Fastener surface treatment national standard GB/T5267.1-2002 threaded fastener electroplating standard, the standard includes: GB/T5267.1-2002 fastener electroplating layer, GB/T5267.2-2002 fastener non- Two standards for electrolytic zinc flake coating. This standard is equivalent to the international standard ISO4042-1999 threaded fastener plating standard.

The main purpose of the surface treatment of fasteners is to improve the corrosion resistance of fasteners to increase the reliability and adaptability of fasteners. The main criterion is corrosion resistance, followed by appearance.

The quality of fastener electroplating is mainly judged from the following aspects:

1. Visual inspection

The surface of the fastener should be smooth, with good gloss, and no missing plating layer. There must be no dirt, pores, pinholes, peeling, burnt plating, dullness, peeling, crust conditions and obvious stripes, as well as pitting, black plating slag. The passivation film is loose, cracked, shedding and severe passivation marks.

2. Coating thickness

The thickness of the fastener's coating is directly related to its corrosion resistance in the atmosphere, but if it is too thick, it will cause thread interference during the installation process. It is recommended that the general coating thickness is 4-12um.

The standard average thickness of hot-dip galvanizing is 54 um (nominal diameter ≤ 3/8 is 43 um), and the minimum thickness is 43 um (nominal diameter ≤ 3/8 is 37 um).

3. Coating distribution

With different deposition methods, the way the plating layer gathers on the surface of the fastener is also different. During electroplating, the coating metal is not evenly deposited on the outer peripheral edge, and a thicker coating is obtained at the corners. In the threaded part of the fastener, the thickest coating is located on the top of the thread, gradually thinning along the side of the thread, and the thinnest deposit at the bottom.

Hot-dip galvanizing is just the opposite. The thicker coating is deposited on the inner corners and the bottom of the thread. The metal deposition tendency of the mechanical coating is the same as that of the hot-dip coating, but it is smoother and the thickness is much more uniform on the entire surface.

4. Hydrogen embrittlement

During the processing and treatment of fasteners, especially in the pickling and alkali washing before plating and subsequent electroplating, the surface absorbs hydrogen atoms and generates hydrogen during the deposition process. When the fastener is tightened, the hydrogen transfers to the most concentrated part, causing the pressure to increase beyond its strength and produce tiny surface cracks, and the hydrogen penetrates into the newly formed cracks. This pressure-break-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, fasteners should be heated and baked within 3 hours after plating to allow hydrogen to seep out of the plating layer, usually at a temperature of about 200 ℃, and the processing time is based on the required tensile strength. Decided.

Since mechanical galvanizing is non-electrolyte, which actually eliminates the threat of hydrogen embrittlement, hydrogen embrittlement rarely occurs in hot-dipped fasteners.Electroplating is the process of applying a metal coating on another piece of metal (or another conductive surface) through an electro-deposition process. In electroplating, the deposited metal becomes part of the existing product with the plating/coating.