41653 Corrosion
Exercise 3
Exercise 3
The Pourbaix Diagram
Purpose:
The purpose of the exercise is to compare measured corrosion data with the theoretically calculated Pourbaix diagrams for a given chemical solution. The Exercise is carried out with steel samples (St. 37).
Part of the exercise has already been carried out in the Virtual Laboratory Exercise 3, where polarization measurements are carried out for a number of different pH values. All these measurements are carried out in a chloride
free environment.
All participants in the course are going to examine how different concentrations of chloride can influence the corrosion performance of steel.
We will choose a salt concentration:
(I)
(II)
(III)
Which corresponds to sea water in the high concentration
Which corresponds to brackish water in the medium concentration
Which corresponds to the maximum allowed level in drinking water (this value is currently 250
mg/liter).
Find the salt content in mg/l from a typical waterworks in the Northern Zealand http://www.esrumvand.dk.
Notice: Data from all Danish waterworks can be found on the internet.
Introduction:
The Pourbaix diagram is a diagram that shows how a metal will behave thermodynamically in aqueous solutions
with a given values of potential and pH i.e. if it corrodes, passivates or becomes immune. We have already used
these diagrams in exercise 2.
The diagram is calculated based on electrochemical equilibriums where the physical and chemical equations for
free energy, equilibrium potential and concentration are linked together. The possible corrosion products in a given
system are calculated.
See: Uhlig UHLIG'S CORROSION HANDBOOK SECOND EDITION Edited by R. Winston Revie CANMET Materials Technology Laboratory Ottawa, Ontario, Canada
Chapter 6: Simplified Procedure for Constructing Pourbaix Diagrams
Chapter 7: Pourbaix Diagrams for Multielement Systems
If the metal is thermodynamically stable it is described as immune. If soluble corrosion products are formed the
material is corroding and if solid corrosion products, which prevent further corrosion, are formed the material is
passivating.
For an equilibrium solubility less than 10-6 M a material is described as insoluble.
Below are shown Pourbaix diagrams for iron, copper, and zinc. At Campusnet (lecture 2) you will find more. You
can on this website calculate simple diagrams with one metal without complexing ions
http://www.crct.polymtl.ca/ephweb.php?lang=en
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41653 Corrosion
Exercise 3
The corrosion speed can be determined from a polarization curve.
The measured current (the corrosion current) is relatively small when the metal is passivated (typically 10
µA/cm2), and the current is relatively large in the corrosion area.
For the given conditions the curve shows the extension of the active area and the placement of the passivation potential which must be exceeded to obtain anodic protection. From the curve the critical current density necessary
for passivation icr and the corrosion speed in passivated condition at ipassiv can also be read
Knowledge of the critical values gives the possibility of judging the risk for corrosion in practical,
based on potential measurements and knowledge of the cathode reaction involved.
Pourbaix diagrams can theoretically be calculated for pure metals in water and the polarization curves
give the possibility for constructing practical Pourbaix diagrams for alloys in different environments.
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41653 Corrosion
Exercise 3
Procedure
Mount the equipment as shown in the instruction video and fill the corrosion cell with 400 ml of solution and stir.
Buffer solutions
1 molar sodium sulphate/sulphuric acid
1 molar sodium acetate/acetic acid
1 molar sodium carbonate/ bicarbonate
Na2SO4/H2SO4
(CH3COONa/CH3COOH)
(Na2CO3/NaHCO3)
pH = 2.0
pH = 6.0
pH = 10.0
The above mentioned solutions are “doped” with a given amount of KCl which you decide.
You will get potassium chloride to add to the test solution for increasing the chloride content. It is particularly interesting to study the conditions in the weak-acidic area and in the alkaline area. Therefore
you must (all groups) carry out 9 measurements (for 3 pH values and 3 chloride concentrations)







When all sodium chloride (NaCl) is dissolved, the working electrode is mounted in the cell and the
potentiostat is connected.
The free corrosion potential can be read after 5 minutes.
The polarization is carried out from about 100 mV cathodic in relation to the corrosion potential.
