Bases
We have seen that acids increase the rate of rusting. Next, find out how a base affects rusting.
Activity 12.3.6
You will need: two boiling tubes, two cleaned iron nails, sodium hydroxide (NaOH) solution
- Put a cleaned iron nail into each boiling tube. Add ordinary cold water to one tube and an equal volume of sodium hydroxide solution to the other.
- Leave for about two days and observe.
Figure to be added
The observation
The nail in the tube of ordinary water has rusted, while the nail in the sodium hydroxide tube has not. This confirms that bases are a factor that reduces the rate of rusting.
Why a base slows it
Look again at the cathode reaction of rusting: oxygen and water are reduced to OH- ions. A basic solution is already full of OH- ions, and it has almost no H+ ions to take electrons from the iron as an acid does. In such conditions a thin, tight layer forms on the iron surface that shields it, and the iron hardly corrodes. So the three activities form a pattern: acid speeds rusting, a base slows it, and salt — neither acid nor base — speeds it by making the water conduct.
The rapid corrosion of iron, such a useful metal, is a harmful situation. So steps must be taken to control the corrosion of products made of iron — the subject of the next lesson.
Bases slow rusting
A nail in sodium hydroxide solution does not rust while one in plain water does.