anodes may be made of magnesium alloy (usually used in soil), aluminum 

 alloy (usually used in seawater) , or zinc alloy (used in either soil or 

 water). Alloys are used because in the cases of the three metals mentioned, 

 certain alloys of each result in greater efficiency (i.e., one or more of 

 higher output current, higher output voltage, or longer life) than is 

 possible when using the pure unalloyed metal. 



Aluminum or zinc anodes are used in seawater or freshwater. Magnesium 

 may also be used, but its cost is higher and its life per newton is shorter. 

 Zinc and magnesium anodes for use in soil are installed with a chemical 

 backfill completely surrounding the anode. The backfill material should be 

 uniform to provide current efficiency. (With a nonuniform backfill, the 

 anode will supply increased currents where the backfill has low resistivity, 

 thus wasting the anode more rapidly in these areas.) A uniform backfill 

 will also aid in keeping the anode continuously moist and will prevent 

 anode contact with adverse materials that may be in the soil. Low back- 

 fill resistivity has the same effect as increasing anode size, thereby 

 decreasing effective anode resistance to earth. 



Magnesium anodes in prepackaged backfill are available in many sizes. 

 The anode with surrounding backfill is contained in a cloth sack or porous 

 tube. Backfill commonly supplied with prepackaged magnesium anodes is 75 

 percent gypsum, 20 percent bentonite alloy, and 5 percent sodium sulfate, 

 but other mixtures usually are available on request. Magnesium anodes are 

 normally furnished with a 3-meter (10 foot) No. 10 or No. 12 AWG copper 

 lead wire with Type TW insulation. 



Zinc anodes are normally not sold in prepackaged backfill or with leads 

 attached due to difficulty in handling the much heavier anodes. If not 

 provided with prepackaged backfill, galvanic anodes in soil should be 

 installed completely surrounded by well-tamped chemical backfill material 

 to ensure best utilization of anode material. Chemical backfill material 

 for either magnesium or zinc anodes is shown in Table 51. No backfill of 

 any kind is used with anodes in water. 



Zinc and aluminum anodes, without backfill, are particularly useful in 

 seawater where the low resistivity electrolyte permits good current output 

 in spite of the relatively low anode driving potential. Magnesium is not 

 suitable for seawater use because of the low efficiency caused by the 

 tendency of magnesium to "self-corrode" in the low resistivity electrolyte. 



Whether or not a galvanic anode system will work depends on the electrical 

 circuit resistance and on the current required for protection. The circuit 

 resistance is determined almost entirely by the resistivity of the electrolyte 

 environment. Galvanic anodes work best in low resistivity electrolytes such 

 as seawater with a resistivity of 16 to 20 ohm-centimeters. Good performance 

 usually is obtained through 1 000 ohm-centimeters. They have been made to 

 work in resistivities as high as 2 000 ohm-centimeters when conditions permit 

 use of the limited output current. The output current is necessarily 

 limited by the relatively low driving potentials of all types of galvanic 

 anodes. Table 52 shows approximate data for some common anode materials. 



Current required for protection is based on current density require- 

 ments, usually stated in milliamperes per unit of area. Bare steel in 



347 



