rust - Rust prevention

The product of corrosion, especially of iron-containing materials. It consists mainly of iron(III) oxide (Fe₂O₃) or hydrated forms.

Iron alloy phases
Austenite (γ-iron; soft) Pearlite (88% ferrite, 12% cementite)
Types of Steel
Plain-carbon steel (up to 2.1% carbon) Stainless steel (alloy with chromium) HSLA steel (high strength low alloy) Tool steel (very hard; heat-treated)
Other Iron-based materials
Cast iron (>2.1% carbon) Wrought iron (almost no carbon) Ductile iron

Rust is the substance formed when iron compounds corrode in the presence of oxygen and water.

Iron is found naturally in the ore haematite as iron oxide, and metallic iron tends to return to a similar state when exposed to air, (hydrogen, oxygen, nitrogen, etc.) and water.

Iron is the main component of steel and the corrosion of steel is observed more frequently, since iron is rarely used without alloying in the present day. On the surface of the metal, iron is oxidized to iron(II):

Fe → Fe

The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen. They reduce the oxygen and water to hydroxide ions:

4e

The hydroxide ions react with the iron(II) ions and more dissolved oxygen to form iron oxide. The hydration is variable, however in its most general form:

Fe → Fe(OH)2 4Fe(OH)2 + O2 → 2(Fe2O3.xH2O) + 2H2O

Hence, rust is iron(III)oxide, which is formed by the dehydration of iron(II) hydroxide.

Rust prevention

Hydrated iron oxide is permeable to air and water, allowing the metal to continue to corrode - internally - even after a surface layer of rust has formed. Given sufficient hydration, the iron mass can eventually convert entirely to rust and disintegrate. Corrosion of aluminium is different from steel or iron, in that aluminium oxide formed on the surface of aluminium metal forms a protective, corrosion resistant coating, a process known as passivation. Stainless steel similarly resists rusting by forming a passivation layer of chromium(III) oxide. Zinc also provides cathodic protection to metal that itself is unplated, but close enough that any water touching bare iron is also in contact with some zinc.

There are several other methods available to control corrosion and prevent the formation of rust, colloquially termed rustproofing. Cathodic protection makes the iron a cathode in a battery formed whenever water contacts the iron and also a sacrificial anode made from something with a more negative electrode potential, commonly zinc or magnesium. The electrode itself doesn't react in water, but only to provide electrons to prevent the iron rusting.

Bluing is a technique that can provide limited resistance to rusting for small steel items, such as firearms; However, if concrete covered steel does corrode, the rust formed can cause the concrete to spall and fall apart.

To prevent rust corrosion on automobiles, they should be kept cleaned and waxed.