How to choose the surface treatment of fasteners?

Almost all fasteners on the market are made of carbon steel and alloy steel. The main purpose of surface treatment is aesthetics and anti-corrosion. The surface treated coating must be firmly attached and cannot fall off during installation and removal. And the surface treatment has a great influence on its fastening performance. Therefore, when selecting the surface treatment process, factors such as the torque of the fastener and the consistency of the preload should be considered. When carrying out the surface treatment of fasteners, Shimai should not only consider the structural design and manufacturing process, but also pay attention to the manufacturability of the assembly, and even the requirements of environmental protection and economy. The following is a brief introduction to some commonly used fastener coatings based on the above factors for the reference of fastener practitioners.

Phosphorization

Phosphorization is a process of chemical and electrochemical reactions to form phosphate chemical conversion film. The formed phosphate conversion film is called phosphating film. The thickness of the film layer is generally 10-15 microns, and the friction coefficient is about 0.05.

The main purpose of phosphating is to provide protection to the base metal and prevent the metal from being corroded to a certain extent; used for primer before painting to improve the adhesion and anti-corrosion ability of the paint film layer; Use for lubrication.

A basic principle is that phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. Oil should be applied after phosphating, and its corrosion resistance has a great relationship with the performance of the oil applied. For example, if you apply ordinary anti-rust oil after phosphating, the neutral salt spray test is only 10 to 20 hours. With high-grade anti-rust oil, it can reach 72 ~ 96 hours. But its price is 2 ~ 3 times that of general phosphating oil.Drywall screws phosphating is divided into black phosphated and gray phosphated. Black phosphorus drywall screws have stronger corrosion resistance and a more beautiful surface than oxidized drywall screws. However, due to process reasons, long pickling time will reduce screw strength and toughness, easy to break.

Nickel Plated

A method of plating a layer of nickel on a metal or some non-metals by electrolytic or chemical methods is called nickel plating. Nickel plating is divided into electroplated nickel and electroless nickel plating.

It is mainly used in places where anti-corrosion and good conductivity are required. Such as the lead-out terminals of vehicle batteries.

Dacromet

Advantage:

Dacromet is a new type of surface treatment technology. Compared with the traditional electroplating process, Dacromet is "green electroplating". The advantages are as follows:

1) Super corrosion resistance

2) No hydrogen embrittlement

3) High heat resistance

4) Good adhesion and recoating performance

5) Good permeability

6) No pollution

Defect:

1) Dacromet contains chromium ions harmful to the environment and the human body, especially hexavalent chromium ions have carcinogenic effects.

2) Dacromet has higher sintering temperature, longer time, and large energy consumption.

3) The surface hardness of Dacromet is not high, and the wear resistance is not good, and the products of Dacromet are not suitable for contact and connection with copper, magnesium, nickel and stainless steel parts, because they will cause contact corrosion Affect the surface quality and anti-corrosion performance of products.

4) The surface color of the Dacromet coating is single, only silver-white and silver-gray, which is not suitable for the individual needs of automobile development.

5) The conductivity of Dacromet coating is not very good, so it is not suitable for conductive connection parts, such as grounding bolts of electrical appliances.