41653 Corrosion
Exercise 2
Exercise 2
Single electrode polarization curve and
measurement of corrosion speed
using electro-chemical methods
The purpose of this exercise is to obtain knowledge about the following points
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Test of potentiostate in accordance to the international standard ASTM G5-87 (The standard is
available as a PDF file for all students at the course – You can read more about and why standards are important also for students http://www.astm.org/studentmember/index.html look at this
streaming video http://www.astm.org/studentmember/Images/yop3_swf_FL8_480x360.swf and
understand why you need to have knowledge about standards. We have the total collection of
ASTM standards with relationship to corrosion as PDF-files and you can make electronic search
in the files to find exact the standard you need. During the course you will learn to understand
the professional terms used in standards.
The influence of the scanning speed on the reliability of the measurement results.
Possible errors when recording polarization curves.
Calculation of corrosion speed at a given potentials.
Studies of chosen materials’ polarization curves and thermo dynamical aspects. (Some of the exercises are done in the Virtual Laboratory)
Explanatory exercise (an experiment to demonstrate the performance of stainless steel at varying
potential)
The exercise 2 takes three Wednesday afternoons to carry out. In the beginning the student can feel that
he or she lacks a bit of theory. Make sure to read the text for every lecture - especially in the beginning of
the course. Experience from earlier years shows that it should not be a problem for the students. We are
keen to help.
After exercise 2 you should have the following capabilities:
Recording polarization curves using the equipment and be able to judge, if the curves are fulfilling the
international standards (here you have to validate the equipment in accordance to an ASTM standard).
You must also be able to explain the importance of the scanning speed (mV/min) for the reliability of the
results and be able to diagnose typical errors when recording polarization curves. We will explain you
about the typical errors during the lab. exercises.
It is important quickly to become familiar with the calculation of corrosion speeds once the corrosion current is known. It is not hard when Faraday’s laws are used. (See Bardal page 8 and 9). In eq. 2.1.a and
2.1b please remember that 1 Coulomb has the unit [ampere  second]. If you get information about that the
corrosions current is 1 micro A/cm2 for a metal you should immediately have a feeling about the corrosion
speed in micron pr year. We will use such key numbers again and again.
You need to obtain practical experience regarding determination of corrosion potential and studies of
electro-chemical reactions, which take place during polarization of different electrode materials in two
different solutions primarily; One 0.5 M sodium acetate solution (NaAc) with and without NaCl (3 %),
pH = 6.0 added. It is important that this exercise gives a fundamental understanding of the chemical reactions at the electrodes, and that they can be related to thermo-dynamical conditions. Make it also clear
what the difference is between the term equilibrium potential and corrosion potential. Ask if you are in
doubt.
(If you are not familiar with calculating molecular weight you can find the atomic weights for the elements here http://www.webqc.org/periodictable.php and a calculator for the molecular weight here
http://www.webqc.org/mmcalc.php or http://www-jmg.ch.cam.ac.uk/tools/magnus/MolWeight.html You
can read much more about molecular weight in Wikipedia http://en.wikipedia.org/wiki/Molecular_mass .
It is very important that you get 100% familiar with that topic, because all concentration of ions in test
solutions has to be converted to mole/liter before we can start the thermodynamic calculations of Pourbaixdiagrams at the PC- see later – ask if you have questions or it feels a little strange)
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41653 Corrosion
Exercise 2
When all measurements are finished you should evaluate the consequence of galvanic coupling of selected materials regarding estimation of corrosion speeds.
When exercise 2 is finished we expect that you are able to compare the polarization curves and Pourbaix
diagrams for a given metallic material. You may also be able to predict the consequences of a change in
pH/chemical composition for the influence of corrosion risk at a given potential for a given material.
After this exercise you should “nearly” be able to choose any arbitrary metal in a given environment and
give a qualified suggestion for the material’s corrosion properties using the thermo-dynamical data and a
calculated Pourbaix diagram. Feel free to ask if you are in doubt.
The last part of exercise 2 will be to observe an interesting experiment which will tell you ”everything”
about the corrosion properties of stainless steel with active and passive areas. This experiment will give
an understanding of the phenomenon of passivation and explain what is going on in the trans-passive area
at a high anodic polarization.
Exercise 2 is a fast, efficient introduction to measuring corrosion properties and to predict corrosion
speeds under simple circumstances. It is important to understand what you learn, since you will have to
use it for “svendestykket” where you have to work independently with a task given by the industry.
