Study of Corrosion

Study of corrosion - Philippe Delsate & Pierre Hautier

Playful Science, Wednesday January 28, 2009, Jemeppe-sur-Sambre

Study of corrosion

Philippe Delsate - Pierre Hautier Corrosion is a reality that should be broached during a chemistry class devoted to oxidation-reduction reactions. If the students don't have to prepare reagents by themselves, then one laboratory period should be enough. Iron corrosion will be examined by revealing Fe++ and OH– ions. To avoid an excessively fast migration of these ions, reactions will be executed in a solid medium (agar or gelatin). The solid medium allows analyzing the experimental results on the next day (or in other classrooms). The water used in the diverse solutions is always de-mineralized water.

1. Identification Tests

In a test tube, dissolve a little K3Fe(CN)6 in some mL of water. In a second test tube, dissolve a bit of iron (Fe II) salt in some mL of water. Add some drops of K3Fe(CN)6 solution and report your observations. In a third test tube, dissolve some zinc salt in some mL of water. Add a few drops of K3Fe(CN)6 solution and report your observations. We will also reveal a basic medium using phenolphthalein that will turn purple.

2. Preparation

2.1 Eliminating the protection of the iron nails

Scour 7 iron nails by plunging them for a moment into HCl 1 mol.L–1.

2.2 Protecting the iron nails with a copper layer

Fill half of a 100 ml beaker with an aqueous solution of CuSO4. Put a copper blade into the solution taking care to bend the top edge back; secure it with an alligator clip connected to the positive terminal of a direct current generator. Bind one end of a pickled iron nail with another alligator clip connected to the negative terminal of the same generator. Turn the generator on. Plunge the nail into the solution, taking care not to plunge the


clip. Set the generator at approximately 10V. Let the current flow for at least 5 minutes. This galvanizing is similar to the electrolytic purification of the copper. Place another pickled iron nail at the bottom of the beaker, taking care not to put it into contact with the first nail nor with the copper blade.

2.3 Preparation of the experimental medium Bring 200ml of water to the boil; dissolve 3,5 g of agar in it; let boil until the solution becomes clear. Prepare the cell (§ 6). Add 0,1 g of phenolphthalein into the remaining solution, as well as 0,2 g of K3Fe(CN)6 and 7 g of NaCl. Be careful! The solution is foaming when it starts to boil: it's essential to withdraw it from the hot plate. Turn the hot plate off. The waste heat will be used to keep the agar solution warm during the preparation.

2.4 Preparation of the other nails Scour some zinc clippings; dry it and wrap it in the middle, around a pickled iron nail. Bend the pickled iron nail in the middle using a clamp.

Make a saw cut onto the pickled iron nail and onto a zinc plated iron nail.

3. Study of corrosion

Pour 30 ml of the agar solution into each Petri box. Wait for a couple of minutes. Just before solidification, place the different nails into the boxes taking care not to put them into contact with each other. box n° 1: 1 not pickled iron nail 1 pickled iron nail 1 steel nail box n° 2: 1 brass nail 1 zinc plated iron nail 1 zinc plated iron nail marked by a saw cut box n° 3: 1 pickled iron nail bent in the middle 1 pickled iron nail marked by a saw cut

1 pickled iron nail protected by zinc clippings (anodic protection or trapping metal)

box n° 4: 1 pickled iron nail protected by a deposit of copper 1 pickled iron nail, protected by an electrolytic deposit of copper and marked by a saw cut

box n° 5: 1 pickled iron nail; connect 2 wires to the direct current generator, and stick the other end of one of these two wires into the frost, on both sides and far enough from the nail.

Wait for a couple of minutes, and record your observations in the table on the next page. You can also try to predict the reactions that will occur in the different boxes...


4. Observations

type of nail Observations

Iron

Pickled iron

Steel

Brass

Zinc plated iron

Cut and zinc plated iron

Bent and pickled iron

Cut and pickled iron

Pickled iron wrapped into zinc clippings

Copper plated and pickled iron

Electrolytically copper plated and pickled iron

Pickled iron subject to an electric field


5. Conclusions

6. A Cell in the agar

Pour 20 ml of the agar solution into a test tube, with 5 g of CuSO4.5 H2O; shake to dissolve it. In another test tube, repeat the operation with 3,2 g of ZnSO4. Create two parts into the Petri box using a cardboard. Place a 10 cm copper wire in one part of the box (one end of the copper wire must be out of the box) ; add the solution of CuSO4. Once cold, repeat the operation with a zinc ribbon and ZnSO4solution. Remove the cardboard and let cool down.

Connect the copper wire and the

zinc ribbon to an electrical device (engine, audio card, voltmeter,...) You can see the electrodes evolving over time. Notes It may be necessary to use 2 cells in a row to power a small engine. It would be possible to replace ZnSO4 and Zn by MgSO4 and Mg respectively in order to increase the voltage supplied by the cell, but magnesium is reacting very (even too much?) easy with the water into the cell.

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Study of Corrosion