This pdf is useful for learning all the things about corrosion and the methods of controlling it using different things. It will help people understand the basic chemistry behind the corrosion along with the chemical reactions . galvanic corrosion, electrochemical corrosion, dry corrosion, wet corrosion. it will tell you more about the basic chemistry behind using the anodic and cathodic coatings

1. Corrosion
Most of the metals exist in nature in combined form as their oxides,
carbonates, sulphides etc. in their ores. Metals are extracted from their
ores and for that high amount of energy is required.
Therefore the metals can be regarded as excited state than their ores.
So they have a tendency to revert back to combined state when exposed
to environment and the destruction and deterioration of the metal starts
at surface.
Any process of deterioration (or destruction) and consequent loss of a
solid metallic material, through an unwanted (or unintentional) chemical
or electrochemical attack by its environment, starting at its surface is
called Corrosion
E.g. rusting of iron (Fe3O4), green film on copper surface (CuCO3 + Cu(OH)
2)

2. 

Dry Corrosion
Direct chemical action of environment/atmospheric gases such as
oxygen, halogen, hydrogen sulphide, sulphur dioxide or anhydrous
inorganic liquid with metal surfaces in immediate proximity
Oxidation corrosion
By direct action of oxygen at low or high temperatures on metals, usually
in absence of moisture. Alkali and alkaline-earth metals can be oxidized
at relatively low temperature. At high temperatures, almost all metals
got oxidized (except Ag, Pt etc.)
Mechanism
Oxidation occur first at the surface of the metal and the resulting
metal oxide scale forms a barrier
For oxidation to continue, the metal and oxide ions have to be diffused.

3. Diffusion of metal ions is more rapid than the diffusion of oxide as the
metal ions are usually smaller than the oxide ion and hence have higher
mobility

4. When the oxidation starts, a thin layer of oxide is formed on the metal
surface and the nature of this film decides further action
If the film is
Stable
A stable layer is fine-grained in structure and can get adhered tightly to
the parent metal surface. So it can be of impervious nature (inhibits
penetration of attacking oxygen to the underlying metal) and can
behaves as protective coating
E.g. Oxide films on Al, Sn, Pb, etc.
Unstable
Oxide layer formed, decomposes back into the metal and oxygen
Consequently, oxidation corrosion is not possible in this case (e.g. Ag, Au,
Pt etc.)

5. Volatile
Oxide layer volatilizes as soon as it is formed, thereby leaving the
underlying metal surface exposed for further attack. This causes rapid,
continuous and excessive corrosion (eg. MoO3)
Porous
Having pores or cracks. In such cases, the atmospheric oxygen have
access to the underlying surface of the metals and thereby corrosion
continues unobstructed, till the entire metal is completely converted to
its oxide

6. Corrosion by other gases like SO2, CO2, Cl2 etc.
Protective or nonporous film: eg. AgCl formed by attack of Cl2 on Ag
Non-protective or porous film: eg. dry Cl2 attacks on Sn forms volatile
SnCl4

7. 

Wet Corrosion
It 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 the of separate anodic or cathodic areas/
parts, between which current flows through the conducting solution
At anodic area, oxidation (liberation of electron) occurs, so anodic metal
is destroyed by either dissolving or assuming combined state (such as
oxide etc.). Hence corrosion always occurs at anodic areas. On the other
hand, at cathodic areas, reduction takes place: dissolved constituents in
the conducting medium accepts the electrons and form some ions like
OH-, O2-
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. The electrons set free at anode flow through the
metal and finally consumed in the cathodic reaction.

8. Mechanism
Flow of electron between anodic and cathodic areas
At Anodic area:
Dissolution of metal with liberation of electron (oxidation)
At Cathodic area:
Consumption of electron (reduction) either by
Evolution of hydrogen or
Absorption of oxygen
Evolution of hydrogen: Occurs in acidic environment
Displacement of hydrogen ions from acidic solution. All metals above
hydrogen in electrochemical series can undergo this type of corrosion. In
this type of corrosion, anodes usually have very large areas where
cathodes are of small areas.
Absorption of oxygen
Eg. Rusting of iron in neutral aqueous solution of electrolyte (NaCl
solution) in presence of atmospheric oxygen

9. Surface of iron is coated with a thin film of iron oxide. If some cracks
developed, then anodic areas are created on the surface. Anodic areas
are small surface parts; while nearly the rest of the surface of the metal
forms large cathode
Fe2+ and OH- ions will diffuse and then combine to form the precipitate
of ferrous hydroxide. Smaller Fe2+ will diffuse more rapidly than larger
OH- ions. So corrosion occurs at the anode and rust deposited at or near
the cathode.

