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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