Chrome, zinc.
Reason: zinc plating process after the grounding screws better corrosion resistance, use longer cut beautiful and environmentally friendly.
The main classification of screws include common screws, machine screws, self-tapping screws and expansion screws three. The principle of the screw is the use of the object's beveled circular rotation and friction physics and mathematical principles, step-by-step tightening of the instrumental machine tools.
Expanded Information:
I. Screws appearance quality requirements: screws appearance of the inspection is from the appearance of the plating layer and other aspects of the inspection.
Second, the screw plating thickness inspection
1, gauge method: the amount of micrometers, vernier calipers, plug gauge.
2, magnetic method: measurement of coating layer thickness, is a magnetic thickness gauge on the magnetic substrate of non-magnetic coating layer for non-destructive measurements.
3, microscope method: known as metallographic method, it is the fastener after etching, placed on the metallographic microscope with microscopic eyepiece magnification, measurement of the thickness of the coating layer on the section.
4, timing the liquid flow method: with the ability to dissolve the plating of the solution flow injection in the plating of the local surface, according to the local plating dissolved in the time required to calculate the thickness of the plating. There are also plating drip method, anodic dissolution coulomb method.
Baidu Encyclopedia - Screws