Plating Description

Plating, Baking & Passivating

4 primary reasons for coating fasteners

  • To improve the appearance of the fastener
  • To fight corrosion
  • To reduce friction
  • To reduce scatter in the amount or preload achieved for a given torque.

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:

Electro-Zinc & Clear
Zinc is the most popular of all commercial platings because it is relatively economical and offers good corrosion resistance in environments not subject to excessive moisture. Commercial zinc plating has a standard minimum thickness of 0.00015 inches. However, Class 2A thread allowances in sizes No. 8< and smaller may not accommodate this thickness.

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.

Electro-Zinc & Yellow
Commercial zinc-yellow plating has a standard minimum thickness of 0.00020 inches. However, Class 2A thread allowances in sizes No.8 and smaller may not accommodate this thickness. To avoid any reduction in the strength properties of these screws, a thinner coating may be acceptable.

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.

Electro-Zinc & Wax
A wax lubricant is added to the zinc coatings of certain fasteners to improve the ease of assembly. This is the standard plating for thread rolling screws including the Plastite® and Taptite® II, as well as two-way reversible center-lock nuts. Case hardened parts are still recommended to be baked after plating (see below).
Mechanical Zinc & Clear
Mechanically applying zinc to fasteners reduces the risk of hydrogen embrittlement forming within the parts. This minimizes the need for the precautionary practice of baking the parts soon after plating.

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.

Mechanical Zinc & Yellow
This finish is identical to mechanical zinc but with a yellow chromate finish. This is the standard plating for high-alloy split lockwashers and for tooth lockwashers used with zinc yellow machine screws.
Electro-Zinc & Clear for Sockets
Socket cap screws can receive a zinc plating of 0.0002 inches thickness. A clear chromate finish is applied on top of the zinc to provide additional protection against white corrosion.

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.

Electro-Zinc & Green
Commercial zinc green is the finish applied to machine screws and thread-cutting screws that will be used as ˝grounding˝ screws in electrical applications.
Nickel
Nickel has more of a silver color to it than zinc and has similar corrosion resistant characteristics. It is the standard finish of cap nuts and countersunk finishing washers.
Tin
Tin plating is sometimes used on parts in the food handling industry. Because it resist organic acids. It improves the lubricity of steel and offers a high degree of corrosion resistance.
Silver
Silver is an excellent conductor of electricity. Its benefits are both decorative and protective as it resists thread galling when mated parts are under extreme pressure or exposed to extreme heat.
Cadmium & Wax
Cadium plating results in a smoother surface and greater resistance to white oxidation spots than zinc plating. However, cadium is much more toxic metal than zinc, which makes the plating process more difficult and costly.

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.

Hot-Dip Galvanized
Hot dip galvanizing is generally the most effective way to apply a sufficient thickness of zinc to threaded fasteners for the zinc to serve as a corrosion protectant in harsh environments. During the galvanizing process, steel reacts with molten zinc, forming layers of zinc-iron alloy layers which are metallurgically bonded to the steel surface.

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.

Baking of Case Hardened Parts
Electroplated screws which are case hardened should be baked for a minimum of 4 hours within the temperature range of 375˚-450˚F no later than 4 hours after the plating operation. However, this process does not guarantee that hydrogen embrittlement will not still be present after baking or that it will not occur at a later date while in service.

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.

Passivation of Stainless Fasteners
This process enhances stainless steel’s corrosion resistance. The fasteners are dipped in a solution which removes ˝free iron˝ from the surface and produces a passive film on the surface of the parts.

NOTE : Passivation does not noticeably improve a stainless fastener’s physical appearance.To achieve a more polished look, a burnishing process can be done.

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