PLATING METALS Zinc Cadmium Aluminum
CHEMICAL CONVERSION COATINGS
OTHER PLATINGS AND COATINGS
Electro-Zinc & Clear
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
coating 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
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 by 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
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 PlastiteR
and TaptiteR 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
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
bluish 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 lock washers made from spring steel to be
plated this way to avoid brittleness after baking.
Mechanical Zinc & Yellow
finish is identical to mechanical zinc but with a yellow chromate finish.
This is the standard plating for high-alloy split lock washers and
for tooth lock washers used with zinc yellow machine screws.
Electro-Zinc & Clear for Sockets
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 agains 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 3750F for 24 hours within 1 hour
of plating, then subjected to a 72-hour stress test.
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
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.
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.
Cadmium & Wax
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.
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 inc.
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.
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
Baking of Case Hardened Parts
screws which are case hardened should be baked for a minimum of 4 hours
within the temperature range of 375-4500F 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.
Approximately 90 percent of all carbon steel fasteners are plated, coated, or furnished with some other type of supplementary finish. Although the principal reason is to protect against corrosion, such treatments also enhance appearance, control installation torqu-tension relationships, minimize thread seizing, and assist product identification.
Platings are the deposition of an adherent metal onto the
surface of a base metal. For
commercial fasteners (non-aerospace), practically all deposition is accomplished
by electroplating, hot-dipping or mechanically.
Other processes – such as, spraying molten metal, vacuum metalizing,
chemical vapor deposition, ion plating and enameling, - are special-purpose and
economically impractical for commercial fasteners.
Electroplating is carried out in a water-based solution
containing a chemical compound of the metal to be deposited.
The parts to be plated are immersed in this bath and an electrical
current passed through which causes the plating metal to precipitate out and be
deposited onto the parts.
Hot-dip galvanizing is accomplished by submerging carbon
steel parts for a few minutes in a bath of molten zinc at about 9600F
(5100C). The result is
an iron-zinc alloy at the steel surface, gradually changing to pure zinc at the
part’s exterior. Hot-dip
aluminizing is a similar process with aluminum substituted for zinc.
In the mechanical deposition process, a metal coating is
applied by impacting particles of the plating metal against the parts and cold
welding a coating to their surface.
Zinc is by far the most widely used plating metal followed
in popularity by cadmium and by aluminum, which has modest use.
Copper, tin, nickel, chromium, lead and silver are used to a lesser
degree – all for special reasons.
Zinc, cadmium and aluminum are favored as plating metals
because in the Galvanic Series (Table 1, page B-xx) they are less noble than
carbon steel, stainless steel, and most other nonferrous metals used in fastener
applications. In an electrochemical
reaction, the plating metal corrodes, and through its sacrifice, the base metal
remains protected. Only after the
plating metal has been significantly lost to corrosion does corrosion of the
base metal begin. Other plating
metals are more noble than carbon steel. When
the coating is breached, the base metal comes under immediate attack.
Zinc is popular as a fastener coating because it is the
least expensive, can be applied in a broad range of thicknesses, has
good-to-excellent corrosion resistance, and is relatively non-toxic.
Zinc plated fasteners require more tightening torque to
develop equivalent preloads in threaded fasteners. Also, zinc coatings without some supplementary protection
develop a dull white corrosion product on their surface which is nicknamed
“white rust”. Because of its
unsightly appearance, most zinc plated fasteners are given a chromate treatment,
which is a chemical conversion process to cover the zinc surface with a hard
non-porous film. This added coating
effectively seals the surface, protects it against early tarnishing and
reinforces the fastener’s resistance to corrosion attack.
Chromate coatings are available clear, iridescent, or in a variety of
Cadmium is extremely toxic and while investigations have established that cadmium plated fasteners are not a high risk hazard, except when in contact with food or beverages, use of cadmium as a plating metal is phasing out.
Cadmium’s environmental protection threat related
primarily to the plating process and the subsequent handling of plating
effluents. The strictness of
government regulations that define effluent disposal controls has discouraged
many job platers from continuing their cadmium plating services.
The high cost of engineering systems to remove cadmium from
effluents, coupled with the declining availability of subcontracting plating
sources, has added greatly to cadmium-plated fastener costs.
Consequently, extensive research effort has been under way during the
past few years to discover alternative plating and coatings with equivalent
performance properties at a reasonable costs.
