Most screw fasteners are surface treated. The surface treatment not only changes the appearance of the screw fastener, but also improves the rust and corrosion resistance of the screw fastener. Generally, the quality of electroplating is mainly measured by its corrosion resistance and appearance. Corrosion resistance is the working environment of the imitation product. It is set as the experimental condition. We use the salt spray machine to test its corrosion resistance and conduct corrosion experiments.
The surface of the screw fastener is not allowed to be partially uncoated, charred, rough, dull, peeled, crusted and marked. It is not allowed to have pinhole pitting, black plating, loose film, cracking, peeling and Severe traces of purification.
The coating distribution of the screw fasteners is carried out by different deposition methods, and the plating layer is differently distributed on the surface of the screw fastener. The plating metal is not uniformly deposited on the peripheral edge during plating, and a thicker coating is obtained at the corner. In the threaded portion of the fastener, the thickest coating is located on the crest 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 just the opposite, the thicker coating is deposited inside the corner and At the bottom of the thread, the mechanical plating of the plated metal tends to be the same as that of hot dip coating, but is smoother and much more uniform over the entire surface.
During the processing and handling of the screw fasteners, especially during pickling and de-washing prior to plating and subsequent electroplating, the surface absorbs hydrogen atoms and the deposited metal coating then captures hydrogen. When the fastener is tightened, hydrogen is transferred toward the most concentrated portion of the stress, causing the pressure to increase beyond the strength of the base metal and causing minor surface cracks. Hydrogen is particularly active and quickly penetrates into the newly formed crack zone. This pressure-fracture-infiltration cycle continues until the fastener breaks. Usually occurs within a few hours after the first stress application.
