Preventing rust

In this experiment, students protect iron nails using painting, greasing and sacrificial protection. The nails are placed in test tubes and covered with corrosion indicator solution. This contains gelatine and so sets to a jelly-like consistency. The indicator changes colour from yellow to blue to show where rusting is taking place. By comparing the amount and position of the blue indicator on each nail, students can assess the effectiveness of the different types of protection.

This can be set up as either a demonstration or a class practical. Students can be told how to carry it out or left to plan it for themselves. If they are to plan it themselves then it would be a good idea to demonstrate the use of the indicator with unprotected iron nails, before they start to think about their plans.

Even as a demonstration, the majority of the work can be done by individual pupils. They can paint the nails, cover them in plastic, wrap them in another metal and so on. It is not necessary to do all the suggested tests – around six, including the control nail, will give the idea. It is probably best to include some familiar methods of preventing rust, such as painting, as well as at least one example of sacrificial protection, such as wrapping with magnesium.

Although the results are obtained quickly for a corrosion practical, it will still take around half an hour for the indicator to change and so it is advisable to have something else planned for the students to do while they wait for the colour to develop.

Equipment

Apparatus

• Eye protection
• Test tubes, at least 6 (see note 1)
• Test tube rack
• Vaseline
• Clingfilm or similar plastic film
• Marker pen or labels for test tubes

Chemicals

• Iron nails, at least 6 (see note 2)
• Galvanised (zinc coated) nails, x2 (see note 3)
• Stainless steel nail, screw or bolt
• Copper foil, small piece
• Magnesium ribbon, about 2 cm
• Zinc foil, small piece
• Cleaning solution, dilute sodium carbonate (see note 4)
• Dilute hydrochloric acid, 1 M, about 10 cm³
• Paint plus a small brush (see note 5)
• Oil (see note 6)
• Gelatine, 5 g
• Potassium hexacyanoferrate(III), 0.2 g

Equipment notes

1. Cheap soda glass test tubes can be used so that stained ones can be discarded after use.
2. The nails for this experiment will ideally be identical and should be iron rather than stainless steel. They can be obtained from a hardware store. They should be cleaned of rust prior to the experiment. This can be achieved with dilute (about 1 M) hydrochloric acid, followed by thorough rinsing in cold water.
3. Clout nails from a hardware store are a good source of galvanised nails. They can be reused.
4. A suitable cleaning solution is a dilute (around 5%) solution of sodium carbonate with a few drops of ordinary detergent added.
5. The paint must not be water soluble and should ideally dry quickly. Acrylic paint or correcting fluid (egTippex) is ideal.
6. Any oil which is safe for pupils is suitable. Cooking oil is probably the easiest to use.

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Iron, Fe(s), copper, Cu(s), magnesium, Mg(s), zinc, Zn(s) – see CLEAPSS Hazcards HC055A, HC026, HC059A and HC107.
• Sodium carbonate solution, Na₂CO₃(aq) – see CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB080.
• Dilute hydrochloric acid, HCl(aq) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.
• Potassium hexacyanoferrate(III), K₃Fe(CN)₆(s) – see CLEAPSS Hazcard HC079.
• Prepare the corrosion indicator shortly before the lesson as it does not keep well. Make a warm solution of 5 g gelatine in 100 cm³ water and then dissolve 0.2 g potassium hexacyanoferrate(III) in it. Keep the solution warm but not hot, otherwise toxic gases can be evolved. If you have access to one, a magnetic stirrer with hotplate is ideal for this but it is not essential. No hazard label is required for the solution.

Procedure

1. Select some nails which do not show any signs of rusting. Clean them thoroughly with the cleaning solution and dry them.
2. Place one nail as a control into a test tube.
3. Treat the other nails as suggested below. Not every test needs to be done. Label each test tube.
4. Wrap one nail in thin plastic film, such as ‘clingfilm’.
5. Paint one nail and let it dry.
6. Coat one nail with Vaseline or other grease, or oil.
7. Wrap a small piece of magnesium ribbon or zinc foil around a section of a nail.
8. Wrap a small piece of copper foil around a section of a nail.
9. Place these nails in separate test tubes.
10. Place a stainless steel nail, bolt or screw into a test tube.
11. Place two galvanised (zinc coated) nails, one which has been scratched with a file to remove a patch of zinc coating, into a test tube.
12. Carefully pour the corrosion indicator into each test tube, completely covering each nail. Leave for at least half an hour.

Teaching notes

After about half an hour it will be possible to see the indicator changing from the starting yellow colour to dark blue in patches on the nails. These dark blue patches indicate areas where rusting is starting.

Rusting is a complex reaction between iron, oxygen and water to form hydrated iron(III) oxides. Initially iron goes into solution as Fe²⁺ ions, losing electrons:

Fe(s) → Fe²⁺(aq) + 2e^–

This is oxidation and occurs on the iron where the protective oxide layer is weakest or damaged. Such areas are called anodic. The Fe²⁺ ions combine with the indicator to form a blue solid.

In the absence of the indicator the Fe²⁺ ions combine with OH^- ions produced at cathodic areas by reduction of oxygen:

O₂(aq) + 2H₂O(l) + 4e^– → 4OH^–(aq)

(By adding a few drops of phenolphthalein indicator solution when making up the gelatine mixture, so-called ‘Ferroxyl indicator’ is obtained. This indicator will show the cathodic areas as well, as the hydroxide ions cause the phenolphthalein to turn pink.)

The iron(II) hydroxide formed is oxidised further by oxygen, to form rust, Fe₂O₃.xH₂O. For more detail on the reactions involved in the rusting process, take a look at this page on the chemistry of rust from Corrosion Doctors.

Typically, the magnesium wrapped nail will rust the least. The magnesium donates electrons to the iron, which slows down the rusting process. This is effective even for the parts of the iron which are not in direct contact with the magnesium. The magnesium corrodes instead of the iron, ‘sacrificing’ itself. This is called sacrificial protection, and is used commercially to protect iron structures in corrosive environments.

The nail wrapped in copper will rust the most. This is due to the opposite process. The more reactive metal, iron, donates electrons to the copper and becomes electron deficient itself. This increases the rate of the rusting.

The other nails will rust in a variable way, depending on how effectively they have been coated. Any chips in the paint, or gaps in the plastic or grease, will leave some of the iron nail exposed to oxygen and water, and these will be the first areas on those nails to rust. It is worth making the comparison with nails in contact with magnesium, which are protected even in areas that are not directly touching the magnesium. Alloying is also an effective method of rust prevention and chips and scratches in the surface are generally not important. As a result, the stainless steel nail will generally not rust much, if at all.

Students could be asked to tabulate the results of the experiment. They could also think about where each method of rust prevention is used in real life, why that method is chosen and how effective it is.