Until these efforts are successful, the need for cadmium as a fastener
plating metal will continue.
Compared to zinc, cadmium provides superior corrosion
protection in marine and other aggressive corrosion atmospheres.
It can be soldered, it doesn’t produce “white rust”, and it has a
smoother appearance with greater luster. Most
importantly, it has lubricity. Lubricity
lowers frictional coefficients and narrows the scatter range of torque-tension
relationships. These properties are
particularly important in applications using prevailing-torque nuts.
Aluminum as an alternative to zinc in the hot-dip
deposition process has two distinct advantages.
It provides superior corrosion resistance in severe industrial and marine
exposures, and it withstands service temperatures to 9000F (4820C)
without discoloration or scaling. Zinc,
at temperatures above 5000F (2600C), tends to alloy or
peel. The main disadvantages of
aluminum are higher cost and limited sources of plating facilities with
equipment and capability.
As a general rule, fastener service life, in a corrosive
atmosphere, is proportional to the thickness of its plating.
The thicker the plating the longer it will survive.
Electroplated fasteners have plating thicknesses ranging
from a “flash” coating of insignificant thickness, to a “commercial”
thickness of 0.00015 in., through to 0.0005 in.
Thicker electroplatings are possible but, from an economics viewpoint,
Hot-dip galvanizing produces much thicker coatings, which
in engineering standards are expressed in terms of ounces of plating metal
deposited per square foot of plated surface.
Standard hot-dip galvanized fasteners have an average plating thickness
of 1.25 oz/sq ft (0.0021 in. in thickness).
Heavier coatings to 2.00 oz/sq ft (0.0034 in.) are feasible, but such
coatings may necessitate adjustments in mating thread fits to a degree that the
fastener’s strength properties may be adversely affected.
Mechanically plated coating thicknesses are available
through the full range offered by either electroplating or hot-dip galvanizing.
For several years, the relative corrosion combating
performance of zinc electroplated and hot-dip galvanized fasteners compared with
mechanically plated fasteners has been under investigation.
A range of exposure environments indicated equivalent performance for
fasteners having the same plating thickness.
Useful service life expectancies of zinc plated fasteners
in various environments are:
- zinc plated with chromate treatment, 0.00015 in. plating thickness: up to 20 year indoors, about 4 years in a rural atmosphere, 2 years in coastal locations, and less than 1 year in heavily polluted industrial atmosphere.
hot-dip galvanized with an average thickness of 1.25 oz/sq ft: over 40
years in a rural atmosphere, 25 to 30 years in coastal locations, and 5 years or
longer in heavily polluted industrial atmospheres.
Survivability is almost a direct
function of plating thickness. However,
plating is expensive. Costs – and
attendant problems increase with increasing plating thickness.
Consequently, the prudent engineer is advised to specify only that
thickness of plating required to satisfy the application.
The build up of plating on fastener surfaces occurs
differently with each of the principal deposition methods.
Electroplating deposits the plating metal unevenly with
exterior edges and corners receiving thicker coatings.
In the fastener’s threaded section, the thickest plating is located at
the thread crests and becomes progressively thinner on the thread flanks, with
the thinnest deposits in the thread roots.
With hot-dip galvanizing, it is just the opposite, with thicker coatings
being deposited at interior corners and in the thread roots.
Because of clogging of thread roots is difficult to control, it is
usually impractical to hot-dip galvanize fasteners of nominal sizes smaller than
3/8 in. Mechanical plating tends to
deposit the plating metal similarly to hot-dip galvanizing but more smoothly and
considerably more uniform in thickness over the entire surface.
Two serious problems are directly attributable to plating
– thread assembly and hydrogen embrittlement.
The addition of a plating to its
surface increases the size of the fastener.
When the plating thickness exceeds certain limits – generally
one-fourth of the specified allowance for the class of thread fit – there is a
distinct possibility the internally and externally threaded parts will not
assemble. When interference between
mating threads is likely, some accommodation must be made prior to plating.
Recommended practices for adjusting thread fits of plated fasteners are
discussed in the earlier discussion of screw threads.
High strength, high hardness
carbon steel fasteners have susceptibility to embrittlement, which evidences
itself in various mechanisms. Plated
and coated fasteners, especially those that are electroplated, are vulnerable to
the one known as hydrogen embrittlement.
Hydrogen embrittlement causes
fastener failures, the actual fracture of the fastener into two separate pieces.