Using the ”scanner” the potential is changed from the starting potential in the anodic direction with
50 mV/min to +2000 mV SCE and/or ian  300 mA.
Current and potential values are collected and transferred into Excel spreadsheets. Results are imaged in a log I/E diagram and I/E diagram.
The Pourbaix diagrams for the used metals can be produced from calculation or by looking it up in
a book. Regarding Fe and chloride you can find an appendix to this instruction about Pourbaix diagrams for different chloride concentrations. If you calculate the diagrams using HSC Chemistry
you must base the calculations on the relevant concentration. You can look at the Pourbaix diagrams in the Pourbaix Atlas before you do the calculations. (Atlas of electrochemical equilibria in
aqueous solutions,. By: Marcel Pourbaix).
Remember to get the results from the other groups
3
41653 Corrosion
Exercise 3
“The Virtual Laboratory”:
On the computers are videos showing the polarization measurements for iron (St 37) in a solutions with
different pH with and without chloride content.
These videos must be compared with the matching polarization curves and the Pourbaix diagrams
should be studied thoroughly before the questions below are answered.
Questions
1.
Can Pourbaix diagrams be used for determination of corrosion speeds? Explain your answer.
2.
Based on the polarization curves determine the corrosion potential, the potential area for passivation, the oxygen development potential (if possible), passivation current density (if possible) in
the solution at pH = 6 for varying chloride content. Remember the curve from the chloride free solutions (The Virtual Laboratory).
3.
If the practical passivation limit is chosen to 1µA/cm2 and 10 µA/cm2 respectively calculate the
corrosion speed in mm/year.
4.
Corrosion conditions (active, passive, immune) and other relevant factors should be commented in
relation to the calculated Pourbaix diagrams. Study the measurement results thoroughly and discuss the importance of your measured data in relation to for example corrosion in reinforced concrete used on environments containing chloride or corrosion in cars caused by NaCl on the roads.
Consider a situation where saltwater and oxygen enters a concrete construction. Answer the following:
1.
At what salt concentration and pH will we start to have a problem? Use the calculated
thermodynamical data for answering this question (Pourbaix diagrams).
2.
Using you measurements state what corrosion speeds you can achieve in practice. We are
interested in obtaining information about the influences that chloride has on the steel’s
passivation capabilities at the chosen pH and chloride values.
3.
Look at the website http://www.astm.org/Standards/C876.htm . Explain what is being
measured and what you can use the results for. Can the describe method give any information about corrosion speeds?
Remarks to question 4
pH in new concrete is very high – more than 10. But when concrete get older and perhaps is damaged serious problems can arise. In Denmark we have built very expensive bridges and tunnels
during the last 15 years. They are reinforced with steel. Read more about the problem at
http://www.azom.com/details.asp?articleID=1318#_Presence_of_Chloride
See also this website
http://www.arminox.com.au/Documents/ProjectProgresso.html#NULL
5.
Explain the shape of the polarization curve for those metals in the Pourbaix diagrams, which show
an inclined borderline between active and passive area.
6.
Would it be safe to use steel in an electrolytic alkaline degreaser (cathodic) where steel plates are
used as the anode material? The polarization is so powerful that it is pass the oxygen development
potential. Expect small amount of chloride (250 mg/l) in the degreaser.
7.
Calculate the Pourbaix diagram for graphite using HSC-Chemistry. Graphite is useable as anode
material in acidic electrolytes. Is this in accordance with the Pourbaix diagram for graphite or not?
Calculate the Pourbaix diagram for chrome on the website http://www.crct.polymtl.ca/ephweb.php?lang=en.
Suggest a method for selective dissolving/etching of a chrome coating on steel so the steel surface is
not affected by the removal of the chrome coating.
