Screws are small in electrical products and are easily overlooked. In addition to specifications, the main considerations when selecting screws are appearance color and corrosion resistance. The screws used in today's small household appliances have higher corrosion resistance requirements than the original AV products due to different use environments. Through recent visits to some electroplating factories, we collected screw samples of various processes, different coatings or coating thicknesses, and different specifications to conduct a bottom-up neutral salt spray experiment comparison and summary, hoping to have a better understanding of the production of induction cookers, soybean milk machines, and kettles. Let's make some reference for the selection of screws used in small household appliances.
Commonly used surface treatment processes for screws include oxidation, electrophoresis, electroplating, and Dacromet. The following is mainly classified and summarized based on the color of the surface treatment of screws.
【1】Black plated screws
Common black screws are mainly used to fix the bottom case or back case of black or other dark-colored electrical appliances. In order to reduce the management of objects, most black screws are also used inside the machine. It mainly includes several processes such as oxide black, black nickel, black zinc and electrophoretic black.
1. Oxidized black:
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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-oiling or over-sealing. It is an oxide film formed by sodium hydroxide and sodium nitrite at high temperatures greater than 100 degrees.
The main component of the oxide film is ferric oxide (Fe3C4). The uniformity of the film is only 0.6-1.5um. The corrosion resistance is relatively poor. The neutral salt spray only lasts about 1-2 hours without oil or sealing. It takes about 3-4 hours after oiling. There are currently no screws using this process for small household appliances. In terms of appearance color, oxide black is close to black zinc and electrophoretic black, but not as bright as black zinc and electrophoretic black.
2. Electroplating:
There are two types of black electroplating: black zinc and black nickel. The process principles are basically the same, but the formula of the electroplating solution and the ladle or passivation solution used for post-processing are different. Zinc has active chemical properties and is easily oxidized and darkened in the atmosphere, eventually producing "white rust" corrosion. After galvanizing, it is treated with chromate to cover the zinc with a chemical conversion film to keep the active metal in a passive state. It is the passivation treatment of the zinc layer. From the appearance, the passivation film can be divided into white passivation (white zinc), light blue (blue zinc), black passivation (black zinc), military green passivation (green zinc), etc.
2.1 Black zinc plating:
The picture electroplating process is: degreasing - cleaning - weak acid etching - electro-galvanizing - cleaning - passivation - cleaning - drying - racking; the final rack is mainly made of black oil (a kind of paint), tinna water, and rubber Water is mixed according to a certain ratio. Its main function is to improve the black uniformity and brightness of the screw surface and improve the corrosion resistance of the screw. After passing through the frame, the neutral salt spray resistance is above 20. If it can be oil-resistant, Corrosion capacity can also be improved.
The neutral salt spray test for black zinc screws lasts for 12 hours. The industry level is relatively high, and no failure has occurred in routine tests. Judging from the appearance, it is darker than black nickel and closer to oxide black and electrophoretic black. The distinguishing method is to rub a few times on white paper to reveal the bluish white color of zinc.
2.2 Black nickel plating:
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The electroplating process is: degreasing-cleaning-weak acid activation-cleaning-copper plating-activation-cleaning-black nickel plating-cleaning-passivation-cleaning-drying-passing rack, the black plated from the black nickel plating solution The nickel layer generally contains 40%-60% nickel, 20%-30% zinc, 10%-15% sulfur, and about 10% organic matter.
During the electroplating process, the sulfur in thiocyanate ions is converted into sulfide ions, and combines with nickel to form black nickel sulfide. The main function of adding a copper plated base in the process is to make it easier to plate nickel in the post-processing process and to improve the corrosion resistance of the screws. Generally, the thickness of the copper base is about 1 micron. Increasing the thickness of the copper base can improve the corrosion resistance, but The ingredients will increase accordingly.
After conducting preliminary experiments on the screws used by various electroplating manufacturers, the industry-level resistance to neutral salt spray is generally 6-12 hours. If the proportion accuracy and raw material quality of various additives used in electroplating are improved, it can be improved. In the nickel plating process, general electroplating manufacturers will oil or seal in order to improve corrosion resistance. Considering the corrosion effect on plastic parts, the screws used cannot be oiled to increase the salt spray time. The neutral salt spray test time is 8 hours. It was unstable in routine experiments, and several batches of routine neutral salt spray failed.
It is easy to distinguish from the appearance because black nickel is darker than other black zinc, oxide black and electrophoretic black.
1. Electrophoresis black:
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The colloidal particles of organic resin are deposited on the parts using electrochemical methods to form organic coating layers of various colors. Electrophoretic black is mostly used in the industry. Take the black process as an example: degreasing - cleaning - phosphating - electrophoretic paint - drying. It can be divided into anodic electrophoresis (the resin becomes negative ions after ionization) and cathodic electrophoresis (the resin becomes positive ions after electrophoresis). Compared with the paint process, the construction performance is better, and the pollution and harm to the environment is reduced. Its neutral salt spray resistance is 300 hours. or above, the cost and corrosion resistance are similar to the Dacromet process.
【2】White plated screws
There are also many white-coated screws that are exposed, mainly including white nickel, white zinc and stainless steel screws without surface treatment.
1. White zinc:
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The electroplating process is: degreasing - cleaning - weak acid activation - zinc electroplating - cleaning - white passivation - cleaning - drying. The difference from black zinc is that there is no difference in passivation and passivation liquid. White passivation is a This kind of colorless and transparent zinc oxide film contains almost no chromium, so the corrosion resistance is worse than that of black zinc, blue zinc and colored zinc. The industry standard is 6-12 hours. Some electroplating manufacturers have improved the preparation of passivation solution. The specific accuracy can resist neutral salt spray for about 20 hours.
Because the neutral salt spray test on screws coated with white zinc surface treatment technology initially showed that the surface coating was corroded and turned white, and red rust appeared after about 40 hours, so the corrosion resistance of white zinc is better than that of white nickel. The appearance is darker than that of white nickel. The original color of zinc is bluish white, which is quite different from that of white nickel.
2. White nickel:
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The electroplating process is: degreasing - cleaning - weak acid activation - cleaning - copper plating - activation - cleaning - nickel plating - cleaning - passivation - cleaning - drying - or sealing. It is basically the same as the black nickel process, mainly the electroplating solution. The formula is different, zinc sulfide is missing and added.
Nickel is a silvery white and slightly yellow metal. To obtain a better appearance, a nickel plating brightener is added. Its corrosion resistance is not much different from that of black nickel. 6-12 hours. Generally, manufacturers will also over-oil or seal during the process. For example, considering the impact on the corrosion of plastic parts, the incoming materials should be focused on controlling whether they are over-oiled.
3. Stainless steel screws:
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The stainless steel screws used are SUS304. SUS202 or SUS201 are also used in the industry, as well as SUS410 (stainless iron screws). Stainless steel screws are made by cleaning off the surface oil during production and forming a protective film. The corrosion resistance of stainless steel screws varies depending on the material. The resistance to neutral salt spray of SUS304 is 48-96 hours. SUS202 can also be surface cleaned and passivated. The resistance to neutral salt spray is above 48, while the stainless iron screw SUS410 we call is resistant to neutral salt for about 20 hours. Its cost is higher than that of all ferrite and surface-treated screws, so it is best not to use stainless steel later. Iron screws.
