Surface treatment of self-tapping screws : Most self-tapping screws are coated with galvanized or zinc phosphate. Cadmium plating is rarely used because of its high cost and toxicity . If appearance is important , nickel or chromium plating can be selected.
One is , the appearance of the disposal will not only increase the scale ( eg reduced cavity, etc. ) and do not like the appearance of disposal will affect the torque , tighten extent , the link between the strength , so when doing experiments to be scored according to galvanizing , There are different rules for cadmium plating or phosphate disposal. When testing the applicable tapping hole diameter , the screws used must be screws with the same appearance.
The second is that high-hardness , small-scale self-tapping screws often fail in use due to hydrogen embrittlement. Self-tapping screws in order to hack into iron , shall not be disposed of carburizing. After carburizing disposal will produce high hardness and high carbon content , when this situation will do plating or pickling attack and hydrogen embrittlement disposal.
Hydrogen embrittlement test : The hydrogen embrittlement test is mainly to test whether the self-tapping screw has residual hydrogen in the screw arrangement and whether it will embrittle the self-tapping screw . ( Generally, the embrittlement does not occur in a while , and must be given the only apparent stress ) plating or cladding of the screws should be charged as shown in table 3 test rules as iron and steel in the flat washer , bearing surface normative flat washer under the screw head distinguished head type , countersunk Type (Oval) screws use a matching chamfered distance steel sheet. The thickness of the flat washer or distance piece installed on the lower bearing surface of the head should be able to fit the maximum invalid tooth length of the screw.
