Fastener surface treatment

Surface treatment is the process of forming a covering layer on the surface of a workpiece by a certain method. The purpose of the surface treatment is to give the surface appearance and anti-corrosion effect of the product. The surface treatment methods are all attributed to the following methods:

1. Electroplating: A part to be electroplated is immersed in an aqueous solution containing a metal compound to be deposited, and an electric current is passed through the bath to deposit and deposit the plated metal on the part. Generally electroplated with galvanized, copper, nickel, chromium, copper-nickel alloys, etc., sometimes boiled (blue), phosphatized, etc. are also included.

2. Hot-dip galvanizing: This is accomplished by immersing the carbon steel parts in a bath of dissolved zinc at a temperature of approximately 510°C. As a result, the iron-zinc alloy on the surface of the steel gradually becomes passivated zinc on the outer surface of the product. Hot dip aluminum plating is a similar process.

3. Mechanical plating: The surface of the product is impacted by the particles of the plated metal, and the coating is cold-welded to the surface of the product.

General screws are mostly electroplated, but they are used for hot-dip galvanizing in hexagonal wood screws such as electricity, highways, etc.; the cost of electroplating is generally 0.6-0.8 yuan per kilogram, and hot-dip zinc is generally 1.5-2 yuan/ Kilograms, the cost is higher.

Electroplating effect:

The quality of electroplating is mainly measured by its corrosion resistance, followed by its appearance. Corrosion resistance is to imitate the working environment of the product, set it to test conditions, and subject it to corrosion tests. The quality of plating products is controlled from the following aspects:

1, appearance:

The surface of the product is not allowed to have local non-plating, scorch, rough, gray, peeling, crusting, and obvious streaks. It is not allowed to have pinhole pits, black plating slag, loose passivation film, cracking, shedding and serious Passivation traces.

2, plating thickness:

The working life of a fastener in a corrosive atmosphere is directly proportional to the thickness of its coating. The general recommended economic plating thickness is 0.00015 to 0.0005 in (4 to 12 um). Hot-dip galvanizing: The standard average thickness is 54 um (approx. 3/8 to 43 um), and the minimum thickness is 43 um (style The diameter ≤ 3/8 is 37 um).

3, plating distribution:

With different deposition methods, the coating is also concentrated on the surface of the fastener. Plating metal is not uniformly deposited on the peripheral edge during plating, and thicker plating is obtained at the corners. In the threaded part of the fastener, the thickest coating is located on the crest of the thread, gradually thinning along the side of the thread, and depositing thinnest at the bottom of the tooth, whereas hot dip galvanizing is the opposite, with thicker deposits deposited on the inner corners and At the bottom of the thread, the mechanically deposited plating metal tends to deposit the same as the hot dip, but it is smoother and the thickness is much more uniform over the entire surface.

4, hydrogen embrittlement:

Fasteners absorb hydrogen atoms on the surface during processing and processing, especially during pickling and caustic washing prior to plating and subsequent electroplating. The deposited metal coating then traps hydrogen. When the fasteners are tightened, hydrogen is transferred toward the most concentrated part of the stress, causing the pressure to increase beyond the strength of the base metal and create a tiny surface crack. Hydrogen is particularly active and penetrates quickly into newly formed fractures. This pressure-rupture-infiltration cycle continues until the fastener breaks. It usually occurs within a few hours after the first stress application.

To eliminate the threat of hydrogen embrittlement, the fasteners are heated and baked as quickly as possible after plating to allow hydrogen to seep out of the coating. Baking is usually carried out at 375-4000F (176-190°C) for 3-24 hours.

Since mechanical galvanization is non-electrolyte, this virtually eliminates the threat of hydrogen embrittlement.