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Cadmium Plating Alternatives
There are
a number of alternatives to Cd that are now used
commercially and are qualified for aerospace use:
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Zn-Ni
electroplate is an aqueous bath that is usually acid,
although alkaline versions are being developed for
commercial use. The process was developed by Boeing and
bath chemicals are available commercially. Because of
hydrogen embrittlement concerns this coating is not yet
recommended for high strength steels.
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Sn-Zn
is also a commercially available bath chemistry,
although not as common as Zn-Ni.
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IVD
aluminum (Ion Vapor Deposited Aluminum) was developed by
McDonnell Douglas Aircraft and has been in use for
aircraft for many years. It is a vacuum process and
does not cause embrittlement – hence it can be used on
high strength steels. As-deposited IVD coatings are
porous and poorly adhered, and have to be glass bead
peened for acceptable performance. Both equipment and
coating services are available commercially.
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Electrolytic aluminum plating has been shown to be an
excellent coating alternative to Cd, usually showing
superior performance. Unlike most electroplating,
however, it must be done using an organic,
toluene-based) bath in a completely enclosed and inert
atmosphere. This makes it much more capital-intensive
to set up than a standard electroplate. Because the
system is completely enclosed there is no worker
exposure to toluene. There is one vendor of this
process in the US (Alumiplate) and one in Europe (Aluminal).
Both license the same original Siemens patents, although
their process details and performance may differ
substantially.
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Thermal spray Al coatings are sometimes used on landing
gear and other large items. On large structures such as
bridges and communications towers, ZnAl is often used.
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Metallic-ceramic coatings (often known by the best-known
trade name SemeTel) are filled with Al flake and are
often used for their combination of abrasion and
corrosion resistance. They must be heat treated
(typically to 375-700°F,
190-370°C),
although some can be air-cured or cured at low
temperature for use on high strength steels. These used
to be very expensive as they could only be applied one
company, but there are now various vendors of the
materials as well as the process.
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Zn-
and Al-filled polymers and ceramics are primarily used for automotive
fasteners as well as for threaded fasteners on equipment
exposed to the elements (especially sea air). These
coatings are usually applied by spray or dip-spin
methods. They are relatively inexpensive and effective,
and can be color coded. They are not suitable for fine
threads or for rivets.
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Stainless steels (sometimes known as Corrosion Resistant
or CRES alloys) are being used increasingly in place of
Cd plated steel, and there are now a number of these on
the market with a range of properties. In addition to
the standard low strength 300 series alloys and higher
strength stainless steels used for fasteners, there are
a number of high strength stainless steels that are
typically used in the aerospace industry. The most
common of these are 15-5PH, 17-4PH and 13-8Mo, and a
newly developed alloy, Ferrium®
S53 (not yet available
commercially) that has the strength of high strength
aerospace landing gear steels.
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