Corrosion handbook
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These two partial reactions can take place on the metal surface in a fairly even
distribution leading to uniform attack (see section 1.2.1). Alternatively, they can
occur locally and separately, leading to localized forms of corrosion such as
pitting corrosion.
The mechanism and the electrochemical nature of the corrosion reaction define
the necessary requirements for atmospheric corrosion to take place:
• a conducting metal
• an electrolyte (a thin, even invisible moisture film on the surface is already
sufficient)
• oxygen for the cathodic reaction
The illustration in Fig. 2 shows the basic corrosion mechanism of iron under a
drop of water. Both metal dissolution and oxygen reduction reactions take place
with slight separation on the surface, while their products (Fe-ions and OH-ions)
react in the water drop to form iron oxide (red rust).
In general, the same scheme applies to other metals such as zinc or aluminum,
but with slightly different chemical reactions in the electrolyte.
With the simple model of the corrosion reaction, as seen in Fig. 3, we are able
to explain many forms of corrosion and also to put measures in place to help
mitigate it. By mitigating these partial reactions, the overall corrosion rate is also
reduced.
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Fig. 2: Requirements for an atmos pheric corrosion reaction.
Fig. 3: Corrosion of iron under a drop
of water, showing metal dissolution,
oxygen reduction and the formation of
rust.
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