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 accomodate 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 accomodate 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 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
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 of
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.
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 surface
imperfections and produces a slight film on the surface of the parts.
Finish
Description
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.
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 whch 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.
Dacrotized
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
maintains 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.
Typically, the oil after-finish is dry
to the touch 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 900°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.
The popularity of black oxide stainless
fasteners is also 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 racks, grills
and windshield wiper assemblies, to name a few.
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