Direct Chemical
Attack is also fundamentally electrochemical in nature.
However, no current flow is detectable, nor are there any
definite anodic or cathodic areas. The theoretical rate of a
chemical attack can be affected by the formation of a protective
film on the metal surface, through secondary reactions involving
the products of corrosion, and the mechanical removal of
protective films, such as by erosion, flexing of the metal
surface, or by temperature changes.
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Dry Oxidation and
Tarnish result when the clean
surfaces of metals are exposed to air or other gasses to form
oxides or other compounds on the surface of the metals. Many of
these films are invisible at room temperature, but at higher
temperatures these films may reach considerable thickness. The
rate of film growth is usually greater at higher temperatures.
At higher temperatures, and more particularly under changes of
temperature, the film may crack or spall to expose fresh metal
to attack. Bending or stressing the metal can induce or increase
the spalling. The presence of sulfur-bearing gasses may greatly
increase the rate of attack. And the presence of moisture will
accelerate attack, and complicate it by permitting electrolytic
corrosion.
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Atmospheric
Corrosion combines electrochemical
attack, direct chemical attack and oxidation. The many
structures created by man which are subject to atmospheric
corrosion, and the many specialized methods which have been
developed to combat it, justify it for consideration as a basic
type of corrosion. Temperature changes, alternate wetting and
drying, and the washing action of the elements of weather modify
the rates of film removal. Rural atmospheres differ from
industrial, and industrial atmospheres differ from one another
to such an extent that special tests and consideration must be
given for the optimum selection of protective coatings and
application methods.
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Crevice
Corrosion is the attack on the
surface of a metal partly shielded from contact with the
corroding fluid, usually by a non-metallic materials. Typically,
this is a concentration cell effect, the shielded area being
anodic.
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Caustic
Embrittlement is usually the
result of steam escaping through a crevice, such as between
boiler plates or pipe flanges. The escaping water, usually in a
fissure of steam, becomes highly concentrate, and the increased
alkalinity of the concentrated water causes failure by stress
corrosion.
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Stress
Corrosion Cracking results when even a very small pit
forms in a metal under stress. The concentrated stress may
either deepen and extend the pit, or crack any protective film
which might tend to form. Under continued exposure to the
corrosive medium and stress, the crack extends by alternate
corrosion and stress failure.
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Hydrogen
Embrittlement and hydrogen attack
results when atomic hydrogen, contained in chemical and refinery
processes or produced electrolytically by the process,
penetrates onto the grain boundaries of steel producing
microcracks, blistering and loss of ductility. The atomic
hydrogen combines into molecules, cannot escape, resulting in
blistering and laminations.
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Pitting is caused by
electrolytic corrosion and is the result of galvanic attack at
the anodes of the metal surface. This can be seen where local
failures in the film on stainless steel may be anodic to the
remainder of the surface. This is more commonly seen on
aluminum, and most commonly on steel in heterogeneous soil.
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In Dezincification, one constituent
of an alloy, such as the zinc in brass or bronze, may be
selectively removed, leaving a porous replica of the original
part. Often the whole alloy is initially dissolved, with one
element redeposited in spongy form.
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Graphitization is most often seen
as the electrolytic corrosion of cast iron, and often takes a
form very similar to the dezincification of brass. Iron is
removed selectively, leaving a replica composed of carbon or
graphite.
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Cathodic
Corrosion. Although the cathode of
an electrolytic cell is not in itself corroded as a direct
result of the electrolytic process, the cathode may be attacked
by the reaction products formed by the process. A typical
example is the corrosion of lead in the very alkaline
environment produced at the cathode of cathodically protected
lead cable.
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Corrosion
Fatigue occurs when a metal is
subjected to alternating stress and relief in a corrosive
environment. Metal failure occurs much more rapidly under the
alternating stress than under either stress or relief alone.
Continuous removal of protective films, and the repeated
exposures of clean metal by small stress failures cause
corrosion fatigue.
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Erosion
Corrosion, as the name implies,
occurs when the corrosion products which would normally afford a
protective film are scoured off by moving fluids, particularly
if the fluids contain abrasive materials. The erosion will
expose clean metal, and develop a physical pattern so obviously
a result of erosion that the corrosive factor may not be
recognized.
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Cavitation or Impingement
Attack is a process which is very similar to erosion. In
cavitation, collapsing gas bubbles in regions of turbulence and
local pressure fluctuations may activate serious corrosion.
Condenser tubes and pump impellers are subject to this type of
attack.
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Fretting
Corrosion. Metal surfaces in close
physical contact, in a corrosive environment, and subjected to
vibration, can accelerate corrosion attack by the continuous
removal of protective films. Machine parts with small relative
motion and high unit loads are subject to fretting
corrosion.
Return to the corrosion tutorial.
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