Anti-corrosion Coating Composite Process for Bolts and Improved Service Life in Humid Environments
What are the design points of the three-layer structure of the bolt anti-corrosion coating composite process?
The three-layer structure of the bolt anti-corrosion coating composite process includes a zinc-aluminum coating, a sealing layer and a lubricating layer, each with different functions and requiring close bonding. The zinc-aluminum coating is the core anti-corrosion layer, mixed with zinc powder and aluminum powder at a ratio of 7:3, coated by thermal spraying process, with a thickness controlled at 80-100μm. The cathodic protection effect of the zinc-aluminum alloy can prevent the bolt substrate from rusting. The sealing layer is made of epoxy resin material with a thickness of 10-15μm, coated on the surface of the zinc-aluminum coating, which can isolate water vapor and oxygen and avoid oxidation failure of the zinc-aluminum coating. The lubricating layer is made of polytetrafluoroethylene material with a thickness of 5-8μm, coated on the surface of the sealing layer, which can not only reduce the friction coefficient during bolt tightening but also further improve the anti-corrosion effect. The total thickness of the three layers of coating should be controlled at 100-120μm; excessive thickness will affect the thread fit accuracy of the bolt, while insufficient thickness cannot achieve the ideal anti-corrosion effect. The bonding strength between the coatings should be ≥5MPa to ensure that the coating will not fall off under long-term vibration load.
What are the anti-corrosion performance differences between composite coated bolts and single galvanized bolts?
The anti-corrosion performance differences between composite coated bolts and single galvanized bolts are mainly reflected in three aspects: salt spray resistance, service life and friction stability. The composite coated bolts can withstand salt spray tests for more than 3000 hours without blistering or peeling of the coating, while the single galvanized bolts can only withstand salt spray for about 1000 hours, after which obvious rust will appear. In the humid ballast bed environment, the service life of composite coated bolts can reach more than 20 years, which is 2-3 times that of single galvanized bolts, greatly reducing the replacement frequency of bolts. The friction coefficient of composite coated bolts is stable between 0.12-0.15, and the preload deviation during tightening is small, while the friction coefficient of single galvanized bolts is easily affected by environmental humidity, with a deviation of more than 0.05, leading to unstable preload. In addition, the composite coated bolts have better acid and alkali resistance, and the corrosion rate in areas with frequent acid rain is only 1/5 of that of single galvanized bolts.
What are the adaptation requirements of composite coated bolts for different humid environments?
The adaptation requirements of composite coated bolts for different humid environments are core to adjusting the coating thickness and material ratio. The coastal high-humidity salt spray environment has the strongest corrosion, requiring the zinc-aluminum coating thickness to be increased to 100μm, the sealing layer to use salt spray-resistant phenolic epoxy resin, and the lubricating layer thickness to be increased to 8μm to ensure that the salt spray resistance time is ≥3500 hours. The corrosion in inland humid and rainy environments is secondary; the zinc-aluminum coating thickness is 80μm, the sealing layer uses ordinary epoxy resin, and the lubricating layer thickness is 5μm, which can meet the requirement of salt spray resistance time ≥3000 hours. The underground tunnel humid environment is characterized by high humidity but no direct sunlight, requiring the sealing layer to add fungicide components to prevent mold growth from damaging the coating, and the zinc-aluminum coating thickness is 90μm, balancing anti-corrosion and anti-mold needs. The low temperature in alpine humid environments will affect the coating toughness, so 5% nickel powder should be added to the zinc-aluminum coating to improve the low-temperature anti-brittle performance of the coating and avoid coating shedding at low temperatures.
What are the key process control points for composite coated bolts?
The key process control points for composite coated bolts are concentrated in substrate pretreatment and coating curing. For substrate pretreatment, the bolts must first be shot-blasted to remove surface oxide scale and oil stains, and the surface roughness is controlled at Ra1.6-Ra3.2μm to increase the contact area between the coating and the substrate. When thermal spraying the zinc-aluminum coating, the spraying temperature should be controlled at 400-450℃, and the spraying distance is 150-200mm to ensure the coating is uniform and free of pores. After coating the sealing layer, it should be cured at 120℃ for 30 minutes to fully crosslink the epoxy resin and improve the sealing effect. After coating the lubricating layer, it should be cured at 80℃ for 20 minutes to avoid decomposition of polytetrafluoroethylene due to high temperature. After processing, a porosity test should be carried out, and the coating porosity should be ≤1%; otherwise, the sealing layer needs to be recoated. In addition, the thread part of the bolt should be masked to prevent the coating from clogging the thread and affecting assembly.
What are the on-site installation and maintenance precautions for composite coated bolts?
The on-site installation of composite coated bolts needs to focus on tightening torque and coating protection. A special torque wrench must be used for installation, and tightening should be strictly carried out according to the design torque. The tightening torque of bolts for high-speed railways is 350-400N·m, and for heavy-haul railways is 400-450N·m to avoid coating damage due to excessive torque. During installation, it is forbidden to use hard tools such as steel wire brushes to clean the bolt surface to prevent scratching the coating; if there is dust on the surface, it can be wiped with a clean cloth. After installation, anti-rust grease should be applied to the connection between the bolt head and the nut to further isolate water vapor. During maintenance, the coating status should be checked regularly with an inspection cycle of 1 year. If small-area damage occurs to the coating, it can be repaired with a special repair agent; if the damaged area exceeds 10%, the bolt should be replaced in time. In addition, avoid contact between the bolt and strong acids and alkalis to prevent chemical corrosion of the coating.