4 primary reasons for coating fasteners
There a variety of coatings and platings that can to used to prevent or delay corrosion in fasteners or enhance the physical look of a fastener.
The following are the most common coating processes:
To avoid any reduction in the strength properties of these screws, a thinner coating may be acceptable.A clear or blueish chromate finish is applied on top of the zinc to provide additional protection against white oxidation spots which can form due to moisture. Electroplating is the most common way of applying zinc coatings to fasteners. It is recommended by certain industry experts that case-hardened parts which are electro-plated should be baked after plating to minimize the risk of hydrogen embrittlement.
Yellow chromate offers a greater degree of protection from white corrosion than does clear chromate. Electroplating is the most common way of applying zinc coatings to fasteners.
A clear or blueish chromate finish is applied on top of the zinc to provide additional protection against white oxidation spots which can form due to moisture.It is common for lockwashers made from spring steel to be plated this way to avoid brittleness after baking.
The manufacturer must be told prior to the thread rolling process that the parts are to be plated.The plated parts are then baked at 375˚ for 24 hours within 1 hours of plating, then subjected to a 72-hour stress test.
The standard most commercial platers use when applying cadium is a minimum thickness of 0.0002 inches.A supplemental wax coating is often added as a lubricant when cadmium is used on prevailing torque lock nuts.
This hard barrier has a low corrosion rate and resists mechanical damage. Bolts and nuts 3/8 inch diameter and smaller shall have a zinc coating with an average thickness of 0.0017 in. With no individual bolt having a coating of less than 0.0017in.
Specialized testing or a substitute part may be required, depending on the application. This heat treatment practice is recommended for tapping screws, drywall screws, SEMS screws, clinch nuts and clinch studs.
NOTE : Passivation does not noticeably improve a stainless fastener’s physical appearance.To achieve a more polished look, a burnishing process can be done.