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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
ACTIVATION POLARIZATION
2H+ + 2e- = H2
H2 = 2H+ + 2e-
rred
roxi
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ACTIVATION POLARIZATION
-- 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.