Factors affecting corrosion & control measures

Prepared by:
Prof Sarala Prasanna Pattanaik
Assistant Professor
Department of Chemistry

• Mechanism of Dry (Chemical) and Wet (electrochemical)
Corrosion
• Types of Electrochemical Corrosion (Differential aeration
corrosion, Galvanic corrosion, Pitting corrosion, Waterline
corrosion, Crevice corrosion, Stress corrosion)
• Factors affecting corrosion: Nature of the metal and Nature
of Corroding medium.
• Protection from corrosion (Sacrificial protection, Cathodic
protection, Metallic coatings, Organic coatings, Proper
designing.)

There are two factors that influence the rate of corrosion which are
involved in the process of corrosion.
1. Nature of the Metal
• Position of metal in Galvanic series
• Purity of the metal
• Physical state of the metal
• Volatility of Corrosion product
• Nature of oxide film formed
• Solubility of corrosion product formed
• Relative areas of Cathode & anode
• Passive character of the metal
Factors affecting Corrosion

2. Nature of Corroding Environment
• Temperature
• Humidity in air
• Effect of pH
• Presence of impurities in atmosphere
• Conductance of the electrolytic medium
• Formation of Oxygen concentration cell
• Presence of Suspended particles in atmosphere
Factors affecting Corrosion

a. Position of metal in Galvanic series:- The extent of galvanic
corrosion depends on the position of the metals in galvanic
series. The metal which is placed at higher position in the series
are more reactive and undergoes corrosion. The rate and
severity of corrosion, depend upon the difference in their
position and greater is the difference, the faster is the corrosion
of the anodic metal.
b. Purity of the metal:- Impurities in a metal cause heterogeneity
and forms tiny/minute electrochemical cells (at the exposed
part) and the anodic part gets corroded. Generally, pure metal
do not undergo any type of electrochemical corrosion. Hence,
more the impurity greater is the rate of corrosion.
Cont ………

c. Physical state of the metal:- Rate of corrosion is influenced by
the physical state of the metal (such as grain size, orientation of
crystals, stress etc). The smaller the grain size of the metal,
higher will be its solubility and greater will be its corrosion.
However, area under stress even in a pure metal, tend to be
anodic and corrosion takes place at these stressed areas.
d. Volatility of Corrosion product:- Higher the volatile nature of the
corrosion product in case of dry corrosion, greater is the rate of
corrosion such as in the case of Mo form volatile oxide layer
(MoO3), Sn forming volatile film of SnCl4 when exposed to Cl2.
Cont ………

e. Nature of oxide film formed:- In aerated atmosphere, practically all
the metals get covered with a thin surface of film of metal oxide. The
ratio of the volume of the metal oxide to the metal is known as
specific volume ratio. Greater the specific volume ratio, lesser is the
oxidation corrosion rate. The specific volume ratio of Al, Ni and Cr are
1.3, 1.6 and 2.0 respectively and consequently the rate of oxidation of
Cr is less as compared to Al and Ni.
f. Solubility of corrosion product formed:- In electrochemical corrosion,
the solubility of the corrosion product in the corroding medium is a
deciding factor for the extent and rate of corrosion. If the corrosion
product is soluble in the corroding medium, corrosion of the metal
takes place at a higher rate. But if the corrosion product is insoluble in
the corroding medium, it forms a protective layer on the metal
surface and inhibits further corrosion of the metal (PbSO4 formed is
insoluble in H2SO4 acts as a barrier when Pb is exposed to H2SO4)
Cont ………

g. Relative areas of Cathode & anode:- When two dissimilar
metals are electrically connected in the presence of an
electrolytic medium, the corrosion of the anodic part is directly
proportional to the ratio of the areas of the cathodic part and
the anodic part. Corrosion is more rapid, severe and highly
localized, if the anodic area is small because the current density
at a smaller anodic area is much greater and the demand for
electrons can be met by smaller anodic area only by undergoing
corrosion more rapidly.
h. Passive character of the metal:- Higher the passive character of
the metal such as Al, Cr, Pb, Sn, Ti, Ni, Co, Mn etc, lesser is the
rate of corrosion as they exhibit outstanding corrosion
resistance.
Cont ………

