Why Corrosion is an Electrochemical Process
Why Corrosion is an Electrochemical Process
Electrochemical Cells Corrosion is electrochemical in nature because the
corrosive chemical reactions involve transfer of charge.
Iron goes into solution as Fe++ ions. As these ions go into solution, the metal
becomes negatively charged (by the electrons left behind) with respect to the
electrolyte.
A potential difference (voltage) is produced between the electrolyte and the
metal. The process in which electrons are given up and positive metal ions are
formed is called oxidation.
The sites at which the oxidation takes place on the surface of the metal become
electrochemical cells made up of micro-electrodes of the two different
substances; the metal and the electrolyte.
These micro-electrodes set up many micro-cells connected through the bulk of the
metal.
If a different metal is used, it will go into solution to a greater (or
lesser) extent producing a larger (or smaller) potential difference between the
metal and electrolyte than was the case for iron, this
form of corrosion
is called galvanic
corrosion or
bimetallic
corrosion.
For example, magnesium and zinc go into solution to a greater extent than iron,
and these metals will be more negative with respect to the electrolyte than
iron.
Nickel, lead, and copper go into solution less readily and produce a
smaller potential difference.
In a Galvanic
Corrosion Chart are reported the potential differences for various metals in
water.
The order of the series can change for different electrolytes (for example, different pH, ions in solution).
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