types of corrosion and factors influencing corrosion

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.



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



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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types of corrosion and factors influencing corrosion

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