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Basics• Corrosion is the natural deterioration of a metal in which metallic

atoms leave the metal or form compounds resulting from chemical attack by its environment

• Example: Corrosion involving water and steel generally results from chemical action where the steel surface oxidizes, forming iron oxide (rust).

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Basics …cont,

Corrosion is a low-temperature oxidation mechanismNormal oxidation is prevented by the natural oxide

coating (eg; α-alumina thin layer on Al)In corrosion, the protective coating does not

automatically formIn the reaction: M++ + O= = MO

The metal ions form at one location (the “anode”)The oxygen forms at another (the “cathode”)The two do not ordinarily develop a good protective coating

Corrosion is an electrochemical processRequires both electrical and chemical contact between

Anode, the metal electrode in an electrolytic cell that dissolves as ions and supplies electrons to the external circuit.

Cathode, the electrode in an electrolytic cell that that accepts electrons

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• Corrosion occurs due to: -- The natural tendency of metals to give up electrons. -- Electrons are given up by an oxidation reaction. -- These electrons then are part of a reduction reaction.

• Corrosion rate depend on -- temperature -- concentration of the reactant & products -- mechanical stress & erosion -- the energy state of the metal

Basics (Cont.)

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ExampleDefine corrosion as it pertains to materials. What are some of the factors that affect the corrosion of metals?

Solution:

Corrosion is the natural deterioration of a metal in which metallic atoms leave the metal or form compounds resulting from chemical attack by its environment

Some of the factors that affect the corrosion of metals are: the temperature and concentration of the reactants and products of a chemical attack; the state of mechanical stress in the metal; the presence of erosion in the metal; and the energy state of the metal.

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Corrosion of metals

For metallic materials, the corrosion process is normally electrochemical, i.e., a chemical reaction in which there is transfer of electrons from one chemical species to another.

As a consequence of oxidation, the metal ions may either:Go into the corresponding solution as ionsForm an insoluble compound with non-metallic elements.

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• Two reactions are necessary: -- oxidation reaction: -- reduction reaction:

reaction) (anodic2eZnZn 2 reaction) (cathodic (gas)H2e2H 2

• Other reduction reactions:-- in an acid solution -- in a neutral or base solution

O2 4H 4e 2H2O O2 2H2O 4e 4(OH)

Zinc

oxidation reactionZn Zn2+

2e-Acid solution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e- in the metal

Ex: Corrosion of Zinc in acid

Zn + 2HCl → ZnCl2 + H2

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• Two outcomes:--Metal sample mass --Metal sample mass

Pla

tin

um

me

tal, M

Mn+ ions

ne- H2(gas)

25°C 1M Mn+ sol’n 1M H+ sol’n

2e-

e- e-

H+

H+

--Metal is the anode (-) --Metal is the cathode (+)

Vmetalo 0 (relative to Pt) Vmetal

o 0 (relative to Pt)

Standard Electrode Potential

Standard hydrogen (EMF) test

Mn+ ions

ne-

e- e-

25°C 1M Mn+ sol’n 1M H+ sol’n

Pla

tin

um

me

tal, M

H+ H+

2e-

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Corrosion of metals (Cont.)

• Galvanic couple: Two metals electrically connected in a liquid electrolyte wherein one metal becomes an anode and corrodes, while the other acts as a cathode.

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• EMF series • Metal with smaller V (i.e., more active) corrodes.Ex: Cd-Ni cell

metalo

-

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

Cd 25°C

more

anodic

more

cath

odic Au

CuPbSnNiCoCdFeCrZnAlMgNaK

+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.262- 2.714- 2.924

metal Vmetalo

DV = 0.153V

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Standard EMF Series

EMF: Electromotive Force

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ExampleThe EMF of a standard Ni-Cd galvanic cell is -0.153 V. If the standard half-cell EMF for the oxidation of Ni is -0.250 V, what is the standard half-cell EMF of cadmium if cadmium is the anode?

