ISSUES TO ADDRESS...• Why does corrosion occur?

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• What metals are most likely to corrode?• How do temperature and environment affect corrosion rate?• How do we suppress corrosion?

CHAPTER 16:CORROSION AND DEGRADATION

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• Corrosion: --the destructive electrochemical attack of a material. --Al Capone's ship, Sapona, off the coast of Bimini.

• Cost: --4 to 5% of the Gross National Product (GNP)* --this amounts to just over $400 billion/yr**

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

Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.

THE COST OF CORROSION

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

Zn Zn2 2e

2H 2e H2(gas)

• 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

Adapted from Fig. 17.1, Callister 6e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

CORROSION OF ZINC IN ACID

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

Plat

inum

met

al, 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 (+) Vmetal

o 0 (relative to Pt) Vmetalo 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

Plat

inum

met

al, M

H+ H+

2e-

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• EMF series • Metal with smaller V corrodes.• Ex: Cd-Ni cell

metalo

-

Ni

1.0 M Ni2+ solution

1.0 M Cd2+ solution

+

Cd 25°C

mor

e an

odic

mor

e ca

thod

ic AuCuPbSnNiCoCdFeCrZnAlMgNaK

+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

V = 0.153V

o

Data based on Table 17.1, Callister 6e.

STANDARD EMF SERIES

Cu ZnZn2+

2e- oxidationreduction

AcidH+ H+

H+H+

H+

H+

H+-+ AnodeCathode

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2H 2e H2(gas) O2 4H 4e 2H2O

CORROSION IN A GRAPEFRUIT

- +

Ni

Y M Ni2+ solution

X M Cd2+ solution

Cd T

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• Ex: Cd-Ni cell with standard 1M solutions

• Ex: Cd-Ni cell with non-standard solutions

VNio VCd

o 0.153 VNi VCd VNi

o VCdo

RTnF lnX

Y-

Ni

1.0 M Ni2+ solution

1.0 M Cd2+ solution

+

Cd 25°C

n = #e-per unitoxid/redreaction(=2 here)F = Faraday'sconstant=96,500C/mol.• Reduce VNi - VCd by

--increasing X --decreasing Y

EFFECT OF SOLUTION CONCENTRATION

• Ranks the reactivity of metals/alloys in seawaterm

ore

anod

ic (a

ctiv

e)m

ore

cath

odic

(iner

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

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Based on Table 17.2, Callister 6e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

GALVANIC SERIES

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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.(see Table17.2) Zn & Mgvery anodic.

• Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows).

• PittingDownward propagationof small pits & holes.

• Crevice Between twopieces of the same metal.

Rivet holes

attacked zones

g.b. prec.

Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)Fig. 17.9, Callister 6e.

Fig. 17.8, Callister 6e.(Fig. 17.8 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)

FORMS OF CORROSION

• Stress & Saltwater... --causes cracks!

• Heat treatment: slows crack speed in salt water!

4m--material: 7150-T651 Al "alloy" (Zn,Cu,Mg,Zr)

Adapted from Fig. 11.20(b), R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials" (4th ed.), p. 505, John Wiley and Sons, 1996. (Original source: Markus O. Speidel, Brown Boveri Co.)

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Adapted from Fig. 17.0, Callister 6e.(Fig. 17.0 is from Marine Corrosion, Causes, and Prevention, John Wiley and Sons, Inc., 1975.)

Adapted from Fig. 11.24,Callister 6e. (Fig. 11.24 provided courtesy of G.H.Narayanan and A.G. Miller, Boeing CommercialAirplane Company.)

“held at 160C for 1hr before testing”

increasing loadcrac

k sp

eed

(m/s

)

“as-is”

10-10

10-8

Alloy 7178 tested in saturated aqueous NaCl solution at 23C

DETERIORATIVE

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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.

Adapted from Fig. 17.13(a), Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)

Adapted from Fig. 17.14, Callister 6e.

CONTROLLING CORROSION

steel

zinczincZn2+

2e- 2e-

e.g., zinc-coated nail

steel pipe

Mg anode

Cu wiree-

EarthMg2+

e.g., Mg Anode

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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.• Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals.• The Galvanic Series ranks the reactivity of metals in seawater.• Increasing T speeds up oxidation/reduction reactions.• Corrosion may be controlled by: -- using metals which form a protective oxide layer -- reducing T

-- adding inhibitors-- painting--using cathodic protection.

SUMMARY

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