10. Galvanic or Bimetallic Corrosion
When two dissimilar metals are connected and exposed to an
electrolyte, the metal higher in electrochemical series undergoes
corrosion.
If enough oxygen is present, ferrous hydroxide will be oxidized to form
ferric hydroxide, which will form yellow rust
If supply of oxygen is limited, the corrosion product will be black
anhydrous magnetite (Fe3O4)
Effect of increasing oxygen content
Forces the cathodic reaction producing more OH- ions
As it removes more electrons, so accelerates the corrosion at anode
Combination of these reactions, will cause more corrosion and rust-
formation
Zinc will dissolve at anodic
areas and oxygen will take
up electrons at cathodic
areas to form OH- ions.

11. Concentration Cell Corrosion
This type of corrosion is due to electrochemical attack on the metal
surface, exposed to an electrolyte of varying concentration or of varying
aeration.
Differential aeration corrosion (most common type of concentration cell
corrosion)
When one part of metal is exposed to a different air concentration from
the other
Generally poor oxygenated parts are anodic.
Therefore parts immersed to greater depth
(less access of oxygen) become anodic. So a
difference of potential is created, which
causes a flow of electron between two
differentially aerated areas of same metal
Corrosion of metals partially immersed in a
solution of a neutral salt just below the
waterline
The parts above and closely adjacent to the
waterline are most strongly aerated (due to
easy access of oxygen) and hence become
Cathodic

12. Iron corrodes under drops of water (or salt solution)
Areas covered by droplets having less access of oxygen, become anodic
with respect to other areas, which are freely exposed to air.
Oxygen concentration cell increases corrosion, but it occurs where the
oxygen concentration is lower. Corrosion may be accelerated in
apparently inaccessible places, because oxygen deficient areas serve
as anodes and therefore cracks or crevices serves as foci for corrosion
Corrosion is accelerated under accumulation of dirt, sand, scale or other
contaminations: Because accumulation of rust, scale etc restricts the
access of oxygen and establishes an anode and undergo localized
corrosion.
Metals exposed to aqueous media corrode under blocks of wood or
glass: Screen some portion of the metal from oxygen access; proceed
to localized attack and thereby corrosion

14. Passivity
Metal or any alloy exhibit a much higher corrosion resistance than
expected from its position in the electrochemical series. This is due to
the formation of a highly protective but very thin film on the surface of
metal or an alloy.
The film is insoluble, non-porous and of “self-healing” nature (if broken,
it will repair itself on re-exposure to oxidizing conditions)
E.g. Ti, Al, Cr, stainless steel (Containing Cr): corrosion resistance (i.e.
passivation) in oxidizing environments, but in reducing environment
they become chemically active. In oxidizing environment the protective
oxide films will be automatically repaired whenever any damage occurs
Al containers can store a concentrated solution of HNO3

15. Pitting Corrosion
Localized accelerated attack results in formation of pinholes, pits and
cavities in metal
This is due to the breakdown or cracking of the protective film on a metal
at specific points. This gives rise to the formation of small anodic and
large cathodic areas.
Metals owing their corrosion resistance to their passive state, show a
marked pitting under the condition leading to the destruction of their
passivity, i.e. those will be the starting points of pitting corrosion. For e.g.
stainless steel and Al show characteristic pitting in chloride solution

16. Intergranular Corrosion
Occurs along grain boundaries
Selective attack at only the grain boundaries, leaving the grain interior
untouched or only slightly attacked. This is due to the fact that the grain
boundaries contain such materials which shows electrode potential more
anodic than that of the grain centre in a particular corroding medium.
For e.g, during welding of stainless steel (an
alloy of Fe, C and Cr), chromium carbide is
precipitated at the grain boundaries, thereby
the region just adjacent to grain boundaries
will have lower amount of Cr composition and
is more anodic w.r.t. the solid solution within
the grain (which is richer in Cr)
Solution of this problem: Heat treatment
method, which dissolves the chromium
carbide precipitated during welding

17. This intergranular corrosion occurs in microscopic scale, without any
apparent external signs. But sudden failure of the material (without any
pre-warning) occurs due to loss of cohesion between grains
Waterline corrosion
When water is stored in steel tank, it is generally found that the maximum
amount of corrosion takes place along a line just beneath the level of
water meniscus. The area above the waterline (highly-oxygenated) acts
as the cathodic will be unaffected by corrosion.
In case of ships, this type of corrosion is accelerated by marine plants
attached to the sides of the ship

18. Crevice Corrosion
Crevice between different metallic objects e.g. bolts, nuts, rivets, in
contact with liquids
Crevice area has lack of oxygen (thus become anodic region and
corrosion takes there). The exposed area acts as cathode.
e.g. at the junction of two metals exposed to a corrosive environment

19. 



