capacity above 600 ℃. Moisture, oxygen, and harmful media in the atmosphere can cause steel structures to rust, gradually damaging structural integrity, shortening service life, and even causing safety hazards. As the core protective measures to ensure the long-term stable operation of steel structures, anti-corrosion and fireproof coating of steel structures directly determines the durability and fire safety of steel structures. It is necessary to strictly follow relevant technical requirements, combine with the actual working conditions and environmental conditions of the project, and construct a comprehensive and multi-level protection system. Based on industry standards and engineering practices such as the "Acceptance Standards for Construction Quality of Steel Structures" (GB50205-2020) and "Fire resistant Coatings for Steel Structures" (GB14907-2018), the technical requirements for anti-corrosion and fireproof coating of steel structures mainly revolve around five core links: preliminary preparation, base treatment, coating construction, quality inspection, and later maintenance. The technical requirements of each link are coordinated and connected to ensure that the protective effect meets the standard.
The anti-corrosion and fireproof coating of steel structures follows the core principles of "prevention first, layered protection, precise control, and long-term adaptation". The core goal is to achieve dual protection of anti-corrosion, rust prevention, fire prevention, and thermal insulation of steel structures through standardized coating construction, ensuring stable performance of steel structures within the designed service life and meeting the requirements of safe operation of the project. Painting construction should take into account both technical specifications and on-site adaptability, combined with the corrosion environment level and fire protection level of the structure, and select targeted painting materials and construction processes to avoid problems such as coating peeling, cracking, and substandard thickness, ensuring the integrity and durability of the protective system.

Pre preparation is the foundation of anti-corrosion and fireproof coating for steel structures, which directly affects the quality and protective effect of subsequent coating. The core technical requirements cover material control, equipment debugging, and technical preparation, and must strictly follow industry standards to control quality hazards from the source. Material control is the core, and the performance of anti-corrosion and fireproof coatings must be accurately matched with engineering conditions, environmental conditions, and design requirements. The use of unqualified materials is strictly prohibited.

Anticorrosive coatings shall be selected according to the corrosion environment level of the steel structure (atmospheric corrosion environment is divided into C1 to C5). Alkyd anti-corrosive coatings can be selected for indoor dry environment. Fluorocarbon, polyurethane and other anti-corrosive coatings with strong corrosion resistance shall be selected for outdoor humid, coastal salt fog or industrial corrosion environment. The coatings shall have good adhesion, weather resistance and anti-aging performance, and their indicators shall meet the requirements of GB50205-2020. Fireproof coatings should be selected according to the fire rating of steel structures. For the first level fire rating, thick fireproof coatings should be used, with a fire resistance limit of not less than 1.5 hours; Thin or ultra-thin fire-resistant coatings can be selected for the second level fire protection level, with a fire resistance limit of not less than 1.0 hour. The coatings must be tested and qualified to ensure the formation of a dense insulation layer in high temperature environments and delay the heating of steel. After the materials enter the site, they need to be sampled and retested to verify the specifications, models, quality certificates, and testing reports. Unqualified materials are strictly prohibited from being put into use and must be stored in a standardized manner. Anti corrosion coatings should be stored in a dry, ventilated, and cool place to avoid moisture and exposure to sunlight; Fireproof coatings must strictly control temperature and humidity according to storage requirements to prevent clumping and deterioration.

Equipment debugging requires comprehensive inspection and calibration of specialized painting equipment, including spraying equipment, mixing equipment, testing equipment, etc. The spraying equipment needs to adjust the pressure and flow rate to ensure that the coating is sprayed evenly without any missed or splashing phenomena; The mixing equipment should ensure that the coating is stirred evenly, avoiding sedimentation and layering; Testing equipment (such as coating thickness gauges and adhesion testers) must be calibrated and qualified to ensure accurate testing data. Technical preparation should be combined with engineering drawings, structural characteristics, and regulatory requirements to develop a special coating construction plan, clarify the number of coating layers, coating thickness, construction process, construction sequence, and quality control standards. At the same time, technical disclosure should be completed to ensure that construction personnel are proficient in the technical requirements of each link and standardize the operation process.

Base treatment is a key prerequisite for coating construction, and the core technical requirement is to ensure that the steel structure base is clean, flat, dry, and has good adhesion, laying the foundation for the close bonding between the coating and the steel structure. This is the core link to avoid coating peeling and cracking in the later stage. The grassroots treatment requires the removal of rust, oxide scale, oil stains, dust, welding slag and other debris on the surface of the steel structure. Common treatment methods include sandblasting rust removal, manual rust removal, and mechanical rust removal, which should be selected according to the degree of grassroots corrosion.

Sandblasting rust removal is suitable for large-area steel structure base treatment, and the rust removal grade should reach Sa2.5 or above to ensure that the steel structure surface is free of visible rust and oxide scale, presenting a uniform metallic luster; Manual rust removal and mechanical rust removal are suitable for local rust removal or complex parts, and the rust removal grade should reach St3 to ensure that the base layer is free of loose rust and oil stains. After rust removal, it is necessary to promptly clean the dust on the surface of the base layer to avoid dust affecting the adhesion of the coating. After cleaning, the first coat of paint should be applied within 4 hours to prevent the base layer from rusting again. For the unevenness and weld defects on the surface of the base layer, putty should be used to fill and polish it flat to ensure that the flatness of the base layer meets the requirements, and to avoid uneven coating thickness and stress caused by uneven base layer, which may lead to cracking. The moisture content of the base layer should be controlled within the allowable range of the specifications, generally not exceeding 8%. Otherwise, drying treatment should be carried out to prevent bubbles and hollowing between the coating and the base layer.