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41653 Corrosion
Exercise 3
Eh (Volts)
Fe - Cl - H2O - System at 25.00 C
2.0
Fe2O3
1.5
Fe(+3a)
1.0
FeO2(-a)
0.5
0.0
Fe3O4
-0.5
Fe(+2a)
FeOH(+a)
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
12
14
C:\HSC4\FeCl25.iep
pH
ELEMENTS
Fe
Cl
Molality
1.000E-06
1.000E-08
Eh (Volts)
Pressure
1.000E+00
1.000E+00
Fe - Cl - H2O - System at 25.00 C
2.0
Fe2O3
1.5
Fe(+3a)
1.0
FeO2(-a)
0.5
0.0
Fe3O4
-0.5
Fe(+2a)
FeOH(+a)
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
C:\HSC4\FeCl25.iep
ELEMENTS
Fe
Cl
12
14
pH
Molality
1.000E-06
1.000E-08
Pressure
1.000E+00
1.000E+00
5
41653 Corrosion
Exercise 3
Eh (Volts)
Fe - Cl - H2O - System at 25.00 C
2.0
Fe(+3a)
Fe2O3
1.5
FeCl2(+a)
1.0
FeO2(-a)
0.5
0.0
-0.5
Fe(+2a)
FeOH(+a)
Fe3O4
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
12
14
C:\HSC4\FeCl25.iep
pH
ELEMENTS
Fe
Cl
Molality
1.000E-06
1.000E-06
Eh (Volts)
Pressure
1.000E+00
1.000E+00
Fe - Cl - H2O - System at 25.00 C
2.0
Fe(+3a)
Fe2O3
1.5
1.0
FeCl2(+a)
FeO2(-a)
0.5
0.0
-0.5
Fe(+2a)
FeOH(+a)
Fe3O4
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
C:\HSC4\FeCl25.iep
ELEMENTS
Fe
Cl
12
14
pH
Molality
1.000E-06
1.000E-04
Pressure
1.000E+00
1.000E+00
6
41653 Corrosion
Exercise 3
Eh (Volts)
Fe - Cl - H2O - System at 25.00 C
2.0
Fe(+3a)
Fe2O3
1.5
1.0
FeCl2(+a)
FeO2(-a)
0.5
0.0
-0.5
Fe(+2a)
FeOH(+a)
Fe3O4
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
12
14
C:\HSC4\FeCl25.iep
pH
ELEMENTS
Fe
Cl
Molality
1.000E-06
1.000E-02
Eh (Volts)
Pressure
1.000E+00
1.000E+00
Fe - Cl - H2O - System at 25.00 C
2.0
Fe(+3a)
Fe2O3
1.5
1.0
FeO2(-a)
FeCl2(+a)
0.5
0.0
-0.5
Fe(+2a)
FeOH(+a)
Fe3O4
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
C:\HSC4\FeCl25.iep
ELEMENTS
Fe
Cl
12
14
pH
Molality
1.000E-06
1.000E-01
Pressure
1.000E+00
1.000E+00
7
41653 Corrosion
Exercise 3
Eh (Volts)
Fe - Cl - H2O - System at 25.00 C
2.0
Fe(+3a)
Fe2O3
1.5
1.0
FeO2(-a)
FeCl2(+a)
0.5
0.0
-0.5
Fe(+2a)
FeOH(+a)
Fe3O4
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
12
14
C:\HSC4\FeCl25.iep
pH
ELEMENTS
Fe
Cl
Molality
1.000E-06
5.000E-01
Eh (Volts)
Pressure
1.000E+00
1.000E+00
Fe - Cl - H2O - System at 25.00 C
2.0
Fe(+3a)
Fe2O3
1.5
1.0
FeO2(-a)
0.5
FeCl2(+a)
0.0
Fe3O4
-0.5
Fe(+2a)
FeOH(+a)
HFeO2(-a)
-1.0
Fe
-1.5
-2.0
0
2
4
6
8
10
C:\HSC4\FeCl25.iep
ELEMENTS
Fe
Cl
12
14
pH
Molality
1.000E-06
1.000E+00
Pressure
1.000E+00
1.000E+00
Please hand in one report for each group.
Please include the following: Course number, exercise number, group number and your names The front page for the report are available at the DVD.
Upload you report to CampusNet , or send it directly to Per Møller pm@epc-info.dk.
You will receive the corrected exercises.
8