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8/12/2019 Types of Corrosion and Factors Influencing Corrosion
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TYPES OF CORROSION AND FACTORS
INFLUENCING CORROSION
Presented by:
TEENA KATHPALLecturer
Pt. J R Govt Polytechnic College
Hoshiarpur
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CORROSION
• “It is the process of deterioration
• ( decay) of a metal as a result of its
reaction with air and water.” • In case of Iron, it is called Rusting and in
case of silver, it is called Tarnishing.
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TYPES OF CORROSION
• 1. DRY or CHEMICAL CORROSION
• 2. WET or ELECTROCHEMICAL
CORROSION
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DRY CORROSION
This type of corrosion occurs by direct
chemical attack of atmospheric gases like O2 ,
halogens, hydrogen sulphide etc. on the metal
surface. it is further of following types:
• (a) Corrosion by O2 or Oxidation corrosion.
• (b) Corrosion by other gases.
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OXIDATION CORROSION
• It is due to chemical attack of O2 on the
metal resulting the formation of metal
oxide layer as:• 2M + nO2-------- 2MOn
(metal oxide)
• This theory is called Wagner’s Theory.
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Nature of Metal oxide layer :
(1) Stable: It acts as a barrier between metal
and O2, So prevents further corrosion.
(2) Unstable: It decomposes back to metal and
O2. Ag, Au, Pt are protected by this manner.
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(3)Volatile: Layer volatalises and metalexposed for further corrosion.
(4) Porous: Corrosion occur through pores
and continues till entire metal gets
corroded.
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CORROSION BY OTHER GASES
• Corrosion is also caused by some other
gases like Cl2 ,H2S, SO2 etc. This type of
corrosion depends on chemical affinitybetween metal and gas.
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WET CORROSION
• This type of corrosion occurs when a
liquid medium is involved. One part
behaves as anode and undergoesoxidation and other part acts as cathode
and undergoes reduction.
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Liquid medium involved acts as
electrolyte. It is further of two types:
(a) Evolution of H2 type corrosion
(b) Absorption of O2 type corrosion
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Evolution of H2 type corrosion
• It occurs in the acidic environment.
At anode:
M-------- M n+ + ne- (oxidation)At cathode:
2H + + 2e- ----- H2 (reduction)
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Absorption of O2 type corrosion
• E.g. Rusting of iron
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If the layer of iron develops some
cracks, this damaged surface acts as
anode while the other intact metal
behaves as cathode. Presence ofmoisture acts as electrolyte.
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The reactions occuring are:
• Fe -------> Fe+2 + 2 e- (oxidation)
1/2O2 (g) + H2O + 2e- ------> 2OH-
(reduction)
Fe+2 + 2OH- --------Fe(OH)2
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In the presence of excess oxygen, Fe(OH)2 oxidises
to Fe(OH)3 as:
4Fe(OH)2 + O2 + 2H2O---- 4Fe(OH)3
This ferric hydroxide formed decomposes tohydrated ferric oxide as:
Fe(OH)3 --------- Fe2O3.xH2O
(Rust)
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OTHER TYPES OF CORROSION
• 1. Galvanic Corrosion.
• 2. Waterline Corrosion.
• 3. Crevice Corrosion.
• 4. Stress Corrosion.
.
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TYPES OF CORROSION (contd.)
• 5. Pitting Corrosion.
• 6. Soil Corrosion.
• 7. High temperature Corrosion.
• 8. Filiform Corrosion
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GALVANIC
CORROSION
• This type of corrosion
occurs when two dis-
similar metals areconnected to each other in
the presence of an
electrolyte. This constitutes
a galvanic cell in which a
metal with lower reduction
potential undergoes
corrosion.
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e.g. when Zn and Cu are connected to each
other then Zn having lower reductionpotential i.e. -0.76V acts as anode and is
corroded while Cu with more reduction
potential i.e 0.34V acts as cathode and
remains protected.
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WATERLINE CORROSION
• This type of corrosion is
observed in the water
tanks where water
remains collected for along time. It is found
just below the line of
water, so called
waterline corrosion.
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CREVICE CORROSION
• This is also known as
Contact corrosion. It
occurs at the place of
contact between metalsand metals or non
metals. This type of
corrosion occurs at
crevices between themetallic objects e.g.
bolts, nuts, washers etc
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PITTING
CORROSION• This type of corrosion
occurs due to cracking
of protective film fromthe surface of metal. It
results in the formation
of pits or holes on themetal, so called pitting
corrosion
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STRESS CORROSION
• It is due to the combined effect of tensile
stress and corrosive environment . This
type of corrosion is observed in the
fabricated articles. During fabrication ,
these are subjected to different type of
stresses like rolling, drawing etc.
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SOIL CORROSION
• This type of corrosion occurs on those
articles which remain burried under soil
e.g. underground water pipes. It occurs
due to moisture, dissolved electrolytes
and presence of micro-organisms.
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HIGH TEMPERATURE CORROSION
• This type of corrosion occurs in those
components which operate at very high
temperature like jet engines, gas turbines
etc. It involves degradation of metal
which results in the deposition of salts or
other compounds on the metal surface.
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It may be of following types:
1. Oxidation: It involves deposition of oxideson the metal surface.
2. Sulphidation: deposition of sulphides.
3. Nitridation: deposition of nitrides.4. Carburization: It involves deposition of
carbon.
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FILIFORM CORROSION
This type of corrosion is observed
under thin coatings in the form of
thread like filaments. It occurs on thesurfaces coated with organic film
which is 0.1mm thick.
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Factors influencing corrosion
1. Nature of metal
2. Nature of corrosive environment.
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1.Nature of Metal:
(1)Purity of metal: Impure the metal, more isthe rate of corrosion. This is because impurities
act as “minute electrochemical cells.”
(2) Physical state of metal: Physical state ofmetal like grain size, stress affects the corrosion
rate. Stressed areas undergo more corrosion.
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(3) Position in Galvanic series:
More the oxidation potential i.e if metal is
higher in the galvanic series, it is more anodic
and hence more is the rate of corrosion.
(4) Nature of Oxide layer : Corrosion ratedepends upon the nature of oxide layer
whether it is stable, unstable, porous etc.
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(5) Passive nature of metal: Some metalslike Ti, Al, Cr show more corrosion resistance
than expected from their position in the
electrochemical series. These are calledpassive metals. This is due to formation of
thin protective layer on metal surface.
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(6) Nature of corrosive products: If the
corrosion products are soluble or volatile in
nature, then corrosion increases.
(7) Relative area of anode and cathode:
smaller the anodic area, more is thecorrosion rate because oxidation of anode
occurs at faster rate.
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2 N t f di i t
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2. Nature of corroding environment:
(1)Temperature: Rate of corrosion
increases with increase in temperature.
(2) Moisture: More the moisture, more is
the rate of corrosion.
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(3) pH Value: Lesser the pH i.e. acidic
conditions, more is corrosion.
(4) Nature of electrolyte: Presence of
salts in the electrolyte increases the rateof corrosion.
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THANK
YOU
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