Chapter - Basics Of Corrosion, PPT, Semester, Engineering - Electronics and Communication Engineering (ECE) PDF Download

BASICS OF CORROSION

Dr. Ramazan Kahraman
Chemical Engineering Department
King Fahd University of Petroleum & Minerals
Dhahran, Saudi Arabia

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What is Corrosion?

Reaction of a metal with its environment
♦ Aqueous corrosion
− reaction with water (usually containing
dissolved ions)
♦ High temperature oxidation
− reaction with oxygen at high temperature
♦ High temperature corrosion
− reaction with other gases

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Examples of Corrosion

Rusting of steel
–corrosion product (rust) is solid but not protective

Reaction of aluminium with water
–corrosion product is insoluble in water, so may be protective

Burning of magnesium in air
–high temperature oxidation

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Corrosion Science and Engineering

♦Corrosion Science
– Study of the chemical and metallurgical processes that occur during corrosion.

♦Corrosion Engineering
– Design and application of methods to prevent corrosion.

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Why is Corrosion Happening?

Because metals want to go back to their stable states.

For Example, Fe is stable when it reacts with oxygen.
So, in the presence of a corrosive environment, Fe tends to separate (decompose) from steel and reacts with oxygen

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Nature of Corrosion

 Formation of cell is essential for corrosion Corrosion cell comprises of the following
–Anode (supplies e- - oxidation reaction)
–Cathode (consumes e- - reduction reaction)
–Electrolyte
–Conductor (electron path)

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Electrodes

Electrodes are pieces of metal on which an electrochemical reaction is occurring

An anode is an electrode on which an anodic or oxidation reaction is occurring

A cathode is an electrode on which a cathodic or reduction reaction is occurring

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Electrochemical Cell

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Electrochemical Cell (cont.)

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Corrosion of a Metal in Acid

Anodic Rxn M M+n + n e-
Cathodic Rxn nH+ + n e- n/2 H2

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Corrosion of a Metal in Aerated Water or Aerated Basic Solutions

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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 (alkali) is a substance that produces excess hydroxyl ions (OH-) when dissolved in water
–examples are NaOH, KOH

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Acids and Bases (cont.)

Note that H+ and OH- are in equilibrium in water:
H2O ⇔ H+ + OHThe

product of [H+] times [OH-] is 10-14, so in pure water both [H+] and [OH-] are 10-7. This leads to the concept of pH, which is defined as -log[H+]
Hence pH = 0 is strong acid, 7 is neutral, and 14 is strong alkali

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Corrosion of Zinc in Acid

● Zinc known as a base or active metal

● Zinc dissolves with hydrogen evolution
Zn + 2HCl → ZnCl2 + H2

But we can separate metal dissolution and hydrogen evolution
Zn → Zn2+ + 2e-
2H+ + 2e- → H2

These are known as electrochemical reactions

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Corrosion of Platinum in Acid

Platinum does not react with acids

Platinum is known as a noble metal

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Zinc and Platinum in Acid – Not Connected

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Connection of Platinum to Zinc
(This is galvanic corrosion which will be studied in detail later)

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External Current Applied to Platinum in Acid

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External Current Applied to Platinum in Alkali

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External Current Applied to Platinum

Hydrogen evolution at one electrode
2H+ + 2e- → H2 (acids)
or 2H2O + 2e- → H2 + 2OH- (alkalis)

A piece of metal in the solution
Oxygen evolution at the other electrode
2H2O → O2 + 4H+ + 4e- (acids)
or 4OH- → O2 + 2H2O + 4e- (alkalis)

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Anodic Reactions

Oxidation reactions
Produce electrons

Examples
Zn → Zn2+ + 2e- zinc corrosion
Fe→ Fe2+ + 2e- iron corrosion
Al→ Al3+ + 3e- aluminium corrosion
Fe2+ → Fe3+ + e- ferrous ion oxidation
H2 → 2H+ + 2e- hydrogen oxidation in acids
H2 + 2OH- → 2H2O + 2e- hydrogen oxidation in water or bases
2H2O → O2 + 4H+ + 4e- oxygen evolution in acids
4OH- → O2 + 2H2O + 4e- oxygen evolution in water or bases

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Cathodic Reactions

Reduction reactions

Consume electrons

Examples
O2 + 2H2O + 4e-→ 4OH- oxygen reduction in water/bases
O2 + 4H+ + 4e- → 2H2O oxygen reduction in acids
2H2O + 2e-→ H2 + 2OH- hydrogen evolution in water/bases
2H+ + 2e- → H2 hydrogen evolution in acids
Cu2+ + 2e- → Cu copper plating
Fe3+ + e- → Fe2+ ferric ion reduction
Sn4+ + 2e- → Sn2+

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Cathodic Rxns in Acidic & Basic Solns

Deaerated Acidic Solutions
2H+ + 2e- → H2

Aerated Acidic Solutions
2H+ + 2e- → H2
O2 + 4H+ + 4e- → 2H2O
(presence of O2 further increases corrosion)

Deaerated Neutral or Basic Solutions
2H2O + 2e- → H2 + 2OH-

Aerated Neutral or Basic Solutions
O2 + 2H2O + 4e- → 4OH-
(this reaction causes higher corr. rate)

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Corrosion Rate

Simplest and most useful technique for corrosion rate determination is the Weight Loss Technique Corrosion Rate = mass / exposed surface area . time
or

Corrosion Rate = avg. corrosion penetration depth / time
( = mass / density . surface area . time )

Common Corrosion Rate Units
– gmd (grams of metal lost per square meter per day)
– mm/y (average millimeters penetration per year)
– mpy (avg. mils penetration per year, 1 mil = 0.001 in)

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Example

A carbon steel test specimen of dimensions 2-in × 3-in × 0.125-in with a 0.25-in hole for suspending in solution is exposed for 120
hours in an acid solution and loses 150 milligrams. Calculate the corosion rate in mpy and mm/y.

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Home Exercise Problems

Prbs. 1, 4, 8, 10 and 11 of Chapter 1
in “Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.

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Faraday’s Law

Charge is related to mass of material
reacted in an electrochemical reaction:

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Faraday’s Constant

One mole of metal (MW g) contains Avogadro’s number
(6×1023) of metal atoms

Hence each mole of metal will produce n times that many number of electrons

Charge on the electron is 1.6 × 10-19 C (coulomb)

Hence one mole of metal will produce a charge of n × 96500 C

96500 C/equivalent is known as Faraday’s constant
(also in units of J/V⋅equivalent)

Conversions: 1 A (ampere) = 1 C/s, 1 J = 1 C⋅V

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Faraday’s Law

So, if Q is known, mass loss by corrosion can be determined.

The details of corrosion rate determination by electrochemical techniques will be covered later.

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References
 “Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.
Web Site of Dr. R. A. (Bob) Cottis.