ZINC
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 and 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.

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 resists organic acids. It improves the lubricity of steel and offers a high degree of corrosion resistance.

CADMIUM and WAX
Cadmium plating results in a smoother surface and greater resistance to white oxidation spots than zinc plating. However, cadmium is a much more toxic metal than zinc, which makes the plating process more difficult and costly. The standard most commercial platers use when applying cadmium is a minimum thickness of .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 0.0014 in.. Bolts and nuts over 3/8 inches diameter , and all sizes if washers shall have a zinc coating with an average thickness of 0.0021 in. with no individual bolt having a coating of less than 0.0017 in..

BLACK PHOSPHATE
This is the standard finish for most drywall screws, particle board screws and retaining rings. It can have either a dull or bright appearance. No additional oil treatment is added. NOTE: A part with a black phosphate coating actually gains its color from that of the substrate (the plain metal) prior to the phosphate finish. This is why a hardened part, such as a drywall screw, will be darker than a machine screw would be if both received the same “black phosphate” coating.

BLACK OXIDE
This is a “conversion coating” which means it is formed by a chemical reaction with the metal to form an integral surface, as opposed to an “applied coating” like zinc which bonds to the metal. An oil finish is applied as a rust inhibitor.

CHROME PLATING
Chrome plating can be done two ways: 1) “hard chrome” finish deposits a thick layer of chrome on the part. This gives the fastener a very hard finish and superior wear resistance but does not offer much protection to corrosion; 2) a “nickel-chrome” finish is achieved by applying a flash of chrome on top of the nickel plating. This offers resistance to tarnishing and corrosion.

BAKING of CASE HARDENED PARTS
Electroplated screws which are case hardened should be baked for a minimum of 4 hours within the temperature range of375-450F 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 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.