Impressed current anodes differ from galvanic anodes in two important 

 aspects: (i) the galvanic potential difference between anode and protected 

 substructures is of no importance, and (21 the impressed current anode 

 should be as inert as possible, i.e., have a very low consumption rate for 

 long life. Consumption rates for various metals including those commonly 

 used for impressed current anodes are shown in Table 40. Carbon or 

 graphite, high silicon cast iron, magnetite, lead-silver, and platinum all 

 have critical current densities above which rapid consumption may occur. 

 The consumption rates listed for these materials assume operation below 

 critical current densities. 



As with galvanic anodes in soil, impressed current anodes in soil 

 should be installed completely surrounded by backfill material. Impressed 

 current anodes in water do not require backfill. Impressed current anodes 

 in soil require a carbonaceous backfill, well-tamped to eliminate air 

 pockets and to provide the best possible electrical contact, both with the 

 anode and with the soil. Two functions are thereby served: 



(a) The very low resistivity of the backfill material has 

 the effect of increasing the anode size with a consequent 

 reduction in resistance to surrounding soil; and 



(b) most of the current flows to the backfill from the 

 anode by direct electrical contact so that most of the electro- 

 lytic consumption is at the soil contact with the outer surface 

 of the backfill column. 



Resistivity of carbonaceous backfill should not exceed 50 ohm- centimeters. 

 There are at least three materials in present use: coal coke breeze, 

 calcined petroleum coke breeze, and manmade or natural graphite flakes or 

 particles. All of these are basically carbon in low resistivity form. The 

 term "breeze" is loosely defined as being a finely divided material. For 

 backfill use, specific sizes are obtainable. Coke breeze should be procured 

 by specification with particle size and resistivity being most important. 

 Particle size should not exceed 9.5 millimeters (0.375 inch) and not more 

 than 10 percent dust should be included. Petroleum coke breeze must be 

 calcined to produce resistivity of 50 ohm- centimeters or less. Graphite 

 flakes should not be used because of possible gas blockage problems (accumu- 

 lation of gas around the cathode from the cathode reaction) . 



(3) Comparison of Anode Types . Cathodic protection for a given 

 subsurface structure may be provided by either galvanic anodes or by impressed 

 current anodes as long as respective limitations are recognized. Some of 



the more important characteristics of each method are listed in Table 54. 



(4) Connections Between Anodes and Structures . In the case of 

 galvanic anodes, the connecting wire from the anode is part of the cathode 

 and bare areas resulting from cut or broken wire insulation or poorly 

 insulated splices will be part of the protected structure and suffer no 

 electrolytic damage. The opposite is true of impressed current anode 

 systems. If any of the insulation is less than perfect, current discharge 

 into the surrounding electrolyte (soil or water) will occur and the wire 

 will corrode too quickly, producing failure of part or all of the system. 

 This means that top-quality insulation must be used for all buried or 



351 



