Bolt Corrosion Protection Coating Technology and Long-Term Protection Knowledge
What are the characteristics of commonly used anti-corrosion coating technologies for bolts?
The hot-dip galvanizing coating thickness is ≥80μm, and the zinc layer forms a metallurgical bond with the substrate, with excellent anti-corrosion performance, which can pass a 1000-hour salt spray test and is suitable for ordinary atmospheric environments. Dacromet coating is composed of zinc-aluminum powder and binder, with a thickness of 5-12μm, strong corrosion resistance, passing a 2000-hour salt spray test, and no hydrogen embrittlement risk, suitable for high-strength bolts. Fluorocarbon coating adopts polytetrafluoroethylene material, with a thickness of 20-30μm, acid and alkali resistance, UV resistance, passing a 3000-hour salt spray test, suitable for highly corrosive environments such as coastal areas and industrial zones. Mechanical galvanizing coating thickness is 20-50μm, with uniform coating, suitable for bolts of complex shapes, slightly lower anti-corrosion effect than hot-dip galvanizing, and moderate cost. Zinc-nickel alloy coating contains 10-15% nickel, with a thickness of 10-20μm, and its corrosion resistance is 3-5 times that of hot-dip galvanizing, suitable for heavy-load and high-humidity environments.
Why is Dacromet coating preferred for high-strength bolts?
High-strength bolts (10.9 grade and above) have high hardness after heat treatment. The traditional hot-dip galvanizing process is prone to hydrogen embrittlement, leading to bolt fracture. Dacromet coating does not require pickling, which can avoid hydrogen embrittlement risk. Dacromet coating has excellent high-temperature resistance, maintaining stable performance in environments below 200℃, suitable for heat-prone parts such as track joints. The coating has strong adhesion to the bolt surface, reaching level 1 of GB/T 9286 standard, not easy to fall off, and can withstand train vibration and impact. Dacromet coating has good lubricity, and the torque coefficient is stable when installing bolts, avoiding torque deviation caused by coating shedding. Its anti-corrosion life can reach more than 15 years, twice that of traditional hot-dip galvanizing, which can reduce the replacement frequency of high-strength bolts.
How to select anti-corrosion coatings for bolts in coastal areas?
Coastal areas have high salt spray concentration, so fluorocarbon coating bolts are preferred, which have the best salt spray corrosion resistance, can effectively resist seawater salt erosion, and extend service life. Secondly, zinc-nickel alloy coating bolts can be selected, with strong corrosion resistance and lower cost than fluorocarbon coating, suitable for large-scale applications. If hot-dip galvanizing coating is adopted, an additional topcoat treatment is required, with a topcoat thickness of ≥10μm to enhance anti-corrosion effect and make up for the deficiency of hot-dip galvanizing in salt spray environments. Ordinary mechanical galvanizing coating is prohibited, as its anti-corrosion performance is insufficient to cope with the strong corrosive environment in coastal areas, and early rust is prone to occur. When selecting, it is necessary to check the coating salt spray test report to ensure it passes a salt spray test of more than 3000 hours and meets the usage requirements.
What are the key points of construction quality control for bolt coatings?
Before coating, the bolt surface must be thoroughly cleaned to remove oil, rust and oxide scale. Sandblasting is used to make the surface roughness reach Ra2.5-5μm to enhance coating adhesion. The hot-dip galvanizing process needs to control the zinc liquid temperature at 440-460℃ and the zinc dipping time at 3-5 minutes to avoid bolt performance degradation due to excessive temperature. Dacromet coating construction must ensure uniform application, avoiding missing coating and sagging, and the drying temperature is controlled at 200-220℃ to ensure complete curing of the coating. Fluorocarbon coating needs to adopt electrostatic spraying process, with a spraying pressure of 0.3-0.5MPa to ensure uniform coating thickness without pinholes and bubbles. After coating, adhesion and thickness testing should be carried out; adhesion is tested by cross-cut method, and thickness is measured by coating thickness gauge. Unqualified products need to be reworked.
How to evaluate the long-term protection effect of bolt coatings?
Salt spray test is the core evaluation method. A neutral salt spray test chamber is used to simulate the corrosive environment, record the time when rust appears on the coating, and ≥2000 hours is qualified for long-term protection. The on-site coupon test needs to hang coated bolt samples in the actual service environment, observe the rust situation regularly, test once a year for 5 consecutive years, and evaluate the actual use effect. The coating wear test simulates the bolt installation and disassembly process, measures the coating wear loss, and ≤5μm/time is qualified to ensure that the anti-corrosion performance can be maintained after multiple disassembly and assembly. Electrochemical testing evaluates the corrosion current density by measuring the polarization curve of the coating; the smaller the current density, the better the anti-corrosion effect. Long-term tracking of the service status of installed bolts, recording the time when rust appears, and counting the average service life, as a direct basis for the long-term protection effect of the coating.