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 MIL-HDBK-419A
Anything that prevents the existence of either of the above conditions will prevent corrosion. For example, in
pure water, hydrogen gas will accumulate on the cathode to provide an insulating blanket to stop current flow.
Most water, however, contains dissolved oxygen which combines with the hydrogen to form additional molecules
of water. The removal of the hydrogen permits corrosion to proceed. This principle of insulation is employed in
the use of paint as a corrosion preventive. Paint prevents moisture from reaching the metal and thus prevents
the necessary electrolytic path from being established.
7.8.1.1 Electrochemical Series. The oxidation of metal involves the transfer of electrons from the metal to
the oxidizing agent, In this process of oxidation, an electromotive force (EMF) is established between the metal
and the solution containing the oxidizing agent. A metal in contact with an oxidizing solution containing its
own metal ions establishes a fixed potential difference with respect to every other metal in the same condition.
The set of potentials determined under a standardized set of conditions, including temperature and ion
concentration in the solution, is known as the EMF (or electrochemical) series. The EMF series (with hydrogen
as the referenced potential of 0 volts) for the more common metals is given in Table 7-6. The importance of
the EMF series is that it shows the relative tendencies of metals to corrode. Metals high in the series react
more readily and are thus more prone to corrosion. The series also indicates the magnitude of the potential
established when two metals are coupled to form a cell. The farther apart the metals are in the series, the
higher the voltage between them. The metal higher in the series will act as the anode and the one lower will
act as the cathode. When the two metals are in contact, loss of metal at the anode will occur through oxidation
to supply the electrons to support current flow. This type of corrosion is defined as galvanic corrosion. The
greater the potential difference of the cell, i.e., the greater the dissimilarity of the metals, the greater the
rate of corrosion of the anode.
7.8.1.2 Galvanic Series.
The EMF series is based on metals in their pure state -- free of oxides and other films -- in contact with a
standardized solution. Of greater interest in practice, however, is the relative ranking of metals in a typical
environment with the effects of surface films included. This ranking is referred to as the galvanic series. The
most commonly referenced galvanic series is listed in Table 7-7. This series is based on tests performed in sea
water and should be used only as an indicator where other environments are of concern.
Galvanic corrosion in the atmosphere is dependent largely on the type and amount of moisture present. For
example, corrosion will be more severe near the seashore and in polluted industrial environments than in dry
rural settings. Condensate near the seashore or in industrial environments is more conductive even under equal
humidity and temperature conditions due to increased concentration of sulfur and chlorine compounds. The
higher conductivity means that the rate of corrosion is increased.
7-31
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