Platings
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Coatings
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Electro-Zinc & Clear
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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)*.
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Electro-Zinc & Yellow
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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.
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Electro-Zinc & Wax
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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)*.
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Mechanical Zinc
& Clear
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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.
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Mechanical Zinc
& Yellow
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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.
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Electro-Zinc & Clear for Sockets
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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 manufacture must be told
prior to the thread rolling process that the parts are
to be plated. The plated parts are then baked at 375F
for 24 hours within 1 hour of plating, then subjected
to a 72-hour stress test.
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Black Phosphate
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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.
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Black Phosphate & Oil
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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.
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Nickel
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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.
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Cadmium & Wax
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Cadmium plating results in a smoother surface and greater
resistance to white oxidation spots than zing plating.
However, cadmium is a much more toxic metal than zinc,
which makes the plating process more difficult and costly.
The standard most commercial platters 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.
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Hot-Dip Galvanized
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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 protestant
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 zinc coating with an average thickness
of 0.0021 in. with no individual bolt having coating
of less than 0.0017 in.
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Dacrotized
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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.
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*Baking of Case Hardened Parts
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Electroplated screws which are case hardened should
be baked for a minimum of 4 hours within the temperature
range of 375-450F no later than 4 hours after the plating
operation. However, this process does not guarantee
that hydrogen embrittlement will not 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.
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