Screw commonly used surface treatment process oxidation, electrophoresis, electroplating, dacro four categories, the following mainly to the surface treatment of the screw color to summarize the classification.
Black coated screw
Common black screws are mainly used to fix the bottom shell or back shell of black or other dark electrical appliances, and black screws are also used in the machine to reduce the management of materials. Its main oxidation black, black nickel, black zinc and electrophoresis black and other processes.
1.Black oxide:
It is divided into normal temperature blackening and high temperature blackening. Taking normal temperature blackening as an example, the process flow is: chemical oil removal - hot water washing - cold water washing - rust removal acid etching - cleaning - blackening - cleaning - oil or oil sealing. It is an oxide film formed in sodium hydroxide and sodium nitrite at a high temperature of more than 100 degrees.
The main component of the oxide film is ferric oxide (Fe3C4). The average degree of the film 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 is only about 1-2 hours, and after oil, it is about 3-4 hours. Small appliances do not use this process of screws. In terms of appearance color, oxidized black is close to black zinc and electrophoretic black, but not as bright as black zinc and electrophoretic black.
2.Electroplating:
Black electroplating has two kinds of black zinc and black nickel, its process principle is basically the same, only the formula of electroplating solution and post-treatment with the rack or passivation liquid is different. The chemical properties of zinc is active, easy to oxidize and dark in the atmosphere, and finally produce "white rust" corrosion, galvanized after chromate treatment, in order to cover a layer of chemical conversion film on zinc, so that the active metal is in the blunt 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), etc.
（1）Black zinc plating:
Electroplating process is: oil removal - cleaning - weak acid etching - galvanizing - cleaning - passivation - cleaning - drying - rack; Finally, the rack is mainly black oil (a kind of paint), Tianthat water, rubber water according to a certain ratio of good, its main role is to improve the screw surface black uniformity and brightness and enhance the corrosion resistance of the screw, after the rack anti-neutral salt spray ability in more than 20, such as oil corrosion resistance can also be improved.
The neutral salt spray test of black zinc screws lasted for 12 hours, and the industry level was relatively high. There was no unqualified phenomenon in the routine test. From its appearance, it is darker than black nickel, and is close to oxidized black and electrophoretic black. It is distinguished by grinding several times on white paper to reveal the bluish white of zinc.
（2）Black nickel plating:
Electroplating process: deoiling - cleaning - activation of weak acid - cleaning - copper plating bottom - activation - cleaning - electroplating of black nickel - cleaning - passivation - cleaning - drying - rack, black nickel plating from the black nickel bath, approximately 40%-60% nickel, zinc 20%-30%, sulfur 10%-15%, organic matter 10%.
During electroplating, the sulfur in thiocyanate ions is converted to sulfur ions and is combined with nickel to form black nickel sulfide. In the process of more copper plating bottom, the main effect is easier after the process of nickel plating and can improve the corrosion resistance of the screw, the thickness of the general copper bottom in 1 micron, such as increasing the thickness of the copper bottom can improve the corrosion resistance, but the composition will increase accordingly.
（3）Electrophoresis black:
The colloidal particles of organic resin are deposited on the parts by electrochemical method to form a variety of colors of organic coating layer. The electrophoresis black is used more in the industry, and the black process is taken as an example: oil removal - cleaning - phosphating - electrophoresis paint - drying. It can be divided into anode electrophoresis (resin ionization into negative ions) and cathode electrophoresis (resin electrophoresis into positive ions), compared with the paint process, the construction performance is good, pollution and harm to the environment is reduced, its resistance to neutral salt spray performance in 300 hours or more, the cost and corrosion resistance is similar to the Dacro process.
White coated screw
White coating screws are used to expose many, mainly white nickel, white zinc and stainless steel screws without surface treatment.
1.White zinc:
The electroplating process is: Oil removal, cleaning, weak acid activation, electric galvanized, wash - white passivation, cleaning, drying, with black zinc difference is not too shocked and passivation liquid differences, white passivation is a colorless transparent of zinc oxide film, almost do not contain chromium, so the corrosion resistance is relatively poor black zinc, blue zinc, color zinc, industry standard in 6-12 hours, The electroplating manufacturer can achieve resistance to neutral salt spray for about 20 hours by improving the ratio accuracy of passivation solution.
