CORROSION BY VANA JAGAN MOHAN RAO

1. CORROSION
V.JAGAN MOHAN RAO M.S.Pharm., MED.CHEM
NIPER-KOLKATA
Asst.Professor, MIPER-KURNOOL
EMAIL- jaganvana6@gmail.com

2. INTRODUCTION
• Corrosion is a process of reaction
between metallic surface and its
environment.
• It is an oxidation process.
• It causes loss ofmetal.
• Hence, disintegration of a metal by
its surrounding chemicals through
a chemical reaction on the surface
of the metal is called corrosion

3. • Example: Formation of rust on
the surface of iron, formation of
green film on the surface of
copper.
• The responsible factors forthe
corrosion of a metal the
environmental chemicals,
temperature and thedesign.

4. DIFFERENT THEORIES OF
CORROSION
There are three theories of corrosion:
• Dry or chemical corrosion theory
• Wet or galvanic orelectrochemical
theory

5. DRY CORROSION THEORY
• According to this theory, corrosion on
the surface of a metal is due to direct
reaction of atmospheric gases like
oxygen, halogens, oxides of sulphur,
oxides of nitrogen, hydrogen sulphide
and fumes of chemical with metal.
• Depends on chemical affinity of the
metal towards reactivegas.

6. There are main three reasons of
dry corrosion.
(i)Oxidation corrosion (Reaction with
oxygen)
(ii) Corrosion by other gases
(iii) Liquid metal corrosion

7. (i) Oxidation corrosion
(Reaction with oxygen)
• Some of the metals directly react with
oxygen in the absence of moisture.
• Alkali and alkaline earth metals react
with oxygen at room temperature and
form corresponding oxides, while some
metals react with oxygen at higher
temperature.
• Metals like Ag, Au and Pt are not oxidized
as they are noblemetals.

8. •During oxidation of a metal, metal oxide is
formed as a thin film on the metallic
surface which protects the metal from
further corrosion.
•If diffusion of either oxygen or metal
occur across this layer, further corrosion
is possible.

9. (ii) Corrosion by other
gases (Cl2, SO2, H2S)
• In dry atmosphere, these gases react
with metal and form corrosion products
which may be protective or non-
protective.
• In petroleum industries at high
temperatures, H2S attacks steel forming
FeS scale which is porous and interferes
with normal operations.

10. (iii) Liquid - metal corrosion
• In several industries, molten metal
passes through metallic pipes and
causes corrosion due to dissolution or
due to internalpenetration.
• For example, liquid metal mercury
dissolves most metals by forming
amalgams, thereby corrodingthem.

11. WET CORROSION THEORY
• It is a common type of corrosion of metal in
aqueous corrosive environment.
• This type of corrosion occurs when the
metal comes in contact with a
conducting liquid
• According to this theory, there is the
formation of a galvanic cell on the surface of
metals.
• Some parts of the metal surface act as
anode and rest act as cathode.

12. • The chemical in the environment acts
as an electrolyte.
• Oxidation of anodic part takes place
and it results in corrosion at anode,
while reduction takes place at
cathode.
• Gain of electron isreduction

13. Single metal/Two metal
corrosion

14. Differences between
Dry corrosion
•Corrosion occurs in the
absence of moisture.
•It involves direct attack
of chemicals on the metal
surface.
• The process is slow.
•Corrosion products are
produced at the site of
corrosion.
•The process of corrosion
is uniform.
Wet corrosion
•Corrosion occurs in
presence of conducting
medium.
•It involves formation of
electrochemical
cells.
• It is a rapid process.
•Corrosion occurs at
anode but rust is
deposited at cathode.
•It depends on the size
of the anodic part of
metal.

15. Factors Influencing Rate of Corrosion
• The nature and extent of corrosion
depend on the metal and the
environment-
(i)Nature of the metal(purity)
(ii)Environment (pH, oxidising agents)
(iii)Temperature
(iv)Motion (it may cause erosion)

16. TYEPS OF CORROSION
There are basically eight types of corrosion
I. Uniform corrosion
II. Pitting corrosion
III. Intergranular (Intercrystalline )corrosion
IV. Stress corrosion
V. Crevice corrosion
VI. Galvanic corrosion
VII. Erosion

17. I. Uniform corrosion
• This type of corrosion results in a uniform
and continuous decrease in thickness over
the entire surface area of the metal.
• The rate of uniform corrosion can be
easily determined by measuring the mass
loss, or the quantity of released hydrogen

18. II.Pitting corrosion
• When there is break in protective oxidative
layer, impurities.
• This localized form of corrosion is
characterized by the formation of
irregularly shaped cavities on the surface
of the metal.
• Their diameter and depth depend on several
parameters related to the metal, the
medium and service conditions.
• It is non uniform corrosion, the intensity
and rate of pitting corrosion can not be
assessed by released hydrogen.

