2.3. Factors influencing corrosion rates.- From the
above examples and the many other envi-
ronmental differences which could be visualized,
it might appear that almost no metalwork could
survive burial in soil or immersion in water. Such
is not the case because the rate at which a cell
functions and corrosion occurs is controlled by
several factors; these factors may virtually halt the
cell action. Some of the more important factors
affecting corrosion are inherent or associated with
the metal itself, such as the effective potential of
the metal in the solution, physical and chemical
homogeneity of the metal surface, and the
inherent ability of the metal to form an insoluble
protective film. Environmental factors affecting
corrosion rates are formation of protective
coatings on metal, temperature, influence of
oxygen in the electrolyte, effect of electrode
potential, and others. No attempt has been made
to list these factors in the order of their
importance. The environmental aspect of
corrosion is the more unpredictable and one that
makes it impossible to describe a single, positive
method of controlling a specific corrosion problem
without detailed investigation.
above examples and the many other envi-
ronmental differences which could be visualized,
it might appear that almost no metalwork could
survive burial in soil or immersion in water. Such
is not the case because the rate at which a cell
functions and corrosion occurs is controlled by
several factors; these factors may virtually halt the
cell action. Some of the more important factors
affecting corrosion are inherent or associated with
the metal itself, such as the effective potential of
the metal in the solution, physical and chemical
homogeneity of the metal surface, and the
inherent ability of the metal to form an insoluble
protective film. Environmental factors affecting
corrosion rates are formation of protective
coatings on metal, temperature, influence of
oxygen in the electrolyte, effect of electrode
potential, and others. No attempt has been made
to list these factors in the order of their
importance. The environmental aspect of
corrosion is the more unpredictable and one that
makes it impossible to describe a single, positive
method of controlling a specific corrosion problem
without detailed investigation.
2.4. The galvanic series.- The differing vigor with
which different metals tend to dissolve in
electrolytes provides the driving force for galvanic
cells and gives rise to the galvanic series. This is
a listing of metals in decreasing order of their
corrosion when any two of them are the
electrodes of a complete cell. That is, the metal
higher on the list will be the anode and will be
corroded while the lower will be the cathode and
will be protected in the cell. A galvanic series
tabulation developed by the International Nickel
Company is shown in table 1. This series was
developed by actual field and laboratory tests
using electrolytes likely to be encountered under
operation conditions. It takes into account that
certain metals from protective oxides which cause
these metals to assume more noble positions in
the series than the clean metal would have. This
series, then considers practical corrosion aspects
as well. However, it cannot anticipate all service
conditions and reversals of position which may
occur. (The galvanic series should not be con-
fused with the electromotive series used by
chemists. The latter is referred to standard
conditions which rarely occur in nature, and the
order of the metals in the electromotive series
does not exactly coincide with that of the galvanic
series.)
which different metals tend to dissolve in
electrolytes provides the driving force for galvanic
cells and gives rise to the galvanic series. This is
a listing of metals in decreasing order of their
corrosion when any two of them are the
electrodes of a complete cell. That is, the metal
higher on the list will be the anode and will be
corroded while the lower will be the cathode and
will be protected in the cell. A galvanic series
tabulation developed by the International Nickel
Company is shown in table 1. This series was
developed by actual field and laboratory tests
using electrolytes likely to be encountered under
operation conditions. It takes into account that
certain metals from protective oxides which cause
these metals to assume more noble positions in
the series than the clean metal would have. This
series, then considers practical corrosion aspects
as well. However, it cannot anticipate all service
conditions and reversals of position which may
occur. (The galvanic series should not be con-
fused with the electromotive series used by
chemists. The latter is referred to standard
conditions which rarely occur in nature, and the
order of the metals in the electromotive series
does not exactly coincide with that of the galvanic
series.)
Table 1 .- Galvanized series of metals and alloys *
Corroded end (anodic or least noble):
Magnesium.
Magnesium alloys.
Magnesium alloys.
Zinc.
Aluminum 2S.
Cadmium.
Aluminum 17ST.
Steel or iron. Cast iron.
Chromium-iron (active)
Ni-Resist.
18-8 Chromium-nickel-iron (passive).
18-8-3 Chromium-nickel-molybdenum-iron
(passive).
18-8-3 Chromium-nickel-molybdenum-iron
(passive).
Lead-tin solders
Lead.
Tin.
Lead.
Tin.
Nickel (active)
Inconel (active)
Hastelloy C (active)
Inconel (active)
Hastelloy C (active)
Brass.
Copper.
Bronzes.
Copper-nickel alloys.
Monel.
Copper.
Bronzes.
Copper-nickel alloys.
Monel.
Silver solder.
Nickel (passive).
Inconel (passive).
Inconel (passive).
Chromium-iron (passive).
18-8 Chromium-nickel-iron (passive).
18-8-3 Chromium-nickel-molybdenum-iron
(passive).
Hastelloy C (passive).
18-8 Chromium-nickel-iron (passive).
18-8-3 Chromium-nickel-molybdenum-iron
(passive).
Hastelloy C (passive).
Silver.
Graphite. Gold. Platinum.
Protected end (cathodic or most noble)
*Metals listed together show little tendency to
corrode galvanically when connected.
corrode galvanically when connected.
2.5. Use of the galvanic series.- The metals
grouped together in the galvanic series cremate
cells having low driving force (voltage) when
connected together and little tendency for gal-
vanic corrosion. Therefore, in general, they can
be used in direct contact with each other
grouped together in the galvanic series cremate
cells having low driving force (voltage) when
connected together and little tendency for gal-
vanic corrosion. Therefore, in general, they can
be used in direct contact with each other
without
damaging effects. When coupled as a cell, two
metals from
metals from
3 (FIST4-5)