2. Syllabus
Corrosion:
• Introduction.
• Dry & Wet corrosion and their mechanism.
• Types of corrosion: Pitting corrosion, waterline corrosion,
intergranular corrosion, galvanic and stress corrosion.
• Pilling Bed worth rule.
• Corrosion control : Design and material selection in corrosion
control, Cathodic protection, Protective surface coatings-
• Hot dipping (Galvanizing and tinning)
Energy storage system: Basic principles of batteries and their types,
Contruction,working and application of Lithium ion battery,
Ni-Cd battery.
3. Introduction
Most of the metals are exist in nature in combined form as their
oxides, hydroxide, carbonates, chlorides, sulphides etc. are known as
ores and minerals. (thermodynamically stable state )
During the process of extraction , these ores or minerals are reduced
to metallic state. During extraction of metal, considerable amount of
energy is required. Thus, isolated pure metals can be regarded in
excited state (a higher energy state) then their corresponding ores,
and thus, they have a natural tendency to revert back to combine state
(or lower energy state)
Hence , when the metals are put into use, in various forms, they are
exposed to environment (such as dry gases, moisture, liquids, etc.).
The metal surface began to react with the surrounding or decay
(i.e. conversion in to more stable state metal compounds)
4. Thus the metal have a natural tendency to go back to their natural
thermodynamically stable state which we call as corrosion
Definition:
Any process of destruction and consequent loss of a solid metallic
material, through an unwanted chemical or electro-chemical attack
by its environment, starting at its surface, is called corrosion.
The decay or destruction or deterioration of metal starts at its
surface.
Metal
Corrosion (Oxidation)
Metallurgy (Reduction)
Metallic Compound
(A State of higher energy) (A State of lower energy)
+ Energy
Thus, corrosion is a process reverse of extraction of metals.
5. The most familiar example of corrosion is rusting of iron, when
exposed to environment, a layer of reddish brown layer is formed of
Fe3O4, and metal becomes weak
Formation of green layer of basic carbonate [CuCO3 + Cu(OH)2] on
the surface of copper when exposed to atmosphere containing
moisture and carbon dioxide.
There are some metals, that do not corrode
Silver
Gold
Platinum
6.
7.
8. Types of corrosion
A]Dry & Wet corrosion and their mechanism
1)Dry or chemical corrosion:
This type of corrosion is mainly due to the direct chemical action
of environmental / atmospheric gases such as oxygen, halogen,
hydrogen sulphide, SO2 , N2 ,anhydrous inorganic liquid, oxygen
ion on the metal surfaces.
There are three main types of chemical corrosion
a) Corrosion by Oxygen
b) Corrosion by other gases
c) Corrosion by liquids
9. I) Oxidation Corrosion:
Oxidation corrosion is due to the direct action of oxygen at
low or high temperatures on metals. In the absence of moisture.
At ordinary temperature or low temperature, metals, in general, are
very slightly attacked.
But alkali metals (Li, Na, K, Rb, etc) and alkaline-earths (Be, Ca,Sr, etc)
are even rapidly oxidised even at low temperatures.
At high temperature, almost all metals (except Ag, Au, and Pt) are
oxidised.
10. The reactions in the oxidation corrosion are:
2M 2Mn+ + 2ne- (Loss of electron)
nO2 + 2ne- 2nO2- (Gain of electrons)
Or 2M + nO2 2Mn+ + 2nO2-
Metal oxide
Mechanism:
Oxidation occur first at the surface of the metal and the resulting
metal oxide scale forms a barrier, that tends to restrict further
oxidation.
Thus, for oxidation to continue, either the metal must diffuse
outwards through the scale to the surface or the oxygen must
diffuse inwards through the scale to the underlying metal.
11. Both transfers occur, but the outward diffusion of metal is, much
more rapid than the inward diffusion of oxygen .
As metal ion is appreciably smaller than at the oxygen ion and
consequently, of much higher mobility.
13. Nature of Oxide Layers Form
The nature of oxide layer formed plays an important role in further
oxidation corrosion
Metal+ oxygen Metal oxide
(corrosion product)
1) Stable Layer:
A stable layer is fine grained in structure and can get adhered tightly
to parent metal surface.
