zinc electroplating

NUCOR Fastener
zinc electroplating
TECHNICAL DATA SHEET
At Nucor, we understand that appearance is often as important as
the function of the finish on the fastener itself. That’s why our sources
for electroplating are all certified professionals committed to providing
us with the highest quality finishes available in the U.S.A.
Benefits of Zinc Electroplating: Zinc Electroplating provides
corrosion resistance to the steel fastener by acting as a barrier
and sacrificial coating. Zinc is more electrochemically reactive than
steel, so when exposed to a corrosive environment, the zinc plating
corrodes sacrificially, delaying rust formation on the fastener even
after portions of bare steel are exposed.
How Zinc Electroplating is Applied: Zinc electroplating is a
method of depositing zinc metal on the surface of another metal, such as steel, by immersing the fasteners
in an appropriate plating bath and applying electrical current. Zinc travels through the electrolytic bath from
the zinc anodes and attaches to the surface of the fastener. The thickness of zinc plating depends on the
time spent in the plating bath, the amount of current, and the chemical composition of the bath.
Specifications and Performance: ASTM F1941, the “Standard Specification for Electrodeposited
Coatings on Threaded Fasteners (Unified Inch Screw Threads (UN/UNR))”; and ASTM F1941M, “Standard
Specification for Electrodeposited Coatings on Threaded Fasteners [Metric]” are the preferred standards to
use when specifying zinc electroplating on fasteners. ASTM F1941 and F1941M also cover Cadmium,
Zinc-Iron, Zinc-Nickel and Zinc-Cobalt electrodeposited coatings not covered in B633. The standards include
information on zinc thickness, chromate coatings, salt spray performance and other quality requirements.
The thickness and chromate coatings for ASTM F1941 and F1941M are summarized in the following tables:
Designation
Type
Typical Appearance
A
Clear
Transparent colorless with slight
iridescence
B
Blue-Bright
Transparent with a bluish tinge
and slight iridescence
C
Yellow
Coating
Designation
Thickness
µm (min)
Thickness
inch (min)
Fe/Zn 12
12
.0005
Fe/Zn 8
8
.0003
Fe/Zn 5
5
.0002
D
Opaque
Fe/Zn 3
3
.0001
E
Black
F
Organic
TDS-004B 2-07-11
Yellow iridescent
Olive green, shading to brown
or bronze
Black with slight iridescence
Any of the above plus organic
topcoat
Corrosion Protection: When zinc corrodes, it develops a white powdery product on the surface, analogous
to and just prior to the appearance of red rust on steel. To delay the appearance of this “white rust” on zinc
plating, a chromate surface treatment is applied. This chromate film seals the surface to increase the corrosion
protection of the zinc. The chromate coating can provide a range of appearances, as detailed in the tables
above, with generally increasing corrosion protection from clear to black. The corrosion protection of zinc/chromate
plating can be measured by salt spray hours. Salt Spray corrosion testing (per ASTM B117) is done in a controlled
test chamber at 95°F with continuous exposure to an atomized fog of salt solution. Salt spray test results provide
a method of comparison testing of coatings in a controlled environment, but may not directly predict corrosion
protection in real world conditions. Consult the standards for specific minimum salt spray testing requirements
for the finish thickness and supplementary chromate conversion coating specified or contact Nucor Technical
Support Personnel for more information.
Hydrogen Embrittlement: During the acid cleaning and in the electroplating process, atomic hydrogen can
be absorbed through the surface of the fastener. The electroplated coating traps the hydrogen inside the fasteners
unless they are baked soon after plating to drive the hydrogen out. If the hydrogen remains in the steel, it can
migrate to areas of high stress and cause small microcracks, which rapidly enlarge under load causing a brittle
failure. Both standards recommend hydrogen embrittlement relief baking for fasteners above HRC 40 maximum
hardness levels. We know high strength, high hardness parts are more susceptible to hydrogen embrittlement,
therefore, Nucor Fastener bakes all Grade 8 cap screws and metric property class 10.9 electroplated fasteners to
reduce the chance of hydrogen embrittlement from occurring, before reaching hardness levels requiring baking
by the standards.
Commercial Zinc Electroplating: Nucor offers a 0.0002” minimum thickness of zinc (Fe/Zn 5) on our
electroplated fasteners. While a heavier zinc plating thickness can provide improved corrosion protection, potential
interference with thread fit-ups can result from the build-up of plating on the threads. The geometry of a 60° thread
results in a change to the pitch diameter equal to four to six times the plating thickness due to irregularities in
plating uniformity. Fine pitch and smaller diameter fasteners tend to be less forgiving than coarse pitch and larger
diameter products, so please consult Nucor Technical Support Personnel before specifying plating thicknesses
greater than .0003”, to avoid needing special thread tolerances during manufacturing to allow for the increased
thickness of the plating.
Commercial Chromate Conversion Coatings: Clear, blue-bright, designation A or B trivalent chromates
are normally used for Grade 5 or metric property class 8.8 fasteners. Commercial Grade 8 and metric property
class 10.9 fasteners are usually supplied with yellow chromate. Our standard yellow chromate is hexavalent free
and designated CT. Other colored chromate conversion coatings can be special ordered to suit your application requirements.
So regardless if you want your fastener finish to just look outstanding; or if you want it to perform outstandingly;
let Nucor and our Team of Finish Professionals recommend the right finish for your fastener application today!
TODAY, THAT LITTLE “n” MEANS BIG THINGS!
This Technical Data Sheet is subject to change without prior notification.
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