standards and guidelines for communication sites - Radio And ...

STANDARDS AND GUIDELINES FOR COMMUNICATION SITES DISSIMILAR METALS AND CORROSION CONTROL
Metals may be listed in order of their respective potentials; such a list is called a galvanic series. A
galvanic series of commonly used grounding (earthing) electrode system metals and alloys is given in
Table 4-6 (from TIA/EIA-222-F-R2003 and MIL-HDBK-419A). When a complete circuit exists,
corrosion occurs on the metal listed higher in the galvanic series. The metal listed higher in the galvanic
series (anode) is where current exits and travels through the soil toward the metal listed lower on the
galvanic series (cathode). The galvanic series of commonly used metals and alloys is as follows:
TABLE 4-6 GALVANIC SERIES OF COMMON METALS
Magnesium
Anodic (Active) End
Zinc (material used to galvanize steel)
Aluminum
Steel, Iron
Lead, Tin
Brass, Copper, Bronze
Silver
Graphite
Cathodic (Most Noble) End
The rate of corrosion mainly depends on the conductivity of the soil and the relative position of the
metals in the galvanic series. The higher the soil conductivity (low resistivity), and the further apart the
metals are in the galvanic series, the faster the rate of corrosion (TIA/EIA-222-F-R2003). To some
extent, the rate of corrosion also depends on the relative surface areas of the metals (BS 7430:1998 and
IEEE STD 142-1991). A small anode (such as a galvanized steel guy anchor point) and large cathode
(such as a copper grounding electrode system) should not be installed; in this case, the total current is
confined to a small space and the current density is large, therefore, corroding the galvanized steel
(IEEE STD 142-1991).
General guidance on the suitability of metals for bonding together with neighboring metals is given
below in Table 4-7 (BS 7430:1998); both metals are assumed to be located in the earth. The bond
between the neighboring metals could be located above or below ground.
68P81089E50-B 9/1/05 4-37