Types Of Surface Finish Coating For Steel Fasteners

Fasteners are general term for mechanical parts used for fixing two or more parts. It usually includes: Bolts, studs, screws, nuts, tapping screws, gaskets, which almost made by carbon steel and alloy steel. Surface treatment is necessary for certain parts. The coating must be firm and installed without shedding. For screw thread fasteners, the coating must be thin enough that the thread can still be screwed after plating. Generally, the temperature limit of coating is lower than that of fasteners, so the operating temperature requirements should also be considered. Therefore, the following issues should be considered in surface treatment: material, appearance, service temperature, installation torque-preload consistency. Some commonly used coatings for fasteners are briefly described below for reference. 

 

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Electric galvanized

Electroplating is the most commonly used coating for commercial fasteners due to its low cost and good appearance. Zinc coating is relatively cheap and easy to plating, is widely used to protect iron and steel parts, especially to prevent atmospheric corrosion or decoration, but anti-corrosion performance is low. General electric galvanized salt spray test in 72 hours, there are special sealant to make neutral salt spray test up to more than 200 hours, but the price is expensive, 5~8 times of the general galvanized. It’s easy to produce hydrogen embrittlement in the electrogalvanizing process, so the bolts above 10.9 grade generally do not use galvanizing treatment. The torque - preload consistency of electrical galvanized fasteners is poor and unstable, generally not used for important parts of the connection. This defect can also be avoided by coating lubricant after plating.

 

Phosphating

Many industrial fasteners are treated by phosphating and oiled, generally zinc phosphating and manganese phosphating. Phosphating is cheaper than galvanized and poor corrosion resistance. Oiling should be applied after phosphating. The corrosion resistance is closely related to the performance of the oil. For example, the neutral salt spray test only takes 10~20 hours after applying general anti-rust oil after phosphating, and 72~96 hours for good anti-rust oil. Zinc phosphating lubrication is better than manganese phosphating, manganese phosphating corrosion resistance, good wear resistance. It can be used at 107~204℃. Phosphating torque - preload consistency is very good, often used in some important parts of the connection, such as steel structure connection pair, engine connecting rod bolts, nuts, cylinder head, main bearing, flywheel bolts, wheel bolts and nuts. Phosphating of high strength bolts avoids hydrogen embrittlement, so phosphating surface treatment is generally used for bolts of grade 10.9 or above in the industrial field.

 

Oxidation

Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest coating. However, this coating has little resistance to rust and can only be tested in a neutral salt spray for 3-5 hours even in the presence of oil. Blackened fastener torques - poor consistency of preload and can be improved by greasing the inner thread during assembly. 

 

Hot dip zinc coating

Hot-dipped zinc is a kind of coating which can prevent corrosion by immersing the derusted steel member in the zinc solution melted at 600℃. The process of hot dipping zinc will produce zinc waste and zinc vapor, which will cause heavy pollution. The thickness of zinc layer shall not be less than 65 microns for sheet metal below 5mm, 86 microns that of plate above 5mm. The thick coating makes it difficult to screw the inside and outside threads, and can be hot dipped in zinc after tapping the inside threads after plating or making the threads larger than the standard by about 0.16~0.75mm (M5~M30) during tapping and then hot dip zinc. Although these two methods solve the problem of screwing, they also reduce the anti-corrosion performance. Although these two methods solve the problem of screwing, they also reduce the anti-corrosion performance. At present, there is a new kind of anti-loose thread, the American "spicicot" internal thread, its internal and external thread are not fastened when the gap is large, allowing a thick coating, without affecting the screwing performance while maintaining the anti-corrosion properties and strength of number zero. It cannot be used for fasteners above grade 10.9 due to the temperature of hot dip zinc processing.     

 

Zinc impregnation

Zincizing is a zinc powder solid metallurgical thermal diffusion coating. It has good uniformity, and even the thread and blind hole can get even coating with 10-110 microns thickness coating, and the error can be controlled at 10%. Its binding strength and anticorrosive properties with the substrate in zinc coating (galvanized, hot dip zinc, dacromet) is the best and no pollution, no hydrogen embrittement, good torque - pretension consistent performance, suitable for high anticorrosive requirements of high strength fasteners.

 

Zinc Chrome coating (DACROMET)

This is a new anticorrosive coating with zinc powder, aluminum powder, chromic acid and deionized water as the main components. It has no hydrogen embrittlement and has excellent torque-preload consistency. If the contamination of hexavalent chromium is not considered, it is actually most suitable for high strength fasteners with high anticorrosive requirements.


In addition, there are surface finish methods such as cadmium plating, chromium plating, silver plating, nickel plating, zinc impregnation, etc. Experienced fastener buyers will recommend suitable surface treatment methods for customers according to the working conditions.