1. Corrosion & its Control
Dr. K. V. Nagalakshmi
&
Dr. B. Santosh Kumar
2.
3. Why do metals corrode?
In nature, when metals exists as their compounds
Haematite , Fe2O3.
Bauxite, Al2O3. 2H2O
Copper Sulfite, CuS
Metals are thus obtained by extraction from their
ores by reduction process.
4. Absorption of energy by the ore results metal.
Ore-Lower energy-stable
Metal-Higher Energy-Unstable
Metals have tendency to go to their natural state
(ore)
i.e. Metal (M) oxidation Metal ion (M+n)
The energy required to convert ore to metal is
released when the metal returns to form the original
compound.
Only the rate of energy change may be different.
Metal ion (M+n) Extraction Metal (M)
Corrosion
5. Corrosion -destruction or deterioration and consequent
loss of metallic material by chemical and
electrochemical attack of its environment starting at
the metal surface.
All metals (Except pt, Au, Ag) do corrode to some
extent.
Examples
Rusting of iron due to formation of hydrated ferric
oxide
Tarnishing of silver wires in H2S in the presence of
moisture air due to formation of silver sulphide
A green color layer of Copper carbonate on the
surface of copper
6. Types of Corrosion
1.Dry Corrosion (or) Chemical corrosion
2.Wet Corrosion (or) Electrochemical corrosion
7. Dry corrosion
This is due to direct chemical reaction of atmospheric
oxygen, O2, other gases like SO2, Cl2, H2S and molten
metal or liquid metal.
• The extent of corrosion of a particular metal
depends on the chemical affinity of the metal
towards reactive gas.
• Oxygen is mainly responsible for the corrosion of
most metallic substances when compared to other
gases.
8. Types of Dry corrosion
1.Oxidation corrosion
2.Corrosion by other gases
3.Liquid Metal Corrosion
9. Oxidation corrosion
Some of the metals directly react with oxygen in
the absence of moisture.
Oxidation Product is Metal oxide.
Alkali and alkaline earth metals react with oxygen
even 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 noble metals.
10. Mechanism
On exposure to atmosphere., metal gets oxidized to
form metal ions
(i) M (S) → Mn+ + ne-
Electrons lost by metal are taken up by oxygen to
form oxide ions
(ii)
𝟏
𝟐
O2(g) + 2e- → O2
2M +
𝒏
𝟐
O2 → 2Mn+ + nO-2 → M2On (Metal Oxide)
11. During oxidation of a metal, metal oxide is
formed as a thin film on the metallic
surface.
Metal oxide --- Protective or non protective
Thus, the extent of corrosion depends on
the nature of layer of metal oxide formed.
12. Types of Oxide layers
1. Stable oxide layer:
Oxides of Pb, Al and Sn are stable and hence inhibit further
corrosion. They form a stable, tightly adhering oxide film.
2. Unstable oxide layer:
Au, Ag, Pt form unstable oxide layer which decomposes soon
after the formation, thereby preventing further corrosion.
3. Porous oxide layer:
In case of porous oxide film, atmospheric gases pass through
the pores and react with the metal and the process of
corrosion continues to occur till the entire metal is converted
into oxide.
Eg-Alkali metals &alkaline earth metals
4. Volatile oxide layer: As soon as the film forms, leaves the
surface, so corrosion continues till entire metal is corroded.
Molybdenum forms a volatile oxide film of MoO3 where
continues corrosion takesplace.
13. Pilling – Bed Worth’S Rule
A protective and Non-Porous metal oxide layer is formed if
molar volume of metal oxide is greater than the volume of
metal from which it is formed.
A non protective and Porous metal oxide layer is formed if
molar volume of metal oxide is lesser than the volume of metal
from which it is formed.
Specific Volume Ratio =
𝐌𝐨𝐥𝐚𝐫 𝐕𝐨𝐥𝐮𝐦𝐞 𝐨𝐟 𝐨𝐱𝐢𝐝𝐞 𝐥𝐚𝐲𝐞𝐫 𝐟𝐨𝐫𝐦𝐞𝐝
𝐌𝐨𝐥𝐚𝐫 𝐕𝐨𝐥𝐮𝐦𝐞 𝐨𝐟 𝐦𝐞𝐭𝐚𝐥
> 1 Protective layer
< 1 Non-protective layer
14. 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.
In nuclear reactors, Na metal is used as coolant
leads to corrode Cd rods
15. Corrosion by Other gases
In dry atmosphere, gases like Cl2, SO2, H2S etc
react with metal and form corrosion products which
may be protective or non-protective.
Eg : Dry Cl2 reacts with Ag and forms AgCl which is
a protective layer, while SnCl4 is volatile.
