Rust
From Wikipedia, the free encyclopedia
| Iron alloy phases |
|---|
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Austenite (γ-iron; hard) |
| Types of Steel |
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Plain-carbon steel (up to 2.1% carbon) |
| Other Iron-based materials |
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Cast iron (>2.1% carbon) |
Rust is the oxide that is formed by open-air oxidation of iron. The chemical composition of rust is mainly iron(III) oxide (Fe2O3), and under wet conditions may include iron(III) oxide-hydroxide (FeO(OH)). Rusting is the common term for corrosion of iron and its alloys, such as steel. Although oxidation of other metals is equivalent, these oxides are not commonly called rust.
As rust has higher volume than the originating mass of iron, its buildup may force adjanced parts apart - a phenomenon known as rust jacking.
Contents |
[edit] Chemistry
When iron is exposed to air, redox reactions form iron(III) ions
- 2 Fe → 2 Fe3+ + 6 e−
The electrons released reduce the oxygen to oxide ions:
- 1.5 O2 + 6 e- → 3 O2-
Fe2O3 is therby formed from the Fe3+ and O2-. The oxidation of iron is exothermic giving off energy when rust forms.
Under wet conditions, the ions also react with water and form hydroxide ions:
- 4 e− + O2 + 2 H2O → 4 OH−
In this case, FeO(OH) is formed.
Rusting can often be undone through electrolysis, effectively reversing the above reactions.
[edit] Rust prevention
Hydrated rust 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. This is also true of magnesium, copper and zinc.
Galvanization consists of coating metal with a thin layer of another such metal. Typically, zinc is applied by either hot-dip galvanizing or electroplating. Zinc is traditionally used because it is cheap, easy to refine and adheres well to steel. 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. The zinc layer acts as a galvanic anode rusting in preference. Galvanization often fails at seams, holes and joints, where the coating is pierced. More modern coatings add aluminium to the coating as zinc-alume, aluminium will migrate to cover scratches and thus provide protection for longer. These rely on the aluminium and zinc oxides protecting the once-scratched surface rather than oxiding as a sacrificial anode.
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; for it to be successful, water-displacing oil must be rubbed onto the blued steel.
- Corrosion control can be done using a coating to isolate the metal from the environment, such as paint. Large structures with enclosed box sections, such as ships and modern automobiles, often have a wax-based product (technically a slushing oil) injected into these sections. This may contain rust inhibiting chemicals as well as forming a barrier. Covering steel with concrete provides protection to steel by the high pH environment at the steel-concrete interface. However, if concrete covered steel does corrode, the rust formed can cause the concrete to spall and fall apart. This creates structural problems.
To prevent rust corrosion on automobiles, they should be kept cleaned and waxed. The underbody should be sprayed to make sure it is free of dirt and debris that could trap moisture. After a car is washed, it is best to let it sit in the sun for a few hours to let it air dry. In winter, or in salty conditions, cars should be washed more regularly as salt (sodium chloride) can accelerate the rusting process.
[edit] See also
[edit] External links
- A Primer on Rust, thorough rust prevention and removal information
