for basic chemical engineering-chemistry

1. What is Corrosion?
Corrodere (Latin) – To eat away
• Corrosion may be defined as the gradual destruction
of metals by the chemical or electrochemical reaction
with the environment. During corrosion, the metals
are converted to their metallic compounds at the
surface.
• The loss of materials due to corrosion has become a
great problem. The most common example for
corrosion is the rusting of iron when it is exposed to
atmospheric conditions. The rusting is due to the
formation of hydrated ferric oxide on the surface.
Another example is the formation of green film of
basic copper carbonate on the surface of Cu, when
exposed to moist air containing CO2.
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2. Why study Corrosion?
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1. Materials are Precious resources
2. Engineering design is incomplete without knowledge
of corrosion
3. Applying knowledge of corrosion protection can
minimize disasters
4. Corrosion- may contaminate stored food, dairy
products , etc
5. Corrosion products cause pollution
6. Artificial implants for the human body ?
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3. Types of Corrosion
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1. Direct Chemical Corrosion or Dry Corrosion
This type of corrosion occurs mainly through the direct chemical
action of atmospheric gases such as O2, halogens, H2S, CO2,
SO2, N2, H2 or liquid metals on metal surface in the absence of
moisture.
2. Electrochemical Corrosion or Wet Corrosion
This type of corrosion occurs when :
a. A metal is in contact with a conducting liquid
b.Two dissimilar metals or alloys are immersed partially in a
conducting solution.
This corrosion is due to the existence of separate anodic and
cathodic areas between which current flows through the
conducting solution.
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4. Direct Chemical Corrosion or Dry
Corrosion
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and other gases
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According to electrochemical theory, corrosion of metals
occurs due to the following changes, when they are exposed to
the environment.
1) A large number of minute galvanic cells are formed which
acts as anodic and cathodic areas.
2) At anode the metal undergoes oxidation and electrons are
liberated which migrates towards cathodic region
3) Oxygen of the atmosphere undergoes reduction at cathodic
area in the presence of moisture forming hydroxyl ions at the
cathode.
Electrochemical Corrosion or Wet Corrosion
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6. Types of Electrochemical Corrosion
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1. Galvanic corrosion or Bimetallic corrosion
When two dissimilar metals or alloys are electrically connected and
exposed to an electrolyte, the metal higher in electrochemical
series under goes corrosion. Thus when Zn and Cu are connected, Zn
being higher in the series act as anode and undergoes corrosion and
Cu which is lower in the series act as cathode and gets protected.
Ex: Steel pipe connected to
copper plumbing and
Lead – antimony solder
around copper wire.
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7. Pitting Corrosion
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Pitting corrosion is a localized accelerated attack resulting in
the formation of pin holes, pits and cavities on the metal
surface. It is due to the breakdown or cracking of the
protective film on the metal at specified points. This gives rise
to the formation of small anodic and large cathodic areas. Once
a small pit is formed the rate of corrosion will be increased.
Presence of external impurities like sand, dust, water drops
etc on the surface of the metal can also be a cause for this
type of corrosion. In this case, the small part below the
impurity acts as the anodic area while the rest of the metal
acts as the cathode area. Due to corrosion a small pit is
formed at the anodic area which grows gradually.

8. Concentration Cell Corrosion
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This type of corrosion is due to electrochemical
attack on the metal surface exposed to an
electrolyte of varying concentrations. Metal in
contact with lower concentration will act as anode
and undergoes corrosion. It is observed in chemical
plants and storage tanks and also in marine
structures like ships. Differential aeration corrosion
is the most important.
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9. Water Line Corrosion
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It is another type of differential aeration corrosion. Corrosion in
storage tanks, H2O tanks, marine structures etc is called water
line corrosion. When water is stored in an iron tank, it is found
that maximum corrosion occurs along a line just below the water
level. The area above the H2O line is highly oxygenated and acts
as cathode while the area just below the H2O line is less
oxygenated and acts as anode and undergoes corrosion.
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The important methods used to control corrosion are,
1.Design and selection of the materials.
2. Protective coatings.
a) Organic coatings – paints and enamels.
b) Inorganic coatings
i) Metal coatings – anodic and cathodic
ii) Surface conversion coatings – anodizing, phosphating.
3) Corrosion inhibitors.
4) Cathodic protection.
5) Anodic protection.
Corrosion Control or Protection from
Corrosion
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11. Inorganic Coatings
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i) Metal coatings – anodic and Cathodic
ii) Surface conversion coatings – anodizing, phosphating.
i) Metal coatings
Deposition of protective metal over the surface of base
metal(metal to be procted from corrosion) is known as metallic
coatings. It is divided into Anodic and Cathodic metal coatings.
a) Anodic metal coatings:
Anodic metal coatings involve coating the base metal with more
active metals, which are anodic to the base metal.
Example: Galvanization
b) Cathodic coatings: Cathodic coatings involve coating a base metal
with more noble metals, which are cathodic to the base metal.
Metals such as Copper, Nickel, Tin, and Silver etc are coated on
Iron. One of the disadvantage of Cathodic coatings is if coating
ruptures it leads more corrosion because of small anodic area and
large cathodic area. Example: Tinning

