Screw surface treatment process

Screws commonly used surface treatment processes are oxidation, electrophoresis, electroplating, Dacromet four categories, the following is mainly to screw the colour of the surface treatment of the classification summary.

• Black oxide：

Divided into room temperature blackening and high temperature blackening, to room temperature blackening as an example of the process is: chemical degreasing – hot water washing – cold water washing – rust removal and acid etching – cleaning – blackening – cleaning -Over oil or over closed. It is a layer of oxide film formed by sodium hydroxide and sodium nitrite at a high temperature greater than 100 degrees.

The main component of the oxide film is iron tetraoxide (Fe3C4), the film uniformity is only 0.6-1.5um, corrosion resistance is relatively poor, in the case of not over the oil or closed neutral salt spray only 1-2 hours or so, over the oil in 3-4 hours or so. Small appliances do not use this process for the time being screws. Distinguished from the appearance of the colour, black oxide and black zinc and electrophoretic black close, but not as bright as black zinc and electrophoretic black colour.

• Galvanise：

Black electroplating has two kinds of black zinc and black nickel, the principle of the process is basically the same, only the formulation of electroplating solution and post-treatment with different lattice or passivation solution. Zinc is chemically active, easy to oxidise and darken in the atmosphere, and finally produce ‘white rust’ corrosion, zinc plating after chromate treatment in order to cover a layer of zinc on the chemical conversion film, so that the active metal is in a passive state, is the passivation of zinc layer. Passivation film from the appearance can be divided into white passivation (white zinc), light blue (blue zinc), black passivation (black zinc), military green passivation (green zinc)and so on.

• Electrophoresis Black：

Adopting electrochemical method to deposit colloidal particles of organic resin on parts to form organic coating layer of various colours, electrophoresis black is used more in the industry, take black process as an example: degreasing-cleaning-phosphating-electrophoresis paint-drying. Can be divided into anodic electrophoresis (resin ionisation into negative ions) and cathodic electrophoresis (resin electrophoresis into positive ions), and paint process compared to the construction performance is good, pollution and harm to the environment to reduce its resistance to neutral salt spray performance in 300 hours or more, the cost and corrosion resistance and Dacromet process is similar.

• Zinc white：

Electroplating process is: degreasing – cleaning – weak acid activation – electroplating zinc – cleaning – white passivation hair – cleaning – drying, and black zinc difference is no Over lat rack and passivation solution differences, white passivation is a colourless transparent zinc oxide film, almost no chromium, so the corrosion resistance relative to black zinc, blue zinc, coloured zinc is poorer, the industry standard in 6-12 hours, there is this plating manufacturer by improving the precision of the ratio of the passivation solution can be done to resist the neutral salt spray for about 20 hours.

Because of white zinc plating type surface treatment process screws to do neutral salt spray test initially appeared on the surface of the plating corrosion white, red rust phenomenon is in about 40 hours, so white zinc corrosion resistance is better than white nickel. Appearance and white nickel compared to dark, white zinc for the original colour of zinc greenish-white, and white nickel compared to the greater difference.

• White Nickel：

Plating process is: degreasing – cleaning – weak acid activation – cleaning – copper bottom – activation – cleaning – electroplating nickel- Cleaning – passivation – cleaning – drying – or closed, and black nickel process is basically the same, mainly plating solution formula is different, less zinc sulfide and join.Nickel is a silver-white yellowish metal, for better appearance, will join the nickel-plated brightener. Its corrosion resistance and black nickel is not much difference 6-12 hours, the process of general manufacturers will also be over oil or closed, such as considering the impact of corrosion on plastic parts of the incoming material should be used as a focus on controlling whether or not over oil.

• Blue zinc, green zinc：

The process is roughly the same as for white zinc, blue zinc is passivated zinc oxide film contains 0.5-0.6mg/dm2 of trivalent chromium. Green passivation, also known as five-acid passivation, can obtain a thick grass-green film, the passivation solution contains phosphate ions, the resulting glossy grass-green film is a complex, structurally complex protective film of chromates and phosphates.

For its corrosion resistance, blue zinc is better than white zinc, while green zinc is better than blue zinc. The colour of blue zinc is slightly blue and white zinc is relatively close to the industry to use more, the latter can also be used as a product design when the alternative process of screws.

• Enameled zinc (chemistry)：

In the galvanising category of the process of coloured zinc is relatively good corrosion resistance, its coloured passivation process is: galvanising – cleaning – 2% – 3% nitric acid out of the light – cleaning – low chromium coloured passivation – cleaning -Baking aging. Passivation temperature is too low, the film is slow, pale film thin. High temperature, the film is thick and loose, not firmly attached. It is best to control at about 25 degrees to ensure that the same colour is obtained within a certain period of time.

After passivation, it must be baked and aged to improve the adhesion and corrosion resistance of the film. Colour zinc-plated screws by touching the bottom of the resistance to neutral salt spray in more than 48 hours, good control can be done more than 100 hours.

• Dacromet：

It is the abbreviation and translation of DACROMET, i.e. flaky zinc-based chromium salt protective coating, also called zinc-aluminium coating. The basic process is: degreasing – degreasing – coating – preheating – sintering – cooling. The process will generally have 2-4 times from coating to cooling process, because the screws with dip coating such as to achieve a certain thickness needs to be done more times.

Structure is on the metal surface, coated with a layer of Dacromet solution (i.e., containing scales of zinc, aluminium [scales generally size 0.1-0.2X10-15 microns] Cr03 and special organic matter of highly dispersible mixed aqueous solution), by 300 ° C or so heat preservation baking a certain period of time, Dacromet liquid in the hexavalent chromium is reduced to trivalent chromium, resulting in amorphous composite chromate compounds (nCr03) mCr203).

Corrosion resistance is very good neutral salt up to 300 hours or more, the disadvantage of the coating is not uniform, thin position of 5-10um, thick position of 40um or so, it will affect the depth of the screw diameter, so the machine tapping screws and screws with a small diameter of the screws is very good not to use Dacromet process as a surface treatment.