Corrosion & its control measures

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

• Metallic Corrosion: Definition, types and examples.
• Mechanism of Dry or Chemical corrosion.
• Mechanism of Wet or Electrochemical Corrosion.
• Distinction between Dry and Wet corrosion.
• Types of Electrochemical Corrosion: Differential aeration corrosion,
Galvanic corrosion, Waterline corrosion, Pitting corrosion, Crevice
corrosion, Stress corrosion.
• Factors affecting corrosion: Nature of the metal and Nature of
Corroding medium.
• Corrosion control measures: Sacrificial protection, Cathodic
protection, Metallic coatings, Electroplating, Hot dipping, Cladding,
Organic coatings, Proper designing, Distinction between Anodic
and Cathodic coating, Comparison between Galvanizing and
Tinning.

Corrosion can be 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, strength and also causes loss of
useful properties such as malleability, ductility, electrical conductivity and optical
refractivity.
Or
Corrosion can be defined as “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 at its surface is called
corrosion”.
Thus corrosion is a reverse process of extraction of metals.
Except a few metals such as Gold, Silver and Platinum (called noble metal), other
metals are prone to corrosion.
Typical examples of Corrosion are
• Rusting of iron due to formation of hydrated ferric oxide (Fe2O3. 3H2O).
• Tarnishing of silver wares in H2S laden air due to formation of silver sulphide.
• Formation of green film of basic carbonate- [CuCO3 + Cu(OH)2] on the surface of
copper when exposed to moist air containing CO2.

Formation of Iron oxide:
Fe → Fe2+ + 2e– (oxidation)
1/2O2 + 2e– → O2– (reduction)
Overall reaction:- Fe + 1/2O2 → Fe2+ + O2–
Or, 2Fe + O2 → 2Fe2+ + 2O2– → 2FeO
Similarly, in excess supply of oxygen, 4Fe + 3O2 → 2Fe2O3
Rusting of Iron in presence of Electrolyte:-
1/2O2 + H2O + 2e– → 2OH– (reduction)
Overall: Fe + 1/2O2 + H2O → Fe2+ + 2OH– → Fe(OH)2
Or, 2Fe + O2 + 2H2O → 2Fe2+ + 4OH– → 2Fe(OH)2
In the presence of excess Oxygen:
4Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3 or 2Fe2O3. 3H2O
General formula of Rust: Fe2O3. xH2O, x = 2 to 5
Corrosion of Copper when exposed to moist air containing CO2
2Cu + O2 + H2O + CO2 → CuCO3.Cu(OH)2
(Green protective coating)

Metals are electropositive in nature. Except a few metals like Gold, Silver and
Platinum (noble metal), other metals are found in nature as their compounds
(such as oxides, hydroxides, carbonates, bicarbonates, chlorides, nitrates,
sulphates, sulphides, phosphates, silicates etc.) which are called as 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 or ore.
So, metals have a tendency to converted back to the ionic state (ore) and hence
metal atoms are prone to get attacked by environment. This is the main reason for
corrosion of metals.
The mechanism of Corrosion of metals involve the concept of Redox reaction.
Metal
(M)
Mineral
or
ore (Mn+
)
Corrosion
product
(Mn+
)
Extraction
by reduction
+ ne
Corrosion
-ne

Types of Corrosion
 Dry or Chemical Corrosion
 Wet or Electrochemical
Corrosion

Dry or Chemical Corrosion: It involves direct chemical
attack of atmospheric gases like CO2, O2, H2S, SO2, halogens,
and inorganic acid vapours on exposed metallic surface.
Example:- Tarnishing of silver ware in H2S laden air.
Wet or Electrochemical 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 or
an electrolytic medium. Generally, impurity (more active
metal) acts as anode and original metal acts 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.
• It involves the process of adsorption.
• Corrosion occurs uniformly on the entire exposed metallic surface.
• It occurs on both homo and heterogeneous metallic surface.
• Both oxidation and reduction occurs at the exposed metallic
surface without the formation of anodic and cathodic area.
• Extent of dry corrosion depends on the nature of the layer formed
on the metallic surface and also on the attraction or affinity
between the gas and the exposed metal.
 Types
• Oxidation corrosion due to Oxygen
• Corrosion by other atmospheric gases
• Liquid Metal Corrosion

