average soil or quiescent seawater usually is considered to be about 10 

 milliamperes per square meter (1 milliampere per square foot). Any coating 

 present, regardless of quality, can produce a drastic reduction in the 

 current density requirement. Actual current required for a given sub- 

 structure, of course, depends on the subsurface area exposed to the con- 

 tacting soil or water. In the case of bare steel, 93 square meters (1 000 

 square feet) probably would require at least 1 ampere for protection. 

 With wrap-coated steel pipe, approximate current density requirements may 

 be estimated by reference to Table 53. These values are for guidance 

 estimates only and may vary widely depending on specific conditions such as 

 moving seawater. 



Table 53. Current requirements for coated steel 

 or wrapped pipe. 



Age of Wrap or Coating 



Approx. Current Density 

 Required (mA/m 2 ) 



1930 to 1950 

 1950 to 1960 

 1960 to 1970 

 1970 to Present 



1.1 

 0.54 

 0.32 

 0.11 



The actual current requirement requires that a calculation of the total 

 area involved, in square feet, be made. Based on the current requirement, 

 a decision can be made whether a galvanic system or impressed current sys- 

 tem should be used. 



(2) Impressed Current (Inert) Anodes . Driving potential of galvanic 

 anode.s may not be high enough to provide sufficient current for effective 

 cathodic protection. This is particularly true when the subsurface structure 

 to be protected is surrounded by an electrolyte of high resistivity such as 

 is frequently encountered with steel in freshwater or soil. Also there are 

 applications requiring higher current density than can be delivered econom- 

 ically by galvanic anodes. Examples of this would be protection of the 

 external bottom surface of a large oil storage tank or either soil or water 

 sides of a steel sheet-pile bulkhead. To be free from the limitations of 

 galvanic anodes, an external power source may be used to provide "impressed" 

 current for protection. Here, the required direct current driving voltage 

 and current are limited only by the external source power availability. 

 Various power sources may be used; the most common is commercial alternating 

 current power connected to a low voltage transformer-rectifier combination 

 (usually called simply a rectifier) to provide the required direct current 

 with easily adjustable output voltage. This provides the means to counteract 

 the flow of corrosion current to make the substructure "all cathode." A 

 simplified diagram as applied to a pipeline is shown in Figure 96. 



349 



