Corrosion is the process by which something erodes because of a chemical reaction.
Metal corrosion is a reaction of metal with its environment that causes measurable alteration and is part of metal's inherent tendency to revert to an original, more stable form. The red rusting of iron and steel is a visible example of corrosion and other examples include the weathering of copper and the oxidation of aluminium and zinc.
Corrosion can only happen in the presence of an electrolyte, e.g., water. The occurrence of salt (or other contaminants) in the water increases the conductivity of the electrolyte and therefore greatly increases the reaction rate.
Salt contamination will also affect the time of wetness. On a clean surface, water vapour will condense at 100% relative humidity, on a salt-covered surface, a wet film can be formed at a relative humidity level of 75% and more.
Corrosion can also be the result of direct contact with another metal or substance, or the result of run-off from incompatible surfaces or fall-out of corrosive particles. Time of wetness, presence or lack of oxygen, and atmospheric contaminants greatly affect the rate of corrosion.
Differences in electrical potential on the surface of corroding metal create microscopic cells comprising cathodes and anodes. In the case of iron, the positively charged electrons in the anode react with the negatively charged hydroxyl ions in the electrolyte to form iron oxide on the anode. Similar reactions occur with other metals. Polarisation changes on the surface cause anodic areas to become cathodic and vice versa, so that over time the rate of corrosion is relatively uniform over the surface.
The build-up of debris on a cladding surface will promote corrosion. The salts in the debris react with the cladding each time they are wetted, and the deposits themselves impede surface-drying, increasing the time of wetness.