Ch-16 Compatibility Mode

7/28/2019 Ch-16 Compatibility Mode

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CHAPTER 16

Corrosion and Degradation of

Materials


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Corrosion:--the destructive electrochemical attack of a material.--Al Capone's

ship, Sapona,off the coastof Bimini.

THE COST OF CORROSION

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: AnIntroduction to Corrosion Science and Engineering, 3rd ed., John Wileyand Sons, Inc., 1985.**Economic Report of the President (1998).

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


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ELECTROCHEMICAL REACTIONS

Half-cell Reaction (Oxidation)Loss of Electron (s) M = Mn+ + ne-

Charge neutrality is maintained by reduction reaction

Half-cell Reaction (Reduction)

n+ - = (n-1)+ n+ - =

Examples

Acid Solution: 2H+ + 2e- = H2

Acid Solution with dissolved O2 O2 + 4H+

+ 4e-

= 2H2ONeutral or Basic Solution with O2 O2 + 2H2O + 4e

- = 4(OH-)


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

Zn Zn2+

+ 2e

2H++ 2e H2(gas)

oxidation reaction

Zn Zn2+

H+

H+

CORROSION OF ZINC IN ACID

Zinc 2e-c

solution

reduction reaction

H+H+

H2(gas)

H+

H+

H+

flow of e-in the metal

Adapted from Fig. 17.1, Callister6e. (Fig. 17.1 is from M.G. Fontana,Corrosion Engineering, 3rd ed.,McGraw-Hill Book Company,1986.)


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Two outcomes:

--Metal sample mass --Metal sample mass

um

M n+

ne-H2(gas)

2e-

e-e-

+

STANDARD HYDROGEN (EMF) TEST

Mn+

ne-

e- e-

um

,M

H+H+

2e-

Plati

meta

l

ions

25C1M Mn+ soln 1M H+ soln

H+

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

Vmetalo

< 0 (relative to Pt) Vmetalo

> 0 (relative to Pt)

Standard Electrode Potential

ions

25C1M Mn+ soln 1M H+ soln

Plati

metal


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EMF series Metal with smallerV corrodes.

Ex: Cd-Ni cellmetalo

- +

recatho

dic Au

CuPbSnNi

+1.420 V

+0.340- 0.126- 0.136- 0.250

metal Vmetalo

o

STANDARD EMF SERIES

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

Cd 25C

morean

odic

mo

oCdFeCrZnAlMgNaK

- .- 0.403- 0.440- 0.744- 0.763- 1.662- 2.262- 2.714- 2.924

0.153V

Data based on Table17.1, Callister 6e.


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CONDITIONS FOR SPONTANEOUS REACTION

Generalized Reaction

M1 = M1n+ + ne- -V1

o Oxidation

M2n+ + ne- = M2 +V2

o Reduction

For the overall reaction: M1 + M2n+ = M1

n+ + M2

Overall cell Potential : Vo = (V2o V1o ) > 0 :Spontaneous

T and Concentration dependence

V = (V2o V1o) (RT/nF) ln ((M1n+)/(M2n+))

= (V2o V1

o) (0.0592/n) log (M1n+)/(M2

n+))


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Cu

ZnZn2+

-reduction

H+ H+

-+AnodeCathode

ANORTHER EXAMPLE OF CORROSION

Acid

H+ H+

H+

H+

H+2H

+

+ 2e

H2(gas)

O2 + 4H++ 4e 2H2O


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- +

Ex: Cd-Ni cell withstandard 1M solutions

Ex: Cd-Ni cell withnon-standard solutions

V

Ni

oV

Cd

o= 0.153

VNi

VCd=

VNio

VCdo

RT

nF ln

X

Y- +

n = #e-

EFFECT OF SOLUTION CONCENTRATION

Ni

Y M

Ni2+ solution

X M

Cd2+ solution

Cd TNi

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

Cd 25C

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

-- increasing X

-- decreasing Y


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Ranks the reactivity of metals/alloys in seawater

recathodic

(inert)

PlatinumGold

GraphiteTitaniumSilver316 Stainless Steel

Based on Table 17.2, Callister

GALVANIC SERIES

morean

odic

(activ

e)

m

CopperNickel (active)TinLead316 Stainless Steel

Iron/SteelAluminum AlloysCadmiumZincMagnesium

. .

M.G. Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill Book Company,1986.)