The failure occurs in service (i.e., after the fastener has been
installed and tightened in its application), it usually happens within hours,
it’s sudden, there’s no advance warning or visible indication of imminence.
During fastener manufacturing and processing, particularly during acid pickling and alkaline cleaning before plating and then electroplating, atomic hydrogen is absorbed into the fastener’s surface. The deposited metallic coating then entraps the hydrogen. When the fastener is tightened, the hydrogen migrates towards points of highest stress concentration. Pressure builds until the strength of the base metal is exceeded and minute surface ruptures occur. Hydrogen is exceptionally mobile and quickly penetrates into the newly formed cracks.
This pressure-rupture-penetration cycle continues until the fastener fractures, usually within hours of first stress application.
To neutralize the threat of hydrogen embrittlement,
fasteners are thermally baked early as possible after plating.
Time delays seriously jeopardize the effectiveness and benefits of the
baking. The purpose of the baking
– generally at 3750F to 4000F for 3 to 24 hours
dependent on plating type and thickness – is to drive out the hydrogen by
bleeding it through the plating. Baking
is always done prior to chromating or application of any other supplementary
In broad terms, fasteners with hardnesses less than
Rockwell C32 have a low risk of embrittlement.
Those with higher harnesses should always be suspect.
Because mechanical plating is nonelectrolytic, the hydrogen
embrittlement threat is virtually eliminated.
In fact, parts with hardnesses up to Rockwell C55, mechanically plated
without post baking, have performed satisfactorily without evidence of
Hot-dip galvanized fasteners are rarely subject to hydrogen
embrittlement. The primary reason
is that engineering standards strongly discourage the hot-dip galvanizing of
fasteners with hardnesses higher that Rockwell C35 – i.e., fasteners stronger
than SAE Grade 5, ASTM A449, and ASTM A325.
The reason is that galvanized fasteners of higher strengths have a
susceptibility to another embrittlement mechanism known as stress corrosion or
stress corrosion cracking.
Chemical conversion coatings are adherent films chemically
formed on a metal’s surface when immersed in a bath of appropriate solution.
Chemical conversion coatings popularly specified for fasteners are
chromate treatments on electroplated parts (mentioned earlier) and zinc and
manganese phosphate coatings.
Zinc phosphate coatings, or manganese phosphate often used
as permitted alternative, are extensively specified for fasteners, particularly
those intended for use in automotive applications. The phosphate base provides an excellent substrate for
painting and for retention of oils, waxes or other organic lubricating
materials. Most zinc phosphated
fasteners are additional oiled to enhance corrosion resistance and to help
control torque-tension relationships. Dry
zinc phosphate is often used as a base for non-metallic locking elements on
externally threaded fasteners.
The corrosion resistance of zinc phosphated and oiled
fasteners is reasonably good in non-aggressive atmospheres.
Significant improvements are possible through secondary treatments, such
Although phosphate-coated high strength fasteners are not
immune from hydrogen embrittlement, susceptibility and frequency of occurrence
are less than similar fasteners which have been electroplated.
Unlike deposited plating, phosphate coatings do not significantly
increase fastener size. Class 2A/2B
screw thread fits are usually adequate to permit assembly.
Rarely is it necessary to make adjustments in thread size limits prior to
One of the more important considerations when evaluating
the possible us of a phosphate coated fastener is cost. Phosphate and oiled coatings are less expensive than zinc
electroplating with chromate treatment. However,
the packaging and handling of phosphate and oiled coated fasteners has a degree
of sensitivity because the oil may be removed by absorption into the packing
This discussion has concentrated on the basic platings and
coatings applicable to carbon steel fasteners.
There are a great many other plating and coatings available
– mostly special-purpose and each with some unique performance enhancing
feature. There are those which
greatly lengthen service life in corrosive atmospheres; those which provide
protection at elevated temperatures; others which drastically lower frictional
coefficients and reduce torque-tension relationship scatter; some give wear
resistance; others help overcome galling and seizing between mating threads.
Some are multi-purpose – for example, the family of fluorocarbons,
which enhance corrosion protection while simultaneously reducing and more
closely controlling frictional variations. Most
of the special platings and coatings are proprietary, using patented processes
and trademarked materials. All are
more expensive than standard electroplating and phosphate coating.
Plating and coating technology is dynamic with new
developments and improvements being introduced with remarkable frequency.