Polymers

• Polymers: Polymers (Poly: Many, Mers: units) are macromolecules

built-up by the linking together of a large number of small molecules
called monomers.
Monomers
• Monomers: The small molecules which combine with each other to
form polymer molecule are termed monomers.
POLYMERIZATION
• POLYMERIZATION: The chemical process in which a large number of
monomer molecules get joined to form the polymer molecules, is
known as polymerization.

• Polymerization is a process of linking together of two or more simpler

molecules resulting in the formation of new C-C linkage.
Addition Polymerization
• Addition Polymerization: “A reaction that yields the product, which is
exact multiple of monomeric molecule, is termed as addition or chain
polymerization”.
• The addition polymerization reaction must be initiated by the
application of heat, pressure or a catalyst for breaking the double
bond of monomer.
• Monomeric molecules which are taking part in the reaction
usually contain one or more double bonds which by intermolecular
rearrangement may make the molecule bifunctional.
Addition Polymerization

n(CH2=CH2) Polymerization -(-CH2-CH2-)n

Ethylene Polythene
Monomer Polymer
Addition Polymerization

H2C
- -
-CH CH2

n Polymerization

Styrene n
Polystyrene
 Copolymerization
• Copolymerization: A type of addition polymerization in which simultaneous
polymerization of two or more chemically different monomers takes place
forming a polymer containing both monomers linked in a chain called
copolymerization.
• e.g. Formation of GR-S or Buna-S rubber by copolymerization of Butadiene
and styrene in presence of sodium as a catalyst.
Condensation Polymerization:
• Condensation Polymerization: “The reaction of monomers, having
functional groups leading to the production of a polymer with the
elimination of small molecules such as H2O, HCl etc. is called as
condensation polymerization”.
• e.g. Formation of Nylon-66 by the condensation polymerization of
hexamethylene diamine and adipic acid .
Condensation Polymerization
• When monomers contain two or more functional groups, then
polymers obtained are with cross linked structure.
 DIFFERENCE BETWEEN ADDITION
AND CONDENSATION
POLYMERIZATION
Addition Polymerization Condensation Polymerization
1. A reaction that yields the product which is exact multiple 1. The reaction of monomers having functional groups,
of monomeric molecule is termed as addition or chain leading to the production of a polymer with the elimination
polymerization. of small molecules such as H2O, HCl etc. is called
condensation polymerization.

2. In this, the polymer is exact multiple of monomer. 2. As there is loss of small molecules, polymer is not exact
multiple of monomer.
3. Polymers are linear, chain type structure i.e. they are of 2 3. Polymers are of 3 dimensional structures as cross linkages
dimensional structures. are present.
4. Monomer should contain at least one double bond. 4. Monomer should contain at least one functional group.

5. This reaction requires light, heat, pressure or catalyst. 5. There is no such condition.

6. e.g. Formation of Polythene, Polystyrene, GR-S rubber etc. 6. Formation of Nylon-66, Bakelite plastics etc.
 DEGREE OF POLYMERIZATION

• Size of all the polymer molecules in the polymer material is not same. It
depends upon the number of repeating units present in a molecule of
polymer.
• e.g. A polyethylene molecule may contain the repeating units – CH2 CH2 – 200 to 500.
 DEGREE OF POLYMERIZATION

• Definition : Degree of polymerization is the average number of repeating

units in the molecules of the polymer.
• The degree of polymerization increases with time and temperature of polymerization
reaction. It also depends upon the concentration of monomer and the initiator
(catalyst).
• In general strength of polymer increases with increase in DP (or molecular weight) in a
non-linear way.
DEGREE OF POLYMERIZATION

• The DP should be such that the polymer has at

least 20,000 molecular weight, for the polymer to
have desirable strength properties.
• However, too high degree of polymerization
causes high melt viscosity, difficult processing
because of increased chain entanglements.
• If the DP of a polymer is below a limit, then the
polymer is either a friable powder or viscous
liquid.
• e.g. vinyl polymers have useful properties in the
DP range of 200 to 600.
 RELATION BETWEEN DP & MW

• The molecular weight and DP are related by,

• M = nMO
• Where n is DP of the polymer and MO is the molar mass of monomer.
• e.g.

•
• Polyvinyl chloride contains the repeating unit of molecular weight,
• Mo = 62.5, and if the DP (n) in the polymer molecules is 500,
• then the Molecular weight of Polymer will be,

• n Mo = 500  62.5 = 31250

 RELATION BETWEEN DP & MW

• Polyethylene having DP as 600, will have molecular weight = 600  28 =

16800.

• Polyethylene having DP as 400, will have molecular weight = 400  28 =

11200.
 NUMERICALS

• 42 gm of propene gas was polymerized by chain mechanism to form polymer

of molecular weight 21,000. Calculate degree of polymerization.

M = n Mo
n = M/Mo
n = 21000/42
n = 500
 NUMERICALS

• Degree of polymerization (DP) of polyethylene is 625. Calculate its molecular

weight.

M = n Mo
M = 625 x 28
M = 17500
Calculate Molecular weight of polyvinyl
chloride having degree of
polymerization 400
(Mol. Wt. vinyl chloride = 62.5)
M = n Mo
M = 400 x 62.5
M = 25000
 Molecular Weight of
Polymers
• The molecular weight study of polymers has great importance in the
research and industries, for manufacturing commercial polymer products.
• The properties of polymers largely depend upon the molecular weight.
• Generally, higher molecular weight polymers are stronger, tougher, brittle
and more heat resistant while lower molecular weight polymers are softer or
gum-like.
 Molecular Weight of
Polymers
• polymer molecules in a sample are of same chemical type but of different DP,
size and molecular weight.
• As the polymer molecules in a polymer differ in length and sizes, we consider
the ‘average value’ of their molecular weights.
Number Average Molecular Weight :

• “ It is the total mass of all the molecules in a polymer sample divided

by the total number of molecules present ”.

n1 M1, n2 M2, n3 M3
= n1*M1 + n2*M2 + n3*M3/ n1+n2+n3
 A polymer has following population
•
5 molecules have molecular weight each 5000
•
10 molecules have molecular weight each 7500
•
15 molecules have molecular weight each 9000
•
20 molecules have molecular weight each 12000
•
25 molecules have molecular weight each 14000
•
15 molecules have molecular weight each 10000
•
10 molecules have molecular weight each 8000
•
Calculate its number average molecular weight.
•

= 5 x 5000 + 10 x 75000 + 15 x9000 + 20 x 12000 + 25 x 14000 + 15 x 10000 + 10 x 8000

5 + 10 + 15 +20 + 25 +15 + 10

= 10550
A polymer has following population
15 molecules have molecular weight each 8000
20 molecules have molecular weight each 10000
25 molecules have molecular weight each 12000
40 molecules have molecular weight each 18000
Calculate its number average molecular weight.