Types of dry corrosion
Oxidation corrosion
 Occurs
 Due to direct chemical reaction of atmospheric O2 with metal surface forming
metal oxide. It involves the process of adsorption of Oxygen on metallic surface.
 It occurs in the absence of moisture or any electrolytic medium.
 Increases with increase in temperature and may take place at low or high
temperature.
 Extent of Oxidation corrosion depends on the nature of oxide layer formed on the
metallic surface.
 Mechanism
 On exposure to atm., metal gets oxidized to form metal ions.
(i) 2M(s) → 2M+n + 2ne- (Oxidation)
 Electrons lost by the metal are taken up by oxygen to forms oxide ions.
(ii) n/2O2(g) + 2ne- → nO2-(Reduction)
Overall reaction:
2M + n/2O2 → 2M+n + nO2- → M2On
Metal Oxide

Mechanism of Dry or Chemical Corrosion

Nature of metal oxide layer
 Types of Oxide Layers formed
 Stable layer: - Al, Pb, Cu, Cr, Ni, Mn, Sn etc. (Negligible corrosion)
 Unstable layer:- Ag, Au, Pt. (No oxidation corrosion)
 Volatile layer:- Mo (as MoO3). (Excess corrosion)
 Porous layer:- Alkali metals & Alkaline earth metals. (Excess corrosion)
 Pilling – Bedworth 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 (R) = Volume of oxide layer formed
Volume of parent metal exposed
If, R ≥ 1, the oxide layer is said to be protective and non porous.
If, R < 1, the oxide layer is said to be non - protective and porous.

Types of dry corrosion
Corrosion by Other Gases
2Ag + Cl2 → 2AgCl
(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 and the solid metal or alloy usually gets weakened.
• Observed in nuclear reactors where Na metal used as a coolant leads to
corrosion of Cd.
• In such type of corrosion, either the liquid metal dissolves the solid metal
surface or penetrates into the solid surface and weakens the bond.

Passivity or Passivation
Passivity or Passivation is the phenomenon in which a metal or an
alloy exhibits much higher corrosion resistance than expected from
its position in the electrochemical series. Passivity is the result of the
formation of a highly protective but very thin (about 0.0004 mm
thick) and quite invisible film on the surface of metal or an alloy
which make it more noble. This thin film is invisible, non porous and
of such a Self healing nature that when broken, it repairs itself on
exposure to the oxidising environment.
Examples of such passive metals and alloys are: Ti, Al, Cr, Ni, Co, Mn,
Pb, Cu and stainless steel alloys containing Cr (about 13 – 25%) etc.
Outstanding corrosion resistance exhibited by various surgical
instruments is due to the presence of Cr (about 13%) in it.
Similarly, Al is not attacked by Conc. HNO3 due to passivity while Fe is
easily attacked by even dilute HNO3.

Rusting of Iron in presence of Electrolyte:-
1/2O2 + H2O + 2e– → 2OH– (reduction)
Overall:- Fe + 1/2O2 + H2O → Fe2+ + 2OH– → Fe(OH)2
Or, 2Fe + O2 + 2H2O → 2Fe2+ + 4OH– → 2Fe(OH)2
In the presence of excess Oxygen:-
Metals when exposed to the acidic solutions:
4Al + 3H2SO4 → 2Al2(SO4)3 + 3H2
Fe + H2SO4 → FeSO4 + H2
Zn + H2SO4 → ZnSO4 + H2
Zn + 2HCl → ZnCl2 + H2
2Al + 6HCl→ 2AlCl3 + 3H2