In petroleum industries at high temperatures, H2S
attacks steel forming FeS scale which is porous
layer.
16. Electrochemical or Wet Corrosion
It is a common type of corrosion of metal in
aqueous corrosive environment.
This type of corrosion occurs,
(i) when the metal comes in contact with a
conducting liquid,
or
(ii) when two dissimilar metals in contact with
each other are immersed in a electrolytic
solution.
17. According to this theory,
Some parts of the metal surface act as anode and
rest act as cathode. If they are in contact with
conducting liquid, this corrosion takes place.
i.e. formation of a electrochemical cell on the
surface of metals.
18. Mechanism
At anodic area oxidation takes place.
M (S) → Mn+ + ne-
At cathodic area reduction takes place.
Cathodic reaction- (i) Evolution of hydrogen
(ii) Absorption of o2 based on nature of corrosive
environment.
Corrosive product between anode and cathode.
Corrosive product may be soluble or insoluble.
If soluble ,rate of corrosion increases.
If not soluble ,rate of corrosion decreases.
19. Evolution of Hydrogen
Anode-Fe → Fe +2 + 2e-
In acidic medium, Cathodic reaction is Evolution
of Hydrogen
2H+ + 2 e- → H2
Fe → Fe +2 + 2e-
2H+ + 2 e- → H2
Fe + 2H+ → Fe +2 + H2
20. Anode- Fe → Fe +2 + 2e-
In basic or neutral medium, Cathodic reaction is
Absorption of oxygen.
½ O2 + H2O + 2e- → 2OH-
Anode :Fe → Fe+2 + 2e-
Cathode:½ O2 + H2O + 2e- → 2OH-
Fe + ½ O2 + H2O → Fe+2 + 2OH-
Fe+2 + 2OH- → Fe(OH)2
In the presence of excess oxygen,
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)2 → Fe2O3 .H2O
If oxygen supply is limited, Fe3O4 is formed
23. Galvanic Corrosion:- When two different metals are
present in contact with each other in conducting
medium,
metal with lower reduction potential becomes
ANODE, gets corroded.
metal with higher reduction potential becomes
CATHODE
Eg.
25. Differential Aeration corrosion
Occurs due to difference 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.
26. Water line corrosion-Differential aeration
corrosion
29. Passivity
Some metals and alloys exhibit Outstanding
corrosion resistance than expected from their
position in the electrochemical series.
This is due to the formation of protective film
formed on the metal surface which is
Insoluble, non porous, self healing film
Eg- Tl, Cr, Al
31. Factors affecting corrosion
A. Nature of the metal
1. Purity of metal
2. Position of metal in Galvanic series
3. Relative areas of Cathode & anode
4. Passivity of the metal
5. Physical state of metal
6. Solubility of corrosion product.
32. B. Nature of Corroding Environment
1. Temperature of the environment
2. Effect of pH of the medium
3. Humidity in air
4. Presence of impurities.
5. Presence of suspended particles
6. Conductance of the medium
33. Corrosion controlling methods
(Protective measures against corrosion)
Metals can be protected from corrosion by various
methods. Some of them are listed below
1.Proper designing
2. Cathodic protection Method
i) Sacrificial anodic method
ii)Impressed current technique
3. Modifying the environment
4. Using corrosion inhibitors- anodic and cathodic
inhibitors
5. Applying surface coatings
i)Metallic coatings
ii) Organic coatings.
34. Cathodic protection Method:
Principle: The metal to be protected is forced to
behave like a cathode.
There are two types of cathodic protection
Methods:
(a)Sacrificial anodic method
(b)Impressed current method
35. Sacrificial anodic method:
In this technique, a more active metal is connected
to the metal structure to be protected so that all the
corrosion is concentrated at the more active metal
and thus saving the metal structure from corrosion.
36. Magnesium being anodic to iron get corroded.
Since they are sacrificed in the process of saving iron, they are called
sacrificial anodes.
The corroded sacrificial anode is replaced by a fresh one, when
consumed completely.
37. Important applications of sacrificial anodic
protection are as follows:
i. Protection from soil corrosion of
underground cables and pipelines.
ii. Magnesium sheets are inserted into
domestic water boilers to prevent the formation of
rust.
iii This method is used for the protection of ships
and boats.
38. Impressed current cathode method: In this method, an
impressed current is applied in an opposite direction to
the corrosion current, converting the base metal from
anode to cathode.
This can be done by applying the D.C current from a
battery.
Battery is connected to an inert anode and to the base
metal which is to be protected.
The anode is generally inert electrode like graphite,
platinum.
Since in this method current from an external source is
impressed on the system, this is called impressed
current method.
