How to choose the surface treatment process of fasteners?

Almost all fasteners are made of carbon steel and alloy steel, and general fasteners are expected to prevent corrosion. In addition, the coating of surface treatment must adhere firmly.

As for surface treatment, people generally pay attention to beauty and corrosion protection, but the main function of fasteners is fastening connection, and surface treatment also has great influence on fastening performance of fasteners. Therefore, when selecting the surface treatment, we should also consider the factor of fastening performance, that is, the consistency of installation torque and preload.

1. Electroplating

Electroplating of fasteners means that the part of fasteners to be electroplated is immersed in a specific aqueous solution, which will contain some deposited metal compounds, so that after passing through the aqueous solution with current, the metal substances in the solution will precipitate and adhere to the immersed part of fasteners. Electroplating of fasteners generally includes galvanizing, copper, nickel, chromium, copper-nickel alloy, etc.

2. Phosphating

Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. There are two commonly used phosphating methods for fasteners, zinc phosphating and manganese phosphating. Zinc phosphating has better lubricating property than manganese phosphating, and manganese phosphating has better corrosion resistance and wear resistance than zinc plating. Phosphating products such as connecting rod bolts and nuts of engines, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.

3. Oxidation (blackening)

Blackening+oiling is a popular coating for industrial fasteners, because it is the cheapest and looks good before the fuel consumption is exhausted. Because blackening has almost no rust-proof ability, it will rust soon after it is oil-free. Even in the presence of oil, the neutral salt spray test can only reach 3~5 hours.

4. Hot dipping zinc

Hot galvanizing is a thermal diffusion coating in which zinc is heated to liquid. Its coating thickness is 15~100μm, and it is not easy to control, but it has good corrosion resistance, so it is often used in engineering. Due to the temperature of hot-dip zinc processing, (340-500C) it can’t be used for fasteners above grade 10.9. The price of hot-dip galvanizing of fasteners is higher than that of electroplating.

5. Zinc impregnation

Zinc impregnation is a solid metallurgical thermal diffusion coating of zinc powder. Its uniformity is good, and even layers can be obtained in threads and blind holes. The thickness of the coating is 10~110μm, and the error can be controlled within 10%. Its bonding strength and anti-corrosion performance with the substrate are the best among zinc coatings (electro-galvanizing, hot-dip galvanizing and dacromet). Its processing process is pollution-free and the most environmentally friendly. If we don’t consider chromium and environmental protection, it is actually the most suitable for high-strength fasteners with high anti-corrosion requirements.

The main purpose of the surface treatment of fasteners is to make the fasteners acquire anti-corrosion ability, so as to increase the reliability and adaptability of fasteners.

Post time: Dec-08-2022