= 15 x 8000 + 20 x 10000 + 25 x 12000 + 40 x 18000

15+20+25+40

= 13400
A polymer has following population
25 molecules have molecular weight each 18000
30 molecules have molecular weight each 12000
35 molecules have molecular weight each 15000
40 molecules have molecular weight each 17000
Calculate its number average molecular weight.
A polymer has following population
15 molecules have molecular weight each
18000
20 molecules have molecular weight each
20000
30 molecules have molecular weight each
22000
35 molecules have molecular weight each
25000
40 molecules have molecular weight each
27000
Calculate its number average molecular
weight.
PLASTICS
• Plastics are such substances which possess property of plasticity. (i.e.
the property by virtue of which a material undergoes permanent
deformation when subjected to heavy load).
 Properties of Plastics :

• 1. Light in weight
• 2. Not affected by environmental conditions
• 3. Insulators : Plastics generally have low thermal and very low electrical
conductivity.
• 4. Easy workability : Plastics can be easily casted, moulded, drilled, machined.
• 5. They have low thermal coefficient of expansion.
• 6. Transparency : Some of the plastics are transparent and have high refractive
index.
 Properties of Plastics :
• 7. Colours : Plastics can be mixed with pigments to give attractive colours.
• 8. Plastics do not absorb water.
• 9. Mechanical strength : Some of the polymers have very good tensile strength and
toughness.
• 10. Adhesives : Some of the polymers have very strong binding character.
• 11. Absorb sound, vibrations and shocks.
• 12. Plastics have few undesirable properties like they are not generally biodegradable
and produce solid waste and they are costly. They have low thermal stability.
 TYPES OF PLASTICS
• Thermosoftening plastics: These are also known as thermoplastics. They are formed by addition
polymerization. The chemical structure of thermo softening plastics consists of long linear chains of
monomers. They can be represented as
 Thermosoftening plastics

• Thus in thermoplastics, cross links are nearly absent. Therefore on

heating and compressing, linear chains slide over each other. This
sliding brings about an easy softening of the plastics. The bonding
between the two consecutive molecules becomes weak on
heating. The plastic therefore gets converted into a fluid on
heating. The soft fluid material can be easily molded.
• On cooling, these linkages can set again. Now they are called
secondary links. These plastics therefore can be softened by
heating, reshaped, reused as many times as desired.
• e.g. Polythene, Polystyrene, etc
Thermosetting plastics

Thermosetting plastics are formed by condensation polymerization. The

structure is somewhat three dimensional and can be represented as
 Thermosetting plastics

• These plastics during molding process get hardened and once

they have solidified, they cannot be softened. i.e. they are
permanently setting plastics.
• The cross links between the monomers serve as a strong
bonds. Due to these bonds the mass once formed cannot be
broken even on applying heat or pressure. Therefore they
cannot be reused.
• At high temperature they lost their usual property which is
called as thermal degradation.
• e.g. Bakelite, Polyester, Nylon-66, etc.
 DIFFERENCE BETWEEN THERMOSOFTENING AND THERMOSETTING
PLASTICS
Thermosoftening Plastic Thermosetting Plastic

1. These are formed by addition polymerization. 1. These are formed by condensation polymerization.

2. These polymers are of low molecular weight. 2. These polymers are of high molecular weight.

3. These are linear polymers without having cross 3. These are having three dimensional structures.
linkage.

4. They can be softened by heating due to breaking of 4. They cannot be softened as the bonds cannot be
bonds. broken easily but at very high temperature substance
degrades.

5. These are soluble in suitable organic solvents. 5. These are insoluble in organic solvents.

6.e.g. Polythene, Polystyrene, etc. 6.e.g. Polyester, Bakelite, Nylon-66, etc.

 Polyvinyl Chloride (PVC) :
• Preparation :
• Vinyl chloride gas is polymerized by either suspension or emulsion technique. Generally
free radical initiator is used.
• The emulsion process is carried out at about 40C and pressure 5 – 10 atmosphere.
• Suspension process is carried out at 50 – 80C in a pressure reactor.
 Rigid PVC :

• The PVC without plasticizer added is known as rigid PVC and it has following properties :
• i) It is rigid, strong and brittle.
• ii) It has polar structure due to C – Cl bonds and therefore stronger force of
intermolecular attraction. Hence, it has high density (1.33 gms/cc).
• iii) It has softening temperature about148C, Tg = 80C.
• iv) Rigid PVC is non-inflammable because of presence of chlorine atoms on polymer
chain.
• v) It is colourless but can be given attractive colour by compounding.
• vi) It has excellent chemical resistance.
 Rigid PVC :

• vii) It is soluble in mixture of acetone and carbondisulphide.

• viii) It has high crystallinity.
• ix) It decomposes above 200C.
• x) It has good abrasion resistance.
• Uses of Rigid PVC :
• It is used as sheets for reactor lining, for light fitting tubes, in helmets and
refrigerator components, hoods, trays, mudguards, aircraft parts, boats,
gramophone records, bottles etc.
 Plasticized PVC :

• Rigid PVC is strong but brittle therefore it is compounded with a plasticizer upto 30% to
get flexible plasticized PVC.
• It softens at lower temperature and it has much lower Tg.
• Remaining properties are similar to rigid PVC i.e., Noninflammable, good chemical
resistance, decomposes above 200C, density 1.33, good abrasion resistance.
• Use of plasticized PVC :
• It is used for packings, rain-coats, curtains, table-cloth, hosiery goods, irrigation pipes,
cabinates for radio, television and electronic equipments, chemical containers, hand
tools, adhesive tapes, films, wire insulation, footware components, bags, glows, matrices,
conveyer belts etc.
 Polyethylene Terphthalate
(PET) :
• PET is a commercial polyester product also known as terylene or dacron and it is
obtained by condensation reaction between ethylene glycol and dimethyl
terephthalate at high temperature.
Polyethylene Terphthalate (PET) :
• Properties :
• 1. It forms crystalline polymer because of symmetrical structure and polar ester
groups in chain.
• 2. It can be easily converted to fine fibers of very good tensile strength.
• 3. It has good impact strength and better thermal stability.
• 4. Fibres of PET are crease resistant, low moisture absorbing.
• 5. PET is resistant to O2 and acids but is attacked by alkali.
• Applications :
• 1. It is the most common synthetic fabric material.
• 2. It can be blended with cotton, viscose, wool, silk to get better fabrics.
• 3. It is useful for making magnetic recording tapes.
• 4. It can be converted to films and bottles.
• 5. It is used for making buttons, knobs.
COMPRESSION MOULDING
 COMPRESSION MOULDING
• This method is applied to both thermoplastic and
thermosetting resins. The calculated quantity of plastic
ingredients in proper proportion get filled between two half
pieces of mould. Heat and pressure are then applied
according to specification. The cavities get filled with
fluidized plastic. The two halves are closed very slowly.
Finally curing is done. After curing the mould article is taken
out by opening the mould parts.
Compression Moulding
INJECTION MOULDING
INJECTION MOULDING
• This is applicable mainly to thermoplastic resin. In this
method the moulding plastic powder is fed into a
heated cylinder, from where it is injected at a
controlled rate into tightly locked mould by means of
screw arrangement. The mould is kept cold to allow
the hot plastic cure and become rigid. When the
material have been cured sufficiently, half of the
mould is opened to allow the injection of the finished
article without any deformation.
 Injection Moulding
*