39. Impressed current cathode method:
This type of protection is given in the case of buried structures such as tanks and
pipelines.
40. Surface coatings
Corrosion of metal surfaces is a common phenomenon.
To protect a metal surface from corrosion, the contact
between the metal and the corrosive environment is to be
cut off.
This is done by coating the surface of the metal with
continues, non-porous material which is introduced to the
corrosive atmosphere.
Such a coating is referred to as surface coating or
protective coating.
In addition to protective action, such coatings also give a
decorative effect and reduce wear and tear.
41. Metal coatings:
Surfacing coatings made up of metals are known as
metallic coatings.
The metallic coatings often used are of Zn, Sn, Ni, Cr,
Al, & Cu …etc.
These coatings separate the base metal from the
corrosive environment and also function as an effective
barrier for the protection of base metals.
The metal which is coated upon is known as the base
metal.
The metal applied as coating is referred to as coating
metal.
42. They are two types of metallic coatings.
1. Anodic coatings:
In this type of coatings, the metal used for coating
is more anodic than the base metal which is to be
protected.
Ex. Coating of Zn on steel surface is called
“galvanizing”.
2. Cathodic coatings:
In this type of coating, the metal used for coating
is more noble than the base metal.
Ex. Coating of tin on iron called is “Tinning”
43. Methods of application of metallic coatings
The different methods used for metallic coating are listed below.
Metal Cladding : The base metal to be protected and coating metal are sandwiched
by pressing through rollers under the action of heat & pressure.
Hot Dipping : The base metal to be coated is immersed in a bath of the molten
coating metal.
Electroplating : A coating metal is deposited on the base metal by passing direct
current through an electrolytic soln.
Electrolessplating: Without making use of electrical energy, noble metals are
deposited from it’s solution on a catalytically active surface of a less noble metal by
employing reducing agent for forming thin uniform coating on base metal
Metal spraying : The coating metal in molten state is sprayed on base metal by
means of spraying gun.
44. Methods of application of metallic coatings
Hot dipping:
In the process of hot dipping, the metal to be coated is dipped in the molten bath of
the coating metal.
Such hot dip coatings are generally non-uniform.
The common examples of hot dip coatings are galvanizing and tinning.
45. Galvanizing
Galvanizing involves the following steps,
1. Pickling-The iron article is first pickled with dilute sulphuric
acid at 60-900C for about 15 -20 minutes.
to remove traces of rust, dust, etc.
2.Washing- The article is washed with distilled water
3. Drying-dried in a drying chamber
4. Hot dipping-The article is then immersed in the melt of
Zinc which is covered with Ammonium chloride flux to prevent
oxidation of Molten zinc.
5.Removal of excess Zn-The coated base metal is then
passed through rollers to correct the thickness of the film.
7. Annealing-Finally the coated object is annealed at 650 oc
to remove internal strains.
47. Applications of galvanizing
1.It is used to protect roofing sheets, wires, pipes, tanks,
nails, screws
2.Even If the protective layer of the zinc coating has any
cracks, iron being cathodic does not get corroded.
Limitations
1.Galvanised vessels can’t be used for storing foods
especially acidic food as Zn is dissolved by the acid
forming poisonous products.
48. Tinning- The coating of tin on iron is called
tinning
Pickling-The iron article is first pickled with dilute sulphuric
acid at 60-900C for about 15 -20 minutes to remove traces of
rust, dust, etc.
2.Washing- The article is washed with distilled water
3.Drying-dried in a drying chamber
4.Hot dipping-The article is then immersed in the melt of tin
which is covered with zinc chloride flux to prevent oxidation of
Molten zinc.
5.Removal of excess tin - The tin coated article is passed
through a series of rollers immersed in a palm oil bath to
remove the excess tin.
7. Annealing-Finally the coated object is annealed to remove
internal strains.
50. Applications of Tinning
1.These sheets are used for making cooking utensils &
refridgeration equipment
2.Copper wires before insulation by rubber are tinned to
avoid the attack of sulphur of rubber
Limitations:
If the protective layer of the tin coating has any cracks,
iron will corrode.
51. Organic coatings
The following materials are widely used for organic
coatings
Paints/Oil base paints
Varnishes
Enamels
The above are applied on window panes,
gates, doors, vehicles and wooden furniture.
52. Paint is colloidal dispersion of mixture of one or
more pigments in a vehicle.
when paint is applied on a metal surface, the
thinner being volatile evaporates leaving behind
the drying oil, which oxidizes forming a dry
pigmented film on the metal surface
53. Ingredients of organic coatings:
Mainly ingredients are :
Pigments
Vehicle or drying oil
Thinners
Driers
Extenders
Plasticizers
Anti skinning agents
