1.What are the application scenarios of galvanized bolts on rails?
Rail connection core components
Mainly used for joint connection between rails, fixing rails and sleepers, and installing track ancillary facilities. These bolts need to withstand dynamic loads, vibrations and alternating stresses during train operation, and are exposed to complex outdoor environments.
Demand to replace ordinary carbon steel bolts
Traditional carbon steel bolts are prone to rust in environments such as moisture, salt spray, and industrial dust, resulting in loose connections, reduced strength, and even potential safety hazards on the track. The galvanized layer significantly extends the service life of the bolts through electrochemical protection and physical barrier effects.
2.How to choose the galvanizing process?
Hot-dip galvanizing is the main method, cold-dip galvanizing is the auxiliary method
Hot-dip galvanizing
Process: Immerse the bolts in molten zinc to form a composite coating of zinc-iron alloy layer and pure zinc layer, with a thickness of usually 50~80μm, which can be increased to more than 100μm in coastal areas or heavily corrosive areas.
Advantages: Strong coating bonding and excellent corrosion resistance (salt spray test can reach more than 1000 hours without red rust), suitable for key rail connection points exposed outdoors for a long time.
Cold galvanizing
Process: The zinc layer is formed by electrochemical deposition, and the thickness is generally 8~15μm, often combined with passivation treatment.
Application: It is suitable for bolts of auxiliary facilities with low requirements for coating thickness and small load, and the cost is lower than hot-dip galvanizing.
3.What are the key indicators of coating performance?
Thickness uniformity: The coating must be covered on complex structures such as bolt threads and heads to avoid corrosion due to insufficient thickness.
Adhesion strength: The bonding strength between the coating and the substrate must be verified through bending tests and hammer tests to prevent the coating from falling off due to train vibration.
4.What are the core advantages in rail applications?
Significantly improved corrosion resistance
The rail area is often corroded by rain, condensed water, and metal dust (containing electrolytes) generated by rail friction. The zinc coating protects the steel substrate through the "sacrificial anode" effect: when the coating is damaged, the zinc corrodes first, preventing the substrate from rusting.
Reduce maintenance costs and safety hazards
Ordinary carbon steel bolts are "rusted" due to corrosion, which increases the difficulty of disassembly (needs to be cut or replaced), while galvanized bolts can keep the threads lubricated, making it easier to disassemble and replace during maintenance, reducing railway maintenance downtime.
Adapt to dynamic loads and environmental changes
The galvanized layer is firmly bonded to the substrate and can withstand high-frequency vibrations (amplitude can reach 0.1~1mm) when a train passes by. The coating is not prone to cracking or falling off, maintaining connection stability.
5.What are the limitations and countermeasures in the application?
Limitations in high temperature environments: When the temperature of the hot-dip galvanized layer exceeds 200°C, the zinc-iron alloy layer may undergo a phase change, resulting in increased brittleness of the coating, which is not suitable for bolts close to the train brake system.
Response: Use stainless steel bolts or high-temperature resistant coatings instead.
Protection problem of welding area: When welding rails, if the bolts are close to the welding point, the high temperature will cause the galvanized layer to volatilize, generate zinc vapor and weaken the coating protection.
Solution: Remove the bolts before welding, and re-galvanize the connection parts locally or apply zinc powder primer after welding.