All metals corrode due to chemical reactions with the environment – with the exception of four rare metals: iridium, niobium, osmium and tantalum. When iron and iron alloys corrode we call it rusting. The product of corrosion is rust, aka hydrated iron oxide: Fe2O3.xH2O.
WHY DOES RUST FORM?
Oxygen (O) is a highly reactive element. Only one element (flourine) has a higherelectronegativity – that is, more inclined to steal electrons than oxygen. Oxygen will form stable bonds with practically all other elements to form oxides.
Iron (Fe) is an innoble metal, meaning it is inclined to give up electrons (unlike noble metals like gold and platinum, which don’t give up electrons, don’t react and don’t corrode under normal circumstances). Iron is the most abundant element on Earth, by mass.
When oxygen and iron meet, oxygen takes iron’s electrons and they bond, forming iron oxide.
This reaction, oxidation, occurs wherever oxygen meets the surface, creating a thin film on the substrate (less than 0.005 microns / 0.0002 mil thick). This passivating layer prevents further oxygen from contacting and reacting with the metal.
Almost all metals form passivating oxide layers. Copper has its green patina. Silver has its tarnish. On stainless steel, a passivating layer of chromium oxide protects the steel from corrosion.
However, in the case of iron and carbon steel, the passivating oxide layer is not so helpful. Not only does the brittle and bulky iron oxide layer not adhere to the substrate –thus providing no protection for the underlying iron – but is hygroscopic as well, meaning it draws moisture from the air.
The presence of pure water on the surface by itself is not enough to cause corrosion.* But when the water contains dissolved salts, or is acidic or basic, conditions are ripe for the formation of a corrosion cell.
*If pure water enters a crack or crevice, oxygen deprivation can cause a buildup of hydrogen ions, creating an acidic solution that can lead to corrosion.
How Choride Attacks a Passivating Layer
After oxygen, chlorine is next most reactive (electronegative) element. Chloride ions want to donate electrons, and experience strong attraction to the oppositely charged iron ions on the surface.
Pulled towards the surface, the chloride ions penetrate through the passivating iron oxide layer, reacting with it, and causing it to dissolve until the metal surface is exposed to the electrolyte solution and a corrosion cell forms.