Plating is a common surface treatment method for fasteners such as hardware screws. Generally, the quality of plating is mainly measured by its corrosion resistance, followed by appearance. Corrosion resistance is to simulate the working environment of the product, set it as the test condition and test it. The quality of plating of hardware screw products is controlled from the following aspects:
1, appearance: hardware screws products surface are not allowed to have a local without coating, burning, rough, dark, peeling, the condition of crust and stripes obviously, there is no pinhole pitting, black plated slag, passivation membrane loose, crack, fall off and the serious passivation.
2, hydrogen embrittlement: hardware screw fasteners in the process of machining and processing, especially in the acid and alkaline cleaning before plating and subsequent electroplating process, the surface absorbed hydrogen, deposition of metal coating and then capture hydrogen. When the fasteners are tightened, hydrogen is rotated towards the part with the highest concentration of stress, causing the pressure to increase beyond the strength of the matrix metal and causing minor surface rupture. Hydrogen is particularly active and quickly seeps into new cracks.
3. Coating distribution: different deposition methods are adopted, and the aggregation patterns of coating on the surface of fasteners are also different. Hardware screw fasteners product plating coating metal is not uniformly deposited on the outer peripheral edge, corner to obtain a thicker coating. In fasteners, threaded portion, the thickest deposit is located in the top thread, along the thread profile gradually thinning, the thinnest the sedimentation of the tooth to ascend, and the opposite of hot dip galvanized, thicker coating deposition, corner and the bottom of the thread, the mechanical plating coating metal deposition tendency of the same as the hot dip plating, but more smooth and thickness is much more evenly across the surface.
4. Plating thickness: the operating life of hardware screw fasteners in corrosive atmosphere is proportional to its plating thickness. Generally, the recommended thickness of electroplating coatings is 0.00015in ~ 0.0005in(4 ~ 12um).
