Download Definition of Corrosion

Definition of Corrosion
Corrion is a destructive attack of a metal by
chemical or electrochemical reaction with its
environment.
Types of Corrosions
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Uniform Attack
Galvanic Corrosion
Crevice Corrosion
Intergranular Corrosion
Stress Corrosion
High Temperature Corrosion
Uniform Attack
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Uniform attack is a form of electrochemical
corrosion that occurs with equal intensity of
the entire surface of the metal. Iron rust when
exposed to air and water, and silver tarnishes
due to exposure to air.
Galvanic corrosion
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Galvanic corrosion occurs when two metals
having different composition are electrically
coupled in the presence of an electrolyte. The
more reactive metal will experience sever
corrosion while the more noble metal will be
quite well protected. Perhaps the most
infamous examples of this type of combination
such as steel and brass or copper and steel.
Crevice Corrosion
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Crevice corrosion is consequence of
concentration difference of ions or dissolved
gases in an electrolytic solution. A solution
became trapped between a pipe and the flange
on the left. The stagnant liquid in the crevice
eventually had a lowered dissolved oxygen
concentration and crevice corrosion took over
and destroyed the flange.
Intergranular Corrosion
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Occurring along grain boundaries for some
alloy, the heating of the some material causes
chromium carbide to form from the chromium
and carbon in the metal. This leaves a
chromium deficient boundary just shy of the
where the metal was heated for welding.
PREVENTION OF CORROSION
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By Material Selection
By Environmental modification
By Corrosion inhibitors
By Paints
By Coating
By Cthodic protection
By Interaction between the above methods
Principle of Cathodic Protection
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Cathodic Protection is to make the potential of
the whole surface of the steel structure
sufficiently negative with respect to the
surround medium to ensure that no current
flows from the metal into the medium.
Method of Cathodic Protection
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Sacrificial Anode System
Impressed direct current System
Sacrificial Anode System
With the Sacrificial anode method, use is made
of galvanic action to provide the cathodic
protection current. The surface of the structure
is made by connecting it electrically to mass of
less noble metal buried or immersed in to
common electrolyte.
Sacrificial Anode Cathodic Protection
Metallic Conductor
Current
PIPE LINE
Protected pipe
Sacrificial Anode
ZINIC
Impress Direct Current System
Impress direct current method, the structure is
connected to the negative of a direct current
power supply and the positive of the supply is
connected to a number of anodes buried in
ground or impressed in water.
Impress Current Cathodic Protection System
Direct Current Source
Current
Metallic Conductor
PIPE LINE
Protected pipe
Ground Bed Anode
THE ELECTROMOTIVE SERIES
Electrodes
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Electrode Potential, volt
Sodium
-2.712
Magnesium
-2.340
Aluminum
-1.670
Zinc
-0.760
Chromium (Trivalent) - 0.710
Iron (divalent)
-0.440
Nickel
-0.250
Tin
-0.136
Lead
-0.126
Hydrogen
0.000
Copper (divalent)
+0.345
Copper (monovalent) +0.552
Silver
+ 0.800
Platinum
+1.200
Gold (monovalent)
+1.680
Base End or Anodic end
Noble End Cathodic End
THE GALVANIC SERIES
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Magnesium
Anode - Corroding End
Aluminum
Zinc
Least Noble,
Cadmium
Electro - negative
Steel
Cast iron
Chromium (Active)
Stain less Steel (Active)
Soft Solder
Tin
Lead
Nickel
Brass
Bronze
Copper
Silver solder
Chromium (Passive)
Cathode - Protected End
Silver
Graphite
Most Noble,
Gold
Electro - Positive
Platinum
Types of Anodes
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Graphite Anode
High Silicon cast iron Anode
Mixed- metal oxide Anode
Platinum coated Anode
Lead alloy Anode
Types of ground Bed
Deep Ground Bed
Ground bed more then 50feet deep.
 Shallow Ground Bed
Ground bed up to 50 feet depth
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Types of backfill
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Standard metallurgical coke breeze
Petroleum coke backfill
Natural or manufactured graphite
Corrosion cell due to dissimilarity of metals
Protective current to old pipe-Large Cathode
Old pipe
Old pipe
New pipe
Corrosion current to new pipe- Small Anode
RESISTIVITY
A resistor is an element that resist the movement
of electrical charge. The unit of resistance is
the ohm. A resistance of one ohm will allow
one ampere to pass when one volt of potential
difference is applied. Electrical resistivity (ρ)
has the ohm-cm and is physical property of the
material.
R= ρ L/A
R = resistanenc (ohm)
L = length (cm) of the current flow path
A = area (cm2) perpendicular to current flow path
ρ = Resistivity (ohm-cm)
SOIL RESISTIVITY
The resistivity of the soil( electrolyte) largely
determine the magnitude of corrosion current in
any corrosion cell. Thus lower the resistivity
the greater would be the intensity of the
corrosive attack. And if cathodic protection is
required, the soil resistivity will determine the
magnitude of current generated by sacrificial
anode or the resistance to ground of impressed
current anode.
Cathodic Protection
A technique to prevent corrosion
of a metal surface by making it
the cathode of an
electrochemical cell