The surface treatment of rapid prototype fasteners: galvanizing, phosphating, blackening, chrome plating which one should be chosen?
The surface treatment of rapid prototype fasteners: galvanizing, phosphating, blackening, chrome plating which one should be chosen?
Almost all commercial fasteners are made from carbon steel alloy steel, and generally anti-corrosion requirements, therefore, surface treatment coating must be attached firmly, cannot fall off in the process of installation and remove, on the other hand, for threaded fasteners, thin coating also requires enough, make the threaded can still generally after plating coating temperature limit is lower than fastener materials, therefore also take into account the working temperature of fasteners are requiredSince the main function of fasteners is to tighten parts, and the surface treatment also has a great impact on its fastening performance, the selection of surface treatment process, the fastener's torque and the consistency of pre-tightening force and other factors should be considered
A high-level designer should not only consider the structural design and manufacturing process, but also pay attention to the requirements of assembly technology, even environmental and economic requirements. The following briefly introduces some commonly used coatings of fasteners according to the above factors, for the reference of fastener practitioners
Electrogalvanizing
Electric galvanizing is the most commonly used coating business fasteners, it is cheaper, look more good-looking, also can have black army green, however, its anticorrosion performance in general, the anticorrosion performance is the lowest in zinc plating layer of the tu) (general electric galvanized within 72 hours of salt spray test, also have the use of special sealing agent, the neutral salt spray test of 200 hours, but the price is expensive, 5 ~ 8 times that of general galvanized
Electrogalvanizing process is easy to produce hydrogen embrittlement, so the bolts above 10.9 generally do not use galvanizing treatment.Although after plating can be used oven dehydrogenation, but due to the passivation film in more than 60 will be damaged, so dehydrogenation must be carried out after plating passivation so operability is poor, high processing cost in reality, the general production plant will not take the initiative to dehydrogenation, unless the specific customer's mandatory requirements
In order to improve the consistency of the torque preload, the method of coating lubricating material after plating can also be used to improve and improve the consistency of the torque preload
Phosphorization
A basic principle is that phosphating is cheaper than galvanizing, and corrosion resistance is worse than galvanizing.After phosphating should be coated with oil, its corrosion resistance and the performance of the oil has a great relationship.For example, after phosphating coated with general anti-rust oil, the neutral salt spray test is only 10 to 20 hours.Apply high - grade anti - rust oil, can be up to 72-96 hours.But its price is general phosphating besmear 2~3 times.
Two common types of fastener phosphating are zinc and manganese series phosphating.Zinc phosphating lubrication performance than manganese phosphating, manganese phosphating corrosion resistance, wear resistance is better than galvanized.It can be used at temperatures from 225 to 400 degrees Fahrenheit (107 to 204 degrees Celsius).Especially the connection of some important parts.For example, engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating is used for high-strength bolts, which can also avoid hydrogen embrittement. Therefore, phosphating surface treatment is generally used for bolts above grade 10.9 in the industrial field.
Oxidation (blackening)
Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good until the oil runs out.Since blackening has almost no rust resistance, it rusts quickly when it is oil-free.Even in the oil state, the neutral salt spray test can only reach 3~5 hours.
Cadmium plating
Cadmium coating corrosion resistance is very good, especially in the Marine atmosphere environment corrosion resistance is better than other surface treatment.In the process of electroplating cadmium, the waste liquid treatment cost is high, the cost is high, its price is about 15~20 times that of electroplating zinc.So it is not used in the general industry, only for some specific environment.For example, fasteners for oil RIGS and hNA aircraft.
Chromium plating
Chromium coating in the atmosphere is very stable, not easy to change color and luster, hardness and good wear resistance.Chromium plating on fasteners is generally used for decorative purposes.It is rarely used in industries with high corrosion resistance requirements, because good chrome-plated fasteners are as expensive as stainless steel, but when stainless steel is not strong enough, chrome-plated fasteners are used instead.
To prevent corrosion, copper and nickel should be plated before chromium plating.Chromium coatings can withstand temperatures of 1200 degrees F (650 degrees C).But it also has hydrogen embrittlement problem with electric galvanizing.
Nickel plating
The process of plating a layer of nickel on a metal or some nonmetals by electrolysis or chemistry is called nickel plating.Nickel plating consists of electroless nickel plating and electroless nickel plating.
It is mainly used in places with good anticorrosion and electrical conductivity.Such as the vehicle battery terminals, etc.
Hot dip zinc
Hot dip zinc is a thermal diffusion coating of zinc heated to liquid.Its coating thickness is 15~100 m, and not easy to control, but good corrosion resistance, used in engineering.Hot dip zinc processing in the serious pollution, such as zinc waste and zinc steam.Due to the thickness of the coating, the internal and external threads are difficult to screw in the fastener.It is not suitable for fasteners of class 10.9 or above due to the temperature of hot dip zinc processing.
Zinc impregnation
Zincizing is a zinc powder solid metallurgical thermal diffusion coating.Its uniformity is good, and the uniform layer can be obtained in thread and blind hole.The coating thickness is 10~110 m, and the error can be controlled in 10%.Its bonding strength and anti-corrosion properties with the matrix are the best among zinc coatings (electric galvanizing, hot dipping zinc, dacromet).Its processing process pollution-free, the most environmental protection.
