Detailed introduction of fastener surface treatment process

The choice of surface treatment process for fasteners is a problem faced by every designer. There are many types of surface treatment available, but only one principle is “economical and practical”.

As almost all commercial fasteners are made of carbon steel and alloy steel, general fasteners are expected to prevent corrosion. In addition, the coating of surface treatment must be firmly attached and cannot fall off during installation and removal. For threaded fasteners, the coating must be thin enough so that the threads can still be rotated after plating. Generally, the temperature limit of plating is lower than that of fastener materials, so the operating temperature requirements of fasteners should also be considered.

For surface treatment, people generally pay attention to aesthetics and anti-corrosion, but the main function of fasteners is to fasten parts, and surface treatment also has a great impact on the fastening performance of fasteners. Therefore, when selecting surface treatment, the factors of fastening performance should also be considered, that is, the consistency of installation torque and preload.

A high-level designer should not only consider the design, but also pay attention to the assembly process and even environmental requirements.

According to the above factors, the plating commonly used for some fasteners is briefly introduced below for the reference of fastener practitioners.

Galvanization

Electrogalvanizing is the most commonly used coating for commercial fasteners. It is relatively cheap and looks good. It can be black or military green. However, its anti-corrosion performance is general, and its anti-corrosion performance is the lowest among zinc plating (coating) layers. In general, neutral salt spray test of galvanization is conducted within 72 hours, and special sealing agent is also used to make neutral salt spray test more than 200 hours, but the price is expensive, which is 5~8 times of that of general galvanization.

Hydrogen embrittlement is easy to occur in the process of galvanization, so the bolts above 10.9 level are generally not subject to galvanization treatment. Although the hydrogen can be removed in an oven after galvanization, the passivation film will be damaged when the temperature is above 60 ℃, so the hydrogen must be removed after galvanization and before passivation. So the operability is poor and the processing cost is high. In reality, general manufacturers will not actively remove hydrogen unless required by specific customers.

The torque preload consistency of the galvanized fasteners is poor and unstable, so they are generally not used for the connection of important parts. In order to improve the consistency of torque and preload, the method of coating lubricant after plating can also be used to improve the consistency of torque and preload.

Phosphating

One basic principle is that phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. After phosphating, it shall be coated with oil, and its corrosion resistance is greatly related to the performance of the oil coated. For example, after phosphating, general antirust oil is applied, and the neutral salt spray test is only 10-20 hours. It can take 72~96 hours to apply high-grade antirust oil. But its price is 2~3 times that of general phosphating coating oil.

There are two kinds of phosphating commonly used for fasteners, zinc phosphating and manganese phosphating. Zinc phosphating has better lubrication performance than manganese phosphating, and manganese phosphating has better corrosion resistance and wear resistance than zinc plating. Its operating temperature can reach 225 ° F to 400 ° F (107~204 ° C).

Especially the connection of some important components. For example, connecting rod bolts and nuts of the engine, cylinder head, main bearing, flywheel bolts, wheel bolts and nuts, etc.

Phosphating is adopted for high-strength bolts, which can also avoid hydrogen embrittlement. Therefore, phosphating surface treatment is generally adopted for bolts above 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 before the oil is used up. Since blackening has almost no anti rust ability, it will rust soon after there is no oil. Even in the presence of oil, the intermediate salt spray test can only reach 3-5 hours

Cadmium electroplating

The corrosion resistance of cadmium coating is very good, especially in marine atmospheric environment. The cost of waste liquid treatment in the process of cadmium electroplating is high, and its price is about 15-20 times that of electroplating. Therefore, it is not used in general industries, but only for some specific environments. For example, it is used for fasteners for oil drilling platforms and HNA aircraft.

Chromium electroplating

The chromium coating is stable in the atmosphere, not easy to change color and lose luster, and has high hardness and good wear resistance. Chromium plating on fasteners is generally used as decoration. It is rarely used in industries with high corrosion resistance requirements, because good chrome plated fasteners are as expensive as stainless steel. Only when the strength of stainless steel is not enough, chrome plated fasteners can be used instead.

To prevent corrosion, copper and nickel should be plated before chrome plating. The chromium coating can withstand the high temperature of 1200 degrees Fahrenheit (650 ℃). However, hydrogen embrittlement is the same as that of electrogalvanizing.

plate with nickel

It is mainly used in places with good corrosion resistance and conductivity. Such as lead out terminal of vehicle battery.

Hot-dip galvanizing

Hot dip galvanizing is a thermal diffusion coating of zinc heated to liquid. The coating thickness is 15~100 μ m. It is not easy to control, but has good corrosion resistance and is mostly used in engineering. Hot dip galvanizing process is seriously polluted, including zinc waste and zinc vapor.

Due to the thick coating, it is difficult to screw the internal and external threads in fasteners.

Due to the temperature of hot dip galvanizing, (340-500C) cannot be used for fasteners above 10.9.