(Dr.) Mirza Salman Baig
Assistant Professor (Pharmaceutics)
AIKTC, School of Pharmacy,New Panvel
Affiliated to University of Mumbai (INDIA)
• Corrosion is a process of reaction
between metallic surface and its
• It is an oxidation process.
• It causes loss of metal.
• 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
• The responsible factors for the
corrosion of a metal the
temperature and the design.
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
chemicals with metal.
• Depends on chemical affinity of
the metal towards reactive gas.
6. There are main three reasons
of dry corrosion.
(i) Oxidation corrosion (Reaction with
(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
• Metals like Ag, Au and Pt are not oxidized
as they are noble metals.
8. • During oxidation of a metal, metal oxide
is formed as a thin film on the metallic
surface which protects the metal from
• If diffusion of either oxygen or metal
occur across this layer, further corrosion
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-
• 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 internal penetration.
• For example, liquid metal mercury
dissolves most metals by forming
amalgams, thereby corroding them.
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
• According to this theory, there is the
formation of a galvanic cell on the surface
• 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
• Gain of electron is reduction
14. Differences between
• Corrosion occurs in the
absence of moisture.
• It involves direct attack
of chemicals on the metal
• The process is slow.
• Corrosion products are
produced at the site of
• The process of corrosion
• Corrosion occurs in
presence of conducting
• It involves formation of
• 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
15. Factors Influencing Rate of
• The nature and extent of corrosion
depend on the metal and the
(i) Nature of the metal (purity)
(ii) Environment (pH, oxidising agents)
(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
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
18. II. Pitting corrosion
• When there is break in protective oxidative
• 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)
• Boundries in metal alloy
• Boundries contain material which shows
electrode potential more than 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
• 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
• These zones also called crevices
• Metal with more concentration of oxygen
• This type of corrosion is also known as
23. VI. Galvanic corrosion
• When two dissimilar metals with large
difference in electrochemical potential,
are in direct contact in a conducting
• One of the two may corrode.
• This is called galvanic corrosion.
• The other metal will not corrode; it may
even be protected in this way.
• Protective oxy layer 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
• Material selection
• Proper fabrication / designing
• Alteration of environment
• Cathodic (metal which is to be
protected is made cathode)
• Anodic protection
• Surface coatings
30. Cathodic protection
• This is method of reducing or preventing
corrosion of a metal by making it a
cathode in the electrolytic cell.
• This can be achieved by means of an
externally impressed current or
• An electrolyte is needed to ensure the
passage of current through the part to be
• This is effective only in soils or aqueous
media where part to be protected is
• It is not effective in the atmosphere.
(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.
(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
• 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 is sacrificial
protection of steel (Zn on steel).
• 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,
• Thus, protection of the metal will have been
achieved by successfully shifting its corrosion to the
• 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 is initially applied on the metal so as to
• Then the current density is reduced to at the value to ensure
that the passive film does not dissolve this is passivating
• 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 reference electrode.
• The primary advantage is its applicability in extreme corrosive
environments with low current requirements.