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CORROSION RATES

Corrosion Penetration rate (CPR)

W (mg) = Wt. loss at time t (h) ; = density (g/cc) ; K =

constant and A (Sq inch) specimen area

= =

At

KWCPR

=

. .

Corrosion rate

i = Current density i.e current for unit surface area ofthe material corroding

nF

ir=


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PREDICTION OF CORROSION RATES

Polarization-- Displacement of each

equilibrium potential from its

equilibrium value

Overvoltage ()-- Magnitude of displacement

involved in olarization

pp. 572-581 , W. D.

Callister, 5th

edition

Electrochemical cell consisting

of standard Zn and H2 electrode

Potential of Znelectrode EquilibriumPotential

VVV 142.0)763.0(621.0 +==


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ACTIVATION POLARIZATION

Activation Polarization-- Refers to a condition

where the reaction rate is

governed by the step with

slowest rate

-- Activation Energy

Barrier in slowest step

Schematic representation of possible stepsin the hydrogen reduction reaction (Rate

controlled by activation Polarization)

SEQUENCE OF STEPS


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Eq. 1

E uilibrium is d namic


2H+ + 2e- = H2

H2 = 2H+ + 2e-

rred

roxi


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-- At Equilibrium rred = roxi

Plot of activation polarization overvoltage Vs logarithm of Current Density

Depends on electrochemical systems


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CONCENTRATION POLARIZATION

Reaction Rate-- Reaction rate by

diffusion in the

solution

-- The diffusion of H+

to the Interface is rate

controlling

For H2 reduction, Schematic representation of H+

distribution in the vicinity of Cathode (a) Lowreaction Rate (b) High reaction Rate


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Overvoltage Vs Logarithm of Current Density

Concentration Polarization

Overvoltage-- Decrease of

Overvoltage as i iL

c va on- oncen ra on o ar za on

Overvoltage-- Sum of activation and

Concentration contribution


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REACTION RATE LIMITED BY ACTIVATION POLARIZATION

Potential of

uncoupled H2Half cell

Exchange current

density; H2 Half

cell

Ox Rate = Red.

ELECTRODE KINETIC BEHAVIOUR OF Zn IN ACID SOLUTION

Potential of

uncoupled Zn Half

cell

Exchange current

density, Zn Half

cell

Rate at Vc

and ic


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REACTION RATE LIMITED BY ACTIVATION and

CONCENTRATIONPOLARIZATION

Ox Rate = Red.

Rate at Vc and ic

ELECTRODE KINETIC BEHAVIOUR OF METAL, M


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PASSIVITY

i, Decreases,

and remains

i, Increases

SCHEMATIC POLARIZATION CURVE FOR A METAL THAT

DISPLAYS ACTIVE-PASSIVE TRANSITION

Linear


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PASSIVITY

Solution 2

ACTIVE-PASSIVE METAL EXHIBITING ACTIVE AND PASSIVE CORROSION


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Forms

Uniform AttackOxidation & reductionoccur uniformly oversurface.

Selective LeachingPreferred corrosion ofone element/constituent

Stress corrosionStress & corrosionwork togetherat crack tips.

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

PittingDownward propagationof small pits & holes.

FORMS OF CORROSION

corrosion

(e.g., Zn from brass (Cu-Zn)). IntergranularCorrosion alonggrain boundaries,often where special

phases exist.

GalvanicDissimilar metals are

physically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic.

Crevice Between two

pieces of the same metal.Rivet holes

attackedzones

g.b.prec.

Fig. 17.6, Callister 6e. (Fig. 17.6 iscourtesy 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.)


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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

CONTROLLING CORROSION

e.g., remove 2 gas y reac ng w an n or .--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) isfrom M.G. Fontana, CorrosionEngineering, 3rd ed., McGraw-Hill Book Co., 1986.)

Adaptedfrom Fig.17.14,Callister

6e.

steel

zinczinc

Zn2+

2e- 2e-

e.g., zinc-coated nail

steelpipe

Mganode

Cu wiree-

Earth

Mg2+

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 ElectrodePotential are more likely to corrode relative toother metals.

SUMMARY

The Galvanic Series ranks the reactivity of metals inseawater.

Increasing T speeds up oxidation/reduction reactions. Corrosion may be controlled by:

-- using metals which forma protective oxide layer

-- reducing T

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

Ch-16 Compatibility Mode

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