Lecture 1b. Electrochemical Aspect
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Transcript of Lecture 1b. Electrochemical Aspect
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Basic Concepts :
Lecture 1b
rini riastuti corrosion - 2006
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Topics :
Theory of aqueous corrosion
Electrochemical aspect
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Electrochemical Nature of aqueous
corrosion
Metallic corrosion process involve transfer of
electronic charge in aqueous solutions
Thus, it is necessary to discuss the
electrochemical nature of corrosion before
discussing of the various form of corrosion.
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Corrosion of Zinc in Acid
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Electrochemical reactions :
Example of corrosion between zinc and
hydrochloric acid, represented by :
Zn + 2 HCl ZnCl2 + H2 (1)
Zinc reacts with the acid solution forming
soluble zinc chloride and liberating hydrogen
bubbles on the surface.
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Acids and Bases
An acid is a substance that produces excess
hydrogen ions (H+) when dissolved in water
examples are HCl, H2SO4
A base is a substance that produces excess
hydroxyl ions (OH-) when dissolved in water
examples are NaOH, KOH
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Corrosion of Zinc in Acid
Zinc dissolves with hydrogen evolution
Zn + 2HCl ZnCl2+ H2
Zinc known as a base or active metal
One atom of zinc metalplus two molecules of hydrogen
chloride (hydrochloric acid)
reacts to form
goes to
one molecule of zinc chlorideplus one molecule of hydrogen gas
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Corrosion of Platinum in Acid
Platinum does not react with acids
Platinum is known as a noblemetal
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Connection of Platinum to Zinc
Zn Pt
HCl
Zinc and platinum not
connected, no reactionon platinumZinc and platinumconnected, current flowsand hydrogen is evolved
on platinum
Zn + 2HCl ZnCl2+ H2
metal + acid salt + hydrogen
Zn Zn2++ 2e-
metalmetal ions + electrons
2H++ 2e- H2
hydrogen ions + electronshydrogen gas
electrons
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Connection of Platinum to Zinc
Zn + 2HCl ZnCl2+ H2
But we can separate metal dissolution and
hydrogen evolutionZn Zn2++ 2e-
Reactions that involve both
chemical change and the
transfer of charge
2H++ 2e-H2
These are known as electrochemicalreactionsOne atom of zinc metalone zinc ion in solutiontwo electrons in the metal
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External Current Applied to Platinum
in Acid
Pt Pt
HCl
+-
Oxygen evolved on
positive electrode
2H2O O2+ 4H++ 4e-
Hydrogen evolved on
negative electrode
2H++ 2e-H2
Overall reaction
2H2O 2H2+ O2
Acid - chemical species that
produces hydrogen ions in water
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Alkali
Alkali - chemical species that
produces hydroxyl ions (OH-)
in water
Note that H+
and OH-
are inequilibrium in water:
H2O H++ OH-
The product of [H+] times [OH-] is 10-14, so
in pure water both [H+] and [OH-] are 10-7.
This leads to the concept ofpH, which is
defined as -log[H+] Hence pH = 0 is strong acid, 7 is neutral, and
14 is strong alkali
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External Current Applied to Platinum
in Alkali
Pt Pt
NaOH
+-Oxygen evolved on
positive electrode
4OH-O2+ 2H2O + 4e-
Hydrogen evolved on
negative electrode
2H2O + 2e-H2+ 2OH
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Overall reaction
2H2O 2H2+ O2
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External Current Applied to Platinum
Hydrogen evolution at one electrode
2H++ 2e-H2 (acids)
or 2H2
O + 2e- H2
+ 2OH-(alkalis)
A piece ofin the solut
Oxygen evolution at the other electrode
2H2O O2+ 4H+
+ 4e-
(acids)
or 4OH- O2+ 2H2O + 4e- (alkalis)
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Electrodes
Electrodes are pieces of metal on which an
electrochemical reaction is occurring
An anodeis an electrode on which an anodic
or oxidation reaction is occurring
A cathodeis an electrode on which a cathodic
or reduction reaction is occurring
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Anodic Reactions
ExamplesZnZn2++ 2e- zinc corrosion
FeFe2++ 2e- iron corrosion
AlAl3++ 3e- aluminium corrosion
Fe2+ Fe3++ e- ferrous ion oxidation
H2 2H++ 2e- hydrogen oxidation
2H2O O2+ 4H++ 4e- oxygenevolution
Oxidationreactions
Produceelectrons
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Cathodic Reactions
ExamplesO2+ 2H2O + 4e
- 4OH- oxygen reduction
2H2O + 2e-H2+ 2OH
- hydrogen evolution
Cu2++ 2e-Cu copper plating
Fe3++ e-Fe2+ ferric ion reduction
Reductionreactions
Consumeelectrons
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Metal Ion Hydrolysis
Note that metal ions may react with water (a
hydrolysis reaction)
e.g. Al3++ 3H2O Al(OH)3+ 3H+
or 2Al3++ 3H2O Al2O3+ 6H+
Note that in an electrochemical reaction, we
have the same number of each atom on eachside of the equation, and the same overall
charge
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Faradays Law
Charge is related to mass of material reacted
in and electrochemical reaction:
2H++ 2e-H2
Two hydrogen
ions
React with twoelectrons
To produce one molecule
of hydrogen gas
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Faradays Constant
One mole of hydrogen ions (1 g) contains
Avogadros number (6 1023) ions
Hence electrons will react with each mole of
hydrogen ions
Charge on the electron is 1.6 10-19C
Hence one mole of ions requires 96500 C
This is known as Faradays constant
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Faradays Law
(g/mole)metalofweightatomic
(g)oxidisedmetalofmassedtransferrelectronsofnumber
C/mole)(96500constantsFaraday'(C)chargewhere
M
mn
F
Q
M
nFmQ
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Effect of Potential
Electrochemical reactions involve transfer of
charge
Hence, we expect that the voltage of the
metal with respect to the solution will affectelectrochemical reactions
Voltage of metal with respect to solution is
known as theelectrochemical potential
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Corrosion of zinc in acid
When zinc is placed in acid the metal will start
to dissolve and hydrogen will start to be
liberated according to the potential of the
metalConsider the anodic zinc dissolution reaction
Zn Zn2++ 2e-
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Energy: the Rule of Law
The law:energy can neither be created nor
destroyed.
The Rule:all spontaneous changes occur with a
release of free energy from the system to thesurroundings at constant temperature and pressure.
Points to note:
corrosion is a spontaneous process.
free energy is released in the process.
metals returned to its stable state.
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Free energy: the portion of internal
energy available for powering engines or
causes corrosion reaction.
The driving force for corrosion reaction:
chemical energy - energy stored in chemical
bonds of substances -internal energy.
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Transition state theory concerns
A+ B= AB= C + D
The transition sate must be of higher free energy
than the sum of the free energies of the separatespecies.