standards and guidelines for communication sites - Radio And ...
standards and guidelines for communication sites - Radio And ... standards and guidelines for communication sites - Radio And ...
DISSIMILAR METALS AND CORROSION CONTROL CHAPTER 4: EXTERNAL GROUNDING (EARTHING) The following general observations can be made from Table 4-5: • Copper-clad steel or solid copper ground rods are one of the better and commonly used materials for grounding electrodes. However, the adverse effect of dissolved salts, organic acids and acid soils generally should be noted (BS 7430:1998, clause 11). • Copper or copper-clad ground rods should not be used in soils where organic acids are present, unless protective measures are taken, such as encasing the ground rods in a grounding electrode encasement material. Organic acids are commonly found in poorly drained and poorly aerated soils. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27. • Galvanized ground rods should not be used in soils with a redox potential below 200 mV, unless protective measures are taken, such as encasing the ground rods in a grounding electrode encasement material. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27. • Galvanized ground rods should not be used in acidic soils with a pH below 6. 4.5.2 GALVANIC CORROSION COPPER GROUND ROD Galvanic corrosion (also called dissimilar metals corrosion) refers to corrosion damage induced when two dissimilar metals are electrically connected and coupled through an electrolyte (such as soil). When a metal is electrically connected to a dissimilar metal, a difference of potential exists between the two metals. If the dissimilar metals are also in contact with a low resistivity soil, a complete circuit will exist. Current will flow from one metal to the other due to the electrical connection and return path through the soil. This naturally occurring phenomenon is why current is obtained from a battery when its terminals are electrically connected to a load (TIA/EIA-222-F-R2003). See Figure 4-26 for an example of installations with and without galvanic corrosion. LVANIZED GROUND ROD FIGURE 4-26 INSTALLATIONS WITH AND WITHOUT GALVANIC CORROSION 4-36 68P81089E50-B 9/1/05 COPPER GROUND ROD COPPER GROUND ROD
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
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DISSIMILAR METALS AND CORROSION CONTROL CHAPTER 4: EXTERNAL GROUNDING (EARTHING)<br />
The following general observations can be made from Table 4-5:<br />
• Copper-clad steel or solid copper ground rods are one of the better <strong>and</strong> commonly used materials<br />
<strong>for</strong> grounding electrodes. However, the adverse effect of dissolved salts, organic acids <strong>and</strong> acid<br />
soils generally should be noted (BS 7430:1998, clause 11).<br />
• Copper or copper-clad ground rods should not be used in soils where organic acids are present,<br />
unless protective measures are taken, such as encasing the ground rods in a grounding electrode<br />
encasement material. Organic acids are commonly found in poorly drained <strong>and</strong> poorly aerated soils.<br />
See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27.<br />
• Galvanized ground rods should not be used in soils with a redox potential below 200 mV, unless<br />
protective measures are taken, such as encasing the ground rods in a grounding electrode<br />
encasement material. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27.<br />
• Galvanized ground rods should not be used in acidic soils with a pH below 6.<br />
4.5.2 GALVANIC CORROSION<br />
COPPER<br />
GROUND<br />
ROD<br />
Galvanic corrosion (also called dissimilar metals corrosion) refers to corrosion damage induced when<br />
two dissimilar metals are electrically connected <strong>and</strong> coupled through an electrolyte (such as soil). When<br />
a metal is electrically connected to a dissimilar metal, a difference of potential exists between the two<br />
metals. If the dissimilar metals are also in contact with a low resistivity soil, a complete circuit will<br />
exist. Current will flow from one metal to the other due to the electrical connection <strong>and</strong> return path<br />
through the soil. This naturally occurring phenomenon is why current is obtained from a battery when<br />
its terminals are electrically connected to a load (TIA/EIA-222-F-R2003). See Figure 4-26 <strong>for</strong> an<br />
example of installations with <strong>and</strong> without galvanic corrosion.<br />
LVANIZED<br />
GROUND ROD<br />
FIGURE 4-26 INSTALLATIONS WITH AND WITHOUT GALVANIC CORROSION<br />
4-36 68P81089E50-B 9/1/05<br />
COPPER<br />
GROUND<br />
ROD<br />
COPPER<br />
GROUND<br />
ROD
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