Table SI. Chemical backfill for galvanic anodes in soil. 



Back- 

 fill 

 Mix 



Gypsum (CaSOi,) 



Bentonite 

 Clay 

 (pet) 



Sodium 



Sulfate 



Cpct) 



Approx . 

 Resistivity, 

 Ohm-cm 



Hydrated 

 Cpct] 



Molding Plaster 



[plaster of paris) 



(pet) 



1 



2 



3 



it 



25 

 50 



75 



50 



75 

 50 

 50 

 20 



5 



2S0 



2S0 



250 



50 



Utilizes moisture-holding 



'Useful in low soil moisture areas, 

 characteristic of bentonite clay. 



2 Usually used with zinc anodes. 



3 Useful with zinc or magnesium in wet or marshy soils. Prevents 

 rapid migration of backfill from anode surface. 



" Low resistivity; useful in high soil resistivity to reduce anode resist- 

 ance to earth. Type usually furnished with prepackaged magnesium anodes. 



Table 52. Approximate data for common galvanic 

 annode alloys. 





Zinc alloy 

 in soil 1 

 or seawater 

 (Mil Spec. 

 A-18001) 



Magnesium Alloys 

 in Soil 1 



Aluminum 



Alloy 

 in seawater 

 (Galvalum 

 Alloy) 



Standard 

 (H-l Alloy) 



High 



potential 

 (Galvalum 

 Alloy) 



Specific Gravity 



7.14 



1.94 



1.94 



2.77 



Kilonewtons per 

 Cubic Meter 



59.1 



19.0 



19.0 



- 



Theoretical Ampere 

 Hours per Newton 



84 



225 



225 



302 



Current Efficiency 

 (pet) 



95 2 



50 3 



50 3 



95 2 



Actual Ampere 



Hours per Newton 



79 2 



112 3 



112 3 



288 2 



Actual Newtons per 

 Ampere-Year 



111 



76 3 



76 3 



31 



Solution Potential 

 (volts) 



-1.1" 



-l.SS" 



-1.80" 



-l.l 5 



Driving Potential, 

 (volts) 



-0.2S 5 



-0.60 7 



-0.85 7 



-0.25 6 



'Anodes in suitable chemical backfill. 



2 Current efficiency of zinc and aluminum reasonably constant 



over wide range of output. 

 3 Current efficiency of magnesium varies with current density. 



Figures given are for approximately 300 milliamperes per square 



meter of anode surface. 

 "To CuSOt, reference cell in neutral soil. 

 5 To CuSOij reference cell in seawater. 

 "Solution potential, the equilibrium potential of a metal exposed 



in a given solution, minus the polarized potential, the potential 



of a metal after the flow of sufficient current to come to 



equilibrium (-0.85 volts to CuSOc, reference electrode). 

 7 Solution potential minus polarized potential of protected structure 



minus approximately 0.1 volt for anode polarization in service. 



348 