Schematic of thermoplastic Injection molding machine

Injection Moulding
TRANSFER MOULDING
 TRANSFER MOULDING
• Here principle of injection moulding is used for thermosetting plastics. In this
method, moulding powder is placed in a heated chamber, maintained at a
minimum temp. at which the moulding powder just begins to become plastic.
This plastic material is then injected through an orifice into a mould by a
plunger working at a high pressure. Due to increase in the temp. the moulding
powder becomes liquid and consequently it flows quickly into the mould
which is being heated up to the curing temp. required for setting. The mould
article is then ejected mechanically.
Transfer Moulding
EXTRUSION MOULDING
EXTRUSION MOULDING
• It is used mainly for continuous moulding of
thermoplastic material into articles of uniform
cross section like tubes, rods, strips, insulated
electric cables etc. The thermoplastic ingredients
are heated to plastic condition and then pushed by
means of a screw into a die, having the required
outer shape of the article to be manufactured. Here
the plastic mass gets cooled, due to the atmospheric
exposure
RUBBER
• These are high polymers which exhibit the property of elasticity.
(Elasticity: It is the property of material by virtue of which a material undergoes
deformation under stress but returns to its original shape on removal of stress).
• The elastic deformation of rubber is caused by the structure of the polymer,
which contains hydrocarbon isoprene (C5H8) as the constituent monomer. The
polymer of rubber is not in the form of a straight chain but in the form of a spring
or coil.
TYPES OF RUBBER:
There are two types of rubber,
NATURAL RUBBER
SYNTHETIC RUBBER.
NATURAL RUBBER:

Natural rubber consists of basic material latex, which is dispersion

of isoprene. During the processing, this isoprene molecule
polymerizes to form long coiled chains or cis-polyisoprene.
Processing of Latex:
Natural rubber is made from the saps of wide range of plants like Havea Brasillians
and Gauyule. The rubber latex is obtained by making incisions on the bark of the
rubber trees and allowing the saps to flow out into small vessels. Latex is diluted
and filtered to eliminate any dirt present in it. Then it is allowed to stand for few
hours. Coagulants like acetic acid and formic acid are added to make the rubber
soft..
 Havea Brasillians and Gauyule
Tree
Sharp Cuts / Incisions on Bark of
tree
Secretion of milky liquid i.e.
Latex

Collection of Latex
Dilution and Filtration of Latex
Coagulation of latex by adding
accetic acid / Formic acid
Coagulated Mass called Coagulum floats on
surface

Coagulum pressed Coagulum is processed in

through sets of rollers to Smoke for 5 days at 40-50OC
expel water
Crepeed Rubber Smoked Rubber
 Dichopsis Tree
Dried Leaves
collection
Crusing of leaves

Solvent extraction
Gutta Purcha
Rubber
 Drawbacks of Natural
Rubber
• 1. It can be used in the temp. range of only 100C to 600C. At high
temp. it becomes soft and at low temp. it becomes too brittle
• 2. Its tensile strength is very less.
• 3. It has large water absorption capacity.
 Drawbacks of Natural
Rubber
• 4. It is non resistant to non polar solvents like benzene, CCl4.
• 5. It is attacked by oxidizing agents like nitric acid, conc. H2SO4,
Chromic acid etc.
• 6. It swells considerably in organic solvent and gradually disintegrates.
 Drawbacks of Natural
Rubber
• 7. It possesses marked tackiness i.e. when two fresh raw rubber
surfaces are pressed together, they form a single rubber.
• 8. they has little durability.
• 9. When stretched to greater extent, it suffers permanent
deformation.
VULCANIZATION
• To overcome the drawbacks and to improve properties of rubber it is
compounded with some chemicals like sulphur, hydrogen sulphide,
benzoyl chloride etc. The most common is addition of sulphur which
is known as VULCANIZATION.
VULCANIZATION
• The process consists of heating the raw rubber with sulphur to 100-
1400C. The added sulphur combines chemically at the double bonds
of different rubber springs.
VULCANIZATION
VULCANIZATION
• Vulcanization thus serves to stiffen the material by a sort of anchoring
and consequently preventing intermolecular movement of rubber
springs.
• The extent of stiffness is dependent on the amount of sulphur added.
• Tyre rubber- 3 to 5 % sulphur.
• Battery case rubber – 30% sulphur.
 ADVANTAGES OF VULCANIZATION
• 1. Tensile strength increases.
• 2. Water absorption capacity is low.
• 3. Better electrical insulator; 32% sulphur for battery case rubber,
ebonite.
• Resistant to organic solvent.
• Useful temp. range 40 to 1500C
• Slight tackiness
• It becomes hard.
 SYNTHETIC RUBBERS:

• STYRENE-BUTADIENE or GR-S or BUNA-S RUBBER-

n
STYRENE-BUTADIENE or GR-S or
BUNS-S RUBBER-
• Properties:
• It possesses greater ozone resistance but lower in crack spread resistance than
natural rubber.
• It has abrasion resistance and high load bearing capacity.
• It does not break down easily.
• It gets readily oxidized.
• It has greater weather ability as compared to natural rubber.
• It can be vulcanized, similar to natural rubber.
STYRENE-BUTADIENE or GR-S or
BUNS-S RUBBER-
• Applications:
• It is used for making light duty tyres.
• For making shoe soles.
• For making foot ware components.
• For making wire and cable coatings.
• For making gaskets.
• For making tank lining.
• For making carpet backing.
 SYNTHETIC RUBBERS:

• THIOKOL or POLYSULPHIDE
RUBBER:
THIOKOL RUBBER
• Properties:
• Thiokol is having outstanding resistance to swelling and disintegration by organic
solvents.
• Fuel oil, lubricating oil, and kerosene have no effect on thiokols.
• Thiokol has low permeability to gases.
• It cannot be vulcanized and hence does not form hard rubber.
THIOKOL RUBBER
• Applications:
• Fabrics coated with Thiokol are used for balloons, light parts and jackets.
• These are used as solid propellant fuel for rocket motors.
• Thiokols are used for seals, gaskets, cable covering, oil tank linings etc.
Polymers in Medicine and Surgery
(Biomaterials) :
• The surgical and orthopedic operations need biodegradable
polymers. These polymers are not purely synthetic and they are
manufactured with the help of specific bacteria varieties. These
bacteria convert the naturally available material into monomers by
fermentation process. The monomer is then separated the
fermented mass and polymerized. Such polymer obtained contains
electronegative atoms like O, N in the chain and the polymer is
hydrophilic in nature. The polymer when used in the surgery, gets
depolymerised or dissolved by the action of body fluid in the time
of 7 to 10 days by the time the organ heals up naturally.
Polymers in Medicine and Surgery
(Biomaterials) :
• Some biologically important polymers in medical field
are given below;
• Silicon polymer :

