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Summary of screw surface treatment process and corrosion resistance

Summary of screw surface treatment process and corrosion resistance

The commonly used surface treatment processes for screws include oxidation, electrophoresis, electroplating, and Dacromet. The following are mainly classified and summarized by the color of the surface treatment of screws.

  1. Black plated screws
  2. Oxidation black:

It is divided into normal temperature blackening and high temperature blackening. Taking normal temperature blackening as an example, the process flow is: chemical degreasing – hot water washing – cold water washing – rust removal and acid etching – cleaning – blackening – cleaning – over oil or over sealing. It is an oxide film formed by sodium hydroxide and sodium nitrite at a high temperature of more than 100 degrees.

The main component of the oxide film is iron tetroxide, the film uniformity is only 0.6-1.5um, and the corrosion resistance is relatively poor. In the case of no oil or sealing, the neutral salt spray only takes about 1-2 hours. in about 3-4 hours.

  1. Electroplating:

The electroplating process is: degreasing-cleaning-weak acid etching-electro-galvanizing-cleaning-passivation-cleaning-drying-passing frame; in the end, mainly black oil (a kind of paint), Tianna water, rubber water according to a certain amount If the ratio is good, its main function is to improve the black uniformity and brightness of the screw surface and improve the corrosion resistance of the screw. The resistance to neutral salt spray is above 20.

  1. White plated screws

The electroplating process is: degreasing-cleaning-weak acid activation-electro-galvanizing-cleaning-white passivation-cleaning-drying, because of the neutral salt spray test of the screws with white zinc-plated surface treatment process, the initial appearance of surface coating corrosion The phenomenon of whitening and red rust occurs in about 40 hours, so the corrosion resistance of white zinc is better than that of white nickel. Compared with white nickel, the appearance is darker. White zinc is the primary color of zinc, which is bluish white, which is quite different from white nickel.

  1. Coated screws of other colors

Generally only for in-machine use, mainly including blue zinc, color zinc, and Dacromet.

  1. Blue zinc: Its process is roughly the same as that of white zinc. Blue zinc is a passivated zinc oxide film containing trivalent chromium of 0.5-0.6 mg/dm2. For its corrosion resistance, blue zinc is better than white zinc. The color of blue zinc is relatively close to that of white zinc, which is widely used in the industry, and can also be used as an alternative process screw in product design.
  2. Color zinc:

In the galvanizing process, colored zinc has relatively good corrosion resistance. The color passivation process is: galvanizing – cleaning – 2%-3% nitric acid to emit light – cleaning – low chromium color passivation – cleaning – baking Baked and aged. The passivation temperature is too low, the film formation is slow, and the color pale film is thin. The temperature is high, the film is thick and loose, and the adhesion is not strong. It’s best to control around 25 degrees to ensure that you get the same color for a certain amount of time.

After passivation, it must be baked and aged to improve the adhesion and corrosion resistance of the film. The color zinc-plated screw is resistant to neutral salt spray for more than 48 hours, and it can be controlled for more than 100 hours.

  1. Dacromet:

It is the abbreviation and transliteration of DACROMET, that is, flaky zinc-based chromium salt protective coating, also called zinc-aluminum coating. The basic process is: degreasing – degreasing – coating – preheating – sintering – cooling. The process generally has 2-4 times from the coating to the cooling process, because the screw is dipped to reach a certain thickness, it needs to be done several times.

The structure is to coat the metal surface with a layer of Dacromet solution (that is, a highly dispersed mixed aqueous solution containing scaly zinc, aluminum [the general size of the scales is 0.1-0.2X10-15 microns] Cr03 and special organics), after 300° After holding and baking for a certain period of time around C, the hexavalent chromium in the Dacromet solution is reduced to trivalent chromium to form an amorphous composite chromate compound (nCr03 mCr203).

The corrosion resistance is very good. The neutral salt can reach more than 300 hours. The disadvantage is that the coating is uneven. The thin position is 5-10um, and the thick position is about 40um, which will affect the depth of the screw tooth diameter. Smaller diameter screws are fine. Do not use Dacromet as a surface treatment.