NB: It is necessary to read chapter 2+3 in the book Corrosion and Protection by Ejnar Bardal and the
two chapters about Pourbaix diagrams loaded up at CampusNet. You must be able to interpret a Pourbaix diagram to be able to benefit from this exercise. In exercise 3 you shall work more with Pourbaix
diagrams for iron. This exercise is partly in The Virtual Lab which means that you must watch some videos of corrosion experiments and compare it to the theory. Furthermore we recommend strongly, that you
try to calculate a Pourbaix diagram – At the DVD you will find an assignment for calculating the Pourbaix diagram for Zn. It is very important that you exactly know how to calculate one before you start to
do it by software. Only few are using software for such calculation and it is important to be able to evaluate the result. At each laptop in the corrosion lab. you have access to software, which can calculate the
diagrams. Start to ask for The Marcel Pourbaix Atlas at my office
http://www.amazon.com/Atlas-Electrochemical-Equilibria-Aqueous-Solutions/dp/0915567989 . Try to
take some cases from the Atlas and select the same chemical species as the diagrams in the Atlas and look
if you get the same result. The software has thermodynamic data for more than 24.000 compounds. By
using such type of software – be carefully - "Garbish in Garbish out" !!!
Let us discuss the use of the software and read the additional material loaded op at CampusNet - about
how to calculate such diagrams. It is very difficult to give clear advises especially calculating Pourbaix
Diagrams for Multielement Systems. It is very new to do by laptops and only few students do it. Let us
discuss it during the lab. exercises. We have used during 10 years with success and use it at Ph.D. courses international.
Other information
Data measured with the potentiostat is recorded on the lab-PC and you can plot your date to a Stern diagram (Log I/E diagram, Tafel-diagram) using Excel. All potentials are stated in relation to the standard
hydrogen electrode (SHE). To convert SCE electrode data into SHE data you must add +244 mV.
Look at the figure (exercise) showing the converting between different electrode types. Regarding the
operation of the equipment, please refer to the instruction videos at the DVD. It a great advantage always
to refer to SHE, since the Pourbaix diagram refers to SHE.
See also: http://www.consultrsr.net/resources/ref/refpotls.htm
Calculation of Pourbaix diagrams was earlier very labor intensive. As an exercise you must calculate as
mentioned before the Pourbaix diagram for zinc (only to be sure that you know how it is done). This is
time consuming, so we will use the software and we can calculate in minutes what earlier could take
hours using HSC chemistry. http://www.outotec.com/pages/Page____35373.aspx?epslanguage=EN .
During the rest of the course we will make a lot of calculation and be familiar with parts of the software.
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41653 Corrosion
Exercise 2
In the lecture two chapters from the book:
Uhlig UHLIG'S CORROSION HANDBOOK SECOND EDITION Edited by R. Winston Revie CANMET Materials
Technology Laboratory Ottawa, Ontario, Canada
will be available as mentioned before at CampusNet. The name Uhlig is dedicated to the very famous
professor Herbert Uhlig, from MIT which edited this book first time in 1948.
Chapter 6: Simplified Procedure for Constructing Pourbaix Diagrams
Chapter 7: Pourbaix Diagrams for Multielement Systems
In chapter 7 especially the calculation of multielemts diagrams by computers will be discussed. During
the course we will take a lot of discussions how to calculate and what we have to think about.
For a quick calculation you can use the software http://www.crct.polymtl.ca/ephweb.php?lang=en Fact
Sage – which is the same software as Uhlig use in chapter 7. This software is freeware at the internet. The
software can calculate Pourbaix diagrams for all metals at 25 oC and at varying concentration of the metal
ion. The freeware is for demonstration only and cannot calculate cases with multielements systems. If you
want to use Fact Sage for multielements systems it is very costly. We use instead HSC chemistry, which
has a much larger data base.
(Notice that molarity and molality is nearly the same at low concentrations. Read about concentration at
this homepage http://en.wikipedia.org/wiki/Concentration or this homepage
http://environmentalchemistry.com/yogi/chemistry/MolarityMolalityNormality.html and be familiar with
molality, molarity and normality.)
Planning the work
(NB: Read thoroughly before doing the exercise)
Note that the exercise consists of two different polarizations.
