A metal object corrodes when it is exposed to the open air and bad weather. If the metal is iron, we call this change rusting, and the weaker, flaky brown compound that is formed is rust.
What causes rust?
This video shows some examples of objects which have become rusty.
The following experiment shows that water and oxygen are required for rust to occur with iron.
|A||Boiled water and oil layer||No rust. Boiled water has no oxygen and oil stops new oxygen entering.|
|B||Salt water||Severe rust. Salt water is an electrolyte which conducts ions, speeding up rusting.|
|C||Air||Rust. Air and moisture cause normal rusting|
|D||Air and calcium chloride||No rust. Calcium chloride dries out the air.|
Salt solution acts as an electrolyte (any substance containing free ions that allows the substance to conduct electricity) allowing iron to lose electrons more easily and so speeds up the rusting process.
Ferroxyl indicator can be used to show the process of rusting. When iron atoms begin to rust, they lose electrons to form iron ions. Ferroxyl indicator turns blue in the presence of iron ions. This shows that rusting has begun, even if there is no reddish brown rust showing on the surface of the iron.
A pink colour is also produced by the ferroxyl indicator. This shows that the ions being lost by iron are being gained by the water and oxygen that are also involved in rusting.
In this experiment, one iron nail is wrapped in magnesium, another in copper and one left alone. The nail wrapped in magnesium is not corroded.
There is slight corrosion on the normal nail and massive corrosion on the copper wrapped nail. Connecting a more reactive metal from higher in the series protects from corrosion. A lower (less reactive) metal accepts electrons from iron and speeds up the rusting process.
Since oxygen and water are needed for corrosion, the main theories behind protection are based on the prevention of any of these from contacting iron. Physical protection creates a barrier which stops water and/or oxygen from reaching the surface of the metal.
Using paint or grease creates a barrier which physically stops oxygen/water from reaching the metal. An example is shown below.
The disadvantage of this method is that it must be constantly renewed (eg oiling a bike chain).
This process deposits a thin layer of metal on the object being protected. An iron object becomes coated in atoms of a less reactive metal. As the new metal is less reactive, it is slower to corrode.
This process is common with gold, silver, nickel, copper and tin as they are low down in the reactivity series. For instance, tin cans are actually steel (containing iron) coated in tin. Bashed or scratched tins rust even quicker as iron is higher than tin. This is why a bashed tin in a supermarket might be sold off at a cheaper than usual price.
When iron is coated in zinc, the process is called galvanising. The zinc layer stops oxygen/water from attacking the iron. While zinc is more reactive than iron, it still offers a physical barrier but also provides chemical protection.
This video shows what happens during the galvanising process.
This method works by coating iron or steel objects with a metal higher up the reactivity series. Not only does the metal act as a barrier (physical protection) but if it becomes scratched, the more reactive metal corrodes faster, preventing the iron object from losing its electrons to form ions.
Magnesium panels can protect a steel pipeline by this method.
Galvanising (coating in zinc) is also a common method of chemical protection.