Chemistry Reference
In-Depth Information
listing of the metals and alloys arranged according to their stability to cor-
rosion, as measured by their
electrochemical potential
a numerical mag-
nitude used to express the tendency of metals and/or alloys to corrode (see
Table 43). As can be seen in the table, at the top of the galvanic series are
the noble metals, which have a low electrochemical potential, undergo
little corrosion, and are stable under almost any conditions. The more reac-
tive, easily corroded base metals have a high electrochemical potential and
are listed at the bottom of the series. Among the metals of antiquity, gold,
a noble metal that was the most chemically stable metal known at the
time, has the lowest electrochemical potential. The least stable and most
easily corroded metal, zinc, whose electrochemical potential is the highest,
is last on the list.
TABLE 43 The Galvanic Series:
Ancient Metals and Alloys
1. Platinum (the most stable metal; has the highest
electrochemical potential)
2. Gold
3. Silver
4. Copper
5. Bronze
6. Brass
7. Tin
8. Lead
9. Lead-tin (solder) alloy
10. Mild steel; wrought iron
11. Zinc (least stable ancient metal; has the lowest
electrochemical potential)
Under equal environmental conditions, the rate of corrosion of alloys
is generally faster than that of their components. The tendency of bronze,
an alloy of copper and tin, to corrode is greater than that of its separate
(unalloyed) components, copper and tin. Moreover, once a corrosion
process sets up in bronze, it also proceeds, under similar conditions, at a
faster rate than in either copper or tin.
Cathodic Protection
When two objects made of different metals or alloys are in contact with
each other in the presence of an
electrolyte
(a medium that provides a
transport mechanism), an electric current flows between them. The direc-
tion of the current's flow is determined by the electrochemical potential
of the metals in contact: the baser metal, having the higher electrochem-
ical potential value, will be preferentially corroded, whereas the one
having the lower electrochemical potential is more passive and will remain