a. Temperature:- Rise in temperature increases the rate of corrosion
due to increase in the rate of diffusion of ions.
b. Humidity in air:- The rate of corrosion will be more when the relative
humidity of the environment is high. The moisture acts as a solvent
for oxygen, CO2, SO2 etc in the air to produce the electrolyte which is
required for setting up of an electrochemical cell.
c. Effect of pH:- Lesser the pH of the corroding medium, greater is the
extent of corrosion i.e acidic medium is more corrosive in nature.
d. Presence of impurities in atmosphere:- Atmosphere in industrial
areas contains corrosive gases like CO2, SO2, H2S and vapours of HCl,
H2SO4 etc and in the presence of these gases the acidity as well as
electrical conductivity of the liquid adjacent to the metal surface
increases thereby increasing the rate of corrosion.
Cont ………

e. Conductance of the electrolytic medium:- Higher the electrical
conductivity of the liquid in contact with the metal surface, greater is
the rate of corrosion. Saline environment is more corrosive than normal
environment due to the presence of higher concentration of dissolved
salts in the liquid due to which metals nearby sea area is more corroded.
f. Formation of Oxygen concentration cell:- Formation of oxygen
concentration cell due to difference in oxygen concentration leads to
higher extent of corrosion. The rate of corrosion increases with the
increase in oxygen concentration. In such case, less oxygenated part of
the metal acts as anode while more oxygenated part acts as cathode
leading to the formation of an electrochemical cell.
g. Presence of Suspended particles in atmosphere:- If the suspended
particles are chemically active in nature like NaCl, (NH4)2SO4 etc, they
absorb moisture and acts as strong electrolyte, thereby increasing the
rate of corrosion. Similarly, if the suspended particles are chemically
inactive in nature like Charcoal, they also absorb both sulphur gases and
moisture ans slowly enhances the rate of corrosion.

Protective Measures Against Corrosion

There are many methods of protecting metals against corrosion.
• Barrier protection
• Sacrificial protection
• Cathodic protection
• Alloy formation
Barrier protection:
In this case, a thin barrier is developed between the surface of iron and
atmosphere by one of the following methods:
a) Painting of the metallic surface uniformly.
b) Coating the base metal surface with a thin film of some non
corrosive/passive metals like nickel, chromium, tin, 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 over iron)
• Electroplating (coating of Cu, Ni or Cr over iron with the aid of direct
current.
• Tin plating or Tinning (coating of tin over iron)
• Sheradizing (it consists of dusting of Zn powder over iron surface
followed by heating)
• Cladding (sandwiching the base metal with coating metal)

Alloying the metal:
Metal has better resistance to corrosion after forming alloy with other
metals e.g. Stainless steel, in which ordinary steel is alloyed with
chromium, nickel, cobalt, manganese etc.
Alloys are homogeneous solid solutions in which the components are
completely soluble in one another, e. g Fe – Cr – Ni (Stainless steel), Cu
– Ni alloy (Monel metal), Au – Ag alloy, Ag – Cu alloy (Sterling silver), Cu
– Zn alloy (Brass), Cu – Sn alloy (Bronze), Pb – Sn alloy (Solder), Cu – Sn
– Zn (Gun metal), Al – Cu – Mn – Mg alloy (Duralumin), Fe – Al – Ni – Co
(Alnico) etc.
Solid solution alloys are more corrosion resistant.

Sacrificial Anodic Protection:
In this case, the surface of the base metal (say Iron) is covered with a more
electropositive coating metal like Mg, Zn or Al etc. Since this coating metal looses
electrons more readily than iron, rusting is prevented. As long as the coating metal
is present, iron does not get rusted. This type of protection is called ‘Sacrificial
Anodic Protection’ in which the coating metal acts as anode w. r. t. the base metal
and sacrifices itself to protect the base metal.
Cathodic Protection:
It is the 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 forcing it to act as a cathode. Hence, the method is called
‘Cathodic Protection’.

Metallic Coatings:
• Electroplating: A coating metal is deposited on the base metal by passing
direct current through an electrolytic solution through electrolysis. It is widely
adopted to coat the base metal with a protective metallic coating of Zn, Au, Ag,
Cu, Ni, Pb, Sn etc.
• Metal Cladding: The base metal to be protected and coating metal are
sandwiched by pressing through the rollers under the action of heat & pressure
(Alclad sheeting obtained by cladding a sheet of duralumin with aluminium).
• Hot Dipping: The base metal to be coated is immersed in a bath of the
molten coating metal (such as Galvanizing and Tinning).
• Cementation: A uniform surface coating is obtained by heating the base
metal in a powder of coating metal. This method is limited to the coating of low
melting metals like Zn, Pb, Sn etc. This can be applied to fabricated structure
and there is no possibility of damage.
• Metal spraying: The coating metal in molten state is sprayed on base metal
by means of spraying gun. The base metal surface must be rough. This method
is limited to the coating of low melting metals like Zn, Pb, Sn etc.

