Download Definition of Corrosion

CORROSION:TYPES
EFFECTS &
PREVENTION
PRESENTED BY:
GAUTAM AHUJA
1207535
What is Corrosion?
Corrosion is the deterioration of materials by
chemical interaction with their environment. The
term corrosion is sometimes also applied to the
degradation of plastics, concrete and wood, but
generally refers to metals.
TYPES OF CORROSION:Uniform corrosion.
Galvanic corrosion.
Pitting & Crevice corrosion.
Stress corrosion cracking.
Erosion corrosion.
Microbiologically induced corrosion.
UNIFORM CORROSION:Uniform (or general)
corrosion refers to the
relatively uniform reduction
of thickness over the
surface of a corroding
material. It is relatively easy
to measure, predict and
design against this type of
corrosion damage. While
uniform corrosion may
represent only a small
fraction of industrial
corrosion failures, the total
tonnage wasted is generally
regarded as the highest of
all forms.
GALVANIC CORROSION:Galvanic corrosion is an electrochemical
process in which one metal corrodes
preferentially when in electrical contact
with a different type of metal and both
metals are immersed in an electrolyte.
When two or more different sorts of metal
come into contact in the presence of an
electrolyte a galvanic couple is set up as
different metals have different electrode
potentials. The electrolyte provides a
means for ion migration whereby metallic
ions can move from the anode to the
cathode. This leads to the anodic metal
corroding more quickly than it otherwise
would; the corrosion of the cathodic metal
is retarded even to the point of stopping.
The presence of electrolyte and a
conducting path between the metals may
cause corrosion where otherwise neither
metal alone would have corroded.
PITTING & CREVICE
CORROSION:Pitting Corrosion is the
localized corrosion of a
metal surface confined to
a point or small area, that
takes the form of cavities.
Pitting is one of the most
damaging forms of
corrosion. Pitting
corrosion forms on
passive metals and alloys
like stainless steel.The
resulting pits can become
wide and shallow or
narrow and deep which
can rapidly perforate the
wall thickness of a metal.
STRESS CORROSION CRACKING:It is the cracking induced
from the combined
influence of tensile stress
and a corrosive
environment. The impact
of SCC on a material
usually falls between dry
cracking and the fatigue
threshold of that material.
Usually, most of the
surface remains
unattacked, but with fine
cracks penetrating into
the material.
EROSION CORROSION:Erosion corrosion is an
acceleration in the rate of
corrosion attack in metal
due to the relative motion
of a corrosive fluid and a
metal surface. The
increased turbulence
caused by pitting on the
internal surfaces of a tube
can result in rapidly
increasing erosion rates
and eventually a leak.
Erosion corrosion can
also be aggravated by
faulty workmanship.
MICROBIAL CORROSION:Microbial corrosion or biological
corrosion, is the deterioration of
metals as a result of the metabolic
activity of microorganisms.There
are about a dozen of bacteria
known to cause microbial
corrosion of carbon steels,
stainless steels, aluminum alloys
and copper alloys in waters and
soils witho pH 4~9 and temperature
10oC~50 C. These bacteria can be
broadly classified as aerobic &
anaerobic. Sulphate reducing
bacteria (SRB) is anaerobic and is
responsible for most instances of
accelerated corrosion damages to
ships and offshore steel structures.
Iron and manganese oxidizing
bacteria are aerobic and are
frequently associated with
accelerated pitting attacks on
stainless steels at welds.
EFFECTS OF CORROSION:•
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Reduced Strength
Downtime of equipment
Escape of fluids
Lost surface properties
Reduced value of goods
Economic losses
The consequences of corrosion are many and varied
and the effects of these on the safe, reliable and
efficient operation of equipment or structures are
often more serious than the simple loss of a mass of
metal. Failures of various kinds and the need for
expensive replacements may occur even though the
amount of metal destroyed is quite small.
CORROSION PREVENTION:Electrically insulate the two metals from each other. Unless they are
in electrical contact, there can be no galvanic couple set up.
Coating the two materials or if it is not possible to coat both, the
coating shall be applied to the more noble, the material with higher
potential.
Another way is to keep the metals dry and/or shielded from ionic
compounds (salts, acids, bases), for example by painting or
encasing the protected metal in plastic or epoxy, and allowing them
to dry.
Proper selection of materials with known resistance to the service
environment .
Control pH, chloride concentration and temperature
Cathodic protection and/or Anodic Protection.
Regular mechanical cleaning if possible.
Chemical treatment with biocides to control the population of
bacteria.
Complete drainage and dry-storage.
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