Gaurav Chudasama Content

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SEMINAR TOPIC

ON

GALVANIC CORROSION AND ITS

PREVENTION METHODS

PREPARED BY: CHUDASAMA GAURAV B

SUBJECT: ELECTROMETALLURGY AND CORROSION

ROLL NO.: 937

DEPT.: METALLURGICAL AND MATERIALS

ENGINEERING.

FACULTY OF TECHNOLOGY AND

ENGINEERING


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M.S. UNIVERSITY OF BARODA

1. INTRODUCTION

(1.1) corrosion:

Corrosion has been defined as the undesirable deterioration of

metal or alloy.

i.e. interactions of the metal or alloy with its environment that

adversely affects on those properties of metals or alloy that are

preserved.

Most important factors which affect corrosion are:

Composition, microscopic structure, stress, emf potential of metal

or alloy

And chemical nature, concentration of reactive species, pressure,

temperature of environment. And also metal environment

interface.

(1.2) galvanic corrosion:

When two different metallic materials are electrically connected

and placed in a conductive solution, an electric potential willexist. This potential difference will provide a stronger driving force

for dissolution of the less noble (more electrically negative)

material.


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The less resistive metal becomes anode and more resistive metal

becomes cathode.

While the relative differences in potential will change from oneenvironment to another, they remain fundamentally same, since

the potential is related to energy required to oxidize them to

metal ions in the given environment.

(1.3) significance:

the significance of this becomes more apparent when a variety of

materials are listed in order of their electrical potential in a

familiar environment called galvanic corrosion series, in which the

more precious metals of gold and platinum are at the high

potential(more noble), while zinc, magnesium are at the low

potential(less noble).

(1.4) examples:

Galvanized steel in which zinc to sacrificially protect a steel part


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Galvanic corrosion is often experienced in homes where more

copper water tubing is connected to the older existing carbon

steel water lines.

A more spectacular example occurred in yhe statue of liberty thatgalvanic corrosion had taken place between copoper skin and

wrought iron support.


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2. FACTORS

The severity of galvanic corrosion depends on the corrosion

current density at the anode surface, and this in turn depends on

several factors:

(2.1) electrical resistance of the metal path:


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The higher the electrical resistance in the metal path between the

pair of metals, the weaker the corrosion current and the amount

of galvanic corrosion is less.

If corrosion products form in the faying surfaces, they may

increase the resistance and prevent further galvanic action.

(2.2) conductivity of the solution:

The higher the conductivity of the solution, less the electrical

resistance in the solution path, and greater the galvanic current.

e.g. in pure water galvanic corrosion tends to be negligible, but

increase with the hardness of water(since conductivity increase

with hardness), while galvanic corrosion is severe in sea water.

(2.3) cathode to anode area ratio:

The size of the cathode area controls the amount of corrosion

current, while the size of anode area controls the current density.

The lower the ratio of cathode area to anode area, the less severe

the galvanic corrosion means in which there is a large cathode

and small anode.

The influence of cathode to anode area ratio in sea water can be

illustrated by the case of Al and steel rivets in steel and Al plates

as shown in figure.
http://www.google.co.in/imgres?q=galvanic+corrosion&hl=en&sa=G&biw=1024&bih=546&tbm=isch&tbnid=DZBImAeLA8VARM:&imgrefurl=http://www.powerofaluminium.com/html/corrosion_resistance.htm&docid=HobWnnB6Pw2GbM&w=252&h=106&ei=9qF8TuWiO4HqrQeLooHRDw&zoom=1


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fig.(a) fig. ( b )

In case (a), with an Al rivet head in a steel plate, the small rivet

head attempt to feed current to a large area of plate and will soonbe consumed and also due to potential difference.

In case (b), the steel rivet head accepts only a small amount of

current which is trying to flow from large area of Al plate. Thus

there is much less total corrosion and very much less loss of Al in

any one place.

(2.4) polarization:

Polarization is the main factor that prevents prediction of the

severity of corrosion from a galvanic series. Some couple

polarizes strongly so that very little amount of current required

for corrosion and vise versa.

(2.5) distance effect:

Accelerated corrosion due to galvanic effects is usually greatest

near the junction, with attack decreasing with increasing distance

from the junction. two metal corrosion is readily recognized by

the localize attack near the junction.

3. PREVENTION METHODS


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(3.1) select combination of the metals as close together as

possible in the galvanic series.

(3.2) avoid the unfavorable area effect of a small anode and largecathode.

(3.3) insulate dissimilar metals wherever it is possible. it is

important to insulate completely if possible. So they are not in

electric contact. so there can be no galvanic set up. this can be

done by using plastic, Bakelite or other insulating materials.

(3.4) apply coating with caution. Keep the coatings in good repair,

particularly the on the anodic member. Coating the two materials.

If it is not possible to coat both, the coating will be applied to

more noble, the material with higher potential.

(3.5) add inhibitors, if possible, to decrease the aggressiveness of

the environment.

(3.6) in welded joint using weld of the same alloy are even better.

(3.7) install third metal which is anodic to both metals in galvanic

contact. e.g. if copper + cast iron in which Zn is anodic to both.

(3.8) if we do metallic coating on metal, coated metal should be

less noble then base metal called sacrificial coating.


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4. BENIFICIAL APPLICATIONS

Galvanic corrosion has several beneficial applications

(4.1) dry cell batteries:

Dry cells and other primary batteries derive their electric power

by galvanic corrosion of an electrode.

The driving force for corrosion is the potential difference between

the two metals. On this principle dry cell batteries work. The

carbon electrode acts as a cathode (noble) and Zn as anode

which corrodes. The moist paste between electrodes is the

corrosive environment that carries current.

(4.2) catholic protection:

Use one or more sacrificial anodes made of a metal which is more

active than the protected metal. Metals commonly used for

sacrificial anodes including Zn, Mg, Al etc. galvanized steel is the

classic example of cathodic protection of steel.

(4.3) cleaning silver:

Useful application of galvanic corrosion for cleaning silverware in

the home.In this process, a simple electrochemical cleaning

method consist of placing the silver in an Al pan or foil containing

water and baking soda. The current generated by the contact


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between silver and Al causes silver sulfide to be reduced back to

silver. No silver actually removed. The silver is then rinsed and

washes in warm soapy water. It saves wear and tear on the silver.

REFERENCES:

Hand book by A. PHILIP.

Corrosion engineering by MARS G. FONTANA.

GOOGLE SEARCH(Wikipedia)

Corrosion of light metals by W. B. JEPSON.

Corrosion (volume 1) L. L. SHREIR.


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