Organic Coating:
 Apply on metallic surface for protection from corrosion & also to
impart decorative value such as paints, enamel, Varnishes etc.
Corrosion Inhibitors:
 Substances which when added in a small amount in an environment
reduces the rate of corrosion of a metal exposed to that
environment. These are of two types of inhibitors such as Cathodic
& anodic inhibitors.
Using Pure Metal:
 Impurities in a metal cause heterogeneity, which decrease corrosion
resistance of the metal. Hence corrosion resistance of any metal is
improved by increasing its purity.
Using Alloys:
 Corrosion resistance of most metals is best increased by alloying
them with suitable elements. For maximum corrosion resistance,
the alloy should be completely homogeneous.

Organic Coating & its constituents:
Organic coatings are inert barriers applied on metallic surfaces and other construction
material for both corrosion protection and decoration. The most important organic
surface coating is paint.
Paint is a mechanical dispersion of mixture of one or more pigments in a vehicle. This
vehicle is a liquid consisting of non-volatile film forming material and a volatile solvent
called as thinner.
Pigment:- It is a solid substance, which provide colour to the paint. It is also used to
improve the strength and adhesion of the paint, protect against corrosion. It imparts
impermeability to moisture and increases weather resistance.
Common Pigment Colour
White lead, Zinc oxide, lithopone White
Red lead, ferric oxide, Chrome red Red
Chromium oxide Green
Prussian blue Blue
Carbon black Black
Umber Brown Brown

Vehicle (or) drying oil:- It is a film forming constituent of paint. These are the
glyceryl esters of high molecular weight fatty acids. This vehicle or binder
provides desired chemical and physical properties. It determines the adhesion,
cohesion and flexibility of the paint. The most widely used drying oils are linseed
oil, soybean oil, tung oil and dehydrated castor oil.
Thinner:- It reduces the viscosity of the paint to a suitable consistency, suspends
the pigments, dissolves the vehicle and other additives. It increases the
penetration power of vehicle and elasticity of the paint film. It also helps in drying
of the paint as it evaporates easily. Common thinners used are turpentine,
mineral spirits, acetone, benzene, naphtha, toluene, xylene, kerosene, methyl
Ethyl ketone, dimethyl formamide, methylated naphthalene etc.
Driers:- These are the oxygen carrier catalysts. They accelerate the drying of the
oil film through oxidation, polymerization and condensation. The main function of
the drier is to improve the drying quality of the oil film. Common driers used are
resinates, linoleates, tungstates and naphthenates of Co, Cu, Fe, Mn, Pb and Zn.

Filler (or) Extender:- These are often colorless inorganic substances like gypsum
aluminium silicate, barium carbonate, barium sulphate, asbestos, clay, mica,
calcium carbonate, magnesium silicate etc added to the paints with the aim to
improve the Strength, toughness, abrasion resistance and adhesion of the paint
and also to reduce the cost. It also acts as carriers for the pigment color and fill the
voids in the paint film, reduce the cracking of the paint film and improve the
durability of the film.
Plasticizers:- They remain permanently in paints and improves the elasticity of the
paint film which prevents cracking of the film. Commonly used plasticizers are
tricresyl phosphate, triphenyl phosphate, di butyl phthalate etc.
Anti – Skinning Agent:- Anti – skinning agents like oximes, quinones, naphthols,
aromatic amines, polyhydric phenols etc are added to the paint so that skinning
of paint can be prevented when stored for a significant period of time in the
container and can be used for a long period of time.

