Four requirements for non-standard bolt electroplating

Update:23 Feb 2021
Summary:

Most of the standard parts have to be surface treated. […]

Most of the standard parts have to be surface treated. Surface treatment can not only change the appearance of standard fasteners, but also improve the rust and corrosion resistance of standard components. The quality of general electroplating is mainly measured by its corrosion resistance and appearance. The corrosion resistance is to imitate the working environment of the product and set as the experimental condition. We often use the salt spray machine to test its corrosion resistance and conduct corrosion experiments on it. The quality of electroplating products is controlled from the following aspects:

1. The appearance of standard parts after electroplating
The surface of the standard parts is not allowed to have partial non-plating, burnt, rough, gray, skin, crust and obvious streaks, and no pinhole pitting, black plating slag, loose purification film, cracking, shedding and serious Purification traces.

2. The working life of standard parts of coating thickness in corrosive atmosphere is proportional to its coating thickness. The general recommended thickness of economic electroplating coating is 0.00015in~0.0005in (4~12um).
Hot-dip galvanizing: the standard average thickness is 54um (named greater than and equal to 3/8 is 43um), small thickness is 43um (named diameter greater than and equal to 3/8 is 37um)

3. Coating distribution of standard parts
With different deposition methods, the plating method is also different in the standard parts collection. During electroplating, the coating metal is not uniformly deposited on the outer peripheral edge, and a thicker coating is obtained at the corners. In the threaded part of the fastener, the thick coating is located on the top of the thread, and it gradually becomes thinner along the side of the thread, and the deposit is thinner at the bottom of the thread. Hot-dip galvanizing is the opposite, and the thicker coating is deposited on the inner corner and the bottom of the thread. , The metal deposition tendency of the mechanical plating is the same as that of hot-dip plating, but it is smoother and the thickness is much more uniform on the entire surface.

4. Hydrogen embrittlement
In the process of processing and treatment of standard parts, especially in the pickling and sub-washing before plating and subsequent electroplating, the surface absorbs hydrogen atoms, and the deposited metal coating then traps hydrogen. When the fastener is tightened, hydrogen transfers toward the stress-concentrated part, causing the pressure to increase beyond the strength of the base metal and produce tiny surface cracks. Hydrogen is particularly active and quickly penetrates into the newly formed fracture zone. This pressure-rupture-infiltration cycle continues until the fastener breaks. It usually occurs within a few hours after the first stress application.
In order to eliminate the threat of hydrogen embrittlement, screw fasteners should be heated and baked as quickly as possible after plating to make hydrogen seep out from the plating. Baking is usually carried out at 375~4000F (176~190°C) for 3~24 hours.
Screw fasteners need to pay attention to the above issues during electroplating, which are also the main reasons why the quality of screw fasteners is not good.

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