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Chapter 17 - 1

ISSUES TO ADDRESS...

How does corrosion occur?

Which metals are most likely to corrode?

What environmental parameters affectcorrosion rate?

How do we prevent or control corrosion?

Chapter 17:

Corrosion and Degradation of

Materials

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Chapter 17 - 2

DESTRUCTIVE MECHANISM

In metal

there is an actual loss either by dissolution(corrosion) or by formation of non-metalic scale or film

(oxidation).

Ceramic materialrelatively resistant to deterioration,

usually occurs at elevated temperature or rather extreme

environments. Also called corrosion.

In polymerthe term used is degradation. Polymer may

dissolve when exposed to a liquid solvent or they mayabsorb the solvent and swell also electromagnetic radiation

(primarily ultraviolet) and heat may cause alterations in

their molecular structures

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Chapter 17 - 3

Corrosion in metal:- the destructive electrochemical attack on metal.

- ordinarlily begins at the surface- -- Ex:Al Capone's

ship, Sapona,off the coastof Bimini.

Cost:-- 4 to 5% of the Gross National Product (GNP)* spend

in corrosion prevention, maintenance and replacement-- in the U.S. this amounts to just over $400 billion/yr**

* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introductionto Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc.,1985.**Economic Report of the President (1998).

Photos courtesy L.M. Maestas, SandiaNational Labs. Used with permission.

THE COST OF CORROSION

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Chapter 17 - 4

Because metals want to go back to

their stable states.

Usually, metals are stable when they

bind with others.

In the presence of a corrosive

environment, metals tend to reactswith oxygen or other corrosive ions.

WHY CORROSION HAPPEN?

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Chapter 17 -

CORROSION INGREDIENTS

Water Oxygen

Electrolyte (ion transport in water) orElectrical conductor

To prevent corrosion remove one of theingredient

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Chapter 17 -

Corrosive Environments

All environments are corrosive to some degree.

Typical corrosive environments:

Humidity

SteamSeawater

Alkalis

Acids

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Chapter 17 -

Corrosive Environments Include the atmosphere, aqueous solutions,

soils, acids, bases, inorganic solvents, moltensalts, liquid metals.

Atmospheric corrosion accounts for thegreatest losses.

Moisture contain dissolve oxygen, and othercompound such as sodium chloride.

Freshwater contained dissolve oxygen and

other minerals. Sea water is more corrosive than freshwater.

Soil contain moisture, oxygen salt content,

alkalinity and acidity 7

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Chapter 17 -

Effect of pH on Corrosion

Corrosion accelerated under acidic condition Secondary reaction, prevent the oxide from

accumulating in the surface thus drive thereaction into the product

Pitting cause the thermodynamic activity ofFe further accelerating the corrosion process

8

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Chapter 17 - 9

Two reactions are necessary:-- oxidationreaction (give up electron)-- reductionreaction (receive electron)

Zn Zn2 2e

2H2e H2(gas)

Other reductionreactions in solutions with dissolved oxygen:

-- acidic solution -- neutral or basic solution

O2 4H4e 2H2O O2 2H2O4e

4(OH)

Adapted from Fig. 17.1,Callister & Rethwisch 8e.(Fig. 17.1 is from M.G.Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Company, 1986.)

ELECTROCHEMICAL CORROSION

Zinc

Oxidation reactionZn Zn2+

2e-Acidsolution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

H+

H+

flow of e-in the metal

Ex: consider the corrosion of zinc in an acid solution

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Chapter 17 -10

STANDARD HYDROGEN ELECTRODE

Two outcomes:

0o

metalV (relative to Pt)

Standard Electrode PotentialAdapted from Fig. 17.2,Callister & Rethwisch 8e.

-- Corrosion

-- Metal is the anode (-)

Platinum

metal,M

Mn+

ions

ne-H2(gas)

25C1M Mn+ soln 1M H+ soln

2e-

e-e-

H+

H+

-- Electrodeposition

-- Metal is the cathode (+)

Mn+ions

ne-

e- e-

25C1M Mn+ soln 1M H+ soln

Platinum

metal,M

H+

H+2e-

0o

metal V (relative to Pt)

H2(gas)

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Chapter 17 - 11

STANDARD EMF (electromotive force) SERIES

metal

o Metal with smaller

V corrodes.

EMFseries

AuCuPbSn

NiCoCdFeCr

ZnAlMgNaK

+1.420 V+0.340- 0.126- 0.136

- 0.250- 0.277- 0.403- 0.440- 0.744

- 0.763- 1.662- 2.363- 2.714- 2.924

metalV

metal

o

Data based on Table 17.1,Callister 8e.

moreano

dic

more

cathodic

DV =0.153V

o

Adapted from Fig. 17.2,Callister & Rethwisch 8e.

-

1.0 M

Ni2+ solution

1.0 M

Cd2 + solution

+

25C NiCd

Cdo

Nio

Ex: Cd-Ni cell

V < V Cd corrodes

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Chapter 17 -12

GALVANIC SERIES Ranking of the reactivity of metals/alloys in seawater

Based on Table 17.2, Callister &Rethwisch 8e. (Source of Table17.2 is M.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Company, 1986.)

PlatinumGoldGraphiteTitaniumSilver

316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead

316 Stainless Steel (active)Iron/SteelAluminum AlloysCadmiumZincMagnesium

moreanodic

(active)

morecathodic

(inert)

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Chapter 17 -

Corrosion Rates

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Chapter 17 -

PASSIVITY

Some normally active metal and alloys underparticular environment; condition lose their chemicalreactivity and become extremely inert.Passivity

Results from the formation of a highly adherent andvery thin oxide form on metal surface which serve as

protective barrier to further corrosion.

