Corrosion theory and its prevention

1. Power Plant Chemistry
Corrosion Theory & Its Prevention
Prepared By : -
Umar Farooq
Chemist “A” SEC
MSC ( Chemistry ) MBA ( Marketing )
umar7325@yahoo.com
107301@se.com.sa
SEC Shoaiba Jeddah
Saudi Arabia 1

2. Power Plant Chemistry
 Table Of Content
 Corrosion definition
 Corrosion Theory
 Corrosion Mechanism
 Types Of Corrosion
 Factor Of Corrosion
 Corrosion Reaction
 What Is Mean by De polarization
 Effect of Various factor on Corrosion
 Method Of Corrosion Control
 Method Of Corrosion Prevention
 How do Corrosion Inhibitor Work
Different Types of
Corrosion Attack
General Attack Localized or
Pitting Attack
Stress Corrosion
Cracking
Crevic Corrosion
Filiform
Corrosion
Inter granular
Corrosion
Microbial
Corrosion
Galvanic
Corrosion

3. Corrosion
Corrosion may be
defined as the destruction of
metal by chemical or electro
chemical with its environment.
In water system
the principal fact influencing
the corrosion of iron ( Fe )
metal .
 Potential difference
 Dissolved solid ( Electrolyte )
 Depolarizer
 CO2 , Free mineral acidity and
pH are also factor causing
corrosion .
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4. Corrosion Theory
Existence of potential difference with in a metal or
between two metal will cause chemical reaction at the anode and
cathode . for under standing this let us consider a sample cell in
which two electrodes one of iron and another of copper are placed in
water.
When the two electrodes are connected together by a wire a
closed circuit is completed . Electrons will flow from anode to
cathode by wire and from cathode to words by electrolyte .
The production of electrons is due to the following reactions.
Fe Fe ++ + 2e –
At cathode
H2O H+ + OH -
2H + 2e - H2
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5. Corrosion Theory
In the above reaction iron ( Fe ) dissolved in (
H2O ) as ( Fe ) anodic direction. The Fe combine with
OH to form Fe ( OH )2. This process stop after some time
this is due to the formation of a thin film of H2 gas . If O2
is present in water it combine with H at the cathode to
form H2O.
4H + O2 2H2O
It is also present the formation of hydrogen
film or layer over the cathode this is the presence of O2 ,
the corrosion possess enhanced as the anodic or
Cathodic reaction continuous.
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6. Corrosion Theory
Another role of O2 increasing corrosion rate, is that
the corrosion product Fe (OH) 3 are formed by oxidation
of Fe (OH) 3 as shown below.
4Fe (OH) 2 + O2+2H2O 4Fe (OH)3
Basically corrosion is an
electro chemical process. For corrosion reaction to occur
there must be a corrosion cell, consisting of cathode ,
Anode and electrolyte . The electrons of metals flow to
words other point cathode where electron concerning
reaction occur.
The result of this activity is the loss of metal
and after the formation of a deposit.
 Cathode has higher potential
 Anode has lower potential.
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7. Corrosion Mechanism
Simple Corrosion Cell
Corrosion is an electro Chemical
process in which a difference in
electrical potential develops between
two metals or between different parts
of single metal, This voltage can be
measured when a metal is electrically
connected to a standard electrode,
The electrical potential of a metal may
be more or less than a standard. In
which case the voltage is expressed
as either positive or negative . This
difference in potential allows current to
pass through metals causing reaction
at Cathodic and anodic sites. These
sites constitute the corrosion cell.
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8. Types of Corrosion
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The anode is the reign of lower potential and conversely cathode is
the reign of higher potential.
Biological corrosion High temperature corrosion
Cold end corrosion Hydrogen attack corrosion
Cavitations Inter granular corrosion
Dezincification corrosion Stress corrosion
Fatigue corrosion Under deposit corrosion
Pitting corrosion ( Localized
corrosion
Galvanic corrosion
General corrosion ( uniform
corrosion )
Crevice Corrosion
Filiform Corrosion