Wet or Electrochemical Corrosion
 Occurs:-
• When a metal is in contact with moist air or any electrolyte or liquid medium.
• When the parts of a metal become differently aerated or exposed to different concentration of
Oxygen.
• When two different metals are partially immersed in a solution or their respective salt solutions.
• Chemically non uniform surfaces of metals behave like electrochemical cells in the presence of
water containing dissolved O2 & CO2.
• When a metal is stressed at sharp corners, bends, pits, crevices, rivets, joints etc and exposed to a
specific electrolytic medium.
• Corrosion occurs at anodic areas only while cathodic area is said to be protected.
 Mechanism:-
• Involves Oxidation – Reduction or Red - ox reaction taking place separately at anode and cathode
in contact with each other.
• Corrosion occurs through the mechanism of electrochemical reaction.
• It is a fast process and occurs on heterogeneous metallic surface only.
• Electrons move from anode to cathode through the metal or through the corrosive medium.
• Depending on the nature of corroding environment, electrons released at anode are consumed at
the cathodic area by two ways:
 Evolution of H2 type cathodic reaction
 Absorption of O2 type cathodic reaction

Evolution of H2 type Cathodic reaction
Mechanism:-
• Occurs when a metal is exposed to or comes in contact with an acidic medium.
• Metallic part at anodic area undergo dissolution with the release of electrons.
• Electrons flow through the metal from anode to the cathode, where H+ ions of
the acidic solution are liberated as H2.
• Such corrosion involves displacement of H+ ions from the acidic solution by the
metal ions due to which pH of the acidic solution increases. Hence all the metals
above the hydrogen in Electrochemical Series have a tendency to get dissolved in
acidic solutions with evolution of hydrogen.
• In such type of Corrosion, anode is usually large metallic area where as cathode
is small metallic area.
Reactions:- Fe(s) + 2HCl → FeCl2 + H2(g)
Reaction at anode:- Fe(s) → Fe+2 + 2e- (Oxidation)
Reaction at cathode:- 2H+ + 2e- → H2(g) (Reduction)
Overall Reaction:- Fe(s) + 2H+ → Fe+2 + H2(g)

Mechanism of Corrosion by H2 Evolution

Absorption of O2 type Cathodic reaction
Mechanism:-
• Occurs when a metal is exposed to a basic or neutral solution (such as salt solution) in
the presence of oxygen.
• Due to the presence of oxygen, the surface of the metal is usually coated with a thin
layer of oxide. If the oxide layer develops some cracks, anodic area is created while
rest of the well metal part behaves as cathode.
• Oxidation occurs at the anodic area while reduction occurs at cathode and electrons
flow through the metal from anode to the cathode with the absorption of O2.
• In such type of Corrosion, anode is usually small metallic area where as cathode is
large well metal parts.
Reaction at cathode:-1/2O2 + H2O + 2e– → 2OH– (reduction)
Overall Reaction:-Fe + 1/2O2 + H2O → Fe2+ + 2OH– → Fe(OH)2
Or, 2Fe + O2 + 2H2O → 2Fe(OH)2
In the presence of excess Oxygen:- 4Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3 or 2Fe2O3. 3H2O

Mechanisms of Corrosion by O2 Absorbtion

Difference:- Dry & Wet Corrosion
DRY or CHEMICAL CORROSION WET or ELECTROCHEMICAL CORROSION
It occurs due to direct chemical attack of atmospheric gases on the
exposed metallic surface.
It occurs when a metal is exposed to a conducting liquid or electrolytic medium
or moisture due to the formation of tiny electrochemical cells.
It occurs in dry condition in the absence of any conducting liquid or
electrolytic medium or moisture.
It occurs in wet condition in the presence of any conducting liquid or
electrolytic medium or moisture.
It is a slow process. It is a fast process.
It takes place at low or high temperature depending on the nature of the
metal.
It takes place under normal condition of temperature.
It is a self controlled process. It is a continuous process.
Corrosion occurs through the mechanism of adsorption. Corrosion occurs through the mechanism of electrochemical reaction.
Corrosion occurs on both homogeneous and heterogeneous metallic
surfaces.
Corrosion occurs on heterogeneous metallic surfaces only.
Corrosion on exposed metallic surface is uniform and occurs on the entire
surface exposed.
Corrosion on exposed metallic surface is non uniform, highly localised and
occurs at anode only.
Corrosion does not involve the formation of separate anodic and cathodic
area.
Corrosion involves the formation of separate anodic and cathodic area in
contact with each other.
Corrosion product accumulates uniformly on the exposed metallic surface.
Corrosion occurs at anode but the corrosion product accumulates near the
cathode or somewhere in between anode and cathode.
Extent of corrosion depends on the affinity between the metal and gases
and also on the nature of the product formed.
Extent of corrosion depends on the potential difference between the anode and
cathode and also on the nature of the corroding medium.
Oxidation corrosion is an example of dry or chemical corrosion.
Galvanic or Bimetallic corrosion, Differential aeration corrosion, Pitting
corrosion, Waterline corrosion, Crevice corrosion, Stress corrosion etc are
examples of wet or electrochemical corrosion.

