Why should the pin coupling be blackened?

The blackening treatment of the pin coupling is to produce an oxide film on the metal surface to block air and achieve the purpose of rust prevention.

Blackening is a common method of chemical surface treatment. The principle is to produce an oxide film on the metal surface to isolate the air and achieve the purpose of rust prevention.Blackening treatment can be used when the appearance requirements are not high. The surface blackening treatment of steel parts is also called blue.

Purpose of blackening treatment:

(1) Prevent rust on the surface of fasteners;

(2) Increase the gloss of the fastener surface to improve the aesthetics;

(3) Reduce the stress in the quenching process.

Processing principle:

When the pin coupling is immersed in the blackening liquid, first, acid corrosion, metal dissolution, and metal iron are oxidized to divalent Fe2+; copper and iron undergo a substitution reaction, and Cu2+ is reduced to metallic copper and deposited on the metal surface; The addition of an oxidation promoter in the solution accelerates the reduction of the tetravalent selenium in the nitrite to Se2-, and promotes the oxidation and reduction of the metal on the surface of the workpiece.Due to the strong oxidizing property of nitrite, it can quickly oxidize Cu deposited on the metal surface to Cu2+, and react with divalent selenium to form copper selenide.But the matters needing attention are the control of the blackening speed of the bath liquid.The blackening speed of the bath liquid has an important effect on the quality of the film. The higher the blackening speed, the darker the film will be and the looser the adhesion and corrosion resistance will be poor. The lower the blackening speed, the worse the film blackness, but the fine crystallinity. , The corrosion resistance is not improved, usually the speed is controlled within 3-8min to reach a uniform depth; the pH value of the bath liquid is controlled between 1 and 3, practice has proved that the lower the PH value, the faster the blackening speed, and the film layer The darker it is, the looser the balance of the organization, and vice versa; the film is treated.The process of passivation treatment should be cleaned. If there are burrs, a brush can be used to not affect the quality of the film.For immersed in dehydrated anti-rust oil for 10h, the performance will be used later.

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