Microbiological Corrosion
Due to metabolic activity under aerobic or anaerobic conditions
Direct chemical action of sulfuric acid formed by the oxidation of
sulfur or sulfide by microorganism
Generation of local electrochemical cells due to change in pH,
concentration and oxidation potentials
Removal of protective coatings or corrosion inhibitors
Underground or Soil Corrosion
In soil, presence of moisture, bacteria micro-organisms and electrolyte
etc are responsible for corrosion which is further promoted by
differential aeration
Eg. Buried pipelines passing from one type of soil to another suffer
corrosion due to differential aeration: like pipelines passing through clay
and then through sand. Since clay is less aerated than sand hence
corrosion starts.

20. 


Stress Corrosion or Stress cracking
Combined effect of static tensile stresses and the corrosive environment
on a metal. Here highly localized attack is occurring, when overall
corrosion is negligible
The corrosive agents are highly specific and selective such as
Caustic alkalis and strong nitrate solution for mild steel
Traces of ammonia fro brass
Acid chloride solution for stainless steel
This type of corrosion is seen in fabricated articles of certain alloys due to
the presence of stresses caused by heavy working like rolling, drawing or
insufficient annealing. This localized electrochemical corrosion occurs
along narrow paths, forming anodic areas w.r.t. more cathodic areas at the
metal surface.

21. Presence of stress also produces strain, which result in localized zone of
higher electrode potential. This becomes so chemically active that they
are attacked, even by mild corrosive environment and finally results crack
which will propagate further.
Examples of stress corrosions
Season cracking
Stress corrosion of copper alloys (containing small amount of alloying
elements like P, As, Sb etc), whereas the pure metal is resistant to stress
corrosion. Intergranular cracking occurs in an atmosphere containing
traces of ammonia or amines. The attack occurs along the grain
boundaries which become more anodic w.r.t. grain themselves.
Caustic Embrittlement
Stress corrosion occurs in mild steels exposed to alkaline solution at high
temperature and stress (like in steam boilers)

22. Galvanic Series
Electrochemical series does not
account for the corrosion of all metals
and alloys. So a more practical series,
called Galvanic Series is prepared by
studying the corrosion of metals and
alloys in a given environment like sea-
water.

23. Factors influencing corrosion
Nature of the metal
Position in Galvanic Series
When two metals or alloys are in electrical contact, in presence of an
electrolyte, the more active metal (higher in Galvanic series) suffers
corrosion.
The rate depends on their difference in position and more the difference,
faster the corrosion of the metal
Relative areas of the anodic and cathodic parts
When two dissimilar metals or alloys are in contact, the corrosion of the
anodic part is directly proportional to the ratio of the areas of the
cathodic part and the anodic part
So corrosion is more severe and highly localized if the anodic area is
small : (e.g small steel pipe in copper tank)

24. Purity of metal
Impurity in metals form minute/tiny electrochemical cells and the
anodic part gets corroded.
E.g. Zn metal containing impurity like Fe, Pb, undergoes corrosion of Zn
due to formation of local electrochemical cells. The rate increase with
increasing exposure and extent of impurity.

25. Physical State of the metal: Such as grain size, orientation of crystals,
stress etc.
Smaller the grain-size of the metal or alloy, greater will be its solubility
and hence greater the corrosion
Nature of surface film
Get covered with a thin film of metal oxide on surface. The ratio of the
volume of the metal oxide to the metal is known as a “specific volume
ratio”. Greater the specific volume ratio, lesser the oxidation corrosion
rate.
Passivity of the metals like Ti, Al, Cr etc. shows much higher corrosion
resistance than expected from their position in Galvanic series due to
formation of highly protective film. Moreover the film is of “self-healing”
nature, if broken repairs itself.
Solubility of corrosion products
In electrochemical corrosion, if the corrosion product is soluble in the
corroding medium, then the corrosion proceeds faster rate. On contrary,
corrosion of Pb in H2SO4 is suppressed due to formation of PbSO4.

