CHEM 3562 (C31C):
Environmental Degradation of Materials
An Introduction to the Principles and Prevention of Corrosion
Friday 1st February 2008
Lecture 6
Dr. Ann Wilson
Chemistry Building C1 – 3rd Floor Room 313
Tel.: Ext. 2283
Email: Ann.Wilson@sta.uwi.edu
Housekeeping
Site Visit to NGC
Date: Thursday 21st January
Time: 1:00 – 4:00 p.m.
Departure time: TBA
Required: Number of vegetarians
CHEM3562 –EDM-L6 -2 -
1
Basic principles of corrosion
Exchange current density
Electrochemical polarization
Overpotential
Mixed potential theory
Topics
CHEM3562 –EDM-L6 -3 -
Basic principles of corrosion
Me → Me z + + z e −
corrosion rate rc =
ia
m
=
= rc
z F At
mass loss
(unit area ) (unit time)
i = current density (e.g. µA/cm2)
rc: e.g. in mg per cm2 per day
corrosion rate is proportional to the current density during
dissolution
corrosion rate is inversely proportional to the area for the
same dissolving current,
⇒ Current measurements are a sensitive and convenient
tool for studying corrosion in the laboratory and the field!!
Corrosion and Faraday’s law
CHEM3562 –EDM-L6 -4 -
2
Basic principles of corrosion
penetration rate rp =
depth of penetration
unit time
corrosion rate rc = X
mg
g
and δ = Y
2
3
cm day
density cm
r
X mg cm3
rp = c =
δ Y cm 2 day g
= 1437
.386
143.739
X mils
(mpy )
Y year
=
=
X 10 −3 g cm3
Y cm 2 day g
10 X µm
Y day
10 −3 X cm
Y
day
=
Corrosion rate ↔ Penetration rate
CHEM3562 –EDM-L6 -5 -
Basic principles of corrosion
Calculation of the penetration/corrosion rates from the current density
needs the equivalent weight a/z (EW)
rp =
rc
=
δ
i a 1
F z δ
rp =
i EW
F δ
For an alloy the EW value is the weighted average of a/z for
the major alloying components !
How to calculate EW for an alloy:
EW =
1
f z 
Σ  i i 
 ai 
Penetration rates for alloys
CHEM3562 –EDM-L6 -6 -
3
Basic principles of corrosion
current flow NOT possible!
Standard half
cell potential:
- 0.763 V
Standard half
cell potential:
0V
Volts
porous
membrane
Platin
Zn2+; unit acitivity
H2
H+; unit acitivity
gas
inlet
Cell with reversible zinc and hydrogen electrodes
CHEM3562 –EDM-L6 -7 -
Basic principles of corrosion
current flow possible!
Ampere
Volts
porous
membrane
Deviation of the electrode
potential from equilibrium
potential
Zn2+
Platin
H2
Zn2+
Zn2+; unit acitivity
H+; unit acitivity
gas
inlet
Short-circuited cell with a zinc and a hydrogen electrode
CHEM3562 –EDM-L6 -8 -
4
Basic principles of corrosion
Anode (+): net oxidation
Cathode (-): net reduction
Polarisation:
deviation of the electrode
potential from the equilibrium
potential
η = E – Eº (overvoltage)
Zinc, corroding in an acidic solution
CHEM3562 –EDM-L6 -9 -
Basic principles of corrosion
Polarisation:
- abreviated with a η
η = E – Eº
- also called overpotential, overvoltage
- is the potential change, E – E0, from the equilibrium
potential, E0, caused by a net surface reaction rate
for the half cell reaction
Cathodic Polarisation: - abreviated with a η c (negative value)
- electrons are supplied to the metal surface
- buildup in the metal due to slow reaction
rate causes surface potential, E, to become
negative with respect to E0
Anodic Polarisation:
- abreviated with a η a (positive value)
- electrons are removed from the metal
- deficiency results in positive potential
change due to slow liberation of electrons
by the surface reaction
Definition – Polarization (Overpotential, Overvoltage)
CHEM3562 –EDM-L6 -10 -
5
Basic principles of corrosion
Zn ↔ Zn2+ + 2eηa = 200 mV (0.2 V)
η/V
E/V
η = E – Eº
η + Eº = E
+
+
0.2
- 0.56
0
- 0.76
- 0.2
- 0.96
-
-
0.2 V + (-0.76 V) = E
exchange
current
densities
E0 = - 0.76 V vs. SHE
Zn → Zn2+ + 2ei0
- 0.56 V = E
ηc = - 200 mV (- 0.2 V)
η = E – Eº
η + Eº = E
- 0.2 V + (-0.76 V) = E
i/mA cm2
Zn2+ + 2e- → Zn
- 0.96 V = E
Example - Overpotential
CHEM3562 –EDM-L6 -11 -
Basic principles of corrosion
Consider:
2H+ + 2e-
rforward
rreverse
H2
At equilibrium potential:
i forward = i reverse = i o
rforward = rreverse =
exchange
current
densities
i0 a
nF
I0 depends mainly on the nature of the electrode surface
The free energy change and the half cell potential are fundamental
thermodynamic parameters of electrochemical reactions, io is the
fundamental kinetic parameter.
Can not be measured directly
Note: There is no net reaction and no net current, since oxidation
and reduction rates are equal
Exchange current density
CHEM3562 –EDM-L6 -12 -
6
Basic principles of corrosion
2H+ + 2e -
rforward
rreverse
H2
∆ G0 = − n F ∆E0
The surface on which the reaction occurs has no effect on the
electrode potential
io, in contrast, is strongly affected by the type of the surface
⇒ the free energy change remains the same but the kinetics
are varied for the electrode reactions at different interfaces
Effect of reaction surface on electrode potential and exchange
current density
CHEM3562 –EDM-L6 -13 -
Basic principles of corrosion
Polarisation, η, is the potential change, E – Eº, from the equilibrium
half cell electrode potential caused by a net surface reaction rate for
the half cell reaction
Two main types of polarisation:
Activation polarisation
refers to electrochemical reactions that are controlled by a slow
step in the reaction sequence occuring to establish electron
transfer
Concentration polarisation
refers to electrochemical reactions that are controlled by the
mass transport of species involved in the reaction sequence
occuring to establish for the electron transfer
Polarisation – Overpotential/Overvoltage
CHEM3562 –EDM-L6 -14 -
7
Basic principles of corrosion
Me
rforward
Mez+ + ze -
rreverse
Equilibrium disturbed:
η = E − E0
Polarized electrode
Activation energy for
dissolution and
deposition are NOT
ANYMORE equal
→
←
i ≠ i ⇒ i net
Activation Polarisation
CHEM3562 –EDM-L6 -15 -
Basic principles of corrosion
Hydrogen evolution reaction↑
Diffusion of involved
species becomes a rate
limiting step
Concentration Polarisation
CHEM3562 –EDM-L6 -16 -
8
Basic principles of corrosion
Butler-Vollmer and Tafel equation
cathodic and anodic polarisation
Mixed Electrode Potential Theory
E/V
Zn → Zn2+ + 2e+
- 0.56
- 0.76
i0
i/mA cm2
- 0.96
-
Zn2+ + 2e- → Zn
Next
CHEM3562 –EDM-L6 -17 -
9