A: Polarization measurement using stainless steel 420
http://www.suppliersonline.com/propertypages/420.asp#chemistry in sulfuric acid using the standard
ASTM G5-87 (1st exercise day). If the material is not available stainless steel 304 can be used.
http://www.fanagalo.co.za/tech/tech_grade_304.htm
B: Polarization measurement using specimens in buffer with and without chloride (1 polarization 1st exercise day – the rest in the next days)
1st exercise day
Please read instruction carefully – including how to assemble and use the potentiostat and prepare the
test sample. On the DVD this is shown in video clips. You must also be familiar with Excel. View the
sequence about Excel, so all your curves will look similar. That will make comparison of the results easier
– during the course it will sometimes be necessary to share information.
We have done the polarization curve according to the ASTM G5-87 standard for you, because it takes
more than 3 hours. The scanning speed is very low; 0.6 V/hr or 10 mV/min. A special kind of stainless
steel 430 is examined in 1 N H2SO4. This curve is also on the DVD for comparison.
Against the background of the Tafel curve (LogI/E curve) you must determine whether the potentiostat
used in the course is fulfilling the demands to reproducibility described in the ASTM G5-87.
From the same standard you must obtain knowledge about error sources, which is important.
A:
Record a polarization curve using a scanning speed of 50 mV/min, which is 5 times faster than the standard using the stainless steel 420. Determine how the two curves deviate from each other and state the reason. Furthermore you must calculate the corrosion speed in µ/year at a potential of 400 mV vs. SCE for
both scanning speeds.
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41653 Corrosion
Exercise 2
Note:
In A the curves must be shown with the potential-axis mV vs. SCE (to be able to compare with the curve
in the standard)
In B the curves must be shown with the potential-axis mV vs. SHE (measured potential-value + 244
mV).
It is important to get a knowledge of the instrument and error sources since you later must be able to make
the measurements alone, when you do the final assignment ”Svendestykket”. Ask when in doubt.
B:
The next collection of polarizations appears from the lists handed out. You must make another polarization curve the first exercise day.
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41653 Corrosion
Exercise 2
The following materials can be investigated
Samples are distributed among the teams
and the results are shared, so everybody
can answer the questions.
Material
Gold
Silver
Stainless 316
Stainless 304
Copper
Brass 63
Cast Iron
Lead
Nickel
Titanium
Iron
Zinc
Aluminum
Graphite
Tin
Magnesium (AM60)
Conversion table for different
reference electrodes.
NB: The solution for the corrosion measurements is handed out at
the beginning of the exercise along with info about the scanning
speed and scanning areas for the potentiostat.
You must choose a metal from the list of 4 metals along with a test solution and other test details, which
you must investigate (perhaps a very noble metal so the polarization will not take too long).
Oxygen free environment can be made using bubbling of N2
Oxygen saturated environment can be created using bubbling of O2
Using no bubbling one will have an oxygen containing environment.
Remember to state all measurement details on all curves:
Chemical
environment
pH
Containing oxygen/Oxygen free
Temperature
Ecorr
Scanning speed
Material
Remember this, because test results without details are useless – let’s take a case if you forget the scan
rate – you cannot compare two experiments carried out in the same “chemistry”. If you forget to give
info about the reference electrode we have the same problem. Make the rule to write all information as
info to each curve.
We have already measured copper, silver, gold and aluminum to make your work easier. The results are
on you PC in the lab along with the Pourbaix diagrams, polarization curves and video recordings showing
the appearance of the test specimens as function of the potential.
You must work through some of the videos from The Virtual Laboratory. Furthermore you will find
questions in the end of this exercise instruction, where we ask you to pay attention to special observations
which can be made.
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41653 Corrosion
Exercise 2
It is important that you always investigate the surface of the tested specimen in a stereo microscope or a
simple magnifications glass after recording the polarization curve. - How is the surface attacked? Uniform or local pitted. Perhaps the surface has changed color, – all this info is extremely important? – It can
give us information about the material is homogenous or not.
Combining this investigation with SEM analyzes
(http://en.wikipedia.org/wiki/Scanning_electron_microscope) combined with EDS
http://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy of the surfaces will improve the
knowledge. But we will not use this type of equipment for the moment, but later in connection with the
apprentices “Svendestykker”.
Using Pourbaix diagrams that are calculated by a PC at the laboratory for the different chemical solutions
together with E/logI curves (polarization curves) and the above information collected visual gives you a
lot of information about what type of chemical reaction probably takes place. You learn in that way to
ask the “right questions”. In the beginning it is difficult. But after few days you will feel yourself as an
expert!
