• Origin and Theory-Types of Corrosion

• Mechanism of Dry (Chemical) and wet (electrochemical)

corrosion
• Types of electrochemical corrosion (Chemical and
electrochemical, pitting, Intergranular, waterline, stress,
galvanic, Galvanic series, differential aeration corrosion)
• Factors influencing corrosion
• Controlling methods: Protection from corrosion
(Sacrificial protection, Cathodic protection, Protective
coatings, Electroplating and Electroless plating-Chemical
conversion coatings-phosphate, chromate, anodized
• Organic coatings – paints and special paints
Corrosion has been defined as a destructive chemical and electrochemical
reaction of a metal with its environment (like O2, moisture, CO2 etc.) which
disfigures metallic products leading to reduction in their thickness and also
causes loss of useful properties such as malleability, ductility, electrical
conductivity and optical refractivity.

Except few metals such as gold, platinum (called noble metal) are prone to
corrosion.

Typical examples of corrosion

1. Rusting of iron due to formation of hydrated ferric oxide.
2. Tarnishing of silver wares in H2S laden air due to formation of silver
sulphide.
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 INTRODUCTION
 Destruction of a metal by chemical or electrochemical reaction
with its environment.
 Physical or mechanical wearing away of a metal is not called
corrosion but is called erosion
 If corrosion and erosion take place together the destruction of
the metal takes place very much faster
 Corrosion is an unintentional attack on a material by reaction
with a surrounding medium
 The corrosion problem should be very seriously considered in
all those cases where the structure or the equipment is meant to
last many years say 20 years…
Corrosion is enhanced by the presence of

• impurities
• air and moisture
• electrolytes
• strains in metals like dent, scratches etc.
Metals are electropositive in nature. Except few metals like gold, platinum (noble
metal) other metals are found in nature as their compounds (such as oxides,
hydroxides, carbonates, chlorides, sulphides, phosphates, silicates etc.) which are
called their ore.
Metals are thus obtained by extraction from their ores by reduction process.
In nature, when metals exists as their compounds (or ore) they are stable and they
are in the low energy states.
However, during extraction of metals from their ores, free metals are become less
stable and are in the higher energy state than in the ionic state.
So, metals have a tendency to back to the ionic state and hence metal atoms are
prone to get attacked by environment .
This is the main reason for corrosion of metals.

Mineral Extraction Corrosion Corrosion

or Metal
(M) product
ore (Mn+) by reduction -ne
(Mn+)
+ ne
 Types Of Corrosion
 Dry or Chemical Corrosion

 Wet or Electrochemical
Corrosion
• Chemical (or dry) corrosion: It involves direct chemical attack of
atmospheric gases like CO2, O2, H2S, SO2, halogen, moisture and inorganic
acid vapours on metal..
Example, turnishing of silver ware in H2S laden air.

• Electrochemical (or wet)corrosion: It occurs due to setting up of a large

number of tiny galvanic cells in metals in presence of an impurity as well as
in presence of moisture. Generally impurity (more active metal) act as anode
and original metal act as cathode so anode is the area where corrosion
occurs.
Example, rusting of iron in moist atmosphere.
 Dry or Chemical Corrosion
 Occurs
• Due to direct chemical reaction of atmospheric gases
• Due to molten metal in contact with metal surface

 Types
• Oxidation corrosion
• Corrosion by gases
• Liquid Metal Corrosion
 Types of dry corrosion
Oxidation corrosion
 Occurs
 due to direct chemical reaction of atm. O2 with metal surface forming metal oxide
 Absence of moisture
 Increases with increase in temp.
 Mechanism
 on exposure to atm., metal gets oxidized to form metal ions
(i) M (S) M+n + ne-
 Electrons lost by metal are taken up by oxygen to forms oxide ions
(ii) 1/2O (g) + 2e- O2
2

2M + n/2O2 2M+n + nO2- M2On

Metal Oxide
(1.) Oxidation Corrosion: This is carried out by the direct action
of oxygen low or high temperatures on metals in absence of
moisture.
Alkali metals and Alkaline earth metals are rapidly oxidized at low
temperatures. At high temperature all metals are oxidized (except
Ag, Au, Pt).

