Kanebridge Fastener Reference Guide Appendix-A

Plating / Baking / Passivating

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 (see below).

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°F for 24 hours within 1 hour 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 resists organic acids. It improves the lubricity 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
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 inch 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 inch.

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.

Plating / Black Oxide Finish

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 Phosphate & Oil
The most common standard coating of black phosphate and oil is 1100 mg per sq/ft, minimum. The oil serves as a rust
inhibitor and a lubricant. Some fasteners with this plating are required to pass a salt-spray test, the duration and cost of
which must be agreed upon between buyer and seller prior to the sale. Floorboard screws, frame bolts, Grade-GT locknuts and spring nuts are usually supplied with a black phosphate and oil finish.

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. For more
information about black oxide, see below.

Chrome
Chrome plating can be done two ways: 1) A "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.

Dacrotlzed
Dacrotizing is a pollution-free ceramic coating for fasteners used with treated lumber. The coating offers corrosion protection comparable to hot-dip galvanizing without discoloring the wood. Screws with a proper dacrotized coating can typically withstand a 500-hour salt-spray test. Dacrotizing minimizes greatly the risk of hydrogen embrittlement so baking the parts is not required after the finish is applied.

Blue Polymer
Polymer is a barrier coating because it creates a seal around the part which resists corrosion and abrasion. A phosphate-zinc base is initially applied to the fastener, followed by the polymer which bonds to the sub-coating. This finish gives excellent rust protection and is commonly used to coat concrete screws.

Green Ceramic
Ceramic finish is also a barrier coating used to offer corrosion resistance. It is used, though not exclusively, on certain types of construction fasteners and typically provides over 500 salt spray hours of protection.

Black Oxide Finish

Black oxide is a conversion coating (as opposed to an applied coating) because it results from a chemical reaction with the iron present in the metal fastener and forms an integral protective surface. It is a coloring of the base metal which neither removes nor deposits metal; therefore it adds, at the most, 5 to 10 millionths of an inch to the fastener's dimensions. Likewise, it cannot chip, peel or rub off. Also, the item maintain 99% of its conductivity making black oxide a popular finish for electrical parts.

When a black oxide finish is specified, it is interpreted as "Black Oxide and Oil" Other supplementary coatings such as wax or lacquer may be ordered, but fasteners with those finishes are usually not available from a distributor's stock. A waxed coating over the black oxide is often preferred for parts used in the furniture industry and for fasteners bagged in kits.

Typically, a water-displacing oil is used which gives the part a lustrous appearance and improved lubricity without excessive residue. The color is a deep black rather than the grayish-black appearance of a black phosphate, and remains consistent at temperatures as high as 600°F. Black oxide also resists abrasions better than phosphate.

One of the most important advantages of black oxide is that there is almost no risk of hydrogen embrittlement because the process does not involve electro-plating nor does it require an acid-activation. Case hardened parts need not be baked after receiving a black oxide finish.

Black oxide neither enhances nor detracts from a fastener's resistance to corrosion. The post-treatment oil application offers good indoor corrosion protection, but a zinc-plated part is more resistant to rust. However, parts with a black oxide finish will not suffer from white corrosion which can occur over time with electro-plated parts. To avoid the risk of white corrosive particles (which can cause electrical shorts) many in the electronics industry opt for black oxide instead of zinc-plated fasteners.

Steel black oxide parts are not recommended for outdoor uses, but the popularity of black oxide stainless fasteners is growing. In addition to being used for decorative purposes, the finish reduces light glare and reflection, and makes the surface of the fastener smoother. The automotive aftermarket utilize them in tail-light assemblies, luggage rack, grills and windshield wiper assemblies, to name a few.

Heat Treating

Steel fasteners are heat treated to develop the fastening strengths their material structure is capable of reaching. The iron, carbon, manganese and other elements which from the grains of steel are transformed during heat treating to a structure capable of greater load carrying. The as-heat treated hardness, and tensile strength, are two of several metallurgical measures of the finished heat treated fastener.

Heat treating is a "three box" process: Harden-Quench-Temper. The raw fasteners travel into the hardening furnace where a high temperature and an atmosphere with a metered amount of carbon molecules alter the fastener' microstructure. Once "soaked" in and transformed in this environment, the fasteners are rapidly cooled, or "quenched" to shift the fastener metal into the martensitic microstructure right for fastening. To relieve the stresses from the rapid temperature changes needed to quench properly, the final step, tempering, draws the fastener back to their final hardness and tensile strength using a temperature lower than used in hardening. By hardening, quenching and tempering a steel fastener, the finished product develops the mechanical properties to provide fastening ability.