|
Organic
Water Treatment Chemicals
Steam Boilers, Cooling Towers,
Hot And Chilled Closed Systems
Fuel Oil Treatment
(Home of D.M Concentrate)
Corrosion
Types
This
technical tip describes the
various forms of corrosion in
industrial systems and suggests
design and/or mechanical means
for reducing the possibility of
each
Uniform
Attack
Uniform
attack is a form of
electrochemical corrosion that
occurs with equal intensity of
the entire surface of the metal.
Iron rusts when exposed to air
and water (hence the need for
corrosion inhibitors), and
silver tarnishes due to exposure
to air. Potentially very risky,
this type of corrosion is very
easy to predict and is usually
associated with "common
sense" when making material
decisions.
Galvanic
Corrosion
Galvanic
corrosion is a little more
difficult to keep track of in
the industrial world. You'll
notice below that simply adding
a screw of the wrong material
can have severe consequences.
Galvanic corrosion occurs when
two metals having different
composition are electrically
coupled in the presence of an
electrolyte. The more reactive
metal will experience severe
corrosion while the more noble
metal will be quite well
protected. Perhaps the most
infamous examples of this type
of corrosion are combinations
such as steel and brass or
copper and steel. Typically the
steel will corrode the area near
the brass or copper, even in a
water environment and especially
in a seawater environment.
Probably the most common way of
avoiding galvanic corrosion is
to attach a third, anodic metal
to the other two. This method is
often referred to as "using
a sacrificial anode."
Sacrificial anodes should be
checked regularly and replaced
as necessary.
Crevice
Corrosion
Another
form of electrochemical
corrosion is crevice corrosion.
Crevice corrosion is a
consequence of concentration
differences of ions or dissolved
gases in an electrolytic
solution. A solution became
trapped between a pipe and the
flange on the left. The stagnant
liquid in the crevice eventually
had a lowered dissolved oxygen
concentration and crevice
corrosion took over and
destroyed the flange. In the
absence of oxygen, the metal
and/or it's passive layer begin
to oxidize. To prevent crevice
corrosion, one should use welds
rather than rivets or bolted
joints whenever possible. Also
consider nonabsorbing gaskets.
Remove accumulated deposits
frequently and design
containment vessels to avoid
stagnant areas as much as
possible.
Pitting
Pitting,
just as it sounds, is used to
describe the formation of small
pits on the surface of a metal
or alloy. Pitting is suspected
to occur in much the same way
crevice corrosion does, but on a
flat surface. A small
imperfection in the metal is
thought to begin the process,
then a "snowball"
effect takes place. Pitting can
go on undetected for extended
periods of time, until a failure
occurs. A textbook example of
pitting would be to subject
stainless steel to a chloride
containing stream such as
seawater. Pitting would overrun
the stainless steel in a matter
of weeks due to its very poor
resistance to chlorides, which
are notorious for their ability
to initiate pitting corrosion.
Alloy blends with more than 2%
Molybdenum show better
resistance to pitting attack.
Titanium is usually the material
of choice if chlorides are the
main corrosion concern. (Pd
stabilized forms of Ti are also
used for more extreme cases).
Intergranular
Corrosion
Occuring
along grain boundaries for some
alloys, intergranular corrosion
can be a real danger in the
right environment. The heating
of some materials causes
chromium carbide to form from
the chromium and the carbon in
the metals. This leaves a
chromium deficient boundary just
shy of the where the metal was
heated for welding. To avoid
this problem, the material can
be subjected to high
temperatures to redissolve the
chromium carbide particles. Low
carbon materials can also be
used to minimize the formation
of chromium carbide. Finally,
the material can be alloyed with
another material such as
titanium, that forms carbides
more readily so that the
chromium remains in place.
Selective
Leaching
When
one element or constituent of a
metal is selectively corroded
out of a material it is referred
to as selective leaching. The
most common example is the
dezincification of brass, which
can occur in cooling systems
exposed to high chlorine levels.
After leaching has occurred, the
mechanical properties of the
metal are obviously impaired and
some metal will begin to crack.
Erosion-Corrosion
Erosion-corrosion
arises from a combination of
chemical attack and the physical
abrasion as a consequence of the
fluid motion. Virtually all
alloy or metals are susceptible
to some type of
erosion-corrosion as this type
of corrosion is very dependent
on the fluid. Materials that
rely on a passive layer are
especially sensitive to
erosion-corrosion. Once the
passive layer has been removed,
the bare metal surface is
exposed to the corrosive
material. If the passive layer
cannot be regenerated quickly
enough, significant damage can
be seen. Fluids that contain
suspended solids are often times
responsible for
erosion-corrosion. The best way
to limit erosion-corrosion is to
design systems that will
maintain flow velocities within
the limits of the metals being
used and minimize the number of
changes in direction.
Stress
Corrosion
Stress
corrosion can result from the
combination of an applied
tensile stress and a corrosive
environment. In fact, some
materials only become
susceptible to corrosion in a
given environment once a tensile
stress is applied. Once the
stress cracks begin, they easily
propagate throughout the
material, which in turn allows
additional corrosion and
cracking to take place. The
tensile stress is usually the
result of expansions and
contractions that are caused by
violent temperature changes or
thermal cycles. The best defense
against stress corrosion is to
limit the magnitude and/or
frequency of the tensile stress.
|
Corrosion
Types 
