Key Technical Points for Corrosion and Loosening Prevention of Rail Fasteners
What are the main types of corrosion of rail fasteners? What are the differences in high-incidence areas and protection focuses of different corrosion types?
The corrosion of rail fasteners is mainly divided into three types: electrochemical corrosion, atmospheric corrosion, and chemical corrosion, with significant differences in their high-incidence areas and protection focuses. Electrochemical corrosion is caused by the contact between the metal parts of the fastener and the electrolyte solution, which is frequent in coastal high-salt spray areas, humid and rainy areas, and waterlogged areas in tunnels. The key to protection is to block the electrochemical reaction, such as using insulating coatings to isolate the electrolyte and installing anti-corrosion washers to avoid contact between different metals. Atmospheric corrosion is the result of the interaction between fasteners and oxygen, water vapor, and pollutants in the air, which is particularly prominent in industrial pollution areas and areas with severe smog. The core of protection is to improve the oxidation and weather resistance of the fastener surface, and processes such as hot-dip galvanizing and Dacromet coating are commonly used. Chemical corrosion is caused by direct erosion of acid and alkali solutions, mostly occurring in lines around chemical industrial parks or northern winter lines where salt is sprinkled for deicing. The focus of protection is to select acid and alkali resistant materials, such as using stainless steel fasteners or coating the surface with chemical corrosion resistant coatings. In addition, freeze-thaw cycles in alpine regions will accelerate the corrosion process, so anti-freezing and anti-corrosion measures should be combined.
What are the common methods of mechanical anti-loosening and chemical anti-loosening in fastener anti-loosening technology? What are their applicable scenarios?
Fastener anti-loosening technology is mainly divided into two categories: mechanical anti-loosening and chemical anti-loosening. The common methods and applicable scenarios have their own focuses. Mechanical anti-loosening locks the thread pair through mechanical structures, and common methods include: ① Spring washer anti-loosening, which uses the reaction force generated by the elastic deformation of the washer to maintain the pre-tightening force, with simple structure and low cost, suitable for lines with small vibration such as ordinary railways; ② Lock washer anti-loosening, which restricts relative rotation through the cooperation of washer tabs with bolts and nuts, with high anti-loosening reliability, used in key parts of high-speed rail fasteners; ③ Cotter pin and slotted nut anti-loosening, after installation, the pin is inserted into the nut groove and bolt hole to completely lock, suitable for scenarios with severe vibration such as heavy-haul railways. Chemical anti-loosening fixes the thread by chemical bonding force, and common methods include: ① Thread locking adhesive anti-loosening, applying anaerobic adhesive to the thread, which forms a high-strength bonding layer after curing, with long-lasting anti-loosening effect, suitable for bolt connections of various lines; ② Pre-coated anti-loosening adhesive bolts, which are pre-coated with anti-loosening adhesive before leaving the factory, easy to install, and mostly used for high-speed rail fasteners with high standardization. Mechanical anti-loosening is convenient for disassembly and maintenance, while chemical anti-loosening has a more durable anti-loosening effect, and the selection should be based on the line maintenance frequency and vibration intensity.
What technical links are specifically included in the "coating + sealing" composite anti-corrosion system adopted by high-speed rail fasteners? Where are its anti-corrosion advantages reflected?
The "coating + sealing" composite anti-corrosion system for high-speed rail fasteners is an integration of multi-layer protection technologies, which specifically includes four technical links: first, substrate pretreatment, which controls the surface roughness of the fastener at Ra25-50μm through shot blasting derusting to enhance coating adhesion; second, bottom coating, which uses zinc-aluminum pseudo-alloy coating for thermal spraying to form a sacrificial anode protective layer with a thickness of 50-80μm to provide cathodic protection; third, intermediate coating, which is coated with epoxy resin primer with a thickness of 20-30μm to further isolate corrosion media; fourth, surface sealing treatment, which uses silicone sealant to fill the connection gaps of fasteners, and at the same time, anti-corrosion sealing caps are set on key parts such as bolt heads and nuts to achieve full enclosed protection. The anti-corrosion advantage of this system is reflected in the synergy of "double barrier + cathodic protection": the coating system forms a multi-layer physical barrier from the outside to the inside to prevent corrosion media such as water and oxygen from invading; the sacrificial anode effect of the zinc-aluminum coating can protect the substrate from corrosion when the local coating is damaged; the sealing treatment solves the weak link of crevice corrosion, extending the anti-corrosion life of fasteners in harsh environments such as coastal and humid areas from 8 years of ordinary coatings to more than 20 years, and greatly reducing maintenance costs.
What effects will improper control of the pre-tightening torque of fastener bolts have on the anti-loosening performance? How to achieve precise control?
Improper control of the pre-tightening torque of fastener bolts will directly damage the anti-loosening performance. Too small torque will lead to insufficient bolt pre-tightening force, and the thread pair is prone to relative sliding under train vibration, causing loosening; too large torque will cause plastic deformation of the bolt, damage the thread profile, and the pre-tightening force will decrease instead. At the same time, it may cause fatigue failure of fastener components (such as rail clips) due to excessive stress. Precise control needs to start from three aspects: testing equipment, construction technology, and quality verification. In terms of equipment, digital torque wrenches or intelligent torque control systems are used, whose torque accuracy can reach ±3%, and can feed back tightening data in real time. In construction, the three-step method of "initial tightening - re-tightening - final tightening" is strictly followed: initial tightening to 50% of the design torque to eliminate thread gaps, re-tightening to 80% to ensure uniform stress, and final tightening to the design value and maintaining for 3-5 seconds. In the quality verification link, the torque-angle method is used for sampling inspection, and the pre-tightening force is judged by measuring the bolt tightening angle. The sampling ratio of high-speed rail lines is not less than 5%, and that of heavy-haul railways needs to reach 10%. In addition, torque tools should be calibrated regularly, and the torque compensation value should be appropriately increased when the ambient temperature is lower than -10℃ to ensure stable pre-tightening torque.
What special technical measures are there for fasteners in alpine and high-altitude areas in terms of anti-corrosion and anti-loosening?
Alpine and high-altitude areas have harsh environmental characteristics such as low temperature, strong ultraviolet rays, large temperature differences, and heavy wind and snow. The anti-corrosion and anti-loosening of fasteners need to adopt special technical measures of "low temperature resistance + ultraviolet resistance + strong locking". In terms of anti-corrosion: ① Select low-temperature resistant zinc-aluminum coating, which can still maintain good adhesion at -50℃ to avoid brittle cracking and falling off of the coating; ② Use fluorocarbon topcoat for sealing, whose UV aging resistance is 3 times that of ordinary coatings, which can resist strong UV erosion at high altitudes; ③ Use low-temperature elastic sealant for fastener gaps, which will not shrink and crack under extreme temperature differences, preventing wind and snow from invading and causing internal corrosion. In terms of anti-loosening: ① Adopt the combined anti-loosening of "double washers + thread locking adhesive", the double washers include spring washers and lock washers, and the locking adhesive is a low-temperature resistant type, which can still maintain bonding strength at -40℃; ② Bolts are made of low-temperature tough materials, such as 35CrMoA alloy steel, to avoid bolt fracture caused by low-temperature brittleness; ③ Optimize the pre-tightening torque parameters, and increase the pre-tightening torque by 10%-15% according to the change of material elastic modulus at low temperature to ensure stable pre-tightening force. At the same time, ultrasonic testing is used to regularly detect the internal stress of the bolts, and fatigue hidden dangers are found in time to ensure the reliable performance of the fasteners in extreme environments.