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
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:
- The potassium ferricyanide and phenolphthalein should be made by the teacher.
- Potassium ferricyanide is not dangerous unless heated to very high temperatures which is not done in this experiment.
- 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.
- 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.
- 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.
- For dish 1: one straight nail and one bent nail
- For dish 2: one straight nail with copper foil wrapped with two turns
one straight nail with zinc foil wrapped with two turns
- For dish 3: straight nail with aluminum foil wrapped with two turns
straight nail with tin foil wrapped with two turns
- For dish 4: straight nail with magnesium ribbon wrapped with two turns
straight nail with one in. of Mg metal set nearby but not touching nail
- For dish 5: straight nail hammered in middle
straight nail heated red hot in the middle and allowed to cool slowly
- For dish 6: two straight nails with tips bent upward, battery attached by wires
- 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.
- Use alligator clip and any other available method to attach lead wires to the nails in the dish.
- 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.
- Allow the dishes to react overnight. Observe, sketch, and explain the changes observed.
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?
- This may be performed as an overhead demonstration using a water mixture containing a few drops of phenolphthalein and potassium ferricyanide solutions. Any 1.5 V battery should work but a 9 V battery will give almost immediate results.
- Attach two dissimilar pieces of metal to a volt meter using either the agar or water solutions.
- For an extension, ask students to design and perform other possible combinations.
- In preparing agar, it is important to get the mixture hot enough to allow the agar to dissolve, but if it gets too hot the agar will burn. If you have not made agar before, you might want to start out with a small batch for practice.
- It is very convenient to use a microwave to heat the agar mixture, if one is available.
- During the section using the batteries, you might ask the students to switch the electrodes from the battery and notice the fading of the original colors and the creation of the other colors.
- Blue coloration is evidence of the reduction of the ferricyanide ion and pink is the oxidation of the iron nail.
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.