Protection of iron

The aim is usually to cover the iron so that it is isolated from air and water, or to make it the cathode of a cell.

1. Painting or oiling: coating with paint, oil, grease or varnish. It is simple, but not effective for the long term because the coating wears or cracks.
2. Covering with a metal
   • Less active metal (e.g. tin plating of food cans): protects only while the coating is whole. If it is scratched, iron (the more active metal) becomes the anode and tin the cathode, so the iron corrodes faster than unprotected iron.
   • More active metal (e.g. zinc, "galvanising" – dipping steel in molten zinc): if scratched, zinc is higher in the series so it is the anode and corrodes first, while iron is the cathode and is protected. Corrosion of the iron begins only after all the zinc nearby is consumed.
3. Cathodic protection with a sacrificial anode: buried iron pipes and ships' hulls (always in contact with salty water or damp soil) are connected to blocks of a more active metal such as magnesium (or zinc). The magnesium acts as the anode and is corroded instead of the iron, so it is called a sacrificial electrode and is replaced from time to time.