Table XII gives the consumption rates in seawater, cost per pound 

 and cost per ampere-year for magnesium, zinc and aluminmn along with 

 other anode materials suitable for impressed current systems. 



The use of magnesium has been favored when higher driving forces 

 are required. Hosford (1963) states that magnesium anodes in seawater 

 tend to disintegrate rather rapidly unless restricted, therefore, zinc 

 anodes are generally better in seawater environments. 



e. Electrolytic (impressed Current) Syste m. This system is 

 often employed in cathodic protection systems where it is desirable to 

 use relatively high currents and voltages, where numerous galvanic anodes 

 would ordinarily be acquired, where flexibility of voltage and/or current 

 is desired, and where automatic control is desired. The source of cur- 

 rent for impressed current systems is usually a rectifier if alternating 

 current is available. Batteries or other direct current electrical sources 

 can be used if necessary. 



(1) Rectifiers . Rectifiers are used in impressed current 

 systems to transform alternating current into direct current of the 

 proper voltage. Rectifiers are also provided with a means of controlling 

 the amperage. A rectifier consists basically of a circuit breaker, step- 

 down transformer, stack, meters and a weatherproof enclosure. Selenium 

 and silicon are the two materials generally used in constructing rectifier 

 stacks. The selenium rectifier has been used extensively in cathodic 

 protection systems in the past. Selenium rectifiers age with time. The 

 aging consists of an increasing resistance in the direction of current 

 flow and a decreasing resistance in the opposite direction. Selenium 

 rectifiers are generally estimated to last 10 years when operated below 

 maximum rating (National Association of Corrosion Engineers, I965). 

 Silicon rectifiers, however, offer the important advantages of apparently 

 not being affected by age, increased efficiency, and compactness relative 

 to selenium rectifiers. Figures 15 and I6 show efficiency curves for 

 selenium and silicon rectifiers. On an operation basis, the National 

 Association of Corrosion Engineers (1965) lists preferable types of 

 rectifiers as follows: 



Conditions Type Rectifier 



Above 30 volts d-c , single phase Silicon 



Above kk volts d-c, three phase Silicon 



Below 30 volts d-c Selenium 



(2) Impressed-Current-System Anodes . Although there are 

 numerous materials that can be used as anodes in impressed current systems, 

 disadvantages encountered eliminate mahy for practical applications. The 

 types of anodes generally used in impressed current systems protecting 

 structures in seawater are graphite, high-silicon cast iron and various 



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