41653 Corrosion
Exercise 4
Exercise 4
Contact corrosion
The purpose of this exercise is to investigate the problems that appear, when different materials are couples electrically
and submerged in an electrolyte or exposed to atmospheric corrosion.
Contact corrosion is a big problem in daily life and can give rise to unexpected damages if left unattended.
You must first carry out the galvanic coupling ”Examination of the importance of the cathode and anode area for the
galvanic coupling.”. In the time left over during the exercise you must carry out the following:
The questions are related to the virtual laboratory and can be answered without use of experimental equipment in DTU.
It is recommended to answer the questions in the virtual laboratory before carrying out the exercise.
Introduction
Contact corrosion is demonstrated by coupling of noble metals and unnoble metals in 0.1 M HCl while observing the
development of hydrogen at the cathode.
The following materials are to be investigated. Threads and rods (length about 50 mm) pr 10x10 mm specimens: Zn, Al,
Fe, Cu, Pt, Au and graphite.
The coupling of the metals takes place in a Petri dish so the metals are physically forced into contact with each other
and the gas development at the cathode can be observed by a stereo microscope or a magnifying glass. Please state if
the development of gas is strong, medium or weak.
Metal coupling
Mg – Cu (NaCl)
Zn - graphite
Zn - Cu
Zn - Au
Zn – stainless steel
Gas development
Material coupling
Fe - graphite
Al - Cu
Al - Fe
Zn - Pt
Gas development
Start by coupling the two metals
For Mg – Cu put Copper flakes into the thread of the 10x10 specimen Mg.
Hereafter the coupled metal specimens are degreased (cleaned for dirt and oxides – so that the surfaces becomes reactive) for about 5-10 sec in a 0.1 M NaOH solution where the specimen is the cathode (-). Then the specimen is rinsed
and put into the Petri dish.
1
41653 Corrosion
Exercise 4
For test of magnesium a salt water solution is used. For other metals a 0.1 M HCl solution is used.
Observe the development of gas at the cathode in a stereo microscope or through magnifying glass.
Try successively to add a couple of drops of the inhibitor, which has been handed out, in the ordered amount for selected material couplings and observe what happens.
Questions
1.
2.
State the cathode and anode reactions for all material couplings you have tested and explain why the gas formation
is particularly powerful on some materials.
Explain the difference between cathodic and anodic inhibitor and sketch how this influences the Tafel lines for
cathode and anode reaction respectively.
Look at and comment the following internet sites
http://www.metalprotection.com/
Website for commercial corrosion inhibitors. State where corrosion inhibitors are used practically.
How does this type of corrosion inhibitor work?
(look at page 2) in the following PDF file http://www.cortecadvertising.com/pressreleases/pa/pdf/MP02-03.pdf
2
41653 Corrosion
Exercise 4
Investigation of the influence of the cathode and anode area for galvanic coupling
The material (noble/unnoble) is connected via a coulomb meter built into the potentiostat (see description page 4
The test samples are cylindrical and assemble the test specimens that were used for the corrosion test in the standard
specimen holder.
The duration of the experiment is 30 min pr sample. The weight loss of the unnoble materials is registered together with
the electrical charge by integration of the current/time curve. The weight loss that can be assigned to the charge is calculated.
NB Stir thoroughly during the experiment! Note what happens if no stirring.
Each group gets a box of sample combinations (Cu/Zn, Fe/Zn, Cu/Mg or stainless steel 316/Mg).
The surface is cleaned before use. The unnoble specimens are cleaned before they are weighed. Clean with alcohol and
mount in holder immediately before the experiment (do not touch surfaces)
Remember to weigh the unnoble specimen before and after test.
Coulomb meter
Zn
Cu
GOOD stirring
Solution: Citrate buffer with 3 % NaCl, pH = 5.5
3
41653 Corrosion
Exercise 4
Group 1
Zn cm2
4
4
4
4
Cu cm2
0 (not coupled)
4
8
12
Group 2
Zn cm2
4
4
4
4
Group 3
Mg cm2
Cu cm2
Group 4
Mg cm2
4
4
4
4
0 (not coupled)
4
8
12
4
4
4
4
Fe cm2
0 (not coupled)
4
8
12
Stainless steel 316
cm2
0 (not coupled)
4
8
12
Galvanic corrosion measurement using the potentiostat.
This is how the potentiostat is programmed
The electrodes are connected as shown
Measure 30 min for each specimen
4
41653 Corrosion
Exercise 4
The curve on the screen
5
41653 Corrosion
Exercise 4
Look at and comment the following websites:
http://www.zincworld.org/
What is zinc primarily used for?
http://www.astm.org/DATABASE.CART/PAGES/B69.htm
 What is the standard about? Comment the material analysis of rolled zinc plates.
 Where is such a material like the one described in the above mentioned standard, used in practice?
In an information from International Zinc Association - You will then find the following information.
Zinc sheet is used extensively in the building industry for roofing, flashing and weathering applications. Architectural alloys generally
contain copper and titanium and are produced in the form of sheet, strip, plate and rods and are used as such, or cut and formed to
desired shapes, such as gutters, cornices and pipes. Zinc sheet is also used in graphic art to make plates and blocks, as well as battery
cans and coinage.
Today, zinc sheet is typically produced by continuous casting/rolling. Zinc is melted in an induction furnace, and the molten metal is
poured between the two endless bands of a Hazelett machine, where it solidifies. The continuous 'ingot' delivered at the other end can
be more than 1 m wide and from 10 to 20 mm thick. The endless strip is fed continuously to a rolling mill, which reduces the thickness
to the desired level in successive passes, after which it is cut to size and coiled.
Explain why a zinc alloy containing titanium and copper has been developed for use for sheets. Do you have any hesitations knowing that the zinc contains copper and titanium? Explain you answer.
(read more about corrosion inhibitors here) http://en.wikipedia.org/wiki/Corrosion_inhibitor
Sodium nitrite acts as an anodic inhibitor. If we should introduce a possible mechanism for the function of nitrite NO2as corrosion inhibitor the following reaction could be the result of the oxidative behavior of the nitrite at the iron surface.
2Fe++ + 2OH- + 2NO2- = 2NO(g) + Fe2O3 + H2O
Explain how nitrite is used commercially.
State some problems by the use of sodium nitrite as corrosion inhibitor. Could this be a problem?
http://www.tolkmit-industries.de/e_nano_sealer.htm
http://www.tolkmit-industries.de/e_mechanical_plating.htm
Read the sites and try to explain the strategy of the two principles used.
6
41653 Corrosion
Exercise 4
“The Virtual Laboratory”:
On the computers there are 2 videos called ”Kobling Fe/Cu” and ”Kobling Fe/Zn”.
Study those and answer the questions:
Questions for the Fe/Cu coupling
Look at the corrosion process on the video
1.
2.
State the chemical reaction taking place on the Cu surface and the Fe surface respectively.
Estimate the expected coupling potential vs. SHE
Questions for the Fe/Zn coupling
Look at the corrosion process on the video
1. State the chemical reaction taking place on the Zn surface and the Fe surface respectively.
2. State the expected coupling potential vs. SHE. What controls this?
How is the Fe-Zn coupling used in practice?
Solution used in the video
Potassium hexacyanoferrat (III) 0.1 %
NaCl 0.1 M
10 drops of 1 % phenolphthalein
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.
7