The main purpose of the surface treatment of fasteners is to improve the corrosion resistance of the fasteners to increase the reliability and adaptability of the fasteners. The main criterion is corrosion resistance, followed by appearance.
The quality of the fastener plating layer 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, and there must be no dirt, pores, pinholes, peeling, burnt plating, dullness, peeling, skin condition and obvious streaks, 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 corner. 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 of the thread.
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
Fasteners in the process of processing and treatment, especially in the pickling and alkaline cleaning before plating and subsequent electroplating process, the surface absorbs hydrogen atoms and generates hydrogen during the deposition process. When the fastener is tightened, hydrogen transfers to the most concentrated part, causing the pressure to increase beyond its strength and produce tiny surface cracks. 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, the fasteners should be heated and baked within 3 hours after plating to allow hydrogen to seep out of the coating. It is usually carried out at a temperature of about 200°C. The processing time is based on the required tensile strength. Decided.
Because mechanical galvanizing is non-electrolyte, which actually eliminates the threat of hydrogen embrittlement, hydrogen embrittlement rarely occurs in hot-dipped fasteners.