Corrosion Inhibitors:
Substances which when added in a small amount in an environment reduces the rate of corrosion of a
metal exposed to that environment. There are two types of inhibitors as anodic and cathodic
inhibitors.
Anodic inhibitors:- Anodic inhibitors stop the corrosion reaction, occurring at anode, by forming a
precipitate with a newly produced metal ion. These are adsorbed on the metal surface in the form of a
protective film or barrier. Examples:- Chromates, Phosphates, Silicates, Tungstates and other transition
metal ions with high oxygen content.
Cathodic inhibitors:- a) In acidic solutions, the cathodic reaction is evolution of hydrogen.
2H+ + 2e- → H2(g)
Corrosion may be reduced either by slowing down the diffusion of hydrated H+ ions to the cathode. The
diffusion of H+ ions is considerably decreased by organic inhibitors like amines, mercaptans or thiol,
heterocyclic nitrogen compounds, substituted urea and thiourea etc.
b) In neutral solutions, the cathodic reaction is the absorption of Oxygen.
1/2O2 + H2O + 2e– → 2OH–
Corrosion is controlled either by eliminating oxygen from the corroding medium or by retarding its
diffusion to the cathodic area. The oxygen is eliminated either by reducing agents like Na2SO3, Na2S,
NH2NH2 or by deaeration. The inhibitors like Mg, Zn or Ni salts tend to retard the diffusion of OH- ions to
cathodic areas.

By Modifying Environment:
• By lowering the temperature.
• By reducing the moisture content.
• By reducing the acidity of the corroding medium or environment.
Electrochemical Protection or Cathodic Protection:
• Sacrificial 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 to an insoluble anode. The current supplied is in
opposite direction to the corrosion current. Thus, the metal to be protected
act as cathode & get protected.

Electroplating:
The process of depositing or coating a metal on the surface of base metal/
non metal by electrolysis is called electroplating. It is widely adopted to coat
base metals with protective metallic coatings of Zn, Au, Ag, Cu, Ni, Pb, Sn etc.
Process:- The base metal surface is cleaned thoroughly. The article to be
electroplated is made as cathode. The anode is made of pure metal, which is
to be coated on the article. The electrolyte is the salt of the metal to be
coated on the article. A direct current is passed through the electrolyte. The
anode dissolves, depositing the metal ions from the solution on the article at
cathode in the form of a fine thin metallic coating.
Ex: Electroplating of Silver on Iron:
Cathode:- Article to be electroplated (spoon of Fe)
Anode:- A block of Silver (Ag) metal
Electrolyte:- Aqueous solution of AgNO3

Image of Electroplaing Process

Proper Designing:
The design of the material should be such that corrosion, even if it occurs, is
uniform and does not result in intense and localized corrosion”.
• Avoid the direct contact of dissimilar metals in presence of a liquid medium.
• 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.
• If two dissimilar metals in contact have to be used, they should be as close as
possible to each other in the electrochemical series.
• Whenever the direct joining of dissimilar metals is unavoidable, an insulating
fitting like wood, glass, rubber, plastic etc may be applied in between them to
avoid the direct metal to metal contact.
• The anodic metal should not be painted or coated, when in contact with a
dissimilar cathodic metal in the presence of a liquid medium.
• Uniform flow of the corroding liquid is desirable.

Proper Designing:
• A proper design should avoid the presence of crevices between adjacent
parts of structure, even in case of the same metal, since crevices permit
concentration differences.
• Sharp corners, bends and recesses should be avoided, as they are favorable
for the formation of stagnant areas and accumulation of solids.
• The equipment should be supported on legs to allow free circulation of air
and prevent the formation of stagnant pools or damp areas.

Proper Designing:

Metallic Coatings
A metal (say Zn) is coated on the base metal (say Fe) so as to prevent corrosion.
Metallic Coatings are of two types.
 Anodic coatings:- These are produced from coating metals, which are “anodic”
to the base metal. This provides the complete protection to the underlying
base metal as long as the coating remain in contact with the base metal.
However, if any corrosion occurs, it will be concentrated on the coating metal
as it is anodic to the base metal to be protected (becomes cathodic) due to
setting up of a galvanic cell.
Example:- In case of galvanized steel, Zinc, the coating metal being anodic is
attacked; leaving the behind the cathodic metal (iron) unattacked and
protected.
 Cathodic coatings:- These are obtained by coating a more noble metal having
higher electrode potential than the base metal to be coated. The cathodic
coating provides effective protection to the base metal only when they are
completely continuous and free from any pores, breaks or discontinuities.
Example:- Coating of Tin on Iron is a cathodic coating.

