Commonly used fastener surface treatment methods

Update:16-09-2020
Summary:

The surface treatment of fasteners is a process of form […]

The surface treatment of fasteners is a process of forming a covering layer on its surface through a certain method. The purpose is to give it a beautiful surface and anti-corrosion effect. There are the following types:

Electroplating: Immerse the electroplated parts in an aqueous solution containing the deposited metal compound, and pass current through the plating solution to precipitate and deposit the electroplated metal on the parts. Generally, electroplating includes zinc, copper, nickel, chromium, copper-nickel alloys, etc. Sometimes boiled black (blue), phosphating, etc. are also included.

Hot-dip galvanizing: The carbon steel parts are immersed in a plating tank with molten zinc at a temperature of about 510°C. The result is that the iron-zinc alloy on the surface of the steel gradually turns into passivated zinc on the outer surface of the product. Hot-dip aluminum plating is a similar process.

Mechanical plating: The particles of the coated metal are used to impact the surface of the product, and the coating is cold welded to the surface of the product.

In fact, the most important intention is to make fasteners have better anti-corrosion function to increase the reliability and adaptability of its use. Therefore, the quality of fasteners' appearance treatment mainly depends on whether its anti-corrosion function meets the expected standards.

Electroplating

Electroplating is the process of using electrolysis to form a uniform, dense, and well-bonded metal or alloy deposition layer on the surface of the product. Compared with other metals, zinc is relatively cheap and easy to be plated. It can especially prevent atmospheric corrosion. It is a low-value anti-corrosion electroplating layer and is widely used to protect steel parts and for decoration.

 

 

According to different electroplating solutions, electro-galvanization is divided into four categories: cyanide zinc plating, chloride zinc plating, zincate zinc plating, and sulfate zinc plating.

Process flow:

Degreasing-hot water washing-cold water washing-strong corrosion-cold water washing-weak corrosion-cold water washing-neutralization-cold water washing-electrolytic plating / chemical plating-cold water washing- —After plating

Hydrogen embrittlement:

In the fastener production process, especially in the pickling before plating and subsequent electroplating, the surface absorbs hydrogen atoms, and then the deposited metal coating also traps hydrogen atoms. When the fastener is tightened, the hydrogen atoms are transferred to the stress concentration part, causing the pressure to increase to exceed the strength of the base metal, resulting in tiny surface cracks. Then the active hydrogen atoms quickly penetrate into the newly formed cracks, and this "pressure-rupture-infiltration" cycle continues until the fastener is broken. The above usually occurs within a few hours after the first stress.

Hydrogen removal process:

The heating for hydrogen removal is generally between 190-230°C for 4-24 hours, and preferably 1 hour after electroplating and before chromate treatment.

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