Hence, such a layer can be of impervious in nature
(i.e. which cut off penetration of attacking oxygen to
underlying metal).
Behaves as protective coating in nature there by shielding the metal
surface.
e.g. Oxide films of Al,Pb,Cu etc. are stable and
impervious in nature. Thus, further oxidation corrosion is
prevented.
14. 2)Unstable Layer :
This oxide layer formed get decomposed back into metal and
oxygen. Thus, oxidation corrosion is not possible in such cases.
e.g. Au, Pt.
3) Volatile:
The oxide layer volatilizes as soon as it is formed thereby leaving the
underlying metal surface exposed for further attack.
This causes rapid and continuous corrosion, leading to excessive
corrosion, e.g. molybdenum oxide (MbO3)is volatile.
15. 4) Porous:
Having pores or cracks.
In such a case, the atmospheric oxygen have access to the
underlying surface of metal.
Through the pores or cracks of the layers thereby the corrosion
continues unobstructed, till the entire metal is completely converted
into its oxide.
16.
17. 2) Corrosion due to Other Gases:
Gases like SO2, CO2, Cl2, H2S, F2, etc. also react with metals.
The extent of corrosive effect depends mainly on the chemical
affinity between the metal and the gas involved.
The degree of attack depends on the formation of protective or non-
protective films on the metal surface.
(a) If the film formed is protective or non-porous (e.g., AgCl film
resulting from the attack of Cl2 on Ag), the intensity or extent of
attack decreases, because the film formed protects the metal from
further attack.
(b) If the film formed is non-protective or porous, the surface of the
whole metal it gradually destroyed. For example, dry Cl2 gas attacks
on tin (Sn) forming volatile SnCl4.
(c) Similarly, in petroleum industry, H2S at high temperature attacks
steel forming a FeS scale, which interferes with normal operations.
18. 3)Liquid-Metal Corrosion:
Chemical action of flowing liquid metal at high temperature on solid
metal or alloy.
This type of corrosion in devices used for nuclear power and
metallurgical industries.
The corrosion reaction involves either.
1) Dissolution of solid metal by a liquid metal.
2) Interpenetration of liquid metal into a solid metal.
Both these mode of corrosion causes weakening of solid metal.
19. B] Wet or Electrochemical Corrosion
Corrosion of metal taking place through ionic reaction in presence of
moisture is called Wet or Electrochemical Corrosion.
This type of corrosion occurs:-
(1)When two dissimilar metals or alloys are dipped partially in a
solution.
(2) When a conducting liquid is in contact with metal.
Formation of separate ‘anodic’ and ‘cathodic’ areas/parts between
which current flows through conducting solution.
At anodic area, during oxidation reaction liberation of free electron
takes place.
Anodic metal is destroyed by either dissolving or assuming
combined state (such as oxide).
Corrosion always occurs at anodic areas.
20. At anode: M Mn++ ne
Mn+ Dissolves in solution
Form compound
At cathode: At cathodic part dissolved constituents to form ions such as
OH- and O2- etc
The metallic ions (at anodic part) and non-metallic ions (formed at
cathodic part) diffuse towards each other through conducting medium
and form a corrosion product somewhere between anode and cathode.
The electron set free at the anode flow through metal and finally
consumed in the cathodic reaction.
21. Mechanism of Wet or Electrochemical Corrosion
Electrochemical corrosion involves flow of electron current between the
anodic and cathodic areas.
At anodic area, M Mn++ne-
cathodic area the cathodic reaction consumes electrons with either by-
(1) Evolution of hydrogen
(2) Absorption of oxygen Depending on nature of the corrosion
environment
1) Evolution of hydrogen:
This type of corrosion occurs, usually in acidic environment.
Fe Fe2++ 2e- --- (Oxidation)
22. These electron flow through metal from anode to cathode, where H+ ions
are eliminated as hydrogen gas.
2H++ 2e¯ H2 --- (Reduction)
Thus , this type of corrosion causes displacement of H+ ions from acid
solution by metal ions.The overall reaction is,
Fe + 2H+ Fe2++ H2
23. Fig. Mechanism of wet corrosion by evolution of hydrogen
All metals above hydrogen in electrochemical series have tendency to
get dissolve in acidic solution with simultaneous evolution of
hydrogen
24. 2) Absorption of oxygen:
The surface of iron is usually coated with a thin film or iron oxide.
iron oxide film develop some cracks, anodic areas are created on the
surface while the well metal parts acts as a cathode.
