Surface treatment of fasteners: hot rust, galvanizing, phosphating, blackening, which should be chosen?
Almost all fasteners are made of carbon steel, alloy steel, and generally have corrosion resistance requirements, therefore, the surface treatment of the coating must adhere firmly, can not fall off in the process of installation and unloading. On the other hand, for threaded fasteners, the coating is required to be thin enough so that the threads can still be screwed after plating. Generally, the temperature limit of the coating is lower than that of the fastener material, so the operating temperature requirements of the fastener should also be considered.
The main purpose of surface treatment is aesthetics and corrosion protection. Because the main function of fasteners is fastening parts, and the surface treatment also has a great influence on its fastening performance. Therefore, when choosing the surface treatment process, the fastener torque and the consistency of the preload should be considered.
Hot rust treasure
Hot rust Treasure is a kind of coating for building screws, its main performance is anti-corrosion, salt spray time 500-1500H; Colorful, can be modulated according to customer requirements, to meet customer requirements; The baking temperature is relatively low, generally controlled within 200 degrees Celsius, especially building screws or drilling nails
Electrogalvanizing is the most commonly used coating for commercial fasteners. It looks better. However, its anticorrosion performance is average, its anticorrosion performance is the lowest zinc plating (coating).
The electrogalvanizing process is easy to produce hydrogen embrittlement, so bolts above grade 10.9 generally do not use galvanized treatment. Although the oven can be used to dehydrogen after plating, but because the passivation film will be destroyed at more than 60℃, so dehydrogen must be carried out before passivation after plating. Such poor operability, high processing costs. Galvanized fasteners have poor torque-preload consistency and are unstable and generally not used for important parts of the connection. In order to improve the torque-preload consistency, the method of coating lubricating material after plating can also be used to improve and improve the torquE-preload consistency.
After phosphating should be coated with oil, its corrosion resistance and the performance of the oil has a great relationship. Two commonly used fastener phosphating, zinc phosphating and manganese phosphating. Zinc phosphating lubrication performance is better than manganese phosphating, manganese phosphating corrosion resistance, wear resistance is better than galvanized. It can be used at temperatures ranging from 225 degrees Fahrenheit to 400 degrees Fahrenheit (107 to 204 degrees Celsius). Especially the connection of some important parts.
High strength bolts using phosphating, can also avoid the problem of hydrogen embrittling, so in the industrial field of 10.9 grade above bolts generally use phosphating surface treatment.
Blackening + oiling is a popular coating for industrial fasteners that looks good until the oil runs out. Since blackening has little rust resistance, it will rust quickly without oil. Even in the state of oil, the neutral salt spray test can only reach 3 ~ 5 hours.
The electrolysis or chemical method of coating a metal or some nonmetals with a layer of nickel, called nickel plating. Nickel plating Nickel plating and electroless nickel plating.
Mainly used for not only corrosion, but also good electrical conductivity.
Hot dip zinc
Hot dip zinc is a coating of zinc heated to a liquid under heat diffusion. The coating thickness is 15~100μm, and is not easy to control, but the corrosion resistance is good, most used in engineering. In the process of hot dip zinc, there are serious pollution, such as zinc waste and zinc vapor.
Because of the thick coating, it is difficult to screw the inner and outer threads in fasteners. Due to the temperature of hot dip zinc processing, (340-500C) it cannot be used for fasteners of grade 10.9 or above.
Zincizing is zinc powder solid state metallurgical thermal diffusion coating. It has good uniformity, uniform layer can be obtained in thread and blind hole. The coating thickness is 10 ~ 110μm, and the error can be controlled by 10%. It has the best binding strength and anticorrosion performance in zinc coating (electrogalvanizing, hot dip zinc, dacro). Its processing process pollution-free, the most environmentally friendly.
DACROMET is dacromet translation and abbreviation, abbreviation Dacromet, Dacromet, Dicron. It is a new type of anticorrosive coating mainly composed of zinc powder, aluminum powder, chromic acid and deionized water.
There is no hydrogen embrittlement problem, and the torque-preload consistency performance is very good. Most suitable for high strength fasteners with high anticorrosion requirements.