M  M2+ + 2e- (Oxidation)

O2 + 2e -  2O2- (Reduction)
M + O2  M2+ + 2O2- (Metal oxide)
Mechanism:-
1. When temp increases the metal undergoes oxidation and losses e-

2M → 2M+n + 2ne-
Metal Ion
2. Electron are gained by the oxygen molecules forms oxide ions

nO2 + 4ne- → 2n O2-

Oxide Ion
3. Scale of metal oxide formed 2M + nO2 → 2M + 2n O2
Metal Oxide
 Nature of metal oxide layer
 Types of Layers
 Stable Layer: - Al, Pb, Cu, Sn
 Unstable Layer :- Ag, Au, Pt
 Volatile Layer:- Mo
 Porous Layer:- Alkali metals &alkaline earth metals

 Pilling – Bed Worth Rule

A protective and Non-Porous metal oxide layer has volume equal to or
greater than the volume of metal from which it is formed.
A Non-Protective and Porous metal oxide layer has volume lesser than
the volume of metal from which it is formed.
Specific Volume Ratio = Volume of oxide formed
Volume of metal
 Types of dry corrosion
Corrosion by Other Gases
• 2AgCl + Cl2 AgCl
• (Non-Porous layer)

• Fe + H2S FeS + H2
• (Porous Layer)

• Sn + 2Cl2 SnCl4
• (Volatile Layer)

Liquid Metal Corrosion

• Occurs due to the action of flowing liquid metal at high temp on solid metals or
alloys.
• Observed in nuclear reaction where Na metal used as a coolant leads to
corrosion of Cd.
 Wet or Electrochemical corrosion
 The electrochemical corrosion occurs when
i) a conducting liquid is in contact with a metal.
ii) when two dissimilar metals or alloys are immersed
partially in the solution.

 Corrosion occurs due to the presence of anodic and

cathodic areas.

 At anode oxidation reactions takes place

 At cathode reduction reactions takes place
 Occurs when aqueous solution or liquid electrolytes are
present

 Wet corrosion takes place in environments where the relative

humidity exceeds 60 %.

 Wet corrosion is most efficient in waters containing salts,

such as NaCl (e.g. marine conditions), due to the high
conductivity of the solution.
Wet corrosion takes by the following ways.

 Hydrogen gas evolution

 Oxygen gas absorption

 Wet or Electrochemical or Immersed
Corrosion
Occurs
• When a metal is contact with moist air or any liquid medium
• When two diff. metals are partially immersed in a soln.
• Chemically non- uniform surfaces of metals behave like electochemical cells in the presence of
water containing dissolved O2 & CO2
• Always occurs at anodic areas

Mechanism
• Involves oxidation- reduction process
• depending on the nature of corroding enviornment, electrons released at anode are consumed at
the cathodic area by two ways :
 Evolution of H2
 Absorbtion of O2
Mechanisms Of Corrosion by H2
Evolution

 Anode :
 M (S) M+n + ne- (Oxidation)
 Cathode :

 2H+ + 2e- H2 (g) (Reduction)

 Over All Rxn :

 2M (S) + 2n H+ 2M+n + nH2 (g)


Mechanisms Of Corrosion by O2 Absorbtion

 Anode :
 M (S) M+n + ne- (Oxidation)

 Cathode :
 O2 + 4e- + 2H2O 4OH- (Reduction)
 Over All Rxn :
 4M (S) + 2nH2O 4M+n + 4OH-
Rusting of Iron
Formation of rust
 Difference

Dry Corrosion Wet Corrosion

Occurs in the absence of moisture Occurs in the presence of conducting

medium

Involves direct attack of chemicals on the Involves formation of electrochemical

metal surface cells

Slow Process Rapid Preocess

Corrosion products are produced at the Corrosion occurs at anode but rust is
site of corrosion deposited at cathode

Process of corrosion is uniform Depends on the size of the anodic part of

the metal
 Types of electrochemical corrosion
 Galvanic Corrosion:- When two different metals are
present in contact with each other in conducting medium

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 Galvanic corrosion
 This corrosion occurs when different metals are in contact and
exposed to corrosive atmosphere.

 The metal which has higher electrode potential value will form
anode and undergoes corrosion. Example: Zn-Cu galvanic
cell zinc behaves as anode and corrosion occurs, copper
behaves as cathode and protected.
 Conc. Cell or Differential Aeriation
• Corrosion
Occurs due to diff. in potential between differently aerated areas
• Part of metal exposed to air is more oxygenated part & acts as
CATHODE
• Part of metal immersed in electrolyte is poorly oxygenated & acts as
ANODE
Water Line Corrosion
 Water line corrosion
 The concentration of oxygen dissolved in
water is greater at the surface than deeper
down. This causes formation of a
concentration cell.

 Anode is the lower portion .

 Cathode is at the water level.

