Surface Treatment Processes and Corrosion Resistance Enhancement Strategies for Spring Clips
Why do elastic strips not adopt the electro-galvanizing surface treatment process but prefer hot-dip galvanizing?
The core reason why elastic strips do not adopt electro-galvanizing is that the electro-galvanized layer is thin (8-10μm) and has low adhesion strength, which cannot meet the service requirements of elastic strips for long-term elastic deformation. Elastic strips need to undergo repeated elastic deformation during work. The electro-galvanized layer has poor toughness and is prone to peeling and falling off during deformation. Once the coating is damaged, the internal steel will rust quickly. The hot-dip galvanized layer can reach a thickness of 60-80μm and is metallurgically bonded to the steel with extremely high adhesion strength. When the elastic strip undergoes repeated elastic deformation, the coating is not easy to fall off and can continuously provide protection. In addition, the corrosion resistance of the hot-dip galvanized layer is 5-8 times that of electro-galvanizing, which can adapt to the corrosive environment of open lines. Therefore, even if the cost of hot-dip galvanizing is higher than that of electro-galvanizing, it is still the mainstream choice for surface treatment of elastic strips.
Which special environments are suitable for the Dacromet surface treatment process for elastic strip protection?
The Dacromet surface treatment process has excellent salt spray resistance, humidity and heat resistance, and hydrogen embrittlement resistance, and is mainly suitable for the protection of elastic strips in coastal salt spray areas, chemical pollution areas and high-humidity rainy areas. The chloride ion concentration in coastal salt spray areas is high, and the ordinary hot-dip galvanized layer is prone to pitting corrosion. The Dacromet coating has a salt spray resistance time of more than 1000h, which can effectively resist chloride ion corrosion. Corrosive gases such as sulfur dioxide and hydrogen sulfide in chemical pollution areas will accelerate the corrosion of elastic strips. The Dacromet coating has good chemical stability, does not react with acidic or alkaline gases, and has a significant protective effect. Elastic strips in high-humidity and rainy areas are prone to electrochemical corrosion. The Dacromet coating is a uniform zinc-chromium coating without pinhole defects, which can isolate moisture from contact with steel. In addition, the Dacromet coating has no risk of hydrogen embrittlement and is suitable for the surface treatment of high-strength elastic strips.
What are the main protective effects of the sealing coating after the surface treatment of elastic strips?
The sealing coating after the surface treatment of elastic strips is an epoxy or polyurethane coating sprayed on the surface of the hot-dip galvanized or Dacromet coating, which mainly plays three protective roles. First, seal pinholes. The surface of the hot-dip galvanized coating inevitably has tiny pinholes. The sealing coating can fill these pinholes to prevent moisture from penetrating to the steel surface through the pinholes and causing pitting corrosion. Second, improve wear resistance. Micro-friction will occur when the elastic strip contacts the rail and pressure plate. The sealing coating has high hardness, which can reduce coating wear and extend the protection life. Third, enhance weather resistance. The sealing coating can resist ultraviolet radiation and temperature changes, preventing the underlying coating from aging and chalking. The thickness of the sealing coating is generally 15-20μm, which must ensure uniform coverage of all surfaces of the elastic strip, especially the stress concentration parts such as the bending parts of the elastic strip.
What technical problems need to be solved focusly in the surface treatment of elastic strips in alpine regions?
The surface treatment of elastic strips in alpine regions needs to focus on solving three technical problems: coating low-temperature brittle fracture, freeze-thaw cycle damage and ice and snow wear. The temperature in alpine regions can be below -40℃ in winter. The toughness of ordinary hot-dip galvanized coatings decreases at low temperatures, and the elastic strips are prone to brittle fracture and falling off when deformed. It is necessary to select a modified hot-dip galvanizing process with good low-temperature toughness. Freeze-thaw cycles will generate peeling stress between the coating and the steel, leading to coating peeling. It is necessary to improve the coating adhesion strength to resist damage caused by freeze-thaw. In ice and snow weather, the ice and snow on the surface of the elastic strip will rub against the coating, accelerating coating wear. It is necessary to add a wear-resistant sealing layer on the coating surface to improve wear resistance. In addition, deicing salt in alpine regions will aggravate corrosion. The surface treatment of elastic strips should adopt a process with excellent salt corrosion resistance, such as Dacromet + sealing coating.
What are the on-site inspection indicators for the surface coating of elastic strips, and how to judge whether they meet the standard?
The on-site inspection of the elastic strip surface coating mainly includes three core indicators: coating thickness, adhesion strength and appearance quality, which can be judged with simple inspection tools. The coating thickness is detected with a coating thickness gauge. The thickness of the hot-dip galvanized coating should be ≥60μm, the Dacromet coating ≥8μm, and the sealing coating ≥15μm, with uniform thickness and deviation ≤±10%. The adhesion strength is detected by the cross-cut test. Use a blade to cut 1mm×1mm squares on the coating surface, stick them with tape and tear them off. If the coating has no peeling or lifting, it is qualified. The appearance quality is inspected visually. The coating surface should be smooth and uniform, without bubbles, cracks, sagging and exposed bottom, and no coating accumulation at the bending parts of the elastic strip. If the inspection finds that the indicators are unqualified, the elastic strips shall not be put into use and need to be re-surface treated.