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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.corrosion-doctors.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 s-1)

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 m-2)

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

7

8

2

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

(

)

10

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

11

Various forms of corrosion Corrosion mitigation

12

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

13

14

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 acid-base 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 acid-base equilibria

16

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4

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

17

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5

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