19. • Pitting corrosion can be assessed using
three criteria : the density, i.e. the
number of pits per unit area, the rate of
deepening and the probability of pitting
• It is caused by Chloride containing ion.

20. III.Intergranular (weld decay) corrosion
• Boundries in metal alloy
• Boundries contain material whichshows
electrode potential morethan other.
• Impoverished (depleted on one component) is
anodic wrt other.
• Steel can be stabilized by titanium

21. IV. Stress corrosion
• This type of corrosion results from the
combine action of a mechanical stress
(bending, tension) and thermal.
• Unequal rate of cooling for different
parts
• Stress induced by bolts

22. V. Crevice corrosion
• Crevice corrosion is a localized corrosion
in recesses :
• overlapping zones for riveting, bolting or
welding, zones under joints and under
various deposits.
• These zones also calledcrevices
• Metal with more concentration of oxygen
become anode
• This type of corrosion is also known as
deposit attack.

23. VI. Galvanic corrosion
• When two dissimilar metals with large
difference in electrochemical potential,
are in direct contact in a conducting
liquid
• One of the two may corrode.
• This is called galvaniccorrosion.
• The other metal will not corrode; it may
even be protected in this way.
• Protective oxylayer reduce corrosion

24. VIII. Erosion
• Corrosion because of attrition or abrasion of
fluid on metal.
• Corrosion by erosion occurs in moving media.
• This rate of corrosion is related to the flow
speed of the fluid.
• It leads to local thinning of the metal, which
results in scratches, which are always oriented
in the same direction, of the flow direction.
• Insulation can reduce corrosion

25. Prevention (Combat) of corrosion
• Material selection
• Proper fabrication /designing
• Alteration of environment
• Cathodic (metal which is to be
protected is made cathode)
• Anodic protection
• Surface coatings

26. Material selection
• Electrochemical potential
• Strength
• Thermal expansion coefficient

27. Designing
• Joints
• Crevices
• Drainage
• Maintainance

28. Alteration of environment
Reduce
• Temperature
• Moisture
• Turbulance

29. Cathodic/Anodic Protection

30. Cathodic protection
• This is method of reducing or preventing
corrosion of a metal by making it a cathode in
the electrolyticcell.
• This can be achieved by means of an
externally impressed current or
sacrificial anode.
• An electrolyte is needed to ensure the passage
of current through the part to be protected.
• This is effective only in soils or aqueous
media where part to be protected is
immersed.
• It is not effective in the atmosphere.

31. (1) Impressed –current method an external
DC power supply is connected to the metal
be protected.
• Impressed emf means current which is
greater than corrosion current is applied.
• The negative terminal of power supply is
connected to the part to be protected and the
positive to an Auxiliary or inert anode eg:
graphite. Steel scrap, Al, Si-Fe are also can be
used. Si-Fe and graphite are suitable for
ground-beds-buried
• Applications: pipe-lines, underground cables
of Al, Pb; storage tanks, heat- exchangers,
steel-gates exposed to sea water, hulls of
ships, highways and bridges.

32. (2) Sacrificial anode this metal which has
more negative electrode potential than the
structure to be protected is connected
electrically to the part or structure to be
protected.
• The structure is protected at the sacrifice of
another metal. Mg alloys, Zn, Al-5% are
widely used. These anodes are replaced as
soon as consumed.
• Applications: under-water parts of ships,ship
hull, underground pipes, steel water tanks,
water heaters, condenser tubes, oil-cargo-
ballest tanks. Galvanized sheet issacrificial
protection of steel (Zn on steel).

33. • With sacrificial anodes, the protected metal acts
as the cathode, while a more reactive metal is
attached to the protected metal as the anode.
• The redox reactions will occur spontaneously. While
the oxidation reaction takes place at the anode,
consuming
• Thus, protection of the metal will have been
achieved by successfully shifting its corrosion to the
anode.
• The materials used for the anodes are either pure
highly reactive metals like magnesium and zinc

35. Anodic protection
• Metal to be protected is made anode.
• This cause formation of a protective film (oxide layer) on
metals by current.
• An external current isinitially applied on the metal so as to
passivate it.
• Then the current density is reduced to at the value to ensure
that the passive film does not dissolve this ispassivating
current.
• Passivating current is minnimum protective current for
passivation (at this point there is minnimum currosioin)
• In presence of steel titanium become passive.
• A potentiostat is used to maintain the metal at a constant
potential w.r.t a referenceelectrode.
• The primary advantage is its applicability in extreme corrosive
environments with low current requirements.