This elastic polymer is extremely chemical resistant

Polymers in Medicine and Surgery
(Biomaterials) :
• Biodegradable Polymers
• Biopol :

PHBV(biopol) is useful for moulding, films, medical and veternary applications

Polymers in Medicine and Surgery
(Biomaterials) :
• Biodegradable Polymers
• Polycaprolactone :
• It is a synthetic biodegradable organic polymer and can be blended with
PVC, nylon-6, etc to make moulded articles.
Polymers in Medicine and Surgery
(Biomaterials) :
• Biodegradable Polymers
• Poly-lactic acids :
• It is useful for organs regeneration, plastic surgery poly l-
lactic acid, poly dl-lactic acid can be prepared by fermentation of
glucose or fructose and polymerization
Polymers in Medicine and Surgery
(Biomaterials) :
• Biodegradable Polymers
• Polyglycolic acid :
• Glycolic acid is one of the products of fermentation
of sugars. If forms the polymer for organs regeneration,
plastic surgery, orthopedic treatments, surgical sutures.
Polymers in Medicine and Surgery
(Biomaterials) :
• Biodegradable Polymers
• Polydioxane : It is useful in medical and agriculture
Polymers in Medicine and Surgery
(Biomaterials) :
• Applications of biodegradable polymers :
• 1. Packaging : Polymer like biopol, starch filled polyethylene, etc. can be
converted to films, which are useful for packaging, lamination, carrybags,
disposable bottles, etc. These polymers do not pose pollution problem.
• 2. Moulded articles : Biodegradable polymers like biopol, poly
caprolactone, etc. can be used for injection moulding, blow moulding, extrusion
moulding articles of common consumer applications.
• 3. Medical field : Polymers like polylactic acid, polyglycolic acid, polydioxane
are important in biomedical applications like surgical sutures, organs
regeneration, orthopedic treatment, slow release drugs.
• 4. Agriculture : Biodegradable polymers are preferred in agriculture for
netting, twine, etc. for crop yields and reducing crop management cost. They
are also required for controlled release fertilizer, pesticide.
Lubricants
• All material surfaces even though they are smooth, shows many
irregularities in the form of peaks called asperities and valleys.

• When two metal surfaces slide (pressed) over each other, the real
contact takes place between a limited numbers of asperities only. So,
during this contact the deformation of more ductile metal takes place.
Frictional Heat Effects:

Roughness on metal surface Two metallic surfaces in real contact on asperites only
(magnified)
Lubricants
• Lubricants can be defined as, “A Substance introduced between two
surfaces so as to reduce frictional resistance and energy wastage in
all sort of machines”.
• “The process of reduction of frictional resistance between moving or
sliding surfaces by introduction of lubricants between them is called
as lubrication”.
Functions of Lubricants-

• It reduces surface deformation, wear and tear by avoiding direct contact

between rubbing surfaces.
• It reduces loss of energy in the form of heat i.e. acts as coolant.
• As it reduces loss of energy, it increases efficiency of machine.
• It reduces expansion of metal by avoiding local frictional heat.
• It avoids seizures of moving surfaces since it minimizes liberation of heat.
• It reduces maintenance cost of machine.
• Sometimes it acts as seal e.g. lubricants used between piston and
cylinder walls of internal combustion engine prevents leakage of gases.
CLASSIFICATION OF LUBRICANTS-

• Lubricants can be broadly classified on the basis of their physical state

as
• liquid lubricants,
• semi-solid lubricants and
• solid lubricants.
Liquid Lubricants / Lubricating Oils:

• These are mainly lubricating oils and works by mainly a fluid film in
between metal surfaces and by this reduce wear and tear. Further
they act as coolant, sealing agent and corrosion preventer.
• Properties of Good Liquid Lubricant-
• High boiling point.
• Requisite viscosity for working condition.
• High resistance to oxidation.
• Low freezing point.
• Heat stability.
• Non corrosiveness
Liquid Lubricants / Lubricating Oils:

• Animal and Vegetable Oils: Before invention of petroleum

oils, these were common lubricants. They possess ‘good
oiliness’ but they are costly and undergo oxidation easily.
They get hydrolyze when come in contact with moisture or
aqueous medium. So at present they are used as blending
agents with other lubricating oils such as mineral oils for
obtaining good result.
• e.g. Olive, palm, caster oils (Vegetable oils). Whale, Lord oil
(Animal Oils).
Liquid Lubricants / Lubricating Oils:

• Mineral or Petroleum Oils: These are obtained by the

fractional distillation of petroleum at 400OC. These are
widely used lubricants, because they are cheap,
available in abundance and stable under service
condition. But their oiliness is very poor as compared
to animal or vegetable oils. This oiliness can be
increased by adding high molecular weight
compounds such as oleic acid, stearic acid, etc.
 Liquid Lubricants / Lubricating Oils:
• Blended Oils: “When mineral oils are mixed with special additives to behave as good lubricant, the
mixture is called Blended oil”.
• No single oil serve as good lubricant, hence it is blended with another suitable oil i.e. specific
additives are added for achieving desired property.
• Following are some additives for specific property.
• Oiliness Carriers- Oiliness of lubricant can be increased by addition of oiliness carriers like vegetable
oils such as coconut oil, castor oil and some fatty acids such as oleic acid, stearic acid, etc.
• Extreme pressure additives- Organic compounds of sulphur, chlorine, phosphorus and lead.
• Pour point depressor- Phenol and some condensation products of chlorinated wax with
naphthalene. These prevent the separation of wax from lubricating oil.
• Viscosity index improver- Compounds of high molecular weight like hexanol.
• Antioxidants- Retards oxidation of oil by getting themselves preferentially oxidized. Particularly
lubricants used in internal combustion engines, turbines tec.
• Corrosion Preventer- Organic compounds of phosphorus and Antimony.
• Antifoaming Agents- Substances such as glycerol are used for decreasing tendency of lubricating oil
to foam.
• Detergents- Some salts of phenols, carboxylic acids prevent formation of scum deposits.
Liquid Lubricants / Lubricating Oils:

• Synthetic Oils: To meet severe operation conditions such as

operations in air craft, atomic energy plants, rockets synthetic oils are
used.
• Polymerized hydrocarbons, polyglycols and related compounds,
organic amines, imines and amides, silicones, fluorocarbons are some
synthetic oils.
• These synthetic oils possess non inflammability, High flash point, high
viscosity index and good thermal stability even at high temperature.
 Liquid Lubricants / Lubricating Oils:

