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.
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.
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
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.
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.
References
- ^ Interview, David Des Marais.
See also
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External links
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