Types of metal surface treatment
Surface treatment is the process of forming a covering layer on the workpiece surface by a certain method. Its purpose is to give products beautiful surface and anti-corrosion effect. What are the types of metal surface treatment? Let's find out.
Metal surface treatment
Types of metal surface treatment
Types of metal surface treatment
1. Electroplating: The parts receiving electroplating are immersed in an aqueous solution containing deposited metal compounds, and the electroplating metal is precipitated and deposited on the parts with the current passing through the plating solution. General electroplating has galvanized, copper, nickel, chromium, copper nickel alloy, etc., sometimes boil black (blue), phosphating and so on are also included.
2, hot dip galvanizing: through the carbon steel parts immersion temperature of about 510℃ dissolved zinc plating tank completed. The result is that the iron - zinc alloy on the surface of the steel gradually becomes the passivated zinc on the outer surface of the product. Hot dip aluminum plating is a similar process.
3, mechanical plating: through the coating of metal particles to impact the surface of the product, and cold welding the coating to the surface of the product.
General screws use electroplating, but used in electric power, highway and other outdoor hexagonal wood screws with hot zinc; The cost of electroplating is generally 0.6 -- 0.8 yuan per kilogram, hot dip zinc is generally 1.5 -- 2 yuan/kg, the cost is higher.
Electroplating effect:
The quality of electroplating is mainly measured by its corrosion resistance, followed by appearance. Corrosion resistance is to imitate the product working environment, set as the test conditions, to its corrosion test. The quality of electroplating products should be controlled from the following aspects:
1. Appearance:
Product surface is not allowed to have local coating, burnt, rough, gray, peeling, skin condition and obvious stripes, not allowed to have pinhole pitting, black plating slag, passivation film loose, cracking, falling off and serious passivation traces.
2. Coating thickness:
The service life of a fastener in corrosive atmosphere is directly proportional to its coating thickness. The general recommended economic plating coating thickness is 0.00015in ~ 0.0005 in(4 ~ 12um).
Hot-dip galvanizing: The standard average thickness is 54 um(nominal diameter ≤3/8:43 um) and the minimum thickness is 43 um(nominal diameter ≤3/8:37 um).
3. Coating distribution:
With different deposition methods, the coating will accumulate in different ways on the fastener surface. During electroplating, the coated metal is not uniformly deposited on the outer edge, and a thicker coating is obtained at the corner. In the threaded part of the fastener, the thickest coating is located at the top of the thread, gradually thinning along the side of the thread, and the deposit is the thinnest at the bottom of the tooth, and hot dip galvanizing is the opposite, thicker coating deposits inside the Angle and the bottom of the thread, mechanical plating and hot dip plating metal deposit tendency is the same, but more smooth and the thickness on the entire surface is much more uniform.
4. Hydrogen embrittlement:
During the processing and treatment of fasteners, especially during pickling and alkali washing before plating and subsequent electroplating, hydrogen atoms are absorbed on the surface and the deposited metal coating then captures hydrogen. When the fastener is tightened, the hydrogen is diverted towards the most concentrated part of the stress, causing the pressure to increase beyond the strength of the base metal and producing a small surface rupture. Hydrogen is particularly active and quickly seeps into newly formed fissures. This pressure-crack-penetration cycle continues until the fastener breaks. It usually occurs within a few hours of the first stress application.
To eliminate the threat of hydrogen embrittance, fasteners are heated and baked as quickly as possible after plating to allow hydrogen to seeping out of the coating, usually for 3-24 hours at 375-4000f(176-190 ° C).