CORROSION
Corrosion (or more specifically, the prevention of
corrosion) costs Australian industries, homeowners and
vehicle owners many millions of dollars each year.
Oxygen is the second most electronegative element and
it is present in our air (≈ 20%) and it is also found
dissolved in water. Metals have a low electronegativity
and tend to lose their electrons to the oxygen. The
original definition of oxidation was to add oxygen to a
substance.
Only gold (very unreactive metal) is not attacked by
oxygen or non-metal oxides (gases) in the atmosphere.
This makes it valuable for jewellery, and due to its
rarity, it is a form of currency (money exchange).
Our most common building material is steel (iron). It
reacts with oxygen in a redox reaction. The halfequations are as follows:
Fe(s) → Fe2+(aq) + 2eFe2+(aq) → Fe3+(aq) + e-
Oxidation:
(Anodic site)
This is a 2-stage reaction. The overall reaction is:
Fe(s)
Reduction:
→
Fe3+(aq)
+
3e-
O2(g) + H2O(l) + 4e- → 4OH-(aq)
The reduction reaction occurs at the cathodic site.
The Fe3+ formed at the anodic site and the OH- formed
at the cathodic site then combine to form iron (iii)
hydroxide, which then turns into hydrated iron (iii)
oxide (rust).
Fe3+(aq) +
2Fe(OH)3(s)
3OH-(aq) → Fe(OH)3(s)
→ Fe2O3·3H2O(s) Rust
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Rust has the property of absorbing more water and
oxygen and flaking off. This then exposes more metal
underneath.
The oxidation and reduction half-reactions do not have
to occur at the same site as the metal will conduct
electrons from an area of weakness to the oxygen/water
site. For example, a nail can pit (have iron removed)
inside a piece of wood, or a metal pipe can form a leak
underneath a concrete driveway.
Wood
Iron eaten away
O2 & H2O
e-
e-
The electrons move from the site of weakness within the
wood, to the site where the oxygen and water come into
contact with the nail.
My cat,
“Holly”.
My nice car, sitting
in my driveway.
A rusted out water pipe. This is going
to cost me a fortune to fix!!! /
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