• Emulsions: An emulsion is two phase system consisting of a fairly coarse

dispersion of two immiscible liquids, where one being dispersed as fine
droplets in other.
• The dispersed or internal phase is the liquid which forms droplets while the
dispersing phase is the surrounding or continuous or external liquid.
• Two immiscible liquids essentially form an unstable emulsion. To increase
stability of dispersion system a third agent emulsifier is added.
• e.g. i) Oil in water: Used for cutting tools, serve as coolant, used for piston in
marine engine.
• ii)Water In Oil: To lubricate compressor and pneumatic tools, serve as
coolant due to evaporation of water.
 Semisolid Lubricants or Greases:
• Lubricating greases consists of soap dispersed in liquid lubricating oil may be
petroleum oil or synthetic oil.
• These are prepared by saponification of fat with alkali followed by adding hot
lubricating oil under agitation.
• The lubricating property of greases depends up on characteristics of oil used for
its preparation. Highly viscous oil makes greases more suitable for high pressure
low speed lubrication.
• Nature of soap formed during saponification decides properties like sticking
capacity, heat resistance, resistance to water and resistance to oxidation of final
greases formed.
• Greases are classified on the basis of soap used in their manufacture.
 Semisolid Lubricants or Greases:
• Calcium based greases: These are emulsions of petroleum oils with calcium
soaps. These are prepared by adding requisite amount of Ca(OH)2 in hot oil.
These are cheapest and most widely used. They are to be used below 80OC
because above 80OC soap begins to separate out.
• Soda based greases: These are prepared by adding sodium soaps in petroleum
oil. These can be used up to 175OC. These are not water resistant.
• Lithium based greases: These are prepared by mixing lithium soaps with
petroleum oil. These are water resistant but can be used up to 15OC only.
• Axles greases: These are prepared by adding lime or any heavy metal
hydroxide to resin and fatty oils. These are water resistant and suitable for less
delicate equipments working at low speed and high load.
 Semisolid Lubricants or Greases:
• Greases are useful under following working conditions:
• When oil cannot remain in place because of high load, low
speed, sudden jerks etc. Such situations are mostly met with
railway axle boxes.
• In bearings and gears operating at high temperature.
• Greases are used in machines preparing paper, textile,
edible article where dripping and spurting of lubricating oil
is unwanted.
 Solid Lubricants
• Solid lubricants are used where-
• Operating conditions are such that a lubricating film cannot
be secured by using lubricating oil or greases.
• When contamination by oil or greases is unacceptable.
• When operating temperature or load is too high.
• When combustible lubricants are prohibited.
 Solid Lubricants
• Graphite: In this ‘C’ atoms are arranged in number of hexagons in parallel
layer. These parallel layers are bonded with each other by weak force of
attraction. So, they can slide over each other and act as lubricant.
Graphite is used either in powdered form or as
suspension. Suspension of graphite in oil is called
‘Oildag’ and suspension of graphite in water is called
‘Aquadag’. Oildag is useful particularly in internal
combustion engines. On the other hand Aquadag is
useful where lubricant free from oil is needed e.g.
Food industry.
Uses: It is used as lubricant in air compressor, lathes,
railway track joints, open gears, chains, and internal
combustion engines.
Solid Lubricants

Molybdenum Disulphide: It has sandwich like

structure in which layer of ‘Mo’ Atoms lies
between two layers of ‘S’ atom. It forms film
between sliding surfaces. It is stable in air up to
400OC and possesses very low coefficient of
friction. For high speed machines a fine powder
of molybdenum disulphide is sprinkled on the
sliding surfaces, it fills the valleys or low spots in
the metal surface and forms a film.
 PROPERTIES OF LUBRICATING OIL:
• Viscosity: “Viscosity is a property of liquid or fluid by virtue of which
it offers resistance to its own flow”.
• A liquid in a state of steady flow on a surface can be consisting of a
series of parallel layers moving one over another. Any two layers
moves with different velocities, top layer moves faster than next
lower layer due to viscous drag i.e. internal friction.
• Lubricant should possess optimum viscosity under working
condition.
 PROPERTIES OF LUBRICATING OIL:
• Viscosity Index: Viscosity is inversely proportional to the
temperature. As temperature increases viscosity decreases.
• “Rate of change of viscosity of lubricating oil with
temperature is expressed in terms of viscosity index”.
• If viscosity of oil is slightly affected on increasing
temperature, then its viscosity index is high.
• If viscosity of oil falls rapidly with increase in temperature,
then it has low viscosity index.
• Lubricant should have high viscosity index.
PROPERTIES OF LUBRICATING OIL:
• Flash Point: “Flash point is the lowest temperature
at which the lubricating oil gives enough vapors
which when ignited by bringing a small flame near
it gives a momentary flash of light”.
• Lubricant should have high flash point.
PROPERTIES OF LUBRICATING OIL:

• Fire Point: “Fire point is the lowest

temperature at which the lubricating oil
gives enough vapors which catch fire and
burn continuously for at least 5 seconds,
when a small flame is brought near it”.
• Lubricant should have high fire point.
PROPERTIES OF LUBRICATING OIL:

•Cloud Point: “When lubricating oil is slowly

cooled, cloud, fog or haze is formed and
the temperature at which it becomes
cloudy in appearance is known as cloud
point of oil”.
•Lubricant should have low cloud point.
PROPERTIES OF LUBRICATING OIL:

•Pour Point: “Pour point of lubricating oil

is that temperature at which it ceases
(stops) to flow or pour when cooled”.
•Lubricant should have low pour point.
 PROPERTIES OF LUBRICATING OIL:
• Aniline Point: Aniline point of oil gives an indication of the possible
tendency of oil when it comes into contact with packing, rubber sealing
etc. Generally, aromatic hydrocarbons have a tendency to dissolve natural
and certain types of synthetic rubbers. Hence, the aromatic hydrocarbon
content of the oil has much significance from this point of view. This is
usually determined on the basis of aniline point.
• Aniline point is defined as, ‘The minimum equilibrium solution
temperature for equal volumes of aniline and oil sample’.
• A higher aniline point means lower percentage of aromatic hydrocarbon.
• Lubricant should have higher aniline point.
 PROPERTIES OF LUBRICATING OIL:
• Oiliness: Oiliness is a property of lubricant by virtue of which
lubricating oil can stick on the surface of the machine parts
operating under high pressure. Oiliness of oil is most
important property of a lubricant under boundary or thin
film lubrication.
• Mineral oils have very poor oiliness whereas animal oil and
vegetable oil have good oiliness. Hence oiliness of mineral oil
is generally improved by adding small quantities of high
molecular weight fatty acids.
PROPERTIES OF LUBRICATING OIL:
• Acid value: The acid value of lubricating oil is defined as the number
of milligrams of potassium hydroxide required to neutralize the free
acid present in 1 gram of the oil.
Volume of KOH required in ml Normality 56
Acid value = Weight of oil taken in gram

Volume of N/10 KOH required in ml 5.6

Acid value =
Weight of oil taken in gram

1000 ml 1N KOH ≡ 56 gm of KOH

1ml of 1N KOH ≡ 56 mg of KOH
1 ml of N/10 KOH ≡ 5.6 mg of KOH
 PROPERTIES OF LUBRICATING OIL:
• Saponification value: The saponification value of oil is the number of
milligrams of potassium hydroxide required to saponify one gram of
the oil. Volume of KOH consumed in ml Normality 56
Saponification value = Weight of oil taken in gram

Volume of N/2 KOH consumed in ml 28

Saponification value = Weight of oil taken in gram

Saponification value =
(Volume of N/2 HCl required in Blank experiment - Volume of N/2 HCl required in Test experiment) 28
Weight of oil taken in gram

1000 ml 1N KOH ≡ 56 gm of KOH

1ml of 1N KOH ≡ 56 mg of KOH
1 ml of N/2 KOH ≡ 28 mg of KOH
MECHANISM OF LUBRICATION: Fluid film/
Thick Film/ Hydrodynamic / Complete fluid
Lubrication
• In this type, moving or sliding surfaces are separated from each other
by a thick film of fluid (at least 1000 AO thick), so that direct surface to
surface contact will avoid.