Methods of applying metallic coatings
Hot dipping:
It is used for producing a coating of low melting metal such as Zn, Sn,
Pb, Al etc. on iron, steel and copper, which have relatively higher
melting points.
Melting point:- Zn – 4190C, Sn – 2320C, Pb – 3270C, Al – 6590C
Melting point:- Cu – 10840C, Fe – 15400C
The process consists of immersing the base metal in a bath of the
molten coating metal, covered by a molten flux layer (usually ZnCl2 or
NH4Cl). 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.
The two most widely applied hot dipping methods are:
Galvanizing and Tinning

Galvanizing:
It the process of coating iron or steel sheets with a thin coat of Zinc to
prevent the sheets from rusting.
The base metal sheet of iron or steel is cleaned by acid pickling method
with dilute Sulphuric acid for 15 – 20 minutes at 60 – 900C which
removes any scale, rust or impurities and then washed and dried. It is
then dipped in a bath of molten zinc maintained at a temperature of
425 – 4300C and after taking out of bath it is passed between hot rollers
to remove excess zinc to produce a uniform coating and then annealed
(slow cooling). During the process the surface of the bath is covered
with flux (NH4Cl) to prevent any oxide formation.

Tinning:
The process of coating metallic tin over the iron or steel articles is
called tinning. The surface the base metal i.e., iron sheet is cleaned by
acid pickling with dilute Sulphuric acid and passed through a bath of
ZnCl2 flux which prevents any oxide formation and helps the molten
metal to adhere to the iron metal sheet surface. Then the sheet is
passed through the molten tin bath maintained at 250 – 2900C and
pressed between two rollers with a layer of palm oil. The oil will help to
protect the tin coated layer from any oxidation. The rollers also remove
excess tin and produce a uniform coating.
Tin metal possesses good resistance against atmospheric corrosion and
is non toxic. Hence such containers can be safely used for storing food
material.

Metal Cladding
It is the process by which a dense, homogeneous layer of coating
metal is bonded (cladded) firmly and permanently to the base metal
on one or both sides. The choice of the cladding metal depends on
the corrosion resistance required for any particular environment.
Here, the metal to be protected is sandwiched between the two
layers of the protecting metal. The whole combination is pressed by
rollers under the action of heat and pressure.
The cladding materials generally used are corrosion resistant such as
Al, Ni, Cu, Pb, Pt, Ag etc. The base materials on which cladding is
done are Fe, Al, mild steel, Ni and their alloys etc. This method is
widely adopted in air craft and automobile industry for the
manufacture of outer body parts.

Image of Metal Cladding Process

Anodic & Cathodic Coating Comparison
Sl No Anodic Coating Cathodic Coating
1
It protects the base metal sacrificially due
to more electropositive character of the
coating metal.
It protects the base metal due to high
corrosion resistance & noble behavior of the
coating metal.
2
Coating metal is at lower potential than the
base metal.
Coating metal is at higher potential than the
base metal.
3
Corrosion of base metal does not increase
even on breaking of the coating as it heals
its film.
Corrosion of base metal increases, if there is a
break in the coating.
4
Galvanizing i.e. coating of Zn coating
iron/steel is an example of anodic coating.
Tinning i.e. coating of Sn coating iron/steel is
an example of cathodic coating.

Comparison: Galvanizing & Tinning
Sl No Galvanizing Tinning
1
Process of covering iron or steel, with a thin coating
of Zinc to prevent it from corrosion.
Process of covering iron or steel with a thin coating of
Tin to prevent it from corrosion.
2
Zinc protects iron sacrificially Since it is more electro
positive than iron and does not permit iron to pass
into the solution.
Tin protects the base metal iron from corrosion due to
its noble or passive nature and higher corrosion
resistance.
3
In galvanized articles, Zinc continues to protect the
underlying iron by galvanic cell action, even if the
coating of Zinc is broken at any place still Zinc will
undergo corrosion and protects iron.
[Fe (- 0.44V & Zn (- 0.76)]
Tin protects underlying iron till the coat is intact and
continuous. Any break in coating causes rapid
corrosion of iron due to lower reduction potential.
[Fe (- 0.44V & Sn (- 0.14)]
4
Galvanized containers cannot be used for storing
acidic food stuffs as zinc reacts with acidic food
forming poisonous compounds.
Tin coated containers and utensils can be used for
storing any food stuff as tin is non-toxic and protects
metal from corrosion.
5
Ideal temperature of operation is around 4500
C.
(Melting point of Zinc is 4190
C)
Ideal temperature of operation is around 2500
C.
(Melting point of Zinc is 2320
C)
6
Ammonium chloride is used as flux which helps in
prevention of formation of any oxide on the parent
metal surface and help the molten coating metal to
adhere to the parent metal.
Zinc chloride is used as flux which helps in prevention
of formation of any oxide on the parent metal surface
and help the molten coating metal to adhere to the
parent metal.

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.

Factors affecting corrosion & control measures

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