What is Rust?
What is Rust?
What is POR15?
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Scientifically speaking rust is called oxidation, which
occurs when oxygen comes in long-term contact with certain metals, most
commonly iron. Over time, the oxygen combines with the metal at an
atomic level, forming a new compound called an oxide and weakening the
bonds of the metal itself. If the base metal is iron or steel, the
resulting rust is properly called iron oxide. Rusted aluminum would be
called aluminum oxide, copper forms copper oxide and so on.
The chemical composition of rust is mainly hydrated iron(III) oxide
(Fe2O3.nH2O), 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 apart adjacent parts - a phenomenon known as
The rusting of iron is one of the more widely used
examples of corrosion. This electrochemical process requires the
presence of water, oxygen and an electrolyte and leads to the formation
of hydrated iron oxides.
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 aluminum 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. In more corrosive environments (such
as at sea) cadmium may be used. Galvanization often fails at seams,
holes and joints, where the coating is pierced. In these cases the
coating provides cathodic protection to metal, where it acts as a
galvanic anode rusting in preference.
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 oxidizing 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 provides 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
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 road salt (calcium chloride) can accelerate the rusting