Fluid film or hydrodynamic lubrication

Journal bearing
 MECHANISM OF LUBRICATION: Fluid film/
Thick Film/ Hydrodynamic / Complete fluid
Lubrication
Lubricant film covers, fills irregularities of sliding or moving surfaces and forms a thick layer in between them, so that
there being no direct contact between metal surfaces as shown in fig.
The resistance to sliding or moving parts is only due to internal resistance between particles of lubricant moving over
each other but not due to frictional resistance of surfaces.
Nature of Lubricant: Lubricant should have following properties.
i) Lubricant should have minimum viscosity under working conditions.
ii) It should remain in place and separate surfaces.
In such a system, friction depends on viscosity, thickness of lubricant, relative viscosity and area of moving
surfaces. Coefficient of friction in this case is low as 0.01 to 0.03.
In bearing, film of lubricating oil covers irregularities of shaft as well as bearing surface and thus metal surfaces do
not come into contact to each other.
Lubricant Type: Hydrocarbon oils are considered to be satisfactory lubricants for fluid film lubrication. In order to maintain
viscosity of oil in all seasons of year, ordinary hydrocarbon lubricants are blended with selected long chain polymers. Some
antioxidants such as aminophenol are also added to avoid oxidation of hydrocarbon.
Applications: This type of lubrication is provided in delicate instruments such as watches, clocks, light machines e.g.
sewing machine, scientific instruments etc.
 MECHANISM OF LUBRICATION: Thin
film / Boundary Lubrication-
This type of lubrication may takes place under any of following conditions.
i) If relative speed of lubricated surface is not sufficient to produce adequate pressure in lubricant film to carry
the load. This happens when a heavily loaded bearing is stopped or started i.e. at the starting and stopping of
the machine.
ii) If the load on bearing is more than the pressure developed in the lubricant film i.e. very heavy load.
iii) If the load is carried on high spot because of imperfection of the surface.
iv) If the viscosity of the oil is very low.
Thus when a continuous film of lubricant cannot persist and direct metal to metal contact is possible
because of above said reasons, boundary lubrication comes into play.
Under such conditions, clearance space between moving surface is lubricated with oil. A thin layer of
this oil lubricant is adsorbed on both metallic surfaces by physical or chemical process. These
adsorbed lubricant on both metal surfaces. Coefficient of friction in such cases is 0.05 to 0.15.
MECHANISM OF LUBRICATION: Thin
film / Boundary Lubrication-
 MECHANISM OF LUBRICATION: Thin
film / Boundary Lubrication-
Nature of Lubricant: Effectiveness is mainly depends on structure and chemical properties of lubricant oil
used.
i) Lubricant should have long hydrocarbon chains with polar groups. Lateral attraction between chains and
polar groups capable of forming linkage with metallic surfaces.
ii) Lubricant should have good oiliness, low pour point, high resistance to oxidation and heat with high
viscosity index.
In order to increase oiliness of mineral oils, some amount of fatty acid is added. The polar carboxylic
group form layer by strongly adhering to metallic surface and forcing long hydrocarbon chain to orient
outward approximately perpendicular to metallic surface. The first adsorbed layer attracts other molecules
to form next layer.
Graphite and molybdenum disulphide are also used as stable suspension in oil for boundary lubrication.
 MECHANISM OF LUBRICATION:
Extreme Pressure Lubrication
When moving or sliding surfaces are under very high pressure and speed,
then these generates a large frictional heat. This high local temperature
developed at surfaces even causes melting or breaking of film, if ordinary
lubricants are used. In such conditions, ordinary lubricants fail to stick and
may decompose or even vaporize.
To meet these extreme pressure conditions special additives are added to
mineral oils. These are called ‘Extreme Pressure Additives’. These additives
form a film on metal surface which is more durable and capable of
withstanding higher temperatures and having sufficient shear strength to
withstand the mechanical action of heavy loads
 MECHANISM OF LUBRICATION:
Extreme Pressure Lubrication
The important special additives used in extreme pressure lubrication are organic
compounds having active radicals or groups such as
i) Organic compounds containing chlorine e.g. chlorinated ester, chlorinated wax etc.
ii) Compounds containing sulphur e.g. sulphurised oil.
iii)Organic compounds containing Phosphorus e.g. Tricresyl phosphate.
iv)Organic compounds containing lead.

These compounds react with metallic surfaces at high temperature forming metallic
sulphides, chlorides, phosphides. These metallic compounds have high melting points
and serves as good lubricant under extreme pressure and extreme temperature conditions.
If, by chance low shear strength films are broken by rubbing of moving parts, they are
immediately replenished.
 SELECTION OF LUBRICANTS
• In selecting for a particular machine, we should know service
condition requirements i.e. we should know different working
conditions, whether heavy load, high load, extreme pressure, high
temperature, etc. and accordingly we have to select lubricant of
required property.
• Properties of selected lubricant should not change under service
conditions. If properties of chosen lubricant alter during service, it
may cause certain problems. Hence a careful study of various
properties of lubricant and their correct interpretation is necessary
while selecting a proper lubricant.
 SELECTION OF LUBRICANTS
• Lubricants for cutting tools: During the operation such as grinding, turning and
cutting of metals continuously a metal from the surface is removed and a fresh
metal is exposed to cutting tools. This consequently causes high friction and
large amount of heat. Hence cutting tools get damaged. So in these operations
cutting fluids are used as lubricants.
• These cutting fluids are mineral oils of low viscosity to which additives such as
fatty acids are added. Polar groups of additives get attached to surface of
continuously exposed metal.
• So these fluids acts as lubricant by following functions
• Cooling the tools so as to reduce wears.
• Preventing metals from losing its hardness.
• Reducing power consumption.
 SELECTION OF LUBRICANTS
• Lubricants for Internal Combustion Engine: In I.C.
engines lubricants are exposed to high temperature so
lubricant should possess high viscosity index and high
thermal stability. Petroleum oils containing additives
are used as lubricants
SELECTION OF LUBRICANTS
• Lubricants for Gears: Working conditions at gears have
extreme pressure so lubricants using for gear should
possess
• Good oiliness i.e. sticks well to teeth of gear.
• Should not be removed by centrifugal force from place of
application.
• Should possess high resistance to oxidation.
• These lubricants containing extreme pressure additives
such as metallic soaps or chlorine or sulphur or phosphorus
compounds are added.
SELECTION OF LUBRICANTS
• Lubricants for Delicate Instruments: Watches,
Clocks, Scientific instruments, Sewing machines
etc. are not exposed to high temperature or
water or extreme load etc. Therefore for such
purpose, animal or vegetable oils are suitable.
SELECTION OF LUBRICANTS
• Lubrications for Steam Turbines: In steam turbines,
lubricant is exposed to high temperature and
oxidizing conditions. In this, leakage of steam and
cooling water may possible. So by considering these
conditions we have to choose lubricants.
• Blended oils having high oxidation stability, corrosion
resistance, and antifoaming agents etc. are suitable.
SELECTION OF LUBRICANTS