Types of Wet or Electrochemical Corrosion
Wet or Electrochemical Corrosion processes are of the following
types as given below.
 Galvanic or Bimetallic Corrosion
 Concentration Cell or Differential aeration Corrosion
 Waterline Corrosion
 Pitting Corrosion
 Crevice Corrosion
 Stress Corrosion
(a) Season Cracking
(b) Caustic Embrittlement

Galvanic or Bimetallic corrosion
Such type of Corrosion occurs when two different or dissimilar metals having different
electrode potentials (Reduction potential) are electrically in contact with each other in
the presence of a conducting medium or when two different metals are electrically
connected with each other in the presence of their respective salt solutions.
The metal placed higher in position in electrochemical series becomes more active metal
and acts as anode while the other metal placed lower in position in electrochemical
series becomes the more noble/passive metal and acts as cathode.
Hence, the more active metal suffers corrosion while the more noble metal is said to be
protected.
Hence, Galvanic couples are required to be avoided in different applications.
Examples:- Nuts and bolts of the same metal is preffered.
A small steel bolt connected to a copper equipment.
A small steel pipe connected to a large copper tank.
Zn suffers corrosion, when electrically coupled with Cu in the presence of an
electrolytic medium.

Mechanism of Galvanic corrosion
In Galvanic corrosion, more active metal acts as anode and undergoes oxidation while
more noble/passive metal acts as cathode and undergoes reduction. Hence, the more
active metal suffers corrosion while the more noble metal is said to be protected.
Depending on the nature of the conducting medium or electrolytic medium,
the cathodic reaction on the more noble metal may involve Evolution of
Hydrogen type (H2) reaction or Absorption of Oxygen type of reaction (O2).
In Acidic solution:- Evolution of Hydrogen type (H2) reaction
In neutral or slightly alkaline solution:- Absorption of Oxygen type of reaction (O2)
Example of Galvanic Couples:-
i) Zn (- 0.76V) & Cu (+ 0.34V) → Zn undergoes corrosion (more active metal)
ii) Fe (- 0.44V) & Cu (+ 0.34V) → Fe undergoes corrosion (more active metal)
iii) Ni (- 0.24V) & Cu (+ 0.34V) → Ni undergoes corrosion (more active metal)
iv) Zn (- 0.76V) & Fe (- 0.44V) → Zn undergoes corrosion (more active metal)
v) Zn (- 0.76V) & Al (- 1.66V) → Al undergoes corrosion (more active metal)

Minimization of Galvanic corrosion
Galvanic or Bimetallic corrosion can be minimized by adopting the following
methods.
i) By avoiding the use of Galvanic couple in presence of conducting medium.
ii) When the use of Galvanic couple is unavoidable, metals must be as much
close as possible to each other in electrochemical series.
iii) When the direct joining of dissimilar metals is unavoidable, an insulating
material like wood, glass, rubber etc may be used to avoid direct electrical
contact between the metals.
iv) When two dissimilar metals are to be used in contact with an electrolytic
medium, the anodic metal should have as large area as possible and
cathodic metal should have as small area as possible since small anode
and large cathode leads to excessive corrosion.
v) The anodic metal should not be painted or coated when in contact with
cathodic metal since any break in the coating leads to severe localized
galvanic corrosion.

