| Corrosion is the
deterioration of a metal because of a reaction with its
environment. The products of corrosion vary widely, from
the flaky red dust of iron to the relatively benign oxide over
aluminum. In general, corrosion should be avoided or
minimized where possible.
There are many types of corrosion, but a
particular type called galvanic corrosion is most important
for inserts and fasteners. Galvanic corrosion occurs
when dissimilar metals are in contact in the presence of an
electrolytic solution. All metals exhibit different
degrees of "activity" or "nobility" and can be arranged in a
galvanic series of increasing activity (see
Jul/Aug '98 issue of KATO Fastening News).
Gold and platinum are most noble, while zinc and magnesium
are most active. The most common electrolytic solution
encountered is ordinary water. Seawater or salt spray
is more damaging because of high concentrations of dissolved
salts.
The best way to preclude galvanic corrosion
is to use similar potential metals and eliminate the
electrolyte conductor. The active stainless steel
CoilThread® Insert are not passivated. This minimizes
the possibility of galvanic corrosion occurring when they
are installed in aluminum or magnesium parent materials.
Some additional precautions for reducing
galvanic corrosion are:
- Isolate the fasteners from the
electrolyte. This can be done through gasketing or
sealing.
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- Specify cadmium plated inserts. The
cadmium plate provides a sacrificial barrier against
corrosion. In addition, the cadmium plate has
lubricating properties that minimize galling when stainless
steel screws are used.
- Apply corrosion inhibiting pastes or
compounds to the screw. These include zinc chromate
primer (MIL-P-8585) and strontium chromate primer
(MIL-P-23377).
Note: Pastes applied to the CoilThread insert
can become trapped between the wire and the hole and can
cause loss of proper tolerance. It is therefore
recommended to apply the paste only to the screw, not the
insert. If Zinc chromate primer is applied to the
tapped hole, it should be thinned and applied sparingly.
The insert should be installed while the primer is still
wet.
- Specify a dry film lubricant such as
molybdenum disulphide on the inserts. This provides a
secondary barrier against corrosion.
- Where practical, or when it will not
interfere with the completed assembly, the external joint
should be coated with a suitable paint.
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