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Applied Electrochemistry

Dept. Chem. & Chem. Eng.

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Lecture 14Metal Electrochemistry

Dept. Chem. & Chem. Eng.

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Outline

Electrodeposition 1

Corrosion 2

Industrial electrolytic process

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DefinitionCorrosion is the deterioration of materials by chemical interaction with their environment.  The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.

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3

Env

ironm

ents

in C

orro

sion

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1Sheir, L.L., R.A. Jarman, and G.T. Burstein, eds. Corrosion. 3rd ed. Vol. 1. 2000, Butterworth-Heinemann: Oxford.

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* 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.

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**

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Cost of Corrosion(2004) in billion US$5

940 (approximately)510.14Global

.....

.....

.....

3.38Canada

3.53Poland

3.78India

6.75Belgium

7.32Australia

8.51UK

49.26Germany

55.01Former USSR

59.02Japan

200 (approximately)303.76USA

Indirect Cost Direct CostCountry

5Bhaskaran, R., N. Palaniswamy, and N.S. Rengaswamy, Global Cost of Corrosion—A Historical Review, in Corrosion: Materials, Vol 13B, ASM Handbook. 2005, ASM International.

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4Fe + 6H2O + 3O2 4Fe(OH)3

 gives ferric hydroxide 2Fe(OH)3 Fe2O3 3H2O

gives iron oxide (rust) and water

The Rusting Mechanism (Peel)

Basic “rusting” or corrosion requirements1. The metal is oxidized at the anode of an electrolytic cell2. Some ions are reduced at the cathode3. There is a potential or voltage difference between the anode and cathode4. An electrolyte (fluid) must be present5. The electrical path must be completed

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• Two reactions are necessary: -- oxidation reaction: Zn → Zn2+ + 2e-

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

metalo

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

V = 0.153V

o

Data based on Table 17.1, Callister 6e.

Standard EMF

-

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

+

25°C NiCd

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Thermodynamic Driving Force

Like all chemical reactions – ThermodynamicsWhat is the driving force for the reaction? (otherwise stated as what is the electrochemical potential for the reaction)

Dissimilar metals Different cold work states Different grain sizes Difference in local chemistry Difference in the availability of species for a reaction (concentration cells) Differential aeration cells

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eFeFe 22

tsreac

productsoo

a

aRTGQRTGG

tan

ln)ln(

)(22 2 gHeH

For:

)(2 22 gHFeHFe

2

2

22

2

][

][ln

)()(

)()(ln

H

FeG

HaFea

HfFeaGG oo

1

1

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)ln(

)ln(

QRTnFEnFE

or

QRTnFEnFE

o

o

Introduce: The total electropotential is G = -nFE

Where: F = Faraday’s constant (total charge on Avogad

ro’s number of electrons)n = the number of electrons transferredE = The electrode potential

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)ln(QnF

RTEE o

For the Given Example:

2

2

][

][ln

H

Fe

nF

RTEE o

Note: pH = -log10[H+]

Nernst Equation:

The Basic equation which describes ALL corrosion reactions

15Source: www.corrosionsource.com

Pourbaix Diagram

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- +

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

lnXY-

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

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Kinetics Describes Rate of Reaction Evan’s DiagramKinetics Describes Rate of Reaction Evan’s Diagram

CURRENT DENSITY, A/cm²

BB

A

0

0

E

E

io Fe ANODIC

ANODIC

Fe F

e+2 +2e-

H 2H+ +2e-

2

PO

TE

NT

IAL

VO

LT

S (

SH

E)

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

10 10 10 10 10 10 10 10 10-10 -9 -8 -7 -6 -5 -4 -3 -2

CATHODIC

CATHODIC

Fe +2 +2e - Fe

2H + +2e - H2

A

for H2on Fe

io

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M

M+ +

e-

iaO2

Ac

ioH+ Aa

½ O2 + H

2O + 2e - 2OH -

AaioO2

CreviceEffect

No CreviceEffect

PLUS CreviceEffect

icorr in very aggresive environment

Log

E

EM/M+

EO2 /OH +

= Area Inside Crevice (Anodic)= Area Outside Crevice (Cathodic)

Aa << Ac

Aa

Ac

i

+oi H Ac

Area Effects

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Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive Metal

Log i

M M+

[Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967]

Increasing OxidantConcentration

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Passivity is defined as corrosion resistance due to formation of thin surface films under oxidizing conditions with high anodic polariza tion.