Conc. Cell or Differential Aeration Corrosion
• Concentration cell corrosion occurs due to difference in concentration of
the liquid medium around different parts of the metal.
• Differential aeration corrosion is a type of Concentration cell corrosion
which Occurs due to difference in potential between differently aerated
parts of a metal in the presence of an electrolytic medium.
• Part of the metal exposed to higher concentration of air is the more
oxygenated part & acts as Cathode.
• Part of the metal immersed inside the greater depth of the electrolyte is
the poorly oxygenated part & acts as Anode.
• Electron flows from the anode (poor oxygenated part) to the cathode (more
oxygenated part) through the metal while ions migrate to each other
through the electrolytic medium producing the corrosion product.
• At the anodic area, dissolution of metal occurs due to oxidation.
• At the cathodic area, absorption of oxygen type of reduction reaction
occurs producing hydroxide ions.

Mechanism of Differential Aeration Corrosion
In case of Iron metal
Reaction at cathode:- 1/2O2 + H2O + 2e– → 2OH– (Reduction)
Overall Reaction:- Fe + 1/2O2 + H2O → Fe(OH)2 Or 2Fe + O2 + 2H2O → 2Fe(OH)2
In the presence of excess Oxygen:-
Similarly in case of Zn metal
Reaction at anode:- Zn(s) → Zn+2 + 2e- (Oxidation)
Reaction at cathode:- 1/2O2 + H2O + 2e– → 2OH– (Reduction)
Overall Reaction:- Zn + 1/2O2 + H2O → Zn(OH)2 Or 2Zn + O2 + 2H2O → 2Zn(OH)2

Image of Differential Aeration Corrosion
A metal (say Zn or Fe) partially immersed in a neutral salt solution undergoes
differential aeration corrosion due to potential difference between less oxygenated
part (anode) and more oxygenated part (cathode).

A metal (say Fe) partly covered with few drops of water undergoes differential aeration
corrosion as the portion of the surface covered with water becomes less oxygenated part
(anode) than the uncovered more oxygenated part of the metal (cathode).

A metal (say Fe) partly covered with drops of salt solution undergoes differential aeration
corrosion as the part of the surface covered with salt solution becomes less oxygenated
part (anode) than the uncovered more oxygenated part of the metal (cathode).

A part of metal surface (say Fe) covered with extraneous matter such as dust, dirt, sand,
clay etc undergoes differential aeration corrosion as the part of the surface covered with
the extraneous matter becomes less oxygenated Part (anode) than the uncovered more
oxygenated part of the metal (cathode).

Waterline Corrosion
A steel tank containing water kept stagnant for a long time, undergoes differential aeration
corrosion just below the waterline or water level since the concentration of oxygen in the
greater depth becomes less oxygenated Part (anode) than the portion of the tank just
above the waterline and become more oxygenated part of the metal (cathode).

Pitting Corrosion
Pitting corrosion is a special type of differential aeration corrosion which involves localized
accelerated attack resulting in the formation of cavities around the metal. Pitting corrosion
arises due to the formation of cracks, pinholes, pits and cavities on the metal surface due
to which small anodic (pit or cavity) and large cathodic areas (rest part) are created.

Crevice Corrosion
Crevice corrosion is a type of differential aeration corrosion which involves localized attack
resulting in the formation of inaccessible areas at the junction of two metals. Crevice area
has a lack of oxygen and acts as anode and becomes prone to corrosion while the well
exposed parts become more oxygenated and act as cathode resulting in intense corrosion.

