Electroplating is a common method of surface treatment of fasteners such as metal screws. Under normal circumstances, the quality of electroplating is mainly measured by its corrosion resistance, followed by appearance. Corrosion resistance is to imitate the working environment of the product, set as the test condition, and conduct corrosion test. The quality of metal screw product plating is controlled from the following aspects:
1, appearance
The surface of the metal screw products is not allowed to be partially uncoated, burnt, rough, gray, peeling, crust, and obvious streaks. Pinhole pitting, black plating, loose passivation, cracking, shedding, and Severe passivation marks.
2, hydrogen brittle
During the processing and handling of metal screw fasteners, especially during pickling and caustic washing before 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 to the most concentrated portion of the stress, causing the pressure to increase beyond the strength of the base metal and creating a slight surface crack. Hydrogen is particularly active and quickly penetrates into newly formed fissures.
3, plating thickness
The working life of a hardware screw fastener in a corrosive atmosphere is proportional to its coating thickness. The recommended economical plating thickness is 0.00015 in to 0.0005 in. (4 to 12 um). Hot dip galvanizing: the standard average thickness is 54 um (the nominal diameter is ≤ 3/8 is 43 um), and the minimum thickness is 43 um (the nominal diameter ≤ 3) /8 is 37um).
4, coating distribution
Different deposition methods use different ways of depositing the coating on the surface of the fastener. When the metal screw fastener product is plated, the plated metal is not uniformly deposited on the peripheral edge, 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, depositing the thinnest at the bottom of the tooth, while hot dip galvanizing is just the opposite, 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.