26. Volatility of corrosion product
If it volatilizes as soon as possible, leaving the underlying metal exposed
for further attack


Nature of the corroding environment
Temperature
On increasing temperature, the reaction as well as diffusion rate will
increase, thereby corrosion rate will be enhanced.
Humidity of air
Critical humidity: The relative humidity above which the atmospheric
corrosion rate of metal increases sharply. The value of critical humidity
depends on the physical characteristics of the metal as well as the
nature of the corrosion products.
Reason of corrosion in humid environment
Humid atmosphere furnish water to the electrolyte, essential for
setting up an electrochemical corrosion cell
Oxide film on metal surface can also absorb moisture and thus
electrochemical type corrosion can occur

27. Presence of impurities in atmosphere
In industrial areas, corrosive gases like CO2, H2S, SO2 and fumes of HCl,
H2SO4. In presence of these gases, acidity adjacent to metal surface
increases ad its electrical conductivity will also increase. This increase
the corrosion current flowing in local electrochemical cells on the
exposed metal surfaces.
In marine atmosphere, presence of NaCl
Influence of pH: Generally acidic media is more corrosive than
alkaline and neutral media.
Nature of ions present:
Presence of anions like silicate in the medium leads to the formation of
insoluble reaction products (e.g. silica gel) which inhibits further
corrosion. On the other hand, presence of Cl- destroy the protective
surface film, thereby exposing the metal surface for further corrosion.
Presence of even traces of Cu in mine water, accelerates the corrosion
of iron pipes

28. 



Corrosion Control
1. Proper designing
The design of the material should be such that corrosion even if it
occurs, it should be uniform and does not result in intense and localized
corrosion.
Avoid the contact of dissimilar metals in presence of corroding
solution.
When two dissimilar metals are to be in contact, the anodic area
should be as large area as possible; whereas the cathodic metal should
have as much smaller area as possible.
If two dissimilar metals in contact, they should be as close as possible
to each other in Galvanic series.
When the direct joining of dissimilar metals is unavoidable, an
insulating fitting may be applied in between them to avoid direct metal-
metal electrical contact.

29. Corrosion Control
2. Using pure metal
Corrosion resistance of a given metal may be improved by increasing its
purity
3. Using metal alloys
Corrosion resistance may be increased by alloying with suitable elements,
but alloy should be completely homogeneous. Chromium is a good
alloying element for iron, because its film is self-healing.

30. 4. Cathodic Protection
Force the metal (which has to be protected) to behave like a cathode,
so corrosion will not occur.
4.1 Sacrificial anodic protection method
The metal (which has to be protected) is connected by a wire to a more
anodic metal, so that all the corrosion will be concentrated at this more
active metal.
The more active metal gets corroded slowly, while the parent structure
(cathodic is protected). The more active metal is called “Sacrificial
Anode”. The corroded sacrificial anode (Mg, Zn, Al or their alloys) will be
replaced by a fresh one, when consumed completely.

31. 4.2 Impressed current Cathodic Protection
An impressed current is applied in opposite direction
to nullify the corrosion current, and convert the
corroding metal from anode to cathode.
The impressed current is derived from a direct
current source with an insoluble anode (graphite,
scrap iron etc.). Usually a sufficient d.c. current is
applied to an insoluble anode, buried in soil (or
immersed in corroding medium) and connected to
the metallic structure to be protected.
E.g. Useful for large structures for long-term
operations.


5. Modifying the environment
Corrosive nature of the environment can be reduced either by:---
Removal of harmful constituents
Addition of specific substances, which neutralize the effect of corrosive
constituents of the environment

32. (a)
(b)
5.1 Deaeration
Removal of oxygen from aqueous environment reduces metal corrosion
by
adjustment of temperature
Mechanical agitation
5.2 Deactivation
Addition of chemicals capable of combining rapidly with oxygen in
aqueous solution (e.g. Na2SO3, hydrazine hydrate)
5.3 Dehumidification
Reduces the moisture content of air to such an extent that the amount
of water condensed on metal is too small to cause corrosion (e.g.
alumina, silica gel)
5.4 Alkaline Neutralization
Neutralization of acidic character of corrosive environment (due to the
presence of ). Alkaline neutralizers like NH3, NaOH are generally injected
in vapor or liquid form to the corroding system.

