Fasteners are a kind of basic parts with large quantity and wide application.
Their quality and reliability play an important role in the working performance of the main machine and the safety of the structure.
With China gradually becoming a global manufacturing center and the rapid development of domestic machinery, automotive and other industries, the demand for fasteners will continue to grow rapidly.
However, with the deterioration of the natural environment, people have put forward higher requirements for the surface corrosion resistance of fasteners, so the surface treatment of fasteners has also received more and more attention.
The traditional galvanizing processes include electro-galvanizing, hot-dip galvanizing and diffusion galvanizing.
Fasteners include bolts, screws, studs, nuts, self-tapping screws, wood screws, washers, retaining rings, pins, rivets, connecting pairs and assemblies, and other types. The surface galvanization requirements for these fasteners are mainly for two reasons: anti-corrosion and decoration, and anti-corrosion.
Their application environments generally include: indoor (such as home decoration, household appliances, etc.); outdoor (such as power, communication, warehouses, seaports and docks, etc., including rural atmospheric environment, industrial atmospheric environment, marine atmospheric environment).
In recent years, the fields of power, construction, transportation, and warehousing have all put forward higher requirements for fasteners:
(1) Longer service life.
Fasteners should maintain their original appearance after 4 years of service; they should be easily disassembled by hand tools after 6 years of service; and they must ensure safe connection after 10 years of service.
(2) Environmentally friendly.
On July 1, 2006, the EU officially implemented the RoHS (Environmental Protection Directive), prohibiting Cr6+ in electroplating. China’s green environmental protection directive also came into effect on March 1, 2007. It can be said that fastener products that do not comply with environmental protection directives and contain harmful and toxic substances are not allowed to enter the market.
(3) Facilitate automated assembly.
For example, when automotive fasteners are assembled by robots or automated tools, the torque each time should be basically the same, the friction coefficient produced by the coating should not be too large, and the coating on the threaded part should not affect the fit.
The galvanized coating is a typical anodic coating for steel fasteners, which provides electrochemical protection to the base metal. Additionally, due to its stability in the atmosphere and low cost, it is widely used in the anti-corrosion of fasteners.
In the past 20 years, mechanical galvanizing and Dacromet coating technology have been developed. Among the various galvanizing processes, the anti-corrosion effect of the galvanized layer is not closely related to the process itself.
Under the same service environment, its corrosion resistance is determined by the thickness of the galvanized layer. Therefore, when choosing the surface galvanizing treatment of fasteners, we should follow two principles: the thickness of the galvanized layer and economy.
The determination of the thickness of the galvanized layer mainly depends on the service environment.
Generally, for indoor fasteners, both anti-corrosion and decoration are important. Currently, electro-galvanizing is the preferred process. For outdoor fasteners, due to the relatively harsh service environment, galvanizing is mainly for anti-corrosion purposes. Hot-dip galvanizing, zinc diffusion, and mechanical galvanizing can be chosen.
However, at present, hot-dip galvanizing is mostly selected for outdoor fasteners both domestically and internationally. Mechanical galvanizing and zinc diffusion have not been widely promoted, which is related to the relatively backward development level of China’s manufacturing industry and the relevant industry standards.
For example, in China, only general technical specifications have been established for zinc diffusion and mechanical galvanizing. However, in the technical specifications of related industries (such as transportation, power, communication, and vehicles), most have put forward the quality requirements and inspection standards for the galvanized coating of fasteners through hot-dip galvanizing.
The many defects exposed in the process of hot-dip galvanizing treatment of fasteners have also promoted the rapid development of mechanical galvanizing and zinc diffusion.
Mechanical galvanizing and zinc diffusion are selected for threaded fasteners, which are highly efficient, cost-effective, and have good interchangeability. For high-strength bolts, mechanical galvanizing is an appropriate choice as it does not cause hydrogen embrittlement and does not reduce strength due to heating.
For instance, the surface of the hot-dip galvanized layer of anchor chains often has nodules and the thickness is extremely uneven. After switching to mechanical galvanizing, the appearance of the coating has been significantly improved, and the thickness of the coating is easier to control.
For elastic parts such as nail guns and cement nails, the thickness of the coating is not required to be thick (generally ≤ 10 μm), and hydrogen embrittlement must not occur. The tip of the nail is required to have a coating, and the appearance of the coating should be smooth and bright.
Only mechanical galvanizing can meet these requirements. Therefore, when choosing a galvanizing process for fasteners, in addition to considering the thickness and economic principles, it is also necessary to conduct specific analysis of specific problems and make the best choice based on the structural characteristics and performance requirements of the fasteners.
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