Corrosion
resistant alternatives
Cadmium is
a sacrificial coating – i.e. in a corrosive environment the
cadmium dissolves, leaving the underlying steel intact.
Even when scratched, the surrounding coating still protects
the exposed steel. The only coating materials that have
this property are the electronegative elements – Cd, Zn, Al
(Mg and Be as well, but we do not use those as coatings).
You can replace Cd with a barrier coating such as Ni or a
polymer, but once it is scratched the protection is lost.
Cd plating
is forbidden under the European RoHS, ELV and WEEE rules,
and it is a worker and consumer exposure problem as well as
an environmental contamination problem. So very little Cd
plating is used in consumer products. It used to be used
for fasteners in cars and outdoor applications such as
decks, as well as for metal items used outside such as
padlocks. Now Cd has been replaced in most of these
applications, leaving it primarily as a corrosion resistant
coating for aircraft components.
There are
a number of alternatives to Cd that are now used
commercially and are qualified for aerospace use:
-
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.
-
Sn-Zn
is also a commercially available bath chemistry,
although not as common as Zn-Ni.
-
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.
-
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 are a few vendors of this
process in the US and Europe.
-
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.
-
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,
700-1,300°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.
-
Zn-
and Al-filled polymers 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.
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, S53
(not yet available commercially) that has the strength of
high strength aerospace landing gear steels.
Chamber
for IVD aluminum (also known as an Ivadizer)
S53 high strength stainless steel forging |