Passivity

For example Fe is passivated in concentrated nitric acid.

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V o lts : S a tu ra te d C a lo m e l H a lf- C e ll R e fe re n c e E le c tro d e

G a lv a n ic S e rie s - C o n c e n tra te d H y d ro c h lo ric A c id a t 2 5 °C [C ru m a n d S c a rb e rry, C o rro s io n o f N ic ke l B a s e A llo y s C o n fe re n c e P ro c e e d in g s - A S M 1 9 8 5 ]

+0.4 +0.3 +0.2 +0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8M a g n e s iu mM a n g a n e seA lu m in u mZin cC a d m iu mLe a dA llo y 2 5 5 (F e rra liu m )H ig h P u rity Iro nC o p p e rA llo y P E 6 2A llo y 2 6 - 1 , 2 6 - 1 1 / 4C a rb o n S te e lM O N E L a llo y 4 5 1TinA llo y 1 8 - 1 8 - 2A llo y 3 R E 6 0A llo y 1 7 - 4 p HIN C O LO Y a llo y 8 4 0 , 5 0 N i - 5 0 C rB ra ss A llo y sN ic ke l S ilv e r9 0 - 1 0 C o p p e r- N ic ke lB ro n z e A llo y s7 0 - 3 0 C o p p e r- N ic ke lA llo y 2 3 0 (C o rro n e l)A llo y 7 0 C b 3S ta in le ss S te e l 3 0 4 , 3 1 6 , 3 1 6 L, 3 1 7A llo y 2 0C a s t Iro nN i- R e s is t 2A llo y 2 5 4 S L XA llo y 9 0 4 LIN C O N E L a llo y s 6 0 0 , 6 0 1 , 6 9 0 , 7 0 2 , 7 4 8 , X 7 5 0IN C O LO Y a llo y 8 2 5A llo y B , P, P D (Illiu m )A llo y GA llo y 6 X (H A )M O N E L a llo y s 4 0 0 , 4 0 4 , 4 0 5 R , K 5 0 0N ic ke l 2 0 0 , 2 7 0A llo y 7 0 0 (J e s so p )A llo y 6 XS ilv e rA llo y GIN C O LO Y a llo y 8 0 0IN C O N E L a llo y s 6 1 7 , 6 1 8 E , 6 2 5A lu m in u m A llo y 5 0 5 2S ta in le ss S te e l 4 3 0Tita n iu m+ 0 .4 - 0 . 4 8 V P la t in u m 9 2 3 4 5 r1

DISSOLVED O2 (Mg/H2O)

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30

25

20

15

10

5

0

CO

RR

OSIO

N R

ATE (

MPY))

DISSOLVED O2 (PPM)

0 1 2 3 4 5 6 7 8 9 10 11

0 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 7.0 7.7

Effect of Temperature and Dissolved O2

LEGEND

FRESH WATER @ 50°FFRESH WATER @ 90°FFRESH WATER @ 120°F

VELOCITY = 2.5 FPS

pH=7, R=100M-0HMpH=7, R=2500M-0HM

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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

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Types of Aqueous Corrosion Cells

a. General Corrosionb. Localized Corrosion

Pitting Crevice Corrosion Under-deposit Corrosion MIC

c. Tuberculationd. Galvanic Corrosion

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• Material properties• Metallurgical factors• Passivity• Environment

Metallurgical factors• Chemical segregation• Presence of multiple

phases• Inclusions• Cold Work• Non-uniform stresses

Passivity• Example with steel in

nitric acid…dilute solutions will cause rapid attack, strong solutions have little visible effect.

• Surface film can be formed

• Some types of steel may do this with rust

• Aluminum does this• Need to watch

passive film, but can be used for simple protection

Factors Affecting Corrosion

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• Self-protecting metals! --Metal ions combine with O2

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 Figs. 17.13(a), 17.14 Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)

steel

zinczincZn2+

2e- 2e-

e.g., zinc-coated nail

steel pipe

Mg anode

Cu wiree-

Earth

Mg2+

e.g., Mg Anode

Controlling corrosion

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Introduction

Corrosion Processes

Corrosion of Metals

Passivity of Metals

Atmospheric Corrosion of Steels