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 MIL-HDBK-419A
2.10.2 Protection Techniques.
Three basic techniques can be used to lessen the corrosion rate of buried metals. The obvious method is to
insulate the metals from the soil by the use of protective coatings. This interrupts the current path through the
electrolyte and stops the erosion of the anode. Insulation, however, is not an acceptable corrosion preventive
for earth electrodes. The second technique for reducing galvanic corrosion is avoiding the use of dissimilar
metals at a site. For example, if all metals in contact with the soil are of one type (such as iron, lead or
copper), galvanic corrosion is minimized. Each of these materials, however, has unique properties such as
weight, cost, conductivity, ductility, strength, etc., that makes its use desirable, and thus none can be
summarily dismissed from consideration for underground applications. Copper is a desirable material for the
earth electrode subsystem; apart from its high conductivity, the oxidation potential of copper is such that it is
relatively corrosion resistant. Since copper is cathodic relative to the more common structural metals, its
corrosion resistance is at the expense of other metals. Iron electrodes would, of course, be compatible with
water pipes, sewer lines, reinforcing rods, steel pilings, manhole covers, etc., but iron is subject to corrosion
even in the absence of other metals. In addition, the conductivity of iron is less; however, steel grounding rods
are sometimes used by electric utilities for grounding associated with their transmission lines. Because of the
greater conductivity and corrosion resistance of copper, it is normally used for the grounding of buildings,
substations, and other facilities where large fault or lightning currents may occur and where voltage gradients
must be minimized to ensure personnel protection.
The third technique for combating the corrosion caused by stray direct currents and dissimilar-metal unions is
commonly called cathodic protection. Cathodic protection may be implemented through the use of sacrificial
anodes or the use an an external current supply to counteract the voltage developed by oxidation. Sacrificial
anodes containing magnesium, aluminum, manganese, or other highly active metal can be buried in the earth
nearby and connected to an iron piling, steel conduit, or lead cable shield. The active anodes will oxidize more
readily than the iron or lead and will supply the ions required for current flow. The iron and lead are cathodic
relative to the sacrifical anodes and thus current is supplied to counteract the corrosion of the iron or lead.
The dc current is normally derived from rectified alternating current, but occasionally from photovoltaic cells,
storage batteries, thermoelectric generators, or other dc sources. Since the output voltage is adjustable, any
metal can be used as the anode, but graphite and high silicon iron are most often used because of their low
corrosion rate and economical cost. Cathodic protection is effective on either bare or coated structures. If the
sacrifical anodes are replenished at appropriate intervals, the life of the protected elements is significantly
prolonged.
2.10.3 Sacrifical Anodes. Sacrificial anodes provide protection over limited areas. Impressed current cathodic
protection systems use long lasting anodes of graphite, high silicon cast iron or, to a lesser extent, platinum
coated niobium or titanium. The protection of long cable or conduit runs can be provided by biasing the metal
to approximately -0.7 to -1.2 volts relative to the surrounding soil. The external dc source supplies the
ionization current that would normally be provided by the oxidation of the cable sheath or conduit. This dc
current is normally derived from rectified ac and occasionally from photovoltiac cells, storage batteries,
thermoelectric generators, or other dc sources. A layer of insulation such as neoprene must cover the metal to
prevent direct contact with the surrounding soil. Therefore, the technique is not appropriate for protecting
foundations, manholes, or other structural elements normally in contact with the soil. It is most appropriate for
supplying the leakage current that would normally enter the soil through breaks in the insulation caused by
careless installation, settling, lightning perforation, etc.
2-65
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