Due to the neutral salt spray experiment of the screw surface treatment process of white zinc plating, the surface coating corrosion is white, and the phenomenon of red rust is about 40 hours, so the corrosion resistance of white zinc is better than that of white nickel. Compared with white nickel, the appearance of white zinc is darker, and white zinc is the primary color of zinc, which is bluish white.
2.White nickel:
Electroplating process is: oil removal - cleaning - weak acid activation - cleaning - copper plating bottom - activation - cleaning - nickel plating - cleaning - passivation - cleaning - drying - or closed, and the black nickel process is basically the same, mainly the electroplating solution formulation is different, less zinc sulfide and add.
Nickel is a silver-white yellowish metal. For a better appearance, a nickel-plated brightener is added. Its corrosion resistance and black nickel is not much different 6-12 hours, the process of general manufacturers will also be oiled or closed, such as considering the impact of corrosion on plastic parts in the material should be the focus of control whether the oil.
3.Stainless steel screw:
Stainless steel screws for SUS304 material, the industry is also useful SUS202 or SUS201, and SUS410 (stainless iron screws).
Stainless steel screw is the production of surface oil cleaning off and form a protective film, its corrosion resistance according to the different materials have some differences, its resistance to neutral salt spray SUS304 in 48-96 hours, SUS202 good surface cleaning passivation can also be resistant to neutral salt spray more than 48 hours, and we call stainless iron screw SUS410 anti-neutral salt in about 20 hours, Its cost is higher than all ferrite and surface treatment screws, so it is best not to use stainless iron screws.
Screws plated in other colors
Generally only used in the machine, mainly blue zinc, green zinc, color zinc, and dacro and other kinds, blue zinc and green zinc according to different needs are often used for external, blue zinc industry is used more, green zinc less.
1.Blue zinc, green zinc:
The process is roughly the same as white zinc, blue zinc is passivated zinc oxide film containing 0.5-0.6mg/dm2 of trivalent chromium. Green passivation is also called pentaacid passivation, can obtain thick grass green film, passivation liquid contains phosphate ions, the oil grass green film is made of chromate and phosphate compound, the structure is very complex protective film.
For its corrosion resistance, blue zinc is better than white zinc, and green zinc is better than blue zinc. The color of blue zinc is slightly blue and relatively close to white zinc, which is widely used in the industry. It can also be used as an alternative process screw in product design.
2.Color zinc:
In the process of galvanizing, color zinc is relatively good corrosion resistance. Its passivation process is: galvanized - cleaning - 2%-3% nitric acid light - cleaning - low chromium color passivation - cleaning - baking aging. The passivation temperature is too low, the film formation is slow, the color is light and the film is thin. The temperature is high, the film is thick and loose, the attachment is not firm. It's best to keep it around 25 degrees to ensure 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. Color zinc plated screws by the bottom of the resistance to neutral salt spray in more than 48 hours, control can be done more than 100 hours.
3.Dacromet:
DACROMET is short for DACROMET, that is, sheet zinc based chromium salt protective coating, also known as zinc-aluminum coating. The basic process is: degreasing - oil removal - coating - preheating - sintering - cooling. The process will generally have 2-4 times from coating to cooling process, because the screw with dip coating if to reach a certain thickness need to do several times.
Structure is on the surface of the metal, coated with a layer of dacrochrome solution (i.e. containing scaly zinc, aluminum [scale size is 0.1-0.2x10-15 micron]Cr03 and special organic compounds of high dispersion mixed aqueous solution), after 300°C heat preservation and baking for a certain time, the hexalent chromium in dacrochrome solution is reduced to trivalent chromium. The amorphous chromate compound (nCr03 mCr203) was formed.
The corrosion resistance of neutral salt is very good up to 300 hours, the disadvantage is the uneven coating, the thin position is 5-10um, the thick position is about 40um, it will affect the depth of screw diameter, so the machine tapping screws and screws with small tooth diameter are very good not to use Dacro process as the surface treatment.
