corrosion

1. THEORIES OF
CORROSION

2. THEORIES OF CORROSION
There are three theories of corrosion
1. Acid theory
2. Dry or chemical theory of corrosion
3. Galvanic or Electro chemical or wet
theory of corrosion

3. ACID THEORY
• This theory states that the acid cause
corrosion of a metal.
• Iron is corroded by atmospheric carbon
dioxide, moisture, and oxygen
• Corrosion products are

4. ACID THEORY

5. DRY OR CHEMICAL THEORY OF
CORROSION
• This theory states that corrosion is due to the
reaction of atmospheric gases such as oxygen,
halogens, sulphur oxides, nitrogen oxides,
hydrogen sulfide and fumes of chemical
products with metal

6. DRY OR CHEMICAL THEORY OF
CORROSION
• The chemical affinity of the metal to the
reactive gas predict the extent of corrosion
• E.g. Alkali and alkaline earth metals react with
oxygen at room temperature and form
corresponding oxides

7. GALVANIC THEORY OF CORROSION
• When the metal or two dissimilar metals
dipped partly or fully in conducting liquid.
• This cause formation of a galvanic cell on the
metals surface
• This type of corrosion is known as galvanic
corrosion

8. GALVANIC THEORY OF CORROSION
• Some parts of the metal surface act anode
and rest as cathode
• The chemical in the environment and
humidity acts as an electrolyte which cause
oxidation of anodic part

9. GALVANIC THEORY OF CORROSION
• This results in corrosion at anode while
reduction takes place at cathode
• Therefore formation of corrosion product
takes place on the surface of the metal
between anode and cathode

10. CORROSION REACTION BETWEEN
METALS
• Galvanic corrosion occur when current flow
from anode to cathode
• Anode represent more active metal while
cathode represent less active metal

11. CORROSION REACTIONS ON A SINGLE
METAL
• Corrosion occur on a piece of iron in
hydrochloric acid
• Anodic and cathodic areas formed on the
surface of iron owing to surface imperfections
or due to variation in the external
environment

12. CORROSION REACTIONS ON A SINGLE
METAL
Reaction at anode
• Oxidation takes place
with the release of
electrons.
• Positive charged iron
atoms get detached
from the solid surface
and enter in to solution
as +ve ions

13. CORROSION REACTIONS ON A SINGLE
METAL
• At anode:
Fe Fe++ +2e- (indicated
by rough surface
oxidation)

14. CORROSION REACTIONS ON A SINGLE
METAL
Reaction at cathode:
• Reduction of
constituents occurs
with the taking up of
electrons
• The free electron reach
the cathode and react
with some positively
charged species such as
hydrogen ions in the
electrolyte solution

15. CORROSION REACTIONS ON A SINGLE
METAL
• At cathode:
2H+ +2e-  H2
(formation of bubbles
at the surface)
Overall reaction:
Fe+2H2OFe(OH)2 +H2

16. CORROSION REACTION BETWEEN
METALS
• Two metals Zn (anode)
and Cu (cathode) when
dipped in electrolyte
solution, they forms
galvanic cells. When two
electrodes are connected
to wire , spontaneous
reaction occur
• At anode(oxidation)
indicates rough surface
Zn Zn++ + 2e-

17. CORROSION REACTION BETWEEN
METALS
• At cathode(reduction)
indicates formation of
bubbles at surface
2H++ + 2e  H2
• When current flows the
anode metal get
corroded and cathode
metal is protected

18. CORROSION REACTION BETWEEN
METALS
• Sometime cathodic
polarization(deposition
of hydrogen layer on
cathode lower the
corrosion) occur
• The accumulation of a
layer of hydrogen on
the cathode surface
slows down the
corrosion- cathodic
polarisation