• Lubricants for Refrigeration System:

Lubricating oil for refrigeration should have
low cloud and low pour points. Naphthalene
based oils are used.
 CORROSION

Corrosion is unintentional destruction of solid

body due to direct chemical or electrochemical
reaction starting from the surface.
 Cause of corrosion-
• Most of the metals exist in nature in the form of
their oxides, carbonates, sulphates, chlorides,
silicates etc. i.e. they are available in the form of
ores and not in free state. The metals are
extracted from these ores. The extraction process
requires considerable amount of energy.
Therefore, extracted pure metals can be treated
as in excited state than their corresponding ores.
Hence they have natural tendency to return back
to their lower energy state i.e. in the ore form.
• “Thus corrosion is a process of reverse of
extraction of metal”.

The most familiar example of corrosion is rusting of iron.

Corrosion
• Corrosion: “Slow or rapid but continuous destruction of solid metallic
material by an unintentional action of surrounding medium is called
corrosion”.
• CLASSIFICATION OF CORROSION-
• Corrosion is mainly due to the reaction between metal surface and
the medium or environment.
• This can be broadly classified as
• Dry corrosion
• Wet corrosion
 DRY CORROSION-
• This type of corrosion occurs mainly through the direct
chemical action of environment or atmospheric gases such
as O2, halogens, H2S, SO2, N2 or anhydrous inorganic liquids,
with metal surface in immediate proximity.
• Dry corrosion can be sub classified as
• Oxidation corrosion
• Corrosion by other gases
• Liquid metal corrosion.
 Oxidation Corrosion- 2M
nO2 + 2ne-
2Mn+
2nO--
• “The corrosion taking place by 2M + nO2 2Mn+ +
the action of oxygen is called 2nO--
Metal Oxide
oxidation corrosion”. ion ion
• It generally takes place in the Metal oxide
absence of moisture. At low
temperature alkali and alkaline
earth metals are oxidized and at
high temperature except Ag, Au
and Pt, all other metals get
oxidized.
 Oxidation Corrosion-
• i) Stable- If the film is stable, it can get adherent tightly to the parent metal
surface. Hence such a layer can behave as protective layer.
• e.g. Oxide film of Al, Sn, Pb, Cu, etc. are stable and hence they prevent the
further oxidation corrosion.
• ii) Unstable- If the film is unstable i.e. if oxide film formed decomposes back to
the metal and oxygen, then rate of oxidation corrosion decreases.
• e.g. Ag, Au and Pt do not corrode due to formation of unstable oxide layer.
• iii) Volatile- The formation of volatile layer volatilizes as soon as it is formed,
thereby leaving the underlying metal surface exposed for further attack. This
causes rapid and continuous corrosion.
• e.g. Mo.
• iv) Porous- This type of layer has cracks or pores; atmospheric oxygen can enter
through these pores. So, corrosion takes place continuously.
• e.g. Fe.
 Dry Corrosion
• b) Corrosion by other gases-
• Corrosion is also takes place by attack of gases like SO2, CO2, Cl2, H2S, F2 etc., by the
attack of these gases respective compounds like carbonates, sulphates chlorides etc. forms
as a layer.
• The degree of attack depends on the formation of protective or nonprotective films on the
metal surface. Also it depends upon the chemical affinity between the metal and the gas
involved.
•
• c) Liquid metal corrosion-
• This corrosion occurs when liquid metal is allowed to flow over solid metal at high
temperature. It tends to weakening of the solid metal due to its dissolution in liquid metal.
• “The scaling of metals, which takes place at elevated temperature under dry condition, is
known as high temperature corrosion”.
 WET / IMMERSED /
ELECTROCHEMICAL CORROSION-
• This type of corrosion occurs where,
• A conducting liquid is in contact with metal or
• When two dissimilar metals or alloys are either immersed or dipped partially in a
solution.
• This corrosion occurs due to existence of separate ‘anodic’ and ‘cathodic’ areas
between which current flows. At anodic areas oxidation takes place and thy get
corrode, while at cathode either H2 gas is evolved or oxygen is absorbed, depending
on nature of corrosive environment.
• Depending on the nature of metallic component and the surrounding medium
there are two mechanisms of wet corrosion.
• Hydrogen Evolution Mechanism
• Oxygen Absorption Mechanism
 Hydrogen Evolution Mechanism-
• This type of corrosion usually occurs in acidic environment such as acid industrial
water and acidic solutions where concentration of dissolved oxygen is low.

Fe Fe++ + 2e- --Oxidation)

2H+ + 2e- H2 -------(Reduction)

Oxygen Absorption Mechanism-

Fe Fe++ + 2e- -------(Oxidation)