Stress Corrosion
Stress corrosion is a type of electrochemical corrosion which is highly specific
and occurs when a metal is subjected to the combined effect of stress and
exposed to a specific electrolytic medium. Generally, the part of the metal
under stress (such as sharp corners, bends, rivets, pits, joints, crevices etc) acts
as anode while the well metal part acts as cathode.
Pure metals are generally resistant to Stress corrosion but fabricated articles
or fabricated metallic structures like alloys of steel, brass etc undergoes stress
corrosion when exposed to some specific electrolytic medium and under
stress.
Examples:-
Brass alloy (Cu – Zn or Cu – Ni) when exposed to ammonia.
Stainless steel (containing 0.1 – 0.4% carbon) exposed to acid chlorides (HCl).
Mild steel in boilers exposed to caustic alkali (Na2CO3) during softening process of
water by lime – soda process.

Types of Stress Corrosion
Stress corrosion can be of two types.
i) Season Cracking – Occurs in Brass exposed to ammonia.
ii) Caustic Embrittlement – Occurs in mild Steel in water softening boilers
exposed to caustic alkali like (Na2CO3) used in Lime – Soda process.
Season Cracking:-
This type of corrosion generally refers to the corrosion of copper alloys such as
Brass (Cu – Zn) when exposed to the action of ammonia. When brass is exposed to
ammonia solution, both copper and zinc form complexes by losing electrons in the
solution due to which dissolution of brass occurs at the boundaries and forms
cracks for stress corrosion. Season cracking is characterized by deep brittle cracks
which penetrate into the affected components. If the concentration of ammonia is
very high, then attack is much more severe.
Reactions:-
Zn → Zn+2 + 2e- & Cu → Cu+2 + 2e-
Zn+2 + 4NH3 → [Zn(NH3)4]+2 & Cu+2 + 4NH3 → [Cu(NH3)4]+2

Images of Season Cracking Corrosion

Caustic Embrittlement
Caustic embrittlement is the phenomenon during which the boiler material becomes
brittle due to the accumulation of caustic substances. This type of boiler corrosion is
caused by the use of highly alkaline water in the high pressure boiler and also due to
stress. Water softened by lime soda process may contain NaOH which is formed by the
hydrolysis of Na2CO3.
This type of corrosion generally Occurs in mild Steel material of water softening boilers
exposed to caustic alkali (Na2CO3) used in lime – soda process at high temperature and
pressure. When water of high alkalinity attack the mild steel near the stressed parts like
bends, sharp corners, joints, rivets, hair – cracks etc., iron of the boiler material suffers
corrosion due to the formation of Fe3O4.
Water containing NaOH flows through the small pores in the stressed areas like bends,
joints, sharp corners, hair-cracks etc by capillary action and when water in these places
evaporates at high temperature, the concentration of NaOH increases as compared to
the unstressed parts and it corrodes the iron of the surrounding area by forming Sodium
ferroate (Na2FeO2). This causes embrittlement of boiler materials particularly at stressed
parts causing even failure of the boiler.

Reactions during Caustic Embrittlement
At high temperature and pressure in steel boiler plants
Na2CO3 + H2O → 2NaOH + CO2
Fe + 2NaOH → Na2FeO2 + H2
(Sodium ferroate)
3Na2FeO2 + 4H2O → Fe3O4 + 6NaOH + H2
or 6Na2FeO2 + 6H2O + O2 → 2Fe3O4 + 12NaOH
(Sodium ferroate)
Concentration Cell Presentation of Caustic embrittlement:-
Caustic embrittlement arises due to the setting up of a concentration cell. With the Iron
surrounded by dil. NaOH acting as the Cathode, while the iron surrounded by conc. NaOH
acting as the anode. The iron in the anodic part gets dissolved or corroded.
Fe (under stress)│Concentrated NaOH ‖ Dilute NaOH│Fe (stress free)
(inaccessible area) (main body)
Anode Cathode
Prevention of Caustic embrittlement:-
1. By using Na2SO4 and Na3PO4 in place of Na2CO3 that blocks the minute hair cracks.
2. By maintaining uniform flow rate of boiler water by avoiding inaccessible areas.
3. Using tannin or lignin as additive in boiler water which blocks the hair cracks.

Images of Caustic Embrittlement

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:
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

Organic Coating:
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 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.

Organic Coating & its constituents:
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 de- aeration. 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.

Corrosion & its control measures

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