Selection of parts for fastener surface treatment(1)

Update:28 Mar 2020
Summary:

1.Electro-galvanizing Electro-galvanized is the most co […]

1.Electro-galvanizing
Electro-galvanized is the most commonly used coating for commercial fasteners. It is cheaper and looks better. It can be black or army green. However, its anti-corrosion performance is average, and its anti-corrosion performance is the lowest among zinc coatings. Generally, the neutral salt spray test of electrogalvanizing is within 72 hours, and a special sealant is also used to make the neutral salt spray test more than 200 hours, but the price is expensive, which is 5-8 times that of general galvanizing.
The galvanizing process is prone to generate hydrogen embrittlement, so bolts above 10.9 grade are generally not treated with galvanizing. Although the oven can be used to remove hydrogen after plating, but the passivation film will be damaged when the temperature is above 60 ° C, so the hydrogen removal must be It is performed before passivation after plating. This has poor operability and high processing costs. In reality, the general production plant will not take the initiative to remove hydrogen unless mandated by a specific customer.
Electrogalvanized fasteners have poor torque-pretension consistency and are unstable, and are generally not used for connection of important parts. In order to improve the consistency of torque-pretension force, the method of applying lubricating substance after plating can also be used to improve and increase the consistency of torque-pretension force.
2.Phosphating
Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. After phosphating, it should be oiled. The corrosion resistance has a great relationship with the performance of the oil. For example, after applying ordinary antirust oil after phosphating, the neutral salt spray test is only 10-20 hours. Coated with high-grade anti-rust oil, it can reach 72 ~ 96 hours. But its price is two to three times that of ordinary phosphating oil.
Two types of fastener phosphating are commonly used, zinc phosphating and manganese phosphating. Zinc-based phosphating has better lubricating performance than manganese-based phosphating. Manganese-based phosphating has good corrosion resistance and wear resistance. Its operating temperature can reach 225 degrees to 400 degrees Fahrenheit (107 ~ 204 ℃).
Many industrial fasteners are treated with phosphating oil. Because it has a good torque-pretension consistency, it can be used to ensure that the design meets the expected tightening requirements during assembly, so it is used more in industry. Especially the connection of some important parts. For example, steel structure joints, engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolt nuts, etc.
Phosphating of high strength bolts can also avoid hydrogen embrittlement, so bolts above 10.9 in the industrial field generally use phosphating surface treatment.