 Due to the poor conductivity of water the

ions just below the water level are available
for reaction, the metal corrodes just below
the water level.
 Pitting Corrosion
• Formed as a result of pit and cavities
• Localized attack and formed by cracking protective coating
 Pitting corrosion
 When corrosion starts on a metal surface
at certain spots they become less open to
oxygen with the result that corrosion
proceeds faster at those very spots making
deeper and deeper cavities.
 Pitting corrosion can be explained on the
basis of differential aeration.
 The pit created becomes deeper, its
bottom becomes less open to oxygen
which makes it more anodic.
 The part which has higher oxygen
concentration is cathodic.
 Types of Corrosion
Crevice Corrosion
• Whenever there is a difference in ionic
concentration between any two local areas of
a metal, a localized form of corrosion know as
crevice corrosion can occur. In a simple
instance, this form of corrosion mostly occurs
in confined spaces (crevices).
• Examples of areas where crevice corrosion
can occur are gaskets, the undersurface of
washers, and bolt heads.
●●●
31
 Types of Corrosion
Stress Corrosion Cracking
• Stress Corrosion Cracking can be
abbreviated to ‘SCC’ and refers to
the cracking of the metal as a result
of the corrosive environment and the
tensile tress placed on the metal. It
often occurs at high temperatures.

●●●
32
 Types of Corrosion
Intergranular Corrosion
• Intergranular corrosion occurs due to
the presence of impurities in the grain
boundaries that separate the grain
formed during the solidification of the
metal alloy.
• It can also occur via the depletion or
enrichment of the alloy at these grain
boundaries.
●●●
33
 Types of Corrosion
Uniform Corrosion
• This is considered the most common
form of corrosion wherein an attack on
the surface of the metal is executed by
the atmosphere.
• The extent of the corrosion is easily
discernible. This type of corrosion has a
relatively low impact on the performance
of the material.
●●●
34
 Factors Affecting Corrosion
 Nature of Metal
• Purity of metal
• Physical state of metal
• Position of metal in Galvanic series
• Nature of oxide fim
• Nature of products of corrosion
• Relative areas of Cathode & anode
 Nature of Corroding Environment
• Temp. & Humidity
• Effect of pH
• Presence of impurities in atm.
• Nature of electrolyte
• Solubility of products of corrosion
 Nature of the metal
1. Purity of the metal: Heterogeneity of the metal is due to the
presence of impurities which form tiny electrochemical
cells at the exposed parts. The anodic parts get corroded.

2.Electrode potentials: metals with higher reduction potentials

do not corrode easily. They are noble metals like gold,
platinum and silver. Whereas the metals with lower
reduction potentials readily undergo corrosion (e.g.. Zn,
Mg, Al etc.).

3.Position of metal in Galvanic series: Metals which possess

low reduction potentials and occupy higher end of galvanic
series undergo corrosion easily.
 When two metals are in electrical contact in presence of an
electrolyte, then the metal which is more active undergoes
corrosion.
The rate of corrosion depends on the difference in their position
in Galvanic series. Greater the difference more will be the extent
of corrosion at anode.

4.Relative areas of anodic and cathodic cells: When two

dissimilar metals or alloys are in contact, the corrosion of the
anodic part is directly proportional to the areas of the cathodic
and anodic parts. i.e, the relative areas o of corrosion is
influenced by cathodic to anodic cells.

5. Physical state of metal: Metals with small grain size have more
tendencies to undergo corrosion. Metal with more stress/strain
also undergoes corrosion easily.
6. Hydrogen over voltage: when a cathode reaction is hydrogen
evolution type, the metal with lower hydrogen over voltage
on its surface is more susceptible for corrosion, since the
liberation
of hydrogen gas is easy at this condition. Hence the cathodic
reaction is very fast which in turn makes anodic reaction fast.
Hence the rate of corrosion increases. Higher the over
voltage, lesser is the corrosion.

7. Nature of surface film: If the corrosion product formed is

more stable, insoluble and non porous, it acts as protective
layer and prevents further corrosion (E.g.. Ti, Al and Cr). If
the corrosion product is porous, volatile and soluble, it further
enhances the corrosion (Fe, Zn and Mg).
 Nature of the environment
1. Temperature: the rate of corrosion reactions increases with
increase in temperature.

2. Humidity in air: the moisture or humidity present in atmosphere

furnishes water to the electrolyte which is essential for setting up
of an electrochemical cell. The oxide film formed has the tendency
to absorb moisture which creates another electrochemical cell.

3. Presence of impurities: Atmosphere is contaminated with gases

like CO2, SO2, H2S; fumes of H2SO4, HCl etc. and other
suspended particles in the vicinity of industrial areas. They are
responsible for electrical conductivity, thereby increasing
corrosion.
4. pH value: pH value of the medium has the greater effect on
corrosion. Acidic pH increases the rate of corrosion.

