The purpose of fastener surface treatment is to improve the corrosion resistance of fasteners, improve the performance of use, and so on, according to the requirements of different quality, choose the surface treatment category. What kind of surface treatment does fastener have?
Blackening + oiling is a popular coating for industrial fasteners, because it's the cheapest and looks good before the fuel consumption goes on. Because of blackening, almost no rust resistance, so no oil, it will soon rust. Under the condition of oil, the salt fog test can only reach 3~5 hours.
Black fasteners torque - preload consistency is also very poor. If you want to improve, you can grease the thread on the inside of the assembly and then spin it.
Two. Zinc penetration
Zinc diffusion as zinc powder solid metallurgical thermal diffusion coating. The uniformity is good, and the uniform layer can be obtained in the thread and blind hole. The thickness of the coating is 10~110 m, and the error can be controlled at 10%. It combines the strength and corrosion resistance of zinc coating on substrate (electricity galvanized, hot dip galvanizing, dacromat) is the best. The process is pollution-free, the most environmentally friendly.
Three. Zinc plating
Zinc plating is the most commonly used coating for commercial fasteners. It's cheaper and looks better. It's black and military green. However, its anti-corrosion properties are general, and its anti-corrosion performance is the lowest in the zinc coating (coating) layer. General electric galvanized neutral salt spray test in 72 hours, but also the use of special sealant, the neutral salt spray test for more than 200 hours, but the price is expensive, is 5~8 times of galvanized.
Electro galvanized process is easy to produce hydrogen, so the bolt above 10.9 level generally do not use galvanized processing. Although plating can be used after the oven to hydrogen, but due to the passivation film in 60 degrees Celsius above will be destroyed, it must be carried out in hydrogen after plating passivation. It is of poor operability and high processing cost. In reality, the general manufacturer will not take the initiative to remove hydrogen unless the mandatory requirements of a particular customer.
The tightening torque of the fastener of galvanized steel is poor and unstable, and it is not used in the connection of important parts. In order to improve the consistency of torque pretightening force, the method of coating lubricating material after plating can also be used to improve and improve the consistency of torque preload.
Four. Cadmium plating
The corrosion resistance of cadmium coating is very good, especially in marine atmosphere environment. In the process of cadmium plating, the cost of waste liquid treatment is large and the cost is high, and the price is about 15~20 times of that of zinc plating. So in the general industry does not use, only for some specific environment. For example, fasteners used in oil drilling platforms and HNA aircraft.
Five. Chromium plating
Chromium coating is stable in the atmosphere, it is not easy to tarnish and tarnish, high hardness, good wear resistance. Chromium coating on fasteners is generally used as decoration. It is seldom used in industrial fields with high corrosion resistance, because good chrome plating fasteners are as expensive as stainless steel, but only with chromium plated fasteners instead of stainless steel strength.