ECHE 789B
Problem Set # 3
Due: Friday, February 9
1. Given an anodic Tafel constant of (1) 0,06 V/decade and (2) 0.12 V/decade
determine the number of electrons transferred during the charge processes.
(T=25oC, symmetry factor  =0.5).
2. Calculate the rate of zinc dissolution (A/m2) if the potential of the Zn anode is
0.898 V vs. SHE. (CZnSO4=1M, Tafel constant=0.045 V/decade, Io=10-2 A/m2,
Eo=0.763V vs. SHE).
3. a) Calculate the Tafel constant for hydrogen evolution on iron. Iron anodic
current is 10 A/m2 and exchange current density is 10-3 A/m2. Potential of iron
electrode with respect to SHE is –0.90 V at pH=0. Recalculate the Tafel
constant for the following iron anodic current densities 20, 30, 40 and 50
A/m2.
b) If the actual value for the Tafel constant is 0.12 V/decade, recalculate the
hydrogen overvoltage on an iron electrode.
4. The potential of Pb electrode (undergoing corrosion) is 0.343 V vs. SHE in 0.02
M PbSO4. Oxygen reduction occurs on the same piece of metal forming a
differential aeration cell. Oxygen reduction occurs at a potential of 0.90 V vs.
SHE in pH=4. Calculate the anodic current if charge transfer only controls the
reaction. (Tafel constants ba=bc=0.10 V/decade, io for both processes=10-1
A/m2).
5. For a hydrogen evolution reaction the exchange current density is 6.8x10-7 A/cm2.
If the reaction is proceeding at an overpotential  =0.1 v, calculate the current
density and the polarization resistance. Take the electrode area as 0.2 cm2 and
 =0.5.
6. The table below lists the kinetic parameters for processes that can occur during
the deposition of iron from acidified 0.05 M ferrous solution. Assuming activation
control, calculate the pH at which the iron deposition current is ten times that of
water discharge.
Reactive species
Fe2+
H3O
H2O
Eo (V) NHE
-0.44
0.00
0.00
log io
PH=9
-(0.5pH+4)
-8.0
1
d  /dlogi
0.12
0.12
0.12
Construction of Evan’s Diagrams
1. Construct an E-log(i) Evan’s Diagram for the corrosion of tin in hydrogen
saturated oxygen free solution in which activity of Sn is 0.4 (molar scale). The
data for the cathodic reaction are as follows:
Given:
ac=1.24
bc=0.11
Assume:
a) charge transfer kinetics
b) high field approximation applies
c) ba=0.1
d) ioa=10-10 mA/cm2
Calculate:
a) corrosion current
b) exchange current density for hydrogen evolution
c) protection current to prevent corrosion for PH2=1 atm.
The corrosion potential for Sn is –0.08V. If the hydrogen evolution reaction is
limited by the diffusion of H+ species to the metal sites, reconstruct the Evan’s
diagram for this case.
2. Using the following data, construct an E-log(i) Evan’s Diagram for the corrosion
of nickel in saturated oxygen solution of a Ni-0.23 (molar scale).
Assume:
a) charge transfer kinetics
b) high field approximation applies
Given:
a) corrosion potential=0.1V
b) exchange current density for nickel dissolution 10-5 A/cm2.
c) Anodic ba=0.12
d) Cathodic bc=-0.1
e) Cathodic intercept a=-0.6
Calculate:
a) corrosion current
b) exchange current density for oxygen on nickel dissolution 10-5 A/cm2
c) the protection current to prevent corrosion for PO2= 1 atm.
d) pH of the solution if the partial pressure of oxygen is 0.5 atm; 1 atm; 2
atm and 3 atm.
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3.
Using the following data, construct an E-log(i) Evan’s Diagram for the corrosion
of copper in saturated oxygen solution of a Cu=10-6M (molar scale) in 1.0 M
NaOH
Assume:
a) charge transfer kinetics
b) high field approximation applies
Given:
c) exchange current density for copper dissolution 10-9 A/cm2.
d) exchange current density for oxygen reduction 10-8 A/cm2.
e) Anodic ba=0.1
f) Cathodic bc=-0.12
g) Cathodic intercept a=-0.7
Calculate:
a) corrosion current
b) Ecorr
c) the protection current to prevent corrosion for PO2= 1 atm.
4. Using the following data, construct an E-log(i) Evan’s Diagram for the corrosion
of cobalt in saturated oxygen solution of a Co-0.3 (molar scale).
Assume:
a) charge transfer kinetics
b) high field approximation applies
Given:
c) corrosion potential=0.2V
d) exchange current density for nickel dissolution 10-7 A/cm2.
e) anodic ba=0.12
f) Cathodic bc=-0.1
g) Cathodic intercept a=-0.8
Calculate:
a) corrosion current
b) exchange current density for oxygen on nickel dissolution 10-5 A/cm2
c) pH of the solution if the partial pressure of oxygen is: 0.5 atm: 1 atm; 2
atm and 3 atm.
d) the protection current to prevent corrosion for PO2= 1 atm.
5.
Using the following data, construct an E-log(i) Evan’s Diagram for the
corrosion of iron in saturated oxygen solution of a Fe-0.3 (molar scale).
Assume:
a. charge transfer kinetics
b. high field approximation applies
Given:
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c) corrosion potential=0.2V
d) exchange current density for nickel dissolution 10-7 A/cm2.
e) ba=0.12
f) Cathodic bc=-0.1
g) Cathodic intercept a=-0.7
Calculate:
a) corrosion current
b) exchange current density for oxygen on nickel dissolution 10-5 A/cm2
c) pH of the solution if the partial pressure of oxygen is: 0.5 atm: 1 atm; 2
atm and 3 atm.
d) the protection current to prevent corrosion for PO2= 1 atm.
6. Using the following data, construct an E-log(i) Evan’s Diagram for the corrosion of
silver in 0.3 M HCl:
Assume:
a) charge transfer kinetics
b) high field approximation applies
Given:
a) ioH2=5x10-7 Am-2
b) ioAg=5x10-6 Am-2
c) bc=-0.09 V
d) ba=-0.12 V
e) EoH+/H2=0.0
f) EoAg+/Ag=0.799
g) [Ag+]=10-26
Calculate:
a) corrosion current
b) corrosion potential
c) the protection current to prevent corrosion
7.
Using the following data construct an E-log(i) Evan’s Diagram for the corrosion of
zinc on hydrogen saturated oxygen free solution of pH=0.8 and a Zn2+=0.15 M
(molar scale).
Assume:
a) charge transfer kinetics
b) high field approximation applies
Given:
a) ioH2=3x10-11 Am-2
b) anodic (  /  log i )=1.24V
c) c=-0.118 V
d) corrosion potential=-0.34V
e) EoH+/H2=0.0
f) EoZn2+/Zn=0.799=-0.763 V
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Calculate:
a)
b)
c)
d)
corrosion current
exchange current density
protection current required to reduce corrosion current to zero
the corrosion rate in mpy
8. Using the following data construct an E-log(i) Evan’s Diagram for the corrosion of
cadmium on hydrogen saturated oxygen free solution of pH=4.0 and a Cd2+=0.1
M(molar scale).
Assume:
a) charge transfer kinetics
b) high field approximation applies
Given:
a) ioH2=3x10-11 Am-2
b) ba=0.1V
c) bc=-0.12V
d) cathodic intercept a=-1.4 V
e) EoH+/H2=0.0
f) EoCd2+/Cd=0.799=-0.563 V vs. SHE
Calculate:
a) corrosion current
b) exchange current density
c) protection current required to reduce corrosion current to zero
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