9. Factor of Corrosion
Dissolved solid electrolyte
 Generally increase in dissolved solid concentration of water makes it
corrosive. Higher solid content increase the conductivity of water. Thus
favoring increase in corrosion . One logical theory involves the effect of ion
in destroying the coating of corrosive production the metal . Higher
concentration of Chloride destroy coating by inhibitors such as chromate.
De Polarizer: Dissolve Oxygen Concentration.
 Oxygen react with hydrogen at the cathode surface forming depolarizing
surface. Thus permitting additional iron to dissolve . The solubility of oxygen
in water depends on the temperature and pressure. Oxygen produce is
identified easily and corrosion produces in the form of small pits or
depression.
pH and free mineral Acidity of oxygen.
 If pH is below 4.3 approximately . The chief controlling factor in promoting
corrosion is pH . In the presence of oxygen pH has no controlling influence
an corrosion rate. In absence of oxygen pH has controlling influence on
corrosion rate.
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10. Corrosion Reaction
Anodic reaction.
 The anodic reaction are typical by
dissolution of metal.
Fe Fe ++ + 2e –
Cathodic reaction.
 In Cathodic reaction evolving electrons
migrate through the metal to cathode ,
where they react in any one of the
several ways. Some typical cathode
reaction are as under .
Typical Cathodic reaction
 Reduction of water occurs in natural
water .
2H2O +2e H2 +2(OH) -
 Hydrogen ion reduction. Important in
acidic solution.
2H + 2e - H2
 Oxygen reduction occurs aerated
acidic solution.
O4+4H+ + 4e 2H2O
 Ferric ion reduction.
Fe +3 +e – Fe +2
 Hence Fe +2 ( Ferrous ion ) produced
at anode combine with OH ions to
form ferrous hydroxide.
Fe + 2(OH)- Fe (OH) 2
 Ferrous hydroxide produced has very
low solubility and is quickly
precipitated as Fe ( OH ) 3 or Fe2O3
as the condition prevail.
Fe (OH) 2 O2+2H2O 4Fe (OH) 2
2Fe (OH) 3 Fe2O3+3H2O
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11. What is mean by depolarization
 It has been noted that some time Cathodic reaction is
reduced due to the formation of thin layer of hydrogen
gas is cathode . Which prevents the flow of electrons
from anode to cathode . This is only possible of oxygen
gas is not present in the media .
H2 + O2 2OH
 Hydrogen gas there fore form the barrier and stop
corrosion reactions. Now if oxygen is present in the
system, it will react with hydrogen gas. And
depolarization occurs. This phenomena is called
depolarization and oxygen gas act as a depolarizer.
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12. Effect of various factors on corrosion
The most important factor are .
 Oxygen and other dissolved gases.
 Dissolve or suspended solid
 Alkalinity or acidity.
 velocity and temperature .
 Microbiological activity ( e.g. algae, fungi, bacteria )
These factors are discussed as .
 Oxygen gas acts as depolarizer and effects directly on
corrosion. It is essential for Cathodic reaction to take
place. However other dissolved gases ( NH3,H2S,SO2 )
effect indirectly on corrosion.
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13. Effect of various factors on corrosion
How do dissolve solid or Suspended solid effect corrosion
 Dissolved solid can effected the corrosion reaction by increasing the
electrical conductivity of water higher the concentration of dissolved
solid greater the conductivity and more like hood of corrosion
.dissolved chlorides and sulphate are practically corrosive by there
corrosive or behavior action they can settle on metal surface to set
localized corrosion cell.
How does water velocity effect corrosion .
 Higher velocity of water increases corrosion by transporting oxygen
to the metal by carrying away corrosion products at faster rate.
Higher velocity can also cause corrosion of mater surface protective
oxide film. When water velocity is low, deposition of suspended solid
can establish localized corrosion cells , there by incensing corrosion
rate.
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14. Effect of various factors on corrosion
How does temperature effect corrosion .
 Below 70C°every 15C° approximately increase in temperature
causes corrosion rate to double above 70C° additional temperature
increases has relatively little effect on corrosion rate in cooling water
system.
How dose acidity or alkalinity effect an corrosion .
 Acidic and slightly alkaline water can dissolve metal and protective
oxide film on metal surface more alkaline water favor the formation
of protective oxide layer.
How does microbial growth effect corrosion .
 Microbial promote the formation of corrosion cells. In addition the by
products of some organisms' such as H2S form anaerobic bacteria
are corrosive.
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15. Different Types of Corrosion attack
General Attack
 When the corrosion is uniformly
distributed over the metal surface.
 The conceder amount of iron
oxide produced by generalized
attack contribute to fouling
problem
 It is recognized by a roughening of
the surface and usually by the
presence of corrosion products
 The mechanism of the attack
typically is an electrochemical
process that takes place at the
surface of the material
.
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General attack corrosion deteriorates a steel lock.
The surface effect produced by most direct
chemical attacks (e.g., as by an acid) is a uniform
etching of the metal.
.

16. Different Types of Corrosion attack
Localized or pitting attack
 Exist when only small areas of
metal surface corrode.
 Pitting is the most serious form of
the corrosion because action is
concentrated in small area
 Pitting may perforate the metal in
a short time.
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Pitting corrosion is localized corrosion that occurs
at microscopic defects on a metal surface. The pits
are often found underneath surface deposits
caused by corrosion product accumulation.

