CORROSION THEORY
DOE-HDBK-1015/1-93
Corrosion
CH-02
Rev. 0
Page 10
Corrosion Theory Summary (Continued)
The reduction step of the oxidation-reduction process is where a
positively-charged ion gains electron(s). The reduction step for most metals,
in an aqueous environment, is the reduction of hydronium ions. An example of
the reduction step is:
H O + e H + H O
3
2
+
-
Passivity is the buildup of a stable, tenacious layer of metal oxide on the surface
of the metal that acts as a barrier separating the metal surface from the
environment. Passivity decreases or stops the corrosion process because of the
formation of the layer.
Polarization is the decrease in cell potential caused by the operation of the
electrochemical cell. Polarization can be in two forms; concentration or
activation. Concentration polarization is associated with the concentration of
ions in solution which shields the metal, thereby causing a decrease in the
electrical potential of the cell. Activation polarization is the formation of a layer
containing absorbed hydrogen atoms that block the metal's surface from the
corrosion process.