5. Amount of oxygen in atmosphere: As the percentage of oxygen

in atmosphere increases, the rate of corrosion also increases
due to the formation of oxygen concentration cell. The decay of
metal occurs at the anodic part and the cathodic part of the
metal is protected.

6.Velocity of ions which flow in the medium: As the velocity of

the diffusion of the ions in the medium increases, the rate of
corrosion increases .
Protective Measures Against Corrosion
 Protective Measures Against Corrosion
 By Modifying Environment :
• By lowering the temp.
• By reducing the moisture
• By reducing the acidic corrosion environment
 Proper Designing :
• Proper designing of the equipment
• Avoid the direct contact of dissimilar metals
• Electrochemical Protection or Cathodic Protection :
• Sacrifical Anodic Protection : Metal to be protected is connected to more anodic metal
to avoid corrosion
• Impressed Current Cathodic Protection : This process consists of connecting the
material to be protected to –ve terminal of DC source& +ve terminal of DC source is
connected toa insoluble anode . The current supplied is in opposite direction to the
corrosion current.Thus the metal to be protected act as cathode & get protected.
 There are many methods of protecting metals against corrosion
• Barrier protection
• Sacrificial protection
• Cathodic protection
• Alloy formation

Barrier protection :
Here, a thin barrier is developed between the surface of iron and
atmosphere by one of the following methods:
a) Painting of the surface
b) Coating the surface with a thin film of some non –corrosive metal like nickel,
chromium copper etc.
Coating the metal : In order to prevent corrosion, resistant coating is
made between metal and environment. Different types of metallic coatings
are
• Galvanizing (thin coating of Zn on iron)
• Electroplating (coating of Cu, Ni or Cr on iron with aid of direct current.
• Tin plating (coating of tin on iron )
• Sheradizing (it consists of dusting of Zn powder on iron surface followed by heating)
• Cladding
Alloying the metal
Metal has better resistance to corrosion after forming alloy with other metal
, e.g. stainless steel, in which ordinary steel is alloyed with chromium and nickel.
There are two kinds of alloys:
Homogeneous alloys are solid solutions in which the components are components
are completely soluble in one another, e.g stainless steel
Heterogeneous alloys are the mixtures of two or more separate phases. The
components of such alloy are not soluble and exist as separate phase.
Solid solution alloys are more corrosion resistant.
•Chromium is used as alloying metals for iron and steel.
• Silicon is also used in making Si-Fe alloy
Sacrificial protection:
In this case, the surface of iron is covered with a more electropositive metal like zinc or
aluminum. Since this metal loses electrons more readily than iron, rusting is prevented.
As long as metal is present, iron does not get rusted. This type of protection is called
‘sacrificial production’.
Cathodic protection (Electrical protection):
It is protection of the parent metal from corrosion by connecting with a more active
metal like Mg, Al, Zn etc. The more electropositive (active) metal acts like anode
(supplies electrons) and parent metal acts like cathode (receives electrons). Thus,
connected metal undergoes corrosion thereby protecting the parent metal from
corrosion by turning it as a cathode. Hence, the method is called ‘cathodic protection’.
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 CATHODIC PROTECTION
 The method of protecting metals and alloys from corrosion making
them completely cathode.

 To achieve this auxilary anode is provided in the corroding medium

which is connected to the structure.

 Cathodic protection is of two types

i) Sacrificial anodic method

ii) Impressed current cathodic method

 Cathodic Protection
Force the metal to be protected to behave like cathode.
(i) Sacrificial anodic protection:
 Metal to be protected from corrosion connected to more
anodic metal
 Commonly used metals Mg, Zn, Al and their alloys
Eg. A ship-hull which is made up of steel is connected to
sacrificial anode (Zn-blocks) which undergoes corrosion leaving
the base metal protected.

Eg. The underground water pipelines and water tanks are also
protected by sacrificial anode method. By referring to the
electrochemical series, the metal with low reduction potential is
connected to the base metal which acts as anode.
Impressed current method:
 Direct current is applied in opposite direction to nullify the
corrosion current
 Converts the corroding metal from anode to cathode.
 SURFACE COATINGS
 The metal surface is covered with a coating to protect it from
corrosion.