1 O + H O + 2e-
2 2 2 2OH- -------(Reduction)

4Fe(OH)2 + O2 + 2H2O 4 Fe(OH)3

FACTORS INFLUENCING CORROSION-
• The rate and extent of corrosion is depends on following two factors.
• Nature of Metal
• Nature of Corroding Environment
FACTORS INFLUENCING CORROSION- Nature
of Metal-
a) Position in galvanic series- The metals securing higher positions in the galvanic series corrode more.
•
More is the difference between the metals in the series more is the corrosion.
• b) Relative areas of the anode and cathode- Corrosion is more rapid if the anodic area is small [e.g.
Small steel pipe fitted in a large copper tank]. Because the current density at a smaller anodic area is
much greater and demand for electrons by large cathodic area can be satisfied by smaller anodic area
only by undergoing corrosion.
• c) Purity of Metal- Impurity in a metal cause formation of tiny electrochemical cells between the
impurity and metal. The anodic part gets corroded. If the metal is more pure then rate of corrosion is
very less.
• d) Physical state of the metal- The physical state such as grain size, orientation of crystals, stress etc.
also influences the corrosion. Smaller the grain size, greater the corrosion.
• e) Nature of the surface film- The film may be stable, unstable, volatile or porous.
• The stable film (e.g. Al, Cu) acts as a protective layer and corrosion rate is less.
• The unstable film (e.g. Ag, Au, Pt.) corrosion is negligible.
• In Mo the film is volatile. Hence every time fresh metal exposed to atmosphere and corrodes more.
• In Fe, base is porous nature of film, the attack of atmosphere is more. Therefore corrosion is more.
• f) Solubility of Corrosion Product- If the corrosion product is soluble in the corroding medium then
corrosion proceeds at faster rate.
FACTORS INFLUENCING CORROSION- Nature
of corroding medium-
• a) Temperature- With increase in temperature of environment, the reaction as
well as diffusion rate increases, hence corrosion rate increases.
• b) Humidity of Air- It is deciding factor in atmospheric corrosion. Corrosion
increases in more humid atmosphere.
• c)Presence of impurity in atmosphere- Atmosphere in the vicinity of industrial
areas contains corrosive gases like CO2, H2S, SO2, fumes of HCL H2SO4 etc. In the
presence of these gases the acidity of liquids adjacent to the metal surface and
its electrical conductivity increases. This consequently increases the corrosion.
• d) Presence of suspended particles in atmosphere- These particles absorbs
moisture from atmosphere and increases corrosion. eg. NaCl, Charcoal, Dust etc.
• e) pH of the solution- The rate of the corrosion is more in acidic solution than
alkaline or neutral solution.
TESTING AND MEASUREMENT OF
CORROSION-
Weight loss method-
• This is a laboratory method in which loss in weight after corrosion is
measured. In this method a clean metallic standard piece is taken. Its
surface area is calculated and weight is measured. Then it is exposed
to corroding media for known time. Afterwords it is cleaned to
remove corrosion products and reweighed.
The rate of corrosion is calculated by the formula

Rate of corrosion = Loss in Weight

2 x surface area x time

The results are generally expressed in mg/cm2.sec or mg/cm2.min or mg/cm2.day.

TESTING AND MEASUREMENT OF
CORROSION-
Electrical resistance method-
• This method is employed for materials to be used in the form of thin
wire or strip.
• In this method, the electrical resistance of material is
measured time to time. The property used is the corrosion of the
metallic material decreases the cross sectional area decreases
electrical resistance increases.
• Hence periodic measurement of electrical resistance can
help to measure the corrosion rate.
PREVENTION OF CORROSION

• CATHODIC PROTECTION-
• The principle involved in this method is to force the metal to be
prevented to behave like a anode. After that as there is no anodic
area the corrosion of metal does not takes place. There are two ways
of Cathodic protection.
• Sacrificial anode method.
• Impressed current method.
Sacrificial anode method-
Impressed (external ) Current
method-
ANODIC PROTECTION-

• In this method, a metal to be protected is passivated by applying

current in a direction that will make it more anodic. Anodic protection
is based on the formation of a protective film on metal by externally
applied anodic current.
• For metals which exhibits active-passive behavior e.g.
Iron, Nickel, Chromium, Titanium and their alloys, If a carefully
controlled anodic currents are applied, they are passivated and rate of
metal dissolution is decreased. To protect the metal anodically a
device called potentiostat is used.
 Potentiostat-
(i) Metals like iron, steel, stainless steel,
aluminium, chromium etc. show passivity
when a certain range of potential is applied
on them.
(ii) Every metal and alloy has a
characteristic passivity range voltage but all
the metals do not have the range wide
enough to employ the anodic protection
technique for corrosion control.
(iii) The metal becomes nonreactive or
corrosion resistant when the voltage applied
on metal corresponds to passivity range.
Potentiostat-
• Principle :
• A metal or alloy having wider range of passivity voltage, is made
anodic and the voltage in the passivity range, is applied over it to
control its corrosion even by strongly corroding media.
Potentiostat-
 Potentiostat-
(i)The metallic installation (reactor, water tank, industrial water coolers,
industrial condensers, etc.) is made anodic by use of reference electrode and
auxiliary electrode, as shown in Fig
(ii) There is a potentiostat for applying the desired voltage and an ammeter to
watch the corrosion current. The minimum or negligible current indicates that
anodic protection is taking place successfully.
(iii) If system goes out of the corrosion control any time, the corrosion current
will be high and the electrical connections are removed immediately.
The cathodic metal taken for protection of steel is generally a high
corrosion resistant metal or platinum and the auxiliary electrode as graphite.
 Potentiostat-
Applications :
(i) Chemical reactors.
(ii) Complex metallic installations on ground or under sea water.
(iii) Industrial water coolers.
(iv) Industrial metal condensers.
(v) Pipe lines for carrying corrosive liquids or solutions. etc.

Advantages :
(i) Low operating cost.
(ii) Applicable to highly corrosive media.
(iii) Reliable to protect complex structures.
(iv) Feasibility of method can be predicted in laboratory.
(v) Protection current gives idea about corrosion rate.

Limitations :
(i) Applicable to only those metals which show active - passive behaviour.
(ii) High installation cost.
(iii) In case system goes out of control, then very high corrosion rate occurs.
 Anodizing

• Metals like Al, Ti etc. and their alloys form a thin oxide film on their surface when
exposed to air. The oxide film for above metals is non porous, adhesive and uniform in
nature thereby providing better corrosion resistance to the base metal. However, the
thickness of naturally formed oxide layer is very small and it does not protect the base
metal properly. To overcome this problem the thickness of oxide film is increased by
electrolysis. This process is known as Anodizing.
• In anodizing, the component to be protected is connected as anode in
electrolytic cell, containing strong electrolyte such as conc. H 2SO4. A small amount of
current is passed which oxidizes the anode. The thickness of the oxide film is
controlled by time and current density. The oxide layer formed is porous in nature
which is then scaled by immersing the article in bath of oil, Wax, Dye or boiling water
etc. Then anodized article can be put to actual use.
•
 Protective Coatings Methods :
• METALLIC COATINGS-The very important technique of protection from corrosion of
metals is coating. This coating may be metallic, nonmetallic or organic.
• Metallic coating are mostly applied on iron and steel. Zn, Sn, Ni, Cr, Al, Pb etc. metals
can be used as coating metal. It can be divided in to two types-
• Anodic coating
• Cathodic coating
• Anodic coating- coating of metals which are anodic to the base metal i.e. the metal
having lower position in galvanic series than the base metal, are used for coating.
e.g. - coating of Sn metal on Fe.
• Cathodic coating- Coating of metals which are Cathodic to the base metal is called
Cathodic protection i.e. the metal having lower position in galvanic series than the
base meta, are used for coatings. e.g. coating of Sn metal on iron.
Methods of applications of metal
coatings-
• HOT DIPPING- Is used for producing a coating of low melting point
metal such Zn (MP = 4190C ), Sn (M.P.=2320C ) Pb, Al etc. on
iron, Steel and Copper which have relatively higher melting points.
• The process is generally consist of immersing the base metal in a bath
of molten coating metal, covered by the molten flux layer.
 GALVANIZING
This is the process of coating iron or steel sheets with a thin coat of zinc to prevent them
from rusting
 In this process tin (Sn) is coated over the iron or steel articles
TINNING
METAL CLADDING