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Electrochemical thermodynamics
Electrochemical corrosion rate measurement techniques
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Electrochemical kinetics Corrosion rate measurements
LPR EIS
· · · · ·
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Various forms of corrosion Corrosion mitigation image source: www.corrosiondoctors.org 1 2
Electrode potential
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Reversible potential
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
· Kinetics perspective:
Reversible potential Mixed / corrosion potential
H+ + e  k f H +  H + e k b H
+
H
+
potential / V
Electrochemical kinetics Corrosion rate measurements
LPR EIS
· Thermodynamics perspective
Electrode potential and free energy change
+
Electrochemical kinetics Corrosion rate measurements
LPR EIS
e

Erev
H
H
+
+e

H
Various forms of corrosion Corrosion mitigation
Various forms of corrosion Corrosion mitigation
io
log (reaction rate) / (A mm log (current density)(/kmol 2)2 s1)
4
3

1
Mixed/corrosion potential
+
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Electrode potential and free energy change
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
H H
+
+e

G = nFE
free energy change = = charge exchanged × × reversible potential difference
Electrochemical kinetics Corrosion rate measurements
LPR EIS
potential / V
Erev (H + H ) Ecorr Erev (Fe / Fe2+ )
Fe
2+
Fe
+2 e
e 2+ + 2 Fe ic anod total tota l ca thod H+ ic +e H Fe
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Various forms of corrosion Corrosion mitigation
icorr
log i / (A m2)
5
Various forms of corrosion Corrosion mitigation
E = Erev ( c )  Erev ( a )

6
Standard electromotive force (emf) potentials
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Various forms of corrosion Corrosion mitigation
Various forms of corrosion Corrosion mitigation
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Reaction feasibility
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Nernst equation
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
image source: http://www.polytechphotos.dk
Zn + 2HCl ZnCl2 + H2
or in terms of half cell reactions
Zn Zn2+ + 2e 2H+ + 2e  H2
aA + mH+ + ne  bB + dH2O
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Electrochemical kinetics Corrosion rate measurements
LPR EIS
E = Erev ( c )  Erev ( a ) = 0 + 0.763 V G = nFE
is negative meaning that reaction is feasible
9
o Erev = Erev +
Various forms of corrosion Corrosion mitigation
Various forms of corrosion Corrosion mitigation
(cA ) (cH+ ) 2.3RT log nF (cB )b cH2O d
a m
(
)
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Nernst equation for hydrogen ion reduction
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Nernst equation for water reduction
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
aA + mH + ne bB + dH2O
H +e H
a=0 n=1 b = 1, B H d=0
+ 
+

H2O + e  H + OH
Electrochemical kinetics Corrosion rate measurements
LPR EIS
at 2.3 RT/F=0.059 V
o rev H + H
25oC
Electrochemical kinetics Corrosion rate measurements
LPR EIS
o Erev ( H 2O H ) = Erev (H + H )  0.059 pH
Various forms of corrosion Corrosion mitigation
Erev (H + H ) = E (
)  0.059 pH
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Various forms of corrosion Corrosion mitigation
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3
Nernst equation for oxygen reduction
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Pourbaix diagram for Fe
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
O 2 + 4H+ + 4e  k f 2H2O
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Erev (O2
H 2O )
o = Erev (O2
H 2O )
 0.059 pH
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Various forms of corrosion Corrosion mitigation
Various forms of corrosion Corrosion mitigation
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Pourbaix diagram for Cr
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Key to features on Pourbaix diagrams
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Various forms of corrosion Corrosion mitigation
Various forms of corrosion Corrosion mitigation
Low potential values represent a reducing environment. High potential values represent an oxidizing environment. Vertical solid lines separate species related by acidbase equilibria Other lines separate species related by redox equilibria where redox equilibria of species not involving hydrogen or hydroxide ions appear as horizontal boundaries while redox equilibria of species involving hydrogen or hydroxide appear as diagonal boundaries becuase they are in part acidbase equilibria
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Key to features on Pourbaix diagrams
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Key to features on Pourbaix diagrams
Introduction Electrochemical thermodynamics
Electrode potential emf Series Nernst equation Pourbaix diagram Reference electrodes
Electrochemical kinetics Corrosion rate measurements
LPR EIS
Long dashed lines enclose the theoretical region of stability of the water to oxidation or reduction. For example dashed line b represents the potential of water saturated with dissolved O2at 1 atm (very well aerated water). Above this potential water is oxidized to oxygen:
Dashed line a represents the potential of water saturated with dissolved H2 at 1 atm pressure (high level or reducing agents in solution). Below this potential water is reduced to hydrogen:
2 H2O O 2 + 4 H+ + 4 e 
Electrochemical kinetics Corrosion rate measurements
LPR EIS
H2O + e  H + OH
Various forms of corrosion Corrosion mitigation
Various forms of corrosion Corrosion mitigation
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