01-Basics of Corrosion

8/12/2019 01-Basics of Corrosion

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BASICS OF CORROSION

Dr. Ramazan Kahraman

Chemical Engineering DepartmentKing Fahd University of Petroleum & Minerals

Dhahran, Saudi Arabia

Reading Material: Chapter 1 in

Principles and Prevention of Corrosion, Denny Jones, Prentice-Hall, 1996.


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What is Corrosion?

Reaction of a metal with its environment Aqueous corrosion

reaction with water (usually containingdissolved 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 solidbut not protective

Reaction of aluminium withwater corrosion product is insoluble in

water, so may be protective

Burning of magnesium in air high temperature oxidation

Rust on an iron surface


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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, Fetends to separate (decompose) from steel and reactswith 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 anelectrochemical reaction is occurring

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

A cathode is an electrode on which acathodic or reduction reaction is occurring


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Electrochemical CellElectrochemical Cell

A C

HCl

ANODE

CATHODE

ELECTROLYTE

electrons

ELECTRONPATH


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Electrochemical Cell (cont.)Electrochemical Cell (cont.)


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MetalM

e -e - H

+

H+ H+

H+

M+n

Cl-Cl-

H2H+

HCl solution

Anodic Rxn M M +n + n e -

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

Corrosion of a Metal in AcidCorrosion of a Metal in Acid


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MetalM

e - H2O

M+n

OH -O 2

Aerated H 2O orBasic Solution

Anodic Rxn M M +n + n e -

Cathodic Rxn (n/2)H 2O + (n/4)O 2 + n e - n OH -

Corrosion of a Metal in Aerated Water orCorrosion of a Metal in Aerated Water or

Aerated Basic SolutionsAerated Basic Solutions


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Acids and Bases

An acid is a substance that producesexcess hydrogen ions (H +) when dissolvedin water

examples are HCl, H 2SO 4A base (alkali) is a substance thatproduces excess hydroxyl ions (OH -) when

dissolved in water examples are NaOH, KOH


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

The product of [H+

] times [OH-

] is 10-14

, so inpure water both [H +] and [OH -] are 10 -7. Thisleads 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

Acids and Bases (cont.)


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Corrosion of Zinc in Acid Zinc known as a base or active metalZinc known as a base or active metal

Zinc dissolves with hydrogen evolutionZn + 2HCl ZnCl 2 + H 2

But we can separate metal dissolutionand hydrogen evolution

Zn Zn 2+ + 2e -

2H + + 2e - H2

These are known as electrochemicalreactions


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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

Zn Pt

HCl

Zinc and platinum not

connected, no reactionon platinum

Zn + 2HCl ZnCl 2 + H 2

metal + acid

salt + hydrogen


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

Zn

APtC

HCl

Zinc and platinum

connected, current flowsand hydrogen is evolvedon platinum

Zn Zn 2+ + 2e -metal metal ions + electrons

(negligible cathodic rxn on Zn relative to that on Pt)

2H + + 2e - H2hydrogen ions + electrons hydrogen gas

electrons


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External Current Applied to Platinum in Acid

Pt Pt

HCl

+-Oxygen evolved onpositive electrode

2H 2O O 2 + 4H + + 4e -

Hydrogen evolved onnegative electrode

2H + + 2e - H2

Overall reaction

2H 2O 2H 2 + O 2


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External Current Applied to Platinum in Alkali

Pt Pt

NaOH

+-

Oxygen evolved on

positive electrode4OH - O 2 + 2H 2O + 4e -

Hydrogen evolved onnegative electrode

2H 2O + 2e - H2 + 2OH -

Overall reaction

2H 2O 2H 2 + O 2


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External Current Applied to Platinum

Hydrogen evolution at one electrode2H + + 2e - H2 (acids)or 2H 2O + 2e - H2 + 2OH - (alkalis)

A piece of metalin the solution

Oxygen evolution at the other electrode2H 2O O 2 + 4H + + 4e - (acids)

or 4OH - O 2 + 2H 2O + 4e - (alkalis)


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Anodic ReactionsOxidation reactions

Produce electronsExamples

Zn Zn 2+ + 2e - zinc corrosion

Fe

Fe2+

+ 2e-

iron corrosionAl Al3+ + 3e - aluminium corrosionFe 2+ Fe 3+ + e - ferrous ion oxidationH2 2H + + 2e - hydrogen oxidation in acids

H2 + 2OH - 2H 2O + 2e - hydrogen oxidation in water or bases2H 2O O 2 + 4H + + 4e - oxygen evolution in acids4OH - O 2 + 2H 2O + 4e - oxygen evolution in water or bases


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Cathodic ReactionsReduction reactions

Consume electronsExamples

O 2 + 2H 2O + 4e - 4OH - oxygen reduction in water/basesO 2 + 4H + + 4e - 2H 2O oxygen reduction in acids2H 2O + 2e - H2 + 2OH - hydrogen evolution in water/bases2H + + 2e - H2 hydrogen evolution in acidsCu 2+ + 2e - Cu copper platingFe 3+ + e - Fe 2+ ferric ion reductionSn 4+ + 2e - Sn 2+


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Cathodic Rxns in Acidic & Basic SolnsDeaerated Acidic Solutions

2H + + 2e - H2Aerated Acidic Solutions

2H + + 2e - H2

O 2 + 4H + + 4e - 2H 2O(presence of O 2 further increases corrosion)

Deaerated Neutral or Basic Solutions

2H 2O + 2e-

H2 + 2OH-

Aerated Neutral or Basic SolutionsO 2 + 2H 2O + 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 TechniqueCorrosion 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 dimensions2-in 3-in 0.125-in with a 0.25-in hole forsuspending in solution is exposed for 120

hours in an acid solution and loses 150milligrams. Calculate the corosion rate inmpy and mm/y.


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Faradays LawCharge is related to mass of materialreacted in an electrochemical reaction:

M Mn+ + ne -

One metal ion

Reacts

To produce one molof metal ion and

n mols of electrons


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Faradays Constant

One mole of metal (MW g) contains Avogadros number(6 10 23 ) of metal atoms

Hence each mole of metal will produce n times thatmany 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 Faradays constant(also in units of J/V equivalent)

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


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Faradays Law

(g/mole)metalof weight(atomic)molecular(g)oxidizedmetalof mass

metalof mol peredtransferr electrons)of (molssequivalentof number

nt)C/equivale(96500constantsFaraday'C)(coulomb,chargewhere

=

=

=

=

=

=

M m

nF Q

M nF Q

m

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.

01-Basics of Corrosion

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