Rusting of iron in aqueous solution of this type of corrosion.
It follows that the anodic area are small, while the rest of the surface of
metal forms large cathode
At the anodic area of metal, Iron dissolves as ferrous ions with
liberation of electron.
Fe Fe2++ 2e- --- (Oxidation)
25. The liberated electrons flows from anodic to cathodic areas through
iron metal where electrons are intercepted by dissolve oxygen as,
O2 + H2O + 2e- 2OH- ---- (Reduction)
The Fe2+ ions (at anode) and OH- ions (at cathode) diffuse and when
they meet, ferrous hydroxide is precipitated.
Fe2+ + 2OH- Fe(OH)2
Enough oxygen is present ferrous hydroxide is easily oxidized to ferric
hydroxide.
4Fe(OH)2 + O2 + 2H2O 4Fe(OH)3
This product is called as yellow rust. If supply of oxygen is limited
corrosion product may be even block anhydrous magnetite Fe3O4.H2O
27. Pitting corrosion:
It is a localized accelerated attack, resulting in the formation of
cavities around which the metal is relatively unattached. Thus
pitting corrosion results in the formation of pin holes pits and
cavities.
Pitting is usually due to the breakdown of protective coating on
metal surface.
This corrosion is based on absorption of oxygen type corrosion.
In which small anodic and large cathodic areas are formed.
Breakdown of the protective film may be caused by:
1) Surface roughness or non uniform finishing.
2) Scratches on outer edges.
Types of corrosion
28.
29. 3) Alternating stresses.
4) Sliding under load.
5) Chemical attack.
The presence of extraneous impurities (like sand, dust etc.)
embedded on surface of metal also leads to the pitting corrosion.
Once a small pit is formed, the rate of corrosion will increased .
30. Intergranular Corrosion
This type of corrosion occurs along grain boundaries and only
where the material, especially sensitive to corrosion attack exist
This type of the corrosion is due the fact that the area just adjacent
to grain boundaries contain material, which show electrode
potential more anodic than that of grain centre in a particular
corroding environment
This may be due to precipitation of certain compounds at the grain
boundaries, thereby leaving the solid metal solution (just adjacent to
the grain boundary) impoverished in one constituent.
The impoverished solid metal solution is anodic with respect to
grain centre's as well as the precipitated compound,
So that it will be attacked preferentially by the corrosive
environment.
The Intergranular corrosion is generally related with alloys.
31.
32. Intergranular corrosion follows the path of grain boundaries and
occurs on microscopic level,
without any external signs of attack.
Thus this causes sudden failure of material without any warning,
due to loss of cohesion forces between grains.
During the welding of stainless steal (an alloy of Fe,C, Cr),
chromium carbide is precipitated at the grain boundaries, there by
Region just adjacent to grain boundaries become depleted in
chromium composition and is more anodic with respect to the solid
solution within the grain (which is richer in chromium). For the
same reason it is also anodic to the chromium carbide so
precipitated.
The problem of intergranular corrosion can be solved by proper
heat treatment.
35. Waterline corrosion
When water is stored in a steel tank for a long time. It is observed
that, the area just below the water line is corroded .
The area above the water line is highly oxygenated and acts as
cathode, where as the area just below the water line becomes
anode and is corroded.
The problem of water line corrosion is concern with the marine
engineering. In case of ships, this corrosion is accelerated by
marine plants attaching themselves to the side of the ships.
36.
37. Stress corrosion
This type of corrosion is seen in fabricated articles of certain
alloys (like high zinc brasses and nickel brasses)
due to the combine effect of static tensile stresses caused by heavy
working like rolling, drawing or insufficient annealing
and the corrosive environment on metal.
Stress corrosion involves in a localized electro-chemical corrosion,
occurring along narrow paths, forming anodic areas with respect to
the more cathodic areas at the metal surface.
Presence of stress produces strains, which results in localized
zones of higher electrode potential.
