Experiment 7


Corrosion of Iron


The objective of this lab is to observe the electrochemical nature of the changes in an iron nail when it corrodes and to investigate methods to protect it.

Review of Scientific Principles:

An understanding of the activity series investigated in experiment 6 suggests that one way of preventing the corrosion of iron is to protect it with a more active metal. Another way to prevent the corrosion of iron is to exclude oxygen and moisture from its surface with a protective coating.


When iron is exposed to the weather, it tends to corrode (rust). Understanding of how this occurs leads to ways of preventing the corrosion.

Time: If the solutions are already prepared, each day will require 15 to 20 minutes.

Materials and Supplies:

Agar 2 to 5 grams/250 ml H2O, enough for 6 to 8 petri dishes

7.5 g NaCl

Iron nails

Copper foil, 2 inch by 1/8 inch, or copper wire

Zinc foil, 2 inch by 1/8 inch

Aluminum foil, 2 inch by 1/8 inch

Tin foil, 2 inch by 1/8 inch

Magnesium ribbon, 2 inch

Magnesium ribbon, 1 inch piece

0.1% phenolphthalein (0.1 g to 50-50 water-alcohol mixture)

0.1 M potassium ferricyanide, K3Fe(CN)6 (0.33 g/100 ml)

3 M HCl (if galvanized nails are used)

1 400 ml beaker (for heating agar solution)

6 petri dishes, either glass or plastic

1 stirring rod

Bunsen burner or hot plate

ring stand, ring clamp, wire gauze if Bunsen burner is used

beaker tongs (optional)

clip lead wires

1.5 V or 9 V battery

General Safety Guidelines:


  1. Add 7.5 g of sodium chloride to 250 ml of distilled water in a 400 ml beaker. Heat this to boiling. Turn off the flame if using a Bunsen burner or turn down the heat if using a hot plate. Add slowly, with constant stirring, 5.0 g of agar.

  2. After the agar has been dissolved, add 5 to 10 drops of the 0.1 M potassium ferricyanide solution and 5 drops of 0.1% phenolphthalein solution.

  3. While the agar mixture is cooling in the beaker, prepare the nails for the petri dishes. Obtain twelve nails that have been soaked in 3 M HCl to remove any zinc coating and six petri dishes. Each dish will have two nails. The following are suggestions for each dish. Your teacher may wish to change the contents of each dish. Get the nails ready but do not place them in the dishes yet.

  4. Pour the agar mixture into the petri dishes to a depth of a little less than a centimeter. Allow it to cool until it just begins to set. Place each nail in the agar.

  5. Use alligator clip and any other available method to attach lead wires to the nails in the dish.

  6. Attach the other ends to a battery. Note which nail is attached to the positive end and which is attached to the negative end of the battery. Observe the reaction and sketch and explain what is happening. After the reaction has been noted, detach the wires.

  7. Allow the dishes to react overnight. Observe, sketch, and explain the changes observed.

Video Clip


1. Compare the colors observed on the straight nail, the bent nail, the hammered nail, and the heated nail.

2. Explain the differences observed between the copper wrapped nail and the zinc wrapped nail.

3. Was there any difference observed between the aluminum wrapped nail and the tin wrapped nail? If so, what was it?

4. Did the magnesium that was near, but not touching, the nail show any protective tendencies?

5. Which end of the battery, the positive or negative, was connected to the nail that turned pink? Was this nail the cathode or the anode?

Teacher Notes:

Answers to Questions:

1. Where the nail was stressed, the agar will turn pink and rust will form on the nail. Other locations around the nail will be blue. Near the tip, end, bent portion, hammered portion and heated portion the agar should be pink.

2. When the nail is removed, less rust should be seen where it was in contact with the zinc. The copper is less active than iron and should not show protective tendencies.

3. The aluminum should show better protective tendencies than the tin.

4. Yes. Since the magnesium is much more active than iron and there is an electrical connection (ion laden agar), the magnesium can show protective tendencies.

5. The agar near the nail connected to the positive end of the battery should turn pink due to the oxidation of the nail. This nail should also show more rust. This nail is the anode. The agar near the nail connected to the negative side of the battery should turn blue, showing the reduction of the ferricyanide. This nail is the cathode.

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