Display by chromium, nickel, titanium, iron, stainlesssteel, aluminum and their alloys.

Aluminum form a tough oxide Al2O3which prevent

further oxidation

Unfortunately Fe2O3 is a flaky , not tough

However, a change in the character of theenvironment may cause a passivated material to

revert to an active state. 14

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Chapter 17 -15

Uniform AttackOxidation & reductionreactions occur uniformlyover surfaces.

Selective LeachingPreferred corrosion of

one element/constituent[e.g., Zn from brass (Cu-Zn)].

Stress corrosionCorrosion at crack tips

when a tensile stressis present.

GalvanicDissimilar metals arephysically joined in thepresence of anelectrolyte. Themore anodic metal

corrodes.

Erosion-corrosionCombined chemical attack andmechanical wear (e.g., pipeelbows).

FORMS OF CORROSION

Formsof

corrosion

Crevice Narrow andconfined spaces.

Fig. 17.15, Callister & Rethwisch 8e. (Fig. 17.15is courtesy LaQue Center for Corrosion

Technology, Inc.)

Rivet holes

IntergranularCorrosion alonggrain boundaries,

often where precip.particles form.

Fig. 17.18, Callister &

Rethwisch 8e.

attackedzones

g.b.prec.

PittingDownward propagation

of small pits and holes.

Fig. 17.17, Callister &Rethwisch 8e. (Fig. 17.17from M.G. Fontana,Corrosion Engineering,3rd ed., McGraw-Hill BookCompany, 1986.)

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Chapter 17 -16

-- Use metals that passivate

- These metals form a thin,adhering oxide layer thatslows corrosion.

Lower the temperature (reduces rates of oxidation and

reduction)

CORROSION PREVENTION (i)

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

Metal oxide

Apply physical barriers -- e.g., films and coatings

Materials Selection-- Use metals that are relatively unreactive in thecorrosion environment -- e.g., Ni in basic solutions

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Chapter 17 -17

Add inhibitors (substances added to solution that decreaseits reactivity)-- Slow oxidation/reduction reactions by removing reactants

(e.g., remove O2gas by reacting it w/an inhibitor).-- Slow oxidation reaction by attaching species to

the surface.

CORROSION PREVENTION (ii)

Adaptedfrom Fig.17.22(a),Callister &

Rethwisch

8e.

Using a sacrificial anode

steelpipe

Mganode

Cu wiree-

Earth

Mg2+

Cathodic (or sacrificial) protection-- Attach a more anodic material to the one to be protected.

Adaptedfrom Fig.17.23,Callister &

Rethwisch

8e. steel

zinczinc

Zn2+

2e- 2e-

e.g., zinc-coated nail

Galvanized Steel

e.g., Mg Anode

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Chapter 17 -18

CORROSION OF CERAMIC

MATERIALS

Extremely immune to corrosion by almost all

environments especially at room temperature.

Corrosion of ceramic materials generally

involve simple chemical dissolution in contact

to electrochemical process found in metals.

Frequently used because of their resistance to

corrosion. Eg. glass is often used to contain liquids for

this reason.

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Chapter 17 -19

DEGRADATION OF POLYMERS

Whereas most metallic corrosions areelectrochemical, polymeric degradation isphysiochemical: that it involve physical as well aschemical phenomena.

Polymers deteriorate by swelling and dissolution. With swelling, the liquid or solute diffuses into and is

absorbed within the polymer: the small solutemolecules fit into and occupy positions amongpolymer molecules thus the macromolecules are

forced apart. This increase in chain separationresults in a reduction of secondary intermolecularbonding forces; as a consequences, the materialbecome softer and more ductile.

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Chapter 17 -20

DEGRADATION OF POLYMERS

Scissionthe severance or rupture of molecular chain bonds.This

courses a separation of chain segments at the point of scission anda reduction in the molecular weight. Bond rapture may result from exposure to radiation or to heat and

from chemical reaction. Certain type of radiationpossess sufficient energy to penetrate a

polymer specimen and interact with the constituent atoms or theirelectrons.

Oxygen, ozone and other substances can causes or acceleratechain scission as a result of chemical reaction

Thermal degradationcorresponds to the scission of molecular chainat elevated temperature; as a consequence some polymer undergo

chemical reactions in which gaseous species are produced. Weatheringwhen exposed outdoor condition deterioration is a

result of oxidation which initiated by ultraviolet radiation from thesun and ,water absorption.

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Chapter 17 -21

Metallic corrosion involves electrochemical reactions

-- electrons are given up by metals in an oxidationreaction-- these electrons are consumed in a reductionreaction

Metals and alloys are ranked according to their

corrosiveness in standard emfand galvanic series. Temperature and solution composition affect corrosion

rates. Forms of corrosion are classified according to mechanism

Corrosion may be prevented or controlled by:-- materials selection

-- reducing the temperature-- applying physical barriers-- adding inhibitors-- cathodic protection

SUMMARY

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Chapter 17 -22

a) Explain the differences between oxidation and reduction

electrochemical reaction

b)Which reaction occurs at the anode and which at the cathode

Quiz

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Chapter 17 -23

a) Explain 5 different form of corrosion and in which

condition it will occur.b) Explain three methods to prevent corrosion

Home work

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Chapter 17 -24

Reading:

Core Problems:

Self-help Problems:

ANNOUNCEMENTS