The observations should be noted and added as informative text on the curves. In theory we can already in
exercise 2 see how a polarization curve can be interpreted by using all the available information.
It is important that you on the first exercise day carry out two polarization curves and start studying another couple in The Virtual Laboratory.
NB: – Make sure that you after the first exercise day can answer as many of the questions as possible so
you don’t have to do all the work at the end.
In A the curves must be shown with the potential-axis mV vs. SCE (to be able to compare with the curve
in the standard)
In B the curves must be shown with the potential-axis mV vs. SHE (measured potential-value + 244
mV).
Exercise day 2
You continue interpreting and recording polarization curves. In total you must record at least 4 curves pr
team and preferably all of them. These must be accepted by the teachers before you can proceed.
You will have sufficient time for the practical work since 3rd exercise day is also dedicated to work in the
lab. It is important to be careful and precise doing the recordings to avoid the errors described in ASTM
G5-87.
Make sure to clean the cell thoroughly and that the counter electrodes are cleaned – preferably in diluted
nitric acid.
You must continue using the Virtual Laboratory and try to understand the chemical reactions taking place
when the different electrodes are polarized anodic and cathodic.
Only when video sequences, polarization curves and Pourbaix diagrams all are included in the work, you
will start being able to explain what really happens on the surface of the electrode.
Observe the video sequences with great attention – If you can observe a change on the electrode surface (a
change in colour or gloss) there will be a cause of the change. This is important in order to be able to explain what happens under the given circumstances.
When corrosion takes place in the real world, exactly what you are observing at the polarization exercises
takes place. Therefore it is important to gain as much understanding as possible, since a good understanding of what you see is a basis for you to later solve real corrosion problems.
When we increase or decrease the potential via a potentiostat and the material either starts to corrode, passivate or is brought to the immune area in a given environment, we can in the real world establish precisely the same conditions by changing the oxidizing or reducing properties of the surroundings. If we are in a
very oxygen rich environment (oxidizing environment) it corresponds to a high potential. If we on the
other hand are in a very oxygen poor environment (when the fishes die for lack of it) it corresponds to a
low potential (see the compendium).
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41653 Corrosion
Exercise 2
Many modern cleaning agents in the food-processing industry are very oxidizing – think about how this
could influence the corrosion speed of for example stainless steel during the cleaning process in a dairy.
What could happen to the material?
The polarization curves recorded by the other teams you will get as finished files because you will need
these to answer question 5.
I can understand the polarization curves as “common property”. It is important that you are thorough
when you carry out your experiments.
In A the curves must be shown with the potential-axis mV vs. SCE (to be able to compare with the curve
in the standard)
In B the curves must be shown with the potential-axis mV vs. SHE (measured potential-value + 244
mV).
All finished polarization curves (Excel files with correct mentioning of all the different parameters) are
uploaded to CampusNet accessible for everyone.
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41653 Corrosion
Exercise 2
Exercise day 3
You should continue the same work as day 2. Yet you need to reserve about 1 hour for the Explanatory
exercise (an experiment to demonstrate the constancy of stainless steel at varying potential). It is important that you understand the point at this exercise and answer the matching questions.
At the end of the day you must have recorded all the polarization curves and have exchanged experimental data with the other groups. Make sure all data is available for the other teams immediately after
the experiments and the acceptance. The first data can be available already after the first exercise day. The
curves (Excel files) are uploaded Campusnet, so everybody can easily access the curves they are interested in. Save the curves, because when we get to ”Svendestykket” the recorded data can be useful to study.
There are some questions for the different exercise. You must answer these from the theory you have
learned and the knowledge you have gained during the exercises. You must hand in the answers before
starting on exercise 3.
NB:
It is important to answer all questions for the succeeding of the lab. exercises. The theory is not understood until you can answer all the questions satisfactory.
Our experience is that it’s not hard, as long as you take things seriously from the beginning and don’t get
behind schedule.
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41653 Corrosion
Exercise 2
Questions for exercise 2
1.
Give the electro-chemical series for all the materials tested by all teams in the solutions respectively.
Account for surprises and give theoretical explanations.
2.
Explain the cause for choosing to represent data in a Stern-diagram (LogI/E). What can you read
out of such a diagram?
3.
Give adequate explanations for the following polarization phenomena:
 Activation polarization
 Diffusion polarization
 Resistance polarization
4.
What is understood by exchange current density and what units are used stating this? Mention two
metals with a high exchange current density. State the practical consequence that a metal has a high
exchange current density. Would it be possible to measure the exchange current density for copper in
a 1 M NaAc added 3 % NaCl pH = 5.5? Explain your answer.
