Corrosion
DOE-HDBK-1015/1-93
CORROSION THEORY
Rev. 0
CH-02
Page 9
Corrosion Theory Summary
Ionization is the process of adding electrons to or removing electrons from atoms
or molecules which creates ions.
Conductivity is a measure of the ability of a substance to allow electron flow.
Corrosion is the deterioration of a material due to interaction with its
environment.
Electrolysis is the decomposition by electric current.
General corrosion is the process whereby the surface of a metal undergoes a slow,
relatively uniform, removal of material.
Corrosion is electrochemical in nature because the corrosive chemical reactions
involve a transfer of charge. The metal ions go into solution causing the metal to
become negatively charged with respect to the electrolyte. The difference in the
charge causes a potential to develop and produces a voltage between the
electrolyte and the metal.
The oxidation step of the oxidation-reduction process is where an atom (in this
case a metal atom) releases electron(s) and becomes a positively-charged ion.
The areas where oxidation takes place become electrochemical cells made up of
two different substances. The oxidation step results in a chemical transformation
that is destructive to the metal. The positive metal ions may go into solution, or
they may combine with any available negative ions or water to form ionic
compounds. An example of the oxidation step is:
Zn Zn
+ 2e
+2
-
The layer of absorbed atomic hydrogen is said to polarize the cell. This type of polarization is
called activation polarization and is sometimes referred to as hydrogen polarization, or cathodic
polarization, because the polarizing reaction occurs at the cathode.
Both concentration and activation polarization decrease the net oxidation-reduction reaction
rate. In corrosion processes, activation polarization usually has the greater effect.
Summary
The important information in this chapter is summarized below.
