For corrosion to form, four ingredients must be present:
- An anode
- A cathode
- An electrolyte
(e.g., moisture)
- An electrical
path (e.g., metal) joining the anode and cathode
The anode
is the part of the metal where corrosion occurs. The anode actually
sacrifices itself as
it releases positively charged metal ions into the electrolyte and
electrons are left behind in the
metal. These electrons flow through the metal to protect the cathode.
The cathode is protected
because various ions or compounds in the electrolyte consume electrons.
An electrolyte
is a solution capable of conducting electrical current in the form
of ionic flow. An electrical path is a connection between the anode
and cathode
where current in the form
of electrons can flow. Free electrons do not flow in the electrolyte,
only in a metal path.
Corrosion occurs because anodes and cathodes are inherent in
all metals, and all metals are
electron conductors. Anodic and cathodic components can be
microscopic in size or rather large
in come cases. Therefore, three of the four requirements for
the corrosion process are inherently
present in every metal. The fourth element required for corrosion
is an electrolyte.
Anodic
and cathodic areas develop where chemicals have deposited, where
there are temperature
differences, and in damp areas. Chlorides and other industrial
contaminants in the electrolyte can
also cause an area to become anodic. The contamination may
be present on the metal surface
before it is coated or insulated. Once these areas become
wet, corrosion begins.
SUMMARY: Corrosion
of metals requires the following conditions:
- An anode, a cathode,
an electrical path, and an electrolyte must all be present.
- The
anode and cathode must be in contact with the same electrolyte.
- The
metal must electrically connect the anode and cathode for electrons
to flow.
- The anodic (oxidation)
and cathodic (reduction) reactions must be equivalent and simultaneous.
