Characteristics of galvanic and rectifier cathodic protection systems 

 compared "by Husock (1962) follows: 



Galvanic 



1. Requires no external power 



2. Fixed driving voltage 



3. Limited current 



Usually used in lower 

 resistivity electrolytes 



In underground applications 

 interference with neighbor- 

 ing structures is usually 

 negligible 



Rectifier 



External power required 



Voltage can be varied 



Can be designed for almost 

 any current requirement 



Can be used in almost any 

 resistivity environment 



Interference with neighboring 

 structures must be considered 



d. Galvanic System . In this system a galvanic corrosion cell 

 is formed by installing an electrode which will be anodic to the metal 

 to be protected (cathode) when the two are connectea in an electrolyte. 

 See Figure lU. The anodes are sacrificed to protect the cathodic metal. 

 The anodes may be replaced periodically, if required. 



(1) Galvanic Anodes for Steel . The principal materials 

 which have been used for galvanic protection of marine structures 

 or steel are magnesium and zinc. Although aluminum is anodic to steel, 

 a surface film forms on the aluminum which hinders its generation of 

 protective current when coupled to steel as sacrificial anodes. Aluminum 

 alloy anodes have been developed and used in seawater applications (Husock. 

 1962). In laboratory tests, alloys of aluminum mercury and zinc have 

 attained efficiences of 95 percent with potentials in the order of 1.05 

 volts. The electrical output per pound of metal consiomed was 1,290 ampere 

 hours (Reding and Newport, I966 ) . A comparison of zinc and magnesium 

 anode characteristics from Husock (1962) is given below. 



Characteristic 



Efficiency (approximate percent) 



Theoretical consumption (lbs/ampere yr) 



Approximate actual consumption (lbs/ampere yr) 



Solution potential* 



Zinc 



Magnesium 



90 



50 



23.5 



8.7 



26.0 



iT.o 



1.1 



1.55 



*Ref erred to a copper sulphate electrode, 



42 