 These may be

i) Metallic

ii) Organic
 METALLIC COATINGS
The metals used for coatings may be placed under two categories

i) metals which are anodic to the metal i.e., metals above it in

galvanic series.

ii) metals which are cathodic to the metal i.e., metals below it in
galvanic series.
Methods of applications of metal coatings:
 Hot dipping: This technique is most widely used to control
corrosion. Hot dipping is used for producing a coating for
low melting metals such as Zn, Sn, pb ,Al.
 on Iron, steel, copper which are having high melting point
usually underwent into corrosion due to their oxidising
property.
 The process in general consisting of immersed a metal in a
bath of its molten coating& covered by a molten layer.
 The flux cleans the base metal& prevent oxidation of metal
coating with molten solution.
 Hot dipping is widely applied either by
(1)GALVANIZING (2)TINNING
 Galvanizing
 It is the process in which iron or steel is protected from
corrosion by coating with a thin layer of zinc.
 Iron or steel is first cleaned by pickling with dil. H2SO4
solution for 15-20 mts at 600-900c. The steel is then washed
well and dried .
 Then the metal sheet is dipped in bath of molten Zinc
maintained at 4250-4300c.
The surface of bath is kept covered with a Zncl2 flux to
prevent oxide formation.
 Then the metal sheet is passed through a pair of hot rollers
which removes excess of Zinc and maintain uniform thickness
of Zinc on metal sheet.
The sheet is subjected to annealing process at 650c and
cooled slowly.

Uses of Galvanization: It is used to protect Iron used for

roofing sheets, wires, pipes, nails, bolts, screws, buckets,
and tubes.
Defects :Galvanised utensils are not used for cooking
because of solubility of Zn.
 TINNING
Tinning is the process of coating of tin over the Iron sheet or
steel articles
 A cleaned iron sheet is passed through a bath of molten flux,
then passes through a tank of molten tin and finally through a
layer of palm oil which protect hot tin coated surface against
oxidation.
Uses: (1)It is widely used for coating of steel, copper, brass &
bronze etc . It is used to store food stuff, ghee oils, kerosene&
pickels and used for refrigerators equipment.
 ELECTROPLATING
Deposition of coating metal on the base metal by passing
direct current through an electrolytic solution which contain
the soluble salts of the coating material.
Pure metal is made as cathode and base metal as anode.
Electrochemically coat metal is deposited on base metal.
This metal gives smooth, fine and uniform coating
It depends on
(i) Temperature

(ii) Current density

(iii) Electrolyte Concentration

(iv) Nature of base metal (v) Time

Applications of Electroplating
 It is most important & frequently used technique in industries to
produce metallic coating

 Both metals & non metals can be electroplated.

 In metals the electroplating increases resistance to corrosion,

chemical attack, hardness, wear resistance and surface
properties.

 In non metals electroplating increases strength and decorates the

surface of non metal like plastics, wood, glass.
Electroless Ni plating:
 Pre treatment of surface:

 (1) CO, Ni, steel and Al can be directly plated.

 (2)Pb, Cd and Sn are first electroplated with Ni prior to

electroless plating.

 (3)Non conductors are activated in Sncl2 & HCl

followed by dipping in Pdcl2,Hcl.on drying thin pd layer
is formed.
 Protective Measures Against Corrosion
 Metallic Coating :
• Electroplating : A coating metal is deposited on the base metal by
passing direct current through an electrolytic soln.
• Metal Cladding : The base metal to be protected and coating metal are
sandwiched by presssing through rollers under the action of heat &
pressure.
• Hot Dipping : The base metal to be coated is immersed in a bathof the
molten coating metal.
• Cementation : A uniform surface coating is obtained by heating the
base metal in a power of coating metal.
• Metal spraying : The coating metal in molten state is sprayed on base
metal by means of spraying gun.
Protective Measures Against Corrosion
 Organic Coating :
• Apply on metallic surface for protection from corrosion & also to
impart decorative value. Such as paints, enamel, Varnishes &
lacquers
 Corrosion Inhibitors :
• Substances which when added in a small amount in an environment
reduces the rate of corrosion of a metal exposed to that
enviornment. These are of two types Cathodic & anodic
 Inorganic Coatings : are produced by chemical or
electrochemical reactions at a metal surface to protect base metal
from corrosion eg. : phophates, oxides etc.
 Using Pure Metal :
 Using Alloys :
 Text books references
1. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006
Dhanpat Rai Publishing Company, NewDelhi.
2. Dara S.S. & Umare S.S. A Text Book of Engineering Chemistry(12th
Edition ) 2008 S.Chand Publishing Company, New Delhi
3. Chawla Shashi: A text book of Engineering Chemistry (3rd Edition)
2010 Dhanpat Rai Publishing Company, New Delhi.
4. Palanna O G : A text book of Engineering Chemistry(4th Reprint)
2012 McGraw Hill, New Delhi
5. Sharma BK, Industrial Chemistry (16th Edition), 2014, Krishna
Prakashan Media (P) ltd. Meerut.