From Wikipedia, the free encyclopedia
A
blacksmith removing rust with sand prior to
welding
Rust damage in automobiles can create hidden
dangers.
Rusting can completely eat away iron. Note the
galvanization on the unrusted
portions.
Rust is a general term for
iron oxides formed by the reaction of
iron
with oxygen. Several forms of rust are
distinguishable visually and by
spectroscopy, and form under different
circumstances.
[1] The chemical composition of rust is typically
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
rust smacking.
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Chemistry
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.
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Net reactions
The overall outcome of rust formation involves reaction of iron
with varying amounts of oxygen and water.
- 2Fe + O2 + 2H2O =
2Fe(OH)2
- 4Fe + O2 + 6H2O =
4Fe(OH)3
- 4Fe + O2 + 2H2O =
2Fe2O3?H2O
- 6Fe + 4O2 =
2Fe3O4
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Mechanism
Pure, solid iron oxidizes in water:
Fe(s) => Fe2+(aq) +
2e-
These electrons will quickly react with the disassociated
hydrogen ions (in H3O+(aq) form) and the
dissolved oxygen in the water (O2(aq)):
4e-(aq) + 4H3O+(aq) +
O2(aq) -> 6H2O(l)
Therefore, as seen from the above equation, the more acidic the
water, the greater will be the rate of
corrosion (since the concentration of
H3O+(aq) will be greater.) At extremely low
pH’s, the
hydrogen ions will react with the
electrons producing hydrogen gas instead:
2H+(aq) + 2e-(aq) ->
H2(g)
Thus, as seen from the above equations, the pH of the solution
(whether it is pure water or water containing electrolytes) rises.
This leads to the formation of OH- ions (in cases where
the body of water is significantly large, the
pH does
not rise as sharply, but this is of no consequence since
OH- ions are always present, even in pure
water.) The
cations then react with the OH- or
even the H+ ions and dissolved oxygen to form a variety
of compounds, which constitute rust:
Fe2+(aq) + 2OH-(aq) ->
Fe(OH)2(s)
4Fe2+(aq) + 4H+(aq) +
O2(aq) -> 4Fe3+(aq) +
2H2O(l)
Fe3+(aq) + 3OH-(aq) ->
Fe(OH)3(s)
From the above equations, it is seen that the pH and amount of
dissolved oxygen can affect the outcome of the reactions. In water
with limited dissolved oxygen Fe3O4(s) is
formed, which is a black solid and commonly called
lodestone:
6Fe2+(aq) + O2(aq) +
12OH-(aq) -> 2Fe3O4(s) +
6H2O(l)
The porous Fe(OH)3 rust can slowly disintegrate into
a crystallized form, which is the familiar red-brown rust:
2Fe(OH)3(s) ->
Fe2O3?H2O(s) +
2H2O(l)
Iron oxide (FeO) can also be formed. The presence
of other ions, such as
calcium or calcium carbonates reacts with the
iron hydroxides and iron
oxides to form a variety of precipitates. Other
metals corrode via similar chemical
processes.
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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 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,
titanium, 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
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 road salt (calcium chloride) can
accelerate the rusting process.
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References
-
^
Interview, David Des Marais.
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See also
Wikimedia Commons has media related
to:
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External links