Solution:

The standard half-cell EMF of the cadmium can be calculated by considering the half-cell reactions:

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Microscopic Galvanic Cell Corrosion of Single Electrodes

The oxidation reaction that will occur at the local anodes is

Zn → Zn2+ + 2e-

If a single electrode of Zn is placed in a dilute solution of air-

free HCl acid

Zn will corroded electrochemically, local anodes & cathodes will develop on surface

due to inhomogeneities in structure & composition

The reduction reaction that will occur at the local cathodes is

2H+ + 2e- → H2

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Microscopic Galvanic Cell Corrosion of Single Electrodes

The oxidation reaction that will occur at the local anodes is

Fe → Fe2+ + 2e- (anodic reaction)

Rusting of iron

A piece of iron immersed in oxygenated water, ferric hyroxide [Fe(OH)3] will form on its surface

The reduction reaction that will occur at the local cathodes is

O2 + 2H2O + 4e- → 4OH- (cathodic reaction)

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• Ranks the reactivity of metals/alloys in seawaterm

ore

anodic

(a

ctiv

e)

more

cath

odic

(i

nert

)

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium

Galvanic series

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ExampleConsider a magnesium-iron galvanic cell consisting of a magnesium electrode in a solution of 1 M MgSO4 and an iron electrode in a solution of 1 M FeSO4. Each electrode and its electrolyte are separated by a porous wall, and the whole cell is at 25°C. Both electrodes are connected with a copper wire.

a)Which electrode is the anode?b)Which electrode corrodes?c)In which direction will the electrons flow?d)In which direction will the anions in the solution move?e)In which direction will the cations in the solution move?f)Write an equation for the half-cell reaction at the anode.g)Write an equation for the half-cell reaction at the cathode.

Fe

1.0 M Fe 2+ solution

1.0 M Mg 2+ solution

Mg 25°C

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Example …cont,

a) The magnesium electrode is the anode.b) The magnesium electrode corrodes since the anode in a

galvanic cell corrodes (oxidizes).c) The electrons will flow from the anode, Mg, to the cathode, Fe.

d) The SO42− anions will flow toward the magnesium anode.

e) The cations will flow toward the iron cathode.f) The oxidation reaction that occurs at the magnesium anode is:

Mg → Mg2++2e−

g) The reduction reaction that occurs at the iron anode is: Fe2+

+2e−→ Fe .

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Passivition

• Passivition; the formation of a film of atoms or molecules on the surface of an anode so that corrosion is slowed down or stopped.

• Examples:

– α-alumina film formed on Al surface

– SS (contains Cr) forms CrO3 on SS surface

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Types of Corrosion

Types of corrosion can be classified according to the appearance of the corroded metal

1) Uniform or general attack corrosion2) Galvanic or two-metal corrosion3) Pitting corrosion4) Crevice corrosion5) Intergranular corrosion6) Stress corrosion7) Erosion corrosion8) Selective leaching or dealloying

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Uniform or general attack corrosion

• Characterized by an electrochemical reaction that proceeds uniformly on the entire metal surface exposed to a corrosive environment.

• Easy to control by– Protective coating– Inhibitors– Cathodic protection

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Galvanic or two-metal corrosion

• Occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte.

• Examples:

– Galvanized steel, steel coated with Zn; Zn is sacrificed to protect steel

– Tin can (food container), steel coated with Sn; Sn is sacrificed to protect steel

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Pitting corrosion

Local corrosion attack resulting from the formation of small anodes on a metal surface that produces holes or pits

Pitting Corrosion

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Crevice corrosion

• Crevice corrosion is a form of localized electrochemical corrosion that can occur in crevices & under shielded surfaces where stagnant solution can exist

• Occurs under gaskets, rivets and bolts between valve disks & seats.

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Intergranular corrosion

• Intergranular corrosion: preferential corrosion occuring at grain boundary or at regions adjacent to the grains boundaries

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Stress corrosion

• Stress-corrosion cracking (SCC); preferential corrosive attack of a metal under stress in a corrosive environment.

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Erosion corrosion

• Can be defined as the acceleration in the rate of corrosion attack in a metal due to the relative motion of a corrosive fluid a metal surface.

• Results wear & abrasion• Characterized by the appearance on the

metal surface of grooves, valley, pits, rounded holes.

Grooves Structure

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Selective leaching or dealloying

• Selective leaching is the preferential removal of one element of a solid alloy by corrosion process

• Eg; – Dezinfication; selective leaching of Zn from copper

in brasses occurs.

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Forms of

corrosion

• Uniform AttackOxidation & reductionoccur uniformly oversurface.

• Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)).

• IntergranularCorrosion alonggrain boundaries,often where specialphases exist.

• Stress corrosionStress & corrosionwork togetherat crack tips.

• GalvanicDissimilar metals arephysically joined. Themore anodic onecorrodes. Zn & Mgvery anodic.

• Erosion-corrosionBreak down of passivatinglayer by erosion

• PittingDownward propagationof small pits & holes.