These becomes so chemically active that they are attacked even by
a mild corrosive environment, resulting in the formation of
Cracks, which grows and propagates in a plant (metal part) ,until
failure occurs.
38.
39. Galvanic corrosion
When two dissimilar metals are electrically connected and exposed
to an electrolyte, the metal higher in electrode-chemical series
undergoes corrosion.
This type of corrosion is called as galvanic corrosion.
Zinc (higher in electro-chemical series) forms the anode and is
attacked and gets dissolved; whereas copper (lower in electro-
chemical series or more noble) acts as cathode.
e.g. if zinc and copper are electrically connected and exposed to
an electrolyte.
40. Mechanism:
In acidic solution, the corrosion occurs by the hydrogen evolution
process, and in neutral or slightly alkaline solution, oxygen
absorption occurs.
The electric current flows from the anode zinc metal to the cathode
metal, copper.
Thus, the corrosion occurs at anode metal; while the cathodic part
is protected.
Zn Zn2n+ + 2e- (oxidation)
41. Examples:
(i)Steels screws in a brass marine hardware
(ii)Lead-antimony solder around copper wire
(iii)A steel propeller shaft in bronze bearing
(iv)Steel pipe connected to copper plumbing
Fig.Galvanic corrosion:
42. Active
(or Anode)
Noble
(or Cathodic)
1. Mg
2. Mg Alloys
3. Zn
4. Al
5. Cd
6. Al Alloy
7. Mild Steel
8. Cast iron
9. High Ni cast iron
10. Pb-Sn Solder
11. Pb
12. Sn
13. Ni-Mo-Fe alloy
14. Brasses
15. Monel (7=Ni,30=Cu,rest=Fe)
16. Silver Solder
17. Cu
18. Ni
19. Cr Stainless Steel
20. 18-8 Stainless Steel
21. 18-8 Mo Stainless Steel
22. Ag
23. Ti
24. Graphite
25. Au
26. Pt
43. Methods of Corrosion Control:
I) Role of design and material selection in corrosion control
I) Material Selection :- Important principles are,
(i) Avoid the contact of dissimilar metals in the presence of a corroding
solution: If this principle is not followed, then corrosion is localized on
the more active metal while the less active metal remains protected.
(ii) When two dissimilar metals are to be in contact, the anodic material
should have as large area as possible; whereas the cathodic metal should
have as much smaller area as possible.
(iii) If two dissimilar metals in contact have to be used, they should be as
close as possible to each other in the electro-chemical series.
44. II) Design Selection:
Prevent the occurrence of in homogeneities , both in the metal and
in the corrosive environment.
i) Thus, a proper design should avoid the presence of crevices.
iv) Whenever the direct joining of dissimilar metals, is unavoidable,
an insulating fitting may be applied in-between them to avoid
direct metal-metal electrical contact.
v) The anodic metal should not be painted or coated, when in
contact with a dissimilar cathodic metal, because any break in
coating would lead to rapid localized corrosion.
45. ii) Sharp corners and recesses should be avoided, because they favour
the formation of stagnant areas and accumulation of solids, etc
iii) Whenever possible, the equipment should be supported on legs to
allow free circulation of air and prevent the formation of stagnant
pools or damp areas.
.
46. v) Uniform flow of a corrosion liquid is desirable, since both
stagnant areas and highly impingement conditions of flowing
liquid should be avoided as practically possible.
V) A proper design should prevent condition subjecting some areas
of structure to stress(cold-worked part). Such an area could set
up a galvanic couple with a non-stressed (not worked areas of
the metal.
47. II) Cathodic protection:
Cathodic protection
The principle involved in this method is to force the metal to be
protected to behave like a cathode, thereby corrosion does not occur,
There are two types of cathodic protections:
I) Sacrificial anodic protection method:
In this protection method, the metallic structure (to be
protected )is connected by a wire to a more anodic metal.
All the corrosion is concentrated at this more active metal.
The more active metal itself gets corroded slowly; while the parent
structure(cathodic) is protected.
The more active metal so-employed is called sacrificial anode.