33. 6. Corrosion Inhibitors
On addition of the inhibitor in small quantities to the aqueous corrosive
environment, effectively decreases the corrosion of a metal.
Two types of inhibitors:
6.1 Anodic inhibitors
Ions of transition elements like chromate, phosphate etc, with a high
oxygen content, suppress the corrosion reaction (at anode) by forming a
sparingly soluble compound by reacting with a newly produced metal ion.
These compounds will then be adsorbed on metal surface and form a
protective film and thereby also reducing corrosion rate.
6.2 Cathodic Inhibitors
(a) In Acidic solution, main cathodic reaction is evolution of hydrogen
Therefore corrosion can be reduced by:
Slowing down the diffusion of hydrated H+ ions to the cathode and/or
Increasing the overvoltage of hydrogen evolution

34. Arsenic or antimony oxides are used as inhibitors, because they deposit
adherent film of metallic arsenic or antimony at cathodic areas, thereby
increasing considerably the hydrogen overvoltage
(b) in Neutral solution:
The corrosion can be controlled either by:
Eliminating oxygen from the corroding medium (by using reducing agents
like Na2SO3 or by deaeration)
By retarding the diffusion to the cathodic areas (by adding Mg, Zn or Ni
salts, which react with OH- ions at cathode, and thereby producing
insoluble hydroxides which will be deposited on the cathodes making self-
barriers)


7. Protective Coatings
Coatings should prevent the penetration of corroding environment to
the material, which need to be protected
Coating must be chemically inert to the corroding environment
The diffusion of H+ ions can be decreased by organic inhibitors (amines,
heterocyclic nitrogen compounds, substituted urea etc.), which are
capable of being adsorbed on the metal surface.

35. 7.1 Anodic Coating
Coating metals are anodic w.r.t. the base metal (i.e. the metal to be
protected).
Eg. Coating of Zn on Fe
Under corrosive environment, if pores, breaks or discontinuities occur in
such anodic coating, a galvanic cell is formed between the coating metal
and the exposed part of base metal
Then the coating metal will be attacked leaving the base metal protected.
So this is basically “Sacrificial Coating”

36. 


7.2 Cathodic coating
Coating of a more noble metal than the base metal. Protection is due
to higher corrosion resistance of that metal than base metal.
This coating provides effective protection to the base metal only when
the protection is completely continuous and free from pores, breaks and
discontinuities.
If such coating breaks, much more corrosion damage can occur
Eg. Coating of tin on iron sheet and the protection will be as long as the
surface of iron is completely covered. If the coating punctured, then tin
will act as cathode and exposed iron will act as anode. A galvanic cell will
be set up and an intense localized attack at the small exposed part will
occur, resulting severe pitting of the base metal.

37. 


(i)
(ii)
Methods of application of metal coating
Hot dipping
For coating of low melting metals like Zn, Sn, Pb, Al etc on iron, steel,
copper etc., which have relatively higher melting points.
Immerse the base metal in a bath of molten coating metal
For good adhesion, the base metal surface must be very clean,
otherwise it cannot be properly wetted by the molten metal.
Two most widely applied hot dipping method:
Galvanizing
Tinning

38. 



(i) Galvanizing:
Process of coating iron or steel sheets with a thin coat of Zn to prevent
rusting
Procedure:
First clean the iron article by dilute H2SO4
Then washed and dried
Dip in the bath of molten Zn. The surface of the bath is covered by a flux
(ammonium chloride) to prevent oxide formation of molten coating metal
Remove excess Zn and produce a thin film of uniform thickness by hot
rollers.

39. 

(ii) Tinning
Process of coating iron or steel articles with a thin coat of Sn
Procedure:
First clean the iron article by dilute H2SO4
Passed through a bath of zinc chloride flux and then pass through a
tank of molten tin and then through a series of rollers and finally through
palm oil, which protect the hot tin coated surface against oxidation.

40. Metal Cladding
Dense, homogeneous layer of coating metal is bonded firmly and
permanently to the base metal on one or both sides.
Corrosion resisting metals (like Ni, Cu, Pb, Ag, Pt etc.) and alloys (like
stainless steel etc.) can be used as cladding materials.
Procedure
Thin sheets of coating metal and base metal are arranged in form of
sandwich
That sandwich then passed through rollers, under the action of heat and
pressure
Eg. Plate of duralumin is sandwiched between two layers of pure Al