Lecture 1b. Electrochemical Aspect

8/12/2019 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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in Alkali

Pt Pt

NaOH

+-Oxygen evolved on

positive electrode

4OH-O2+ 2H2O + 4e-

Hydrogen evolved on

negative electrode

2H2O + 2e-H2+ 2OH

-

Overall reaction

2H2O 2H2+ O2


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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.

Lecture 1b. Electrochemical Aspect

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