What Is Corrosion?
Corrosion is an electrochemical process by which some types of metal are broken down. It commonly happens with anode metal, or a metallic substance that gives up electrons when in contact with a liquid conductor of electrons (electrolyte), oxygen and a cathode (a metallic substance that accepts electrons). Electrons flow through the electrolyte from the anode metal to the cathode, causing the anode metal to disappear or "wear away."
Rust, a Common Example of Corrosion
One of the most familiar forms of metal corrosion is rust---which is a cathode. Rust, or iron oxide (Fe2O3), occurs when iron (an anode) combines with oxygen and water (the electrolyte). This reaction is so efficient and common that pure iron is almost never found naturally.
Salt Speeds Process of Corrosion
The electrons from the anode are transferred to the cathode by the electrolyte, which is, as mentioned above, commonly water. Water, in general, is an excellent conductor of electricity, but certain substances can make it an even better conductor. This will speed up the process of corrosion exponentially. One of these substances is salt. Salt added to water speeds up corrosion of metal.
Not All Metals Corrode
So, in general, anode metals corrode because they are driven by an inevitable electrochemical process. However, other types of metals are more resistant, if not impervious, to corrosion. Gold and platinum do not corrode, for instance, because they have a negligible reaction to moisture or oxygen.