5.
For answering this question you must know the curves recorded by the other teams, or curves given
by the laboratory. We want you to judge whether it is risky to use some of the following material
combinations in contact with each other in water rich environment like the above mentioned (select
three combinations). The corrosion speed must be estimated for the selected combinations and stated
in hours it takes to dissolve 1 m from the electrode. The cathode- and the anode reaction should also
be stated. For the calculations it is assumed, that the area of cathode and anode is the same (remember to state the environment you have chosen for the galvanic coupling). Find 7 examples from practice where such couplings can be found and explain shortly for problems arising out of these conditions.
Gold – Stainless Silver – Stainless
steel
steel
Gold – Nickel
Gold – Silver
Lead – Aluminium
Gold - Magnesium
Zink - Magnesium
Zink - Cobber
Zink - Aluminium
Iron - Graphite
Iron - Aluminium
Iron - Nickel
Iron - Cobber
Titanium - Zink
Titanium - Gold
6.
State two other ways of determining the corrosion speed.
7.
All polarization curves recorded must be interpreted (you must interpret you own curves, the curves
from the other curves and the curves from the Virtual Laboratory). This means that you must explain
the expected electrode reactions taking place. Using the Pourbaix diagrams calculated you must make
the reactions probable. It is important that you always keep an eye on the surface of the tested specimen. A change in colour or topography, the curve’s shape and/or the thermo dynamical calculations
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41653 Corrosion
Exercise 2
can give a lot of useful information. After the measurement it is a good idea to study the surface of
the specimen in a stereo microscope. The observations should be noted and added as informative text
on the curves. Even though you haven’t done all the experiments you must explain the results. You
can discuss mutually.
Internet pages helpful for answering questions
http://www.avogadro.co.uk/definitions/sep.htm
Standard electrode Potential, Eº (definition) Look at page and understand the content. It’s very fundamental and can be read in 2 minutes.
http://www.consultrsr.net/resources/ref/refpotls.htm Information about reference electrodes
http://www.webelements.com/
Investigate for what purposes the more rare noble metals are used.
download/Electrochemical_Dictionary.pdf
Electro chemical dictionary where all terms are explained
Can also be found in the Important Databases section of the USB
http://www.corrosion-doctors.org/
Information about corrosion – This page must be used during the exercise for useful information.
http://www.tannerm.com/index.htm
Look at Electrochemistry
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41653 Corrosion
Exercise 2
Explanatory exercise with stainless steel
View the hole experiment on a video at the laptop.
The purpose of this experiment is to give a feel of the corrosion properties of stainless steel by studying
corrosion products developed in a special cell and in a given environment.
The purpose is to relate the observations from the cell to a polarization curve for stainless steel. The setup consists of 24 electrodes manufactured from stainless steel 304. Between each electrode is placed a 12
ohm resistor – a total of 23 resistors which are given a voltage of 2.4 volt using a rectifier. The liquid in
the container consists of 0.2 mol/l H2SO4, 0.002 mol/l K3Fe(CN)6 + 0.02 mol/l NH4SCN as indicator.
_
+
After a few minutes characteristic colours will appear on the different electrodes. Let the experiment continue for 30 minutes. Clean the electrodes if the experiment is repeated.
Things to explain
1.
State the electro-chemical reaction at each electrode (or group of electrodes). Explain the chemical
reaction that causes the colour and state the placement of the electrodes in relation to the current potential curve for stainless steel.
2.
Determine the free corrosion potential at each of the 24 electrodes. Compare with observations from
exercise 2.
3.
Describe another way to estimate the corrosion speed for the different electrodes.
4.
Could a similar reaction to this occur in real life?
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41653 Corrosion
Exercise 2
“The Virtual Laboratory”:
On the computers in the lab there are videos showing polarization measurements for different materials in
different solutions.
These videos must be compared with the corresponding polarization curves and Pourbaix diagrams and be
studied thoroughly and the corresponding questions must be answered.
Questions for all materials:
You must explain the electro chemical reactions taking place with background in the calculated Pourbaix
diagram and polarization curve.
Concerning cobber polarized in acetate solution:
Explain the cause of that the cobber electrode from about + 1150 mV starts to assume a colour like nonoxidized cobber.
Concerning aluminium electrode:
Why is the aluminium electrode corroded after the cathodic polarization?
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.
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