17. Different Types of Corrosion attack
Localized or pitting attack
 Pitting is a very destructive, localized
corrosion that most commonly occurs
in water-filled systems
 Pitting is caused by localized anodic
sites formed at surface cracks,
scratches, or at points of oxygen
differential on the metal surface.
 Pitting is prevalent when there are very
low flow rates. Pitting can be a major
problem because after the pit initially
forms, flow rate in the pit decreases
further allowing for the acidity and the
oxygen differential of the fluid within
the pit to increase
 This increased acidity and oxygen
differential within the pit accelerate the
corrosion process until it becomes self-
sustaining.
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18. Different Types of Corrosion attack
Galvanic attack
 Galvanic corrosion occurs when two dissimilar
metals come in contact with each other. This
corrosion is caused by the potential difference that
develops between the metals. Galvanic corrosion
occurs when the contact between dissimilar metals
is bare metal to bare metal and they are in the
same physical environment.
 A common area of galvanic corrosion is in cooling
systems that contain both steel and copper
alloys. As the copper components corrode, copper
is released into the water. This dissolved copper
plates out on the steel surfaces and induces
galvanic corrosion on the steel.
 This is an area of concern in power plants, which
use copper alloys in the condenser or the feed
water system. Corrosion of copper alloys in the
feed water components will plate out on the inside
of the steel water wall tubes; eventually leading to
water wall tube failures.
Common example in water system are
 Steel & brass , Zinc & brass , Aluminum & steel ,
Zinc & steel ,
 If galvanic attack occurs the metal named first will
corrode
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19. Different Types of Corrosion attack
Stress Corrosion Cracking
 Stress corrosion cracking in stainless steel is
normally seen in the presence of a high
chloride concentration. Ammonia is the main
cause of stress corrosion cracking in brass
components. Crevices and low flow sites are
the most likely locations for stress corrosion
cracking.
 Stress corrosion cracking is a type of localized
corrosion that occurs in a corrosive
environment where metal stress is
present. This type of corrosion causes
cracking of the metal.
 The problem and indications of stress
corrosion cracking are most often seen in
cooling systems that use brass and/or
stainless steel components. These alloys are
more susceptible to this type of corrosion.
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20. Different Types of Corrosion attack
Crevice Corrosion
 Crevice or contact corrosion is the
corrosion produced at the region of
contact of metals with metals or metals
with nonmetals. It may occur at
washers, under barnacles, at sand
grains, under applied protective films,
and at pockets formed by threaded
joints.
 The mechanism of crevice corrosion is
similar to pitting except that the
corrosion takes place in crevices.
 As with the case of pitting, the solution
in the crevice becomes acidic and an
oxygen differential may
develop. Eventually the corrosion
process becomes self-sustaining.
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21. Different Types of Corrosion attack
Microbial corrosion
 Microbial corrosion (also called
microbiologically -influenced corrosion or
MIC) is corrosion that is caused by the
presence and activities of microbes. This
corrosion can take many forms and can be
controlled by biocides or by conventional
corrosion control methods
Filiform Corrosion
 This type of corrosion occurs on painted or
plated surfaces when moisture permeates
the coating. Long branching filaments of
corrosion product extend out from the
original corrosion pit and cause
degradation of the protective coating.
Inter granular corrosion
 Inter granular corrosion is an attack
on or adjacent to the grain
boundaries of a metal or alloy.
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22. Method of Corrosion Control
 Changing the environment of the metal is
a more practical method of corrosion
control. Three methods of changing the
environment to which a metal is exposed
include removing oxygen, using corrosion
inhibitors, or promoting the formation of a
protective scale.
 Removing oxygen from the water is one way
to alter the environment of the
metal. Oxygen maintains and promotes the
cathodic reaction of the corrosion process.
 Metal surfaces in contact with a higher
oxygen concentration will become cathodic
compared to the surfaces in contact with
the lower oxygen concentration. The
surfaces exposed to the lower oxygen
concentration will act as the anode and
suffer accelerated corrosion in what is called
an oxygen concentration cell.
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23. Method of Corrosion Control
 Oxygen removal will normally reduce the
corrosiveness of water. The oxygen may
be removed through de aeration or by
addition of chemicals called oxygen
scavengers.
 Sodium sulfite is a common oxygen
scavenger used with cooling water
systems. For small, closed-loop systems,
this is an acceptable method. However, for
large systems such as circulating water
systems, chemical costs would be too high.
 As a maintenance consideration, once the
deposits become severe enough to cause
problems, the piping and equipment must
generally be cleaned, removed, replaced, or
repaired.
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24. Method of Corrosion Prevention
Corrosion can be prevented or minimized by one
or more of the following methods.
When designing a new system ,
chose corrosion resistance material to minimize the
effect of aggressive environment.
 Adjust pH
 Apply protective coating , such as paints metal plating or
plastics.
 Protect catholically, using sacrificial anodes ( Cathodic
protection )
 Add protective film forming chemicals inhibitors that the
water can be distributed. To all wetted part of the
system.
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25. How do Chemical Corrosion Inhibitors work
Chemical inhibitors reduced or stop corrosion by
interfering with the corrosion mechanism. Inhibitors usually effect
either anode or cathode and so are called as anodic and Cathodic
inhibitors.
Anodic Inhibitors
The inhibitors' which stop or minimize anodic corrosion
are called anodic inhibitors.
Sodium chromate ( Na2CrO4 ) sodium nitrate ( NaNO2 )
Potassium nitrate, poly phosphate , all hydroxide ( NaOH , KOH )
etc.
Cathodic Inhibitors.
The inhibitors which stop or minimize Cathodic corrosion are called
Cathodic inhibitors e.g. salt of zinc and nickel
ZnCO3,NiCO3,ZnO,NiO etc.
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