Surface treatment of fasteners: galvanized, phosphating, blackening, chrome plating. Which one?

Almost all commercial #fasteners are made of carbon steel and alloy steel. Generally, they have anti-corrosion requirements, so the surface treatment coating must adhere firmly and cannot fall off during installation and removal. On the other hand, for threaded fasteners, the plating layer must be thin enough to #screw the thread after plating still. Generally, the temperature limit of the plate is lower than that of the fastener material, so the operating temperature requirements of the fastener need to be considered.

The main purpose of surface treatment is aesthetics and anti-corrosion. Since the primary function of fasteners is to fasten parts, the surface treatment also dramatically impacts their fastening performance. Therefore, when choosing a #surfacetreatment process, one should consider factors such as the torque and preload consistency of fasteners.

A high-level designer must not only consider the structural design and manufacturing process but also pay attention to the processability of the assembly and even the requirements of environmental protection and economy. The following briefly introduces some commonly used coatings for #fastener practitioners according to the above factors.

Electrogalvanized

Electro galvanizing is the most commonly used coating for #commercial fasteners. It is cheaper, has a better appearance, and can be available in black and army green. However, its anti-corrosion performance is average, and its anti-corrosion performance is the lowest in the zinc plating (coating) layer. Generally, the electro-galvanized neutral salt spray test is within 72 hours, and special sealing agents are also used so that the neutral salt spray test is more than 200 hours. Still, the price is high, which is 5~8 times that of general galvanizing.

The electro-galvanizing process is easy to produce hydrogen embrittlement, so bolts above grade 10.9 generally do not use #galvanized treatment. However, an oven can remove hydrogen after plating because it will destroy the passivation film at 60 °C or more, so hyitust be carried out before passivation after electroplating. Such as poor operability and high processing costs. In reality, general production plants will not actively dehydrogenate unless mandatory by specific customers.

Electrogalvanized fastener torque-preload consistency is poor and unstable and is generally not used for connection in essential parts. To improve the torque-preload texture, the torque-preload character can also be enhanced and improved by coating the lubricating substance after plating.

Phosphating

A fundamental principle: phosphating is cheaper than galvanizing, and corrosion resistance is worse than galvanizing. After phosphating, it should be oiled, and its corrosion resistance has a great relationship with the performance of the oil. For example, apply general anti-rust oil after phosphating, and the neutral salt spray test is only 10~20 hours. Coated with high-grade anti-rust oil, it can reach 72~96 hours. However, its price is 2~3 times that of general phosphating oil.

There are two commonly used fastener phosphating, zinc-based phosphating and manganese phosphating. Zinc phosphating lubrication performance is better than manganese phosphating, and manganese phosphating corrosion resistance and wear resistance is better than galvanized. Its use temperature can reach 225 degrees Fahrenheit to 400 degrees (107~204 °C). In particular, the connection of some essential components. Such as engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolt nuts, etc.

High-strength bolts use phosphating, which can also avoid the problem of hydrogen embrittlement, so bolts above grade 10.9 in the industrial field generally use phosphating surface treatment.

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 after being oil-free. Even in oily, the neutral salt spray test can only reach 3~5 hours.

Cadmium plating

Cadmium coating has good corrosion resistance, especially in the marine atmospheric environment, and the corrosion resistance is better than other surface treatments. The waste liquid treatment cost in the processing process of electronic cadmium plating is high, and the cost is high, and its price is about 15~20 times that of electro galvanizing. Therefore, it is not used in the general industry, only for some specific environments—for example, fasteners for oil drilling platforms and HNA aircraft.

Electrochromic plating

The chromium coating is very stable in the atmosphere, does not easily change color and tarnish, has high hardness, and has good wear resistance. Chrome plating on fasteners is generally used as a decorative function. It is rarely used in industrial fields with high anti-corrosion requirements because suitable chrome-plated pins are as expensive as stainless steel. When the strength of stainless steel is not enough, chrome-plated fasteners are used instead.

To prevent corrosion, copper and nickel should be plated before chrome plating. The chrome coating can withstand temperatures as high as 1200 degrees Fahrenheit (650°C). But it also has the same hydrogen embrittlement problem as electro galvanizing.

Nickel

The method of plating a layer of nickel on metals or some nonmetals by electrolysis or chemical methods is called nickel plating. Nickel plating is divided into electroplating nickel and electroless nickel plating.

It is mainly used in places where it is necessary to be both anti-corrosion and good conductivity. Such as the exit terminal of the vehicle battery.

Hot-dip zinc

Hot-dip zinc is a heat-diffusion coating of zinc heated into a liquid. Its coating thickness is 15~100μm, and it is not easy to control, but it has good corrosion resistance and is primarily used in engineering. The hot dip zinc processing process, including zinc waste and steam, is seriously polluted.

Due to the thick plating, the problem of brutal screwing of internal and external threads is caused in fasteners. Due to the temperature of hot-dip zinc processing (340-500C), it cannot be used for pins above grade 10.9.

Zinc penetration

Zinc penetration is a solid metallurgical, thermal diffusion coating for zinc powder. Its uniformity is good and can obtain a uniform layer in the thread and blind hole. The thickness of the coating is 10~110μm and can control the error at 10%. Its bonding strength and anti-corrosion properties with the substrate are the best in zinc coatings (electro-galvanized, hot-dip zinc, Dacromet). Its processing process is pollution-free and the most environmentally friendly.

Dacromet

Dacromet is a transliteration and abbreviation of DACROMET, abbreviated as Dacromet, Darkrust, and Dacron. Domestic named zinc chromium coating is a new anti-corrosion coating with zinc powder, aluminum powder, chromic acid, and deionized water as the main components.

There is no hydrogen embrittlement problem, and consistent torque-preload performance is good. If chromium valence and environmental protection issues are not considered, it is the most suitable for high-strength fasteners with high corrosion protection requirements.