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Choosing the appropriate surface treatment is a problem faced by every designer. There are many available types of surface treatment. A high-level designer should consider not only the economic and practical aspects of the design, but also the feasibility of assembly, as well as environmental requirements. Below, in accordance with the above principles, some common plating types for fasteners are briefly introduced for reference by those working in the fastener industry. 01 Electro-galvanizing: Electro-galvanizing is the most commonly used coating for commercial fasteners. It is relatively inexpensive, has an attractive appearance, and is commonly available in black and army green. However, its corrosion resistance is average, being the lowest among zinc-plated (coated) layers. Typically, the neutral salt spray test for electro-galvanized surfaces lasts within 72 hours; however, special sealants can be used to extend this test period to over 200 hours, but such sealants are expensive, costing 5 to 8 times more than those used for ordinary galvanizing. Hydrogen embrittlement can easily occur during the electro-galvanizing process; therefore, bolts of grade 10.9 and above are generally not subjected to galvanizing treatment. Although degassing can be carried out in an oven after plating, the passivation film is damaged at temperatures above 60°C; therefore, degassing must be performed after plating but before passivation. This results in poor operability and high processing costs. In reality, manufacturers generally do not remove hydrogen on their own, unless required by specific customers. Fasteners with electro-galvanized plating exhibit poor consistency and instability in terms of torque-to-preload relationship, and are generally not used for connections in critical areas. To improve torque-preload consistency, the method of applying a lubricant after plating can also be used to enhance this consistency. 02 Phosphating: A basic principle. Phosphating is relatively cheaper than galvanizing, but its corrosion resistance is inferior to that of galvanizing. Oil should be applied after phosphating, and the level of corrosion resistance is greatly dependent on the properties of the oil used. For example, when a regular rust-inhibiting oil is applied after phosphating, the neutral salt spray test duration is only 10 to 20 hours. By applying high-quality rust preventive oil, the duration can reach 72 to 96 hours. However, its price is 2 to 3 times that of ordinary phosphating oil. The two commonly used types of phosphating for fasteners are zinc-based phosphating and manganese-based phosphating. Zinc-based phosphating has better lubricating properties than manganese-based phosphating, while manganese-based phosphating offers better corrosion resistance and wear resistance compared to galvanizing. Its operating temperature can range from 225 to 400 degrees Fahrenheit (107–204°C). Especially the connection of some key components. Such as the engine’s rod bolts and nuts, cylinder head, main bearings, flywheel bolts, and wheel bolt nuts, etc. High-strength bolts are phosphated, which also helps to prevent hydrogen embrittlement; therefore, in the industrial sector, bolts of grade 10.9 and above generally undergo phosphating treatment for their surfaces. 03 Oxidation (blackening): Blackening plus coating is a popular finish for industrial fasteners, as it is the cheapest option and keeps them looking good until the coating wears out. Since blackening provides almost no rust prevention, it will rust quickly after being oil-free. Even when there is oil present, its neutral salt spray test duration can only reach 3 to 5 hours. 04 Cadmium plating: The cadmium coating offers excellent corrosion resistance, especially in marine atmospheric environments, where its resistance to corrosion is better than that of other surface treatments. The waste liquid treatment costs in the processing of cadmium plating are high; the expense is approximately 15 to 20 times that of zinc plating. Therefore, it is not used in general industries, but only in certain specific environments. Such as fasteners for oil drilling platforms and marine aircraft. 05 Electroplated chromium: The chromium coating is very stable in the atmosphere; it does not easily change color or lose its luster, and it has high hardness as well as good wear resistance. A chromium plating on fasteners is generally used for decorative purposes. It is rarely used in industrial fields with high requirements for corrosion resistance, as good chromium-plated fasteners are just as expensive as stainless steel; chromium-plated fasteners are used only when the strength of stainless steel is insufficient. To prevent corrosion, copper and nickel should be plated first before chromium plating. The chromium coating can withstand high temperatures of 1200 degrees Fahrenheit (650°C). However, it also suffers from hydrogen embrittlement, just like electro-galvanizing. 06 Nickel plating is mainly used in applications where both corrosion resistance and good electrical conductivity are required. Such as the terminals of a vehicle battery, etc. 07 Hot-dip galvanizing: Hot-dip galvanizing involves heating zinc to a liquid state in order to apply a thermal diffusion coating. Its coating thickness ranges from 15 to 100 μm, and it is difficult to control; however, it has good corrosion resistance and is widely used in engineering applications. The hot-dip galvanizing process is highly polluting, generating zinc waste and zinc vapor, among other things. Due to the thick coating, it caused a problem in fasteners where the internal and external threads could not be screwed together. Due to the temperature requirements of hot-dip zinc plating (340–500°C), it cannot be used for fasteners of grade 10.9 or higher. 08 Zinc Plating: Zinc plating is a solid-state metallurgical thermal diffusion coating made of zinc powder. It has good uniformity, allowing for a uniform layer to be formed within threads and blind holes. The coating thickness is 10–110 μm, with the error able to be controlled within 10%. Its bonding strength to the substrate and corrosion resistance are the best among zinc coatings (electro-galvanizing, hot-dip galvanizing, Dacromet). Its processing process is pollution-free and the most environmentally friendly. 09 Dacromet does not suffer from hydrogen embrittlement, and it exhibits excellent consistency in torque-preload performance. If price of chromium and environmental concerns are not taken into account, Dacromet is actually the most suitable for high-strength fasteners with high corrosion resistance requirements.