• Crevice Between twopieces of the same metal. due to concentration difference

Rivet holesattacked zones

g.b. prec.

Forms of corrosion

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ExampleDefine,

1) Uniform or general attack corrosion2) Galvanic or two-metal corrosion3) Pitting corrosion4) Crevice corrosion5) Intergranular corrosion6) Stress corrosion7) Erosion corrosion8) Selective leaching or dealloying

Solution;

Refer lecture note

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Corrosion Control

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Metallic Selection

• General rules for metallic selection– For reducing or, nonoxidizing conditions such as

air-free acids & aqueous solutions, Ni & Cu alloy are often used.

– For oxidizing conditons, Cr containing alloy are used.

– For extremely powerful oxidizing conditions, Ti & its alloy are commonly used.

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Coatings

• Metallic, inorganic, & organic coatings are applied to metals to prevent or reduce corrosion– Metallic coating eg.; Zinc coating on steel

(sacrificial anode)– Inorganic coating (ceramics & glass) eg.; Glass-

lined steel vessels– Organic coating eg.; paints, varnishes & lacquers

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Design8 engineering design rules that are important to the reduction or prevention of corrosion are:

.

1 Allow for the penetration action of corrosion along with the mechanical strength requirements when determining the appropriate metal thickness

2 Weld rather than rivet containers to reduce crevice corrosion. If rivets are used, choose a rivet material that is cathodic to the

materials being joined .

3 If possible, use galvanically similar metals for the entire structure. Avoid dissimilar metals that can cause galvanic corrosion. If galvanically dissimilar metals are bolted together, separate them with nonmetallic gaskets and washers.

4 Avoid excessive stress and stress concentrations in corrosive environments to prevent stress-corrosion cracking, especially when using susceptible materials such as stainless steels and brasses.

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Design… Cont.,8 engineering design rules that are important to the reduction or prevention of corrosion are:

5 Avoid sharp bends in piping systems to prevent erosion corrosion.

6 Design tanks and other containers for easy draining and cleaning.

7 Design systems for easy removal and replacement of parts that are expected to fail in service, such as pumps in chemical plants.

8 Design heating systems such that hot spots do not occur.

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Alteration of Environment

• 4 general methods of altering an environment to prevent or reduce corrosion are:

– Lower the system temperature to lower the reaction rates and thus reduce corrosion. Certain exceptions exist, such as seawater, for which the temperature should be raised rather than reduced.

– Decrease the velocity of corrosive fluids such that erosion corrosion is reduced while fluid stagnation is avoided.

– Remove oxygen from water solutions unless it is necessary for passivation.

– Reduce the concentration of corrosive ions in a solution which is corroding a metal.

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Cathodic protection • The protection of a metal by connecting it to a sacrificial

anode or by impressing a DC voltage to make it cathode• Eg;

– sacrificial anode– Impressed current

Cathodic protection of an underground tank by using impressed currents

Cathodic protection of an underground pipeline by using sacrificial Mg anode

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Anodic protection

• The protection of a metal that forms a passive film by the application of an externally impressed anodic current

Carefully controlled anodic currents by a device called a potentiostat can be applied to protect metals that passivte.

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• Self-protecting metals! --Metal ions combine with O to form a thin, adhering oxide layer that slows corrosion.

Metal (e.g., Al, stainless steel)

Metal oxide

• Reduce T (slows kinetics of oxidation and reduction)• Add inhibitors --Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). --Slow oxidation reaction by attaching species to the surface (e.g., paint it!).• Cathodic (or sacrificial) protection --Attach a more anodic material to the one to be protected.

Controlling corrosion

steel

zinczincZn2+

2e- 2e-

e.g., zinc-coated nail

steel pipe

Mg anode

Cu wiree-

Earth

Mg2+

e.g., Mg Anode

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Example

1. Describe eight engineering design rules that may be important to reduce or prevent corrosion.

2. Describe four methods of altering the environment to prevent or reduce corrosion.

3. Describe two methods by which cathodic protection can be used to protect steel pipe from corroding.

Solution:

Refer lecture note

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Degradation of Non-metallic Materials

• Ceramics & polymers do not suffer electrochemical attacks but can deteriorated by chemical attack

• Examples:

– Ceramic refractory mater. Can be chemically attacked at high temp. by molten salts

– Organic polymers can be

a) chemically attacked by of organic solvents

b) changes its dimensions or property resulted by absorbing of water

c) deteriorated by combination of oxygen & ultraviolet radiation even at room temperature.

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