48. The corroded sacrificial anode block is replaced by a fresh one, when
consumed completely.
anodic method include protection of buried pipelines, underground
cable, marine structures, ship-hulls, water-tanks, piers,
a)Buried pipeline b)Ship-hull c)Hot water tank
50. II) Impressed current cathodic protection:
In this method, an impressed current is applied in opposite direction to
nullify the corrosion, impressed current is derived from a direct current
source with an insoluble anode
Like graphite, high silica iron, scrap iron, stainless steel or platinum.
Sufficient d.c. current is applied to an insoluble anode, buried in the soil
and connected to the metallic structure to be protected
The anode is, usually, in a backfill (composed of coke, breeze or gypsum)so as to
increase the electrical contact with surrounding soil.
This kind of protection technique is, particularly, useful for large structures for long-
term operations.
This type of cathodic protection has been applied to:
Open water-box, coolers, Water-tanks, buried oil or water pipes
Condensers ,Transmission line towers marine piers, Load-up ships
53. III) Metal coatings or Hot dipping:
This is used for producing a coating of low-melting metal such as
Zn (m.p. =4190C), Sn (m.p. =2320C), Pb,Al, etc. on iron, steel
and copper, which have relatively higher melting points.
The process, in general, consists of immersing the base metal in a
bath of the molten coating-metal, covered by a molten flux layer
(usually zinc chloride).
The flux cleans the base metal surface and prevents the oxidation
of the molten coating-metal.
For good adhesion, the base metal surface must be very clean;
otherwise it cannot be properly wetted by the molten metal.
For good adhesion, the base metal surface must be very clean;
otherwise it cannot be properly wetted by the molten metal.
For good adhesion, the base metal surface must be very clean;
otherwise it cannot be properly wetted by the molten metal.
54. Two most widely applied hot dipping methods are :
a) Galvanising
b) Tinning
55. a) Galvanising:
It is the process of coating iron or steel sheets with a thin coat of zinc
to prevent them from rusting. The process is carried out as follows,
1) The iron or steel article (e.g. sheet, pipe, wire) is first cleaned
by pickling with dilute H2SO4 solution for 15-20 minutes at
60-900C.
2) This treatment also removes any scale, rust (oxide layer) and
impurities.
3) The article is then washed well and dried.
4) It is then dipped in bath of molten zinc, maintained at 425-4300C.
5) The surface of the bath is kept covered with a flux [ammonium
chloride] to prevent oxide formation.
6) When the article is taken out, it is found to have been coated
with a thin layer of zinc.
7) It is then passed through a pair of hot rollers.
56. 10) It is most widely used for protection of iron from atmospheric
corrosion in the form of roofing sheets, wires, pipes, nails, bolts,
screws, buckets, tubes, etc.
8) This process removes any superfluous (excess) of zinc and
produces a thin film of uniform thickness.
9) Then, it is annealed at a temperature of 6500C and finally, cooled
slowly.
57. 2) It may be pointed here that zinc gets dissolved in dilute acids to
form highly toxic (or poisonous) especially acidic ones.
Uses:
1) It is most widely used for protection of iron from atmospheric
corrosion in the form of roofing sheets, wires, pipes, nails,
bolts, screws, buckets, tubes, etc.
3) Hence, galvanized pots are not used for preparing and storing
acidic food products.
58. b)Tinning:
It is a process coating tin over the iron or steel articles.
1) The process consists in first treating steel sheet in dilute sulphuric
acid (pickling) to remove any oxide film.
2) After this, it is passed through a bath of zinc chloride flux.
3) The flux helps the molten metal to adhere to the metal sheet.
4) Next the sheet passes through a tank of molten tin and finally
through a series of rollers from underneath the surface of a layer of
palm oil.
5) The palm oil protect the hot tin-coated surface against oxidation.
59. 7) The rollers remove any excess of tin and produce a thin film of
uniform thickness on the steel sheet.
Fig. Tinning
Fig: Tinning
60. Uses :
1)The possess considerable resistance against atmospheric corrosion.
2) Tin does not forms toxic products with food products, tinning is
widely used for coating steel, copper and brass sheets, used for
manufacturing containers for storing foodstuffs, ghee, oils,
kerosene and packing food materials.
3)Tinned-copper sheets are employed for making cooking pots and
refrigeration equipments.