Coating construction is the core link to achieve anti-corrosion and fire protection. The technical requirements focus on controlling the coating sequence, coating thickness, coating process, and construction environment to ensure that the coating is uniform, dense, and defect free, while also considering the coordinated adaptation of anti-corrosion and fire protection coating. The painting sequence should follow the principle of "from inside to outside, from top to bottom, and from difficult to easy". Priority should be given to painting the internal and concealed parts of the steel structure, as well as weak parts such as welds and nodes, before painting the external and exposed parts to avoid missing concealed parts and ensure full coverage of the painting.

The thickness of the coating must be strictly controlled according to the design requirements. The anti-corrosion coating should adopt the "multi pass thin coating" method, and the thickness of each coating should be uniform. The total thickness should meet the design standards. Generally, the total thickness of the indoor anti-corrosion coating should not be less than 80 μ m, and the total thickness of the outdoor anti-corrosion coating should not be less than 120 μ m; The thickness of fireproof coating should be determined according to the fire rating. The total thickness of thick fireproof coating is 7-45mm, thin type is 3-7mm, and ultra-thin type is 0.8-3mm. The thickness of each coating should be controlled within the allowable deviation range of the specifications to avoid problems such as missed coating, thick coating, and thin coating. The coating process should be selected according to the type of coating. Solvent based coatings can be sprayed, brushed, rolled, and other methods. When spraying, the spraying distance and angle should be controlled to ensure uniform coating; Waterborne coatings need to control the humidity of the construction environment, avoid construction in high humidity environments, and prevent coating wrinkling and peeling.

The construction environment should meet the requirements of the specifications. The temperature for painting construction should be controlled between 5-35 ℃, and the relative humidity should not exceed 85%. Construction should be avoided in rainy, snowy, and windy weather. Rainproof and windproof facilities should be built for outdoor construction to prevent rainwater erosion and sand adhesion from affecting the quality of the coating. The connection between anti-corrosion coating and fireproof coating needs to be standardized. Generally, anti-corrosion coating is completed first, and after the anti-corrosion coating is completely dry, fireproof coating can be carried out to avoid mutual influence between the two coatings and ensure the protective effect. For weak parts such as steel structure nodes, welds, bolt connections, etc., it is necessary to strengthen the coating, increase the coating thickness, and enhance the protective ability.

Quality inspection is a key link in ensuring the quality of coating construction. The core technical requirement is to comprehensively verify the appearance quality, thickness, adhesion and other indicators of the coating through scientific testing methods, to ensure compliance with design requirements and industry standards. Unqualified parts need to be repaired in a timely manner. Appearance quality inspection requires checking whether the coating surface is flat, smooth, free of defects such as bubbles, hollows, cracks, omissions, and sagging, with uniform and consistent color that meets design requirements; For any defects that occur, they need to be repaired in a timely manner to ensure the integrity of the coating.

Coating thickness testing requires the use of a coating thickness gauge and random sampling, with a sampling ratio that meets regulatory requirements. At least 3 points should be tested every 100 square meters, with 3 tests per point and the average taken. The thickness deviation should be controlled within ± 10%, and any non-conforming areas should be repainted to the design thickness. Adhesion testing requires the use of an adhesion tester to test the bonding strength between the coating and the steel structure substrate. The adhesion of the anti-corrosion coating should not be less than 1.5MPa, and the adhesion of the fireproof coating should not be less than 0.3MPa. Areas that fail the testing must have the coating removed and the substrate treated and coated again. In addition, fire-resistant coatings also need to undergo fire resistance limit testing to ensure that they can achieve the designed fire resistance time in high temperature environments. The test results must comply with the requirements of GB14907-2018 standard.

Post maintenance is an important guarantee for maintaining the long-term stability of coating protection effect. The technical requirements mainly focus on coating maintenance, daily inspection, and defect repair to ensure the long-term effectiveness of the protection system. Coating maintenance should be carried out after the coating is completed. During the maintenance period, it is necessary to avoid collisions, friction, and scratches on the steel structure. Subsequent construction is prohibited before the coating is dry. The maintenance time should be determined according to the type of coating, generally not less than 7 days. In high temperature and high humidity environments, the maintenance time should be appropriately extended to ensure complete curing of the coating.

A normalized mechanism should be established for daily inspections, with regular checks on the coating status, focusing on inspecting the coating of steel structure nodes, welds, and exposed parts. Surface dust and debris should be cleaned in a timely manner. If defects such as coating peeling, cracking, and corrosion are found, repair measures should be taken in a timely manner. When repairing, the original coating process should be followed, and the base layer should be treated first, and then layered to ensure that the repaired area is closely connected with the original coating and the protective effect is consistent. For steel structures in harsh environments, it is necessary to increase inspection frequency appropriately, strengthen maintenance efforts, and extend the service life of the protective system.

The technical requirements for anti-corrosion and fireproof coating of steel structures run through the entire construction process, and each link is interrelated and indispensable. It is necessary to strictly follow industry standards such as the "Acceptance Standards for Construction Quality of Steel Structures" and "Fireproof Coatings for Steel Structures", optimize the construction plan based on the actual working conditions of the project, and strengthen technical control. With the application of new coating materials and intelligent construction equipment, the level of refinement and standardization of coating construction continues to improve. Strictly implementing various technical requirements can not only effectively improve the anti-corrosion and fire resistance performance of steel structures, extend the service life of steel structures, but also ensure the safe and stable operation of construction projects, providing strong support for the high-quality construction of various steel structure projects.