12. Surface Conversion Coatings
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Surface conversion coatings are chemical conversion coatings. The
surface layer of the base metal is converted into a compound by
chemical or electrochemical reactions, which prevents the base
metal form corrosion. The coating can be done by chemical dip,
spray or by electrolytic method. The coating helps in the increased
electrical insulation, enhanced adherence for paints and prevention
of corrosion.
Example: Anodizing , Phosphating
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13. Explain the Process of Galvanization
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It is a process of coating the base metal surface with Zinc, tin,
lead, or aluminium metal. Example: coating Zinc on Iron by hot
dipping Method. It involves the following steps.
• The Iron metal surface is washed with organic solvents to
remove oil, grease etc content on the metal surface.
• Then the metal is passed through dilute sulphuric acid to remove
rust and other depositions. Finally the metal is washed with water
and dried.
• The metal is then dipped in molten Zinc and passed through
Ammonium chloride and Zinc chloride flux to prevent oxidation of
Zinc. The excess Zinc is removed by passing through the rollers or
by wiping.
• Uses: Galvanisation is used for roofing sheets, buckets, bolts,
nuts, nails, pipes etc.

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Explain the Process of Tinning
Tinning is a process of coating the base metal with Tin (Sn). It is
carried out by hot dipping method as fallows.
• The base metal surface is washed with organic solvents to
remove oil, grease etc content on the metal surface.
• Then the metal is passed through dilute sulphuric acid to remove
rust and other depositions. Finally the metal is washed with water
and dried.
• The metal is passed through Ammonium chloride and Zinc
chloride flux and then dipped in molten Tin. Finally it is dipped in
palm oil to prevent oxidation of Tin. The excess Tin is removed by
passing through the rollers or by wiping.
Tinning is widely used for coating steel, Cu and brass sheets which
are used for making containers for storing food studs, oils,
kerosene & packing food materials. Tinned Cu sheets are used for
making cooking utensils & refrigeration equipments.

15. Corrosion Control or Protection from
Corrosion
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Cathodic Protection
In this method, the corroding metal is forced to behave like a
cathode. There are two types of cathodic protection.
a. Sacrificial Anodic protection on Galvanic protection
In this method, the metallic structure which is to be protected
from corrosion is connected to a more anodic metal by a wire so
that the entire corrosion is concentrated on this more active
metal. The more active metal loses electrons and get corroded
and this metal is called sacrificial anode. Metals commonly
employed as sacrificial anode are Mg, Zn, Al and their alloys.
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16. Sacrificial Anodic protection on Galvanic
protection
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Applications
Important applications of sacrificial anodic method include
protection of buried pipe lines, underground cables, marine
structures etc.
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17. Impressed Current Cathodic Protection
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In this method, an impressed current is applied in the opposite
direction to nullify corrosion current so as to convert the
corroding metal from anode to cathode. Impressed current can
be derived from a direct current source like battery. An inert
or insoluble electrode like graphite or silica act as anode to
complete the circuit. The surroundings of anode should be
filled with salts and carbon to increased the conductivity.
Applications
This type of cathodic protection
has been applied to water coolers,
water tanks, buried oil and
water pipes, transmission towers
etc.
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18. Use of Corrosion Inhibitors
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Chemicals which are added in small quantities to the corroding
medium in order to reduce the corrosion rate are called corrosion
inhibitors. They reduce corrosion by forming a protective film
either at the cathode or anode. Thus there are two types of
corrosion inhibitors – anodic inhibitors and cathodic inhibitors.
Anodic inhibitors
Chromates (CrO4), phosphate (PO4), and Tungstates (WO4)
transition metals are used as anodic inhibitors. They react with
the newly produced metal ions at the anode forming a protective
film or barrier there by preventing further corrosion.
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Cathodic Inhibitors
These are the substance, which slow down the cathodic reaction.
The cathodic reactions involve liberation of hydrogen in acidic
solution or OH-ions in alkaline and neutral medium. The cathodic
organic inhibitors include amines, thiourea, sulphoxides etc. The
two types of cathodic inhibition reactions are liberation of
hydrogen, absorption of oxygen and formation of hydroxyl ions.
i)Inhibition of Oxygen absorption and Hydroxyl ions
ii)Inhibition of Hydrogen liberation
Use of Corrosion Inhibitors
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20. Cathodic Inhibitors
i) Inhibition of oxygen absorption and hydroxyl ions
H2O + ½ O2 + 2 e-
2 OH-
The formation of OH-
ions can be prevented either by removing O2
from the medium or by decreasing the diffusion of O2 in to the
cathode. O2 is removed either by adding reducing agents like
Na2 SO3, N2H4 etc or by mechanical dearation.
O2 + NH2 – NH2 N2 + 2H2O
2 Na2 SO3 + O2 2Na2SO4
Salts of Zn, Mg or Ni are added to the corroding medium to reduce
the diffusion of O2 towards cathode. These salts react with OH-
ions at the cathode forming insoluble hydroxides which are
adsorbed at the cathode. 2020

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Cathodic Inhibitors
ii) Inhibition of Hydrogen Liberation
2H+
+ 2 e- H2 (g)
Evolution of H2 can be prevented by slowing down the diffusion of
H+
ions to the cathode or by increasing H2 over voltage. Diffusion
of H+
ions can be prevented by adding organic inhibitors such as
amines, urea, thiourea etc. These are adsorbed at the surface as
a film. Arsenic oxide or antimony oxide is added to increase the
H2 over voltage. These oxides form adherent film of metallic
arsenic or antimony at the cathodic areas.
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