and drilled holes.
Corrosion around these areas
gases are generated in a high-pressure chamber and
generally is more harmful to the strength of the
then are bled off through a nozzle or orifice into a low-
assembly than corrosion of unbroken surfaces.
pressure chamber, where the pressure energy is
converted into work.
48. Dissimilar Metals.
a. In some designs of
b. The high-low system is complicated by the two
propellant actuated devices, it is necessary that
pressure levels of operation and the need for an orifice.
dissimilar metals contact each other. Aluminum, for
The ballistician provides the designer with the
example, because of its highly anodic characteristics,
approximate pressures and the orifice size, and the
corrodes appreciably (galvanic action) when it contacts
designer chooses the most efficient configuration † and
another metal (steel) with a lesser anodic solution
the location and mounting arrangement of the orifice.
potential for a prolonged period. Common corrosive
media as rain water, sea water, atmospheric moisture,
c. From an erosion standpoint, high-molybdenum-
or some organic liquids may serve as the electrolytic
content steel or pure molybdenum is best for orifice
solution.
material.
If the device is to be fabricated from
b. Galvanic or electrochemical corrosion is
aluminum, the nozzle may be a threaded insert. If,
characterized by severe local corrosion of the anodic
however, the high-pressure chamber is to be fabricated
metal at the point of contact of the two dissimilar metals,
from a molybdenum-bearing steel it may be possible to
if that contact takes place in the electrolyte. The
dispense with an insert orifice, and fabricate both
corrosion is in the form of a film, often only to a depth of
components as one piece. Ease of fabrication, weight,
a few molecular layers. In some instances, this film has
and cost are the usual factors that apply to decisions in
the power to protect the metal underneath, thereby
such instances. Nozzle erosion may be a special
preventing further corrosion. In addition, if there are any
problem in devices which are produced for repeated
cracks, the film will promote self-healing. However, in
applications, and the feasibility of using high-low
thicker films, which are characteristic of a great number
pressure systems must be weighed carefully.
of common metals under the action of a mild, corrosive
47. Protective Coatings. a. Protective coating can be
agent, this self-healing ability of the film is absent..
used on metallic parts likely to be exposed to corrosion
Where the film is broken, corrosion tends to localize.
environments to reduce the possibility of service
This localizing process eventually causes the metal to
failures. For steel components, cadmium, followed by a
pit, thereby lowering the resistance of the metal to
chromate dip, is used widely as a protective coating.
further stress, since any hole or notch in the metal tends
The chromate finish acts as a sealer and retards or
to intensify the stress at that point.
prevents the formation of white corrosion products on
c. The condition in which the metal is subjected to
surfaces exposed to moisture.
repeated stress in a corrosive medium is known as
b. The metallic surfaces of aluminum alloy parts
corrosion fatigue and tends to reduce the fatigue
usually are anodized to prevent corrosion.
By
strength.
Hence, under the combined action of
anodizing, a controlled film of aluminum oxide is formed
corrosion and stress, a corrodible steel will fail
on the surface which serves to protect the underlying
eventually, regardless of the magnitude of the stress.
base metal from further corrosion. This oxide film is
For nonferrous metals, the effects of corrosion fatigue
bonded with the aluminum and, for this reason, exhibits
are quite varied: copper is unaffected, while nickel,
excellent adhesive properties with the base metal and
brass, aluminum, and duralumin are severely affected.
cannot be readily detached by bending or any other
process used in ordinary fabricating.
d. The use of contacting dissimilar metals should
be avoided to prevent galvanic corrosion and corrosion
c. Dichromate dipping and anodizing not only
fatigue. When it is essential that this combination of
retard corrosion, but also retard erosion. In all cases,
metals be employed, an interposing material, acting as
protective coatings should be relatively nonporous and
a protective layer, should be used.
strongly adherent to the base metal, especially in such
localized stress regions as around notches, grooves,
e. Although contacting similar metals are not
†The envelope dimensions dictate whether a uniaxial assembly with high- and low-pressure chambers on a single axis or
a pair of concentric chambers with the high-pressure chamber within the low-pressure chamber is used.
45
