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FORMS OF CORROSION
O Corrosion may beclassified in differentways
O Wet / Aqueous
corrosion & DryCorrosion
O Room Temperature/High Temperature
CORROSION
WET
CORROSION
DRY
CORROSION
CORROSION
ROOM
TEMPERATURE
CORROSION
HIGH
TEMPERATURE
CORROSION
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WET & DRY CORROSION
O Wet / aqueous corrosion is the major form
of corrosion which occurs at or near room
temperature and in the presence of water
O Dry / gaseous corrosion is significant mainly
at high temperatures
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WET / AQUEOUS CORROSION
Based on the appearance of the corroded
metal, wet corrosion may be classified as
O Uniform or General
O
Galvanic or Two-metalO Crevice
O Pitting
O Dealloying
O Intergranular
O
Velocity-assistedO Environment-assisted cracking
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UNIFORM CORROSION
O Corrosion over the entire
exposed surface at a uniform
rate. e.g.. Atmospheric
corrosion.
O Maximum metal loss by this
form.
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CREVICE CORROSION
O Intensive localized corrosion
within crevices & shielded
areas on metal surfaces .
O Small volumes of stagnant
corrosive caused by holes,
gaskets, surface deposits, lap
joints .
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DEALLOYING
O Alloys exposed to corrosives
experience selective leaching
out of the more active
constituent. e.g.Dezincification of brass.
O Loss of structural stability and
mechanical strength
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PITTING
O A form of extremely
localized attack causing
holes in the metal
O Most destructive form
O Autocatalytic nature
O Difficult to detect and
measure
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INTERGRANULAR CORROSION
O The grain boundaries in metals
are more active than the grains
because of segregation of
impurities and depletion of
protective elements. So
preferential attack along grain
boundaries occurs. e.g. weld
decay in stainless steels
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CAVITATION DAMAGE
O Cavitation is a special case of
Erosion-corrosion.
O In high velocity systems, local
pressure reductions create
water vapour bubbles which
get attached to the metal
surface and burst at
increased pressure, causing
metal damage
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HIGH TEMPERATURE(DRY) CORROSION
O Exposure to high temperatures in air or
other gaseous environments causes sever
material damage.
O The major corrosion processes are: Oxidation
Corrosion in Sulfur environments
Hydrogen attack
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Oxidation-resistant alloys
O The oxide characteristics determine theoxidation resistance of
an alloy
O Most oxides are non-stoichiometric compounds
with structural defects. They may be n-type or p-type
semiconductors whose conductivities couldbe altered by alloy
additions.
O This principle is used in developing high
temperature oxidation resistant alloys likeFe-C
r,Fe-Cr-Al, and Ni-base alloys.
