Single Electrode Polarization Curve And

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

·  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)

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 çIç/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.

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 (LogçIç/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.

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.

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)

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.

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”.