Surface treatment type:
Surface treatment is the process of forming a covering layer on the surface of the workpiece by a certain method, the purpose of which is to give the product a beautiful surface and anti-corrosion effect. The surface treatment methods are attributed to the following methods:
- Electroplating: The parts to be electroplated are immersed in an aqueous solution containing deposited metal compounds, and the current passes through the plating solution to precipitate and deposit the electroplated metal on the parts. General electroplating has galvanized, copper, nickel, chromium, copper-nickel alloy, etc., and sometimes black (blue), phosphating, etc., are also included.
- hot dip galvanizing: through the carbon steel parts immersed in the temperature of about 510℃ in the plating tank of dissolved zinc. 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.
- mechanical plating: by coating metal particles to impact the surface of the product, and the coating cold welded to the surface of the product.
The effect of electroplating: the quality of electroplating is mainly measured by its corrosion resistance, followed by the appearance. Corrosion resistance is to imitate the working environment of the product, set as the test conditions, and perform corrosion tests on it. The quality of electroplating products is controlled from the following aspects:
The surface of the product is not allowed to have local coating, burnt, rough, gray, peeling, peeling and obvious stripes, and is not allowed to have pinhole pitting, black plating slag, passivation film loose, cracking, falling off and serious passivation marks.
- coating thickness:
The working life of a fastener in a corrosive atmosphere is proportional to its coating thickness. Generally recommended economic plating coating thickness is 0.00015in ~ 0.0005in(4 ~ 12um).
Hot-dip galvanizing: The average thickness of the standard is 54um (43um for diameter ≤3/8), and the minimum thickness is 43um (37um for diameter ≤3/8).
- coating distribution:
Using different deposition methods, the coating on the surface of the fastener is also different. 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 thinnest deposit at the bottom of the tooth, while the hot dip galvanizing is the opposite, the thicker coating is deposited in the inner corner and the bottom of the thread, the mechanical plating metal deposit tends to be the same as the hot dip plating, but it is smoother and the thickness is much more uniform across the surface.
- Hydrogen embrittlement
During the processing and treatment of fasteners, especially in the pickling and alkali washing before plating and the subsequent electroplating process, the surface absorbs hydrogen atoms, 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. The hydrogen moves around and soon seeps into the newly formed fissure. This pressure-crack-penetration cycle continues until the fastener breaks. It usually occurs within a few hours of the first stress application.
In order to eliminate the threat of hydrogen embrittlement, fasteners should be heated and baked as quickly as possible after plating to allow hydrogen to seep out of the coating, and baking is usually carried out at 375-4000F (176-190 ° C) for 3-24 hours.
Since mechanical galvanizing is non-electrolyte, this virtually eliminates the threat of hydrogen embrittlement.