ajesh ppt
TRANSCRIPT
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BE IV(MET.& MTRLS ENGG.)
METALLURGICAL & MATERIALS ENGINEERING DEPARTMENT
FACULTY OF TECHNOLOGY AND ENGINEERING
M.S UNIVERSITY OF BARODA
VADODARA
SEMINAR TOPICON
Galvanic Corrosion parametersPREPARED BY :
AJESH BABU
ROLL NO-901
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WHAT IS GALVANIC CORROSION?
Galvanic corrosion occurs when twometals or alloys with differentelectrochemical potentials or with
different tendencies to corrode are inmetal-to-metal contact in a corrosiveelectrolyte.
It is a form of Wet corrosion
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Galvanic reaction is exploitedin dry cell to generate avoltage.
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Four essential things necessary for
galvanic corrosion : - Electrolyte corrosive medium that conducts
electricity.
Anode
one which is less positive or negative emfvalue.
Cathode one which is more positive or positive
emf value. Metallic pathway for the flow of electrons from
anode to cathode.
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Factors affecting Galvanic Corrosion :-
Position of metal in galvanic series
Nature of the environment
Area , distance and geometric effects
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Position of metal in galvanic series:-
Further apart the metals in galvanic series, the
greater is the chance of galvanic corrosion.
Active metal should not be joined with the passive
metal
for e.g Al should not be joined with steel as Al
being more active would tend to corrode.
Metals selected should be close to each other in
galvanic series to minimize galvanic corrosion
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NATURE OF ENVIRONMENT :-
The environment surrounding the metal must be
given due consideration.
For e.g water containing Cu ions (seawater) forms
galvanic cell on steel surface & if water is acidic(salt) galvanic reaction is accelerated because of
increased ionization of electrolyte.
In cold climate galvanic corrosion of buried materialis reduced because of increased resistivity of soil &
in warm climate it is reversed.
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Temperature :- Galvanic corrosion increases with
the increase in temperature
Concentration :- Galvanic corrosion increases
with the increase in concentration
Oxygen Availability :- Galvanic corrosion also
depends upon the oxygen availability & scarcity.
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Area, Distance & Geometric Effects
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Effect of Area :- The rate of Corrosionincreases with the ratio of cathodic to anodic
areas.
e.g . the metal with smallest area has the largestcurrent density & is more damaged if
corrosion occurs at it.
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Steel rivets on a copper bar submerged in 3% sodium chloride solution at the start of the experiment
Steel rivets on a copper bar submerged in 3% sodium chloride solution after six months
Steel rivets on a copper bar submerged in 3% sodium chloride solution after ten months
The GALVANIC TABLE indicates that iron is anodic with respect tocopper and therefore is more rapidly corroded when placed in contactwith it. This effect is greatly accelerated if the area of the iron is small incomparison to the area of the copper, as shown below.
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However, under the reverse conditions when the area of the iron is very large
compared to the copper, the corrosion of the iron is only slightly accelerated.
Copper rivets on a steel bar submerged in 3% sodium chloride solution at the start of the experiment
Copper rivets on a steel bar submerged in 3% sodium chloride solution after six months
Copper rivets on a steel bar submerged in 3% sodium chloride solution after ten months
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Effect of Distance :- If two different metalsare far away from each other, there would be
no risk of galvanic corrosion because of very
little current flow.
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Effect of Geometry :- As current does notflow around the corners geometry of circuit
affects the degree of galvanic corrosion. Any
obstacle to polarization would accelerate
galvanic corrosion.
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Reference :- Nace Volume 1 Principles of Corrosion Engineering &
Corrosion Control Zaki Ahmad
GoogleWikipedia
Electrochemistry & corrosion science Nestor
PerezAn introduction to Electrometallurgy Sharan
& Narain