Corrosion under water is not difficult to control by cathodic protection 

 by itself or as a supplement to an organic coating such as coal tar enamel 

 or a vinyl system. For bare steel a current density of 3 milliamperes per 

 square foot should suffice. In time, this can be reduced gradually, per- 

 haps to as little as 1 milliampere per square foot. A useful criterion of 

 protection is ihe measurement of the potential of the pile v.dth reference to 

 a saturated calomel half cell, /hen the steel has been polarized to a 

 potential of 0.8 volt or higher, it may be assumed that corrosion has been 

 practically arrested. The applied current can be adjusted on this basis, 



A rugged pure zinc electrode may be substituted for the fragile calomel 

 half cell for potential measurements, ''ith zinc as the reference, the 

 potential of the pile should be held not more than 0,2 volt more noble 

 than the zinc. 



..Tien supplementary organic coatings are used, the current requirements are 

 reduced considerably - to an extent determined by the permeability of the 

 c&ating, the number of holidays in it originally and the number of bare 

 spots that develop during pile driving and in service as a result of acci- 

 dental damage or effects of marine organisms. Organic coatings may also 

 be damaged by the application of too much current for protection and the 

 consequent generation of excessive alkali, and especially hydrogen which 

 may blister the coatings severely-*-. This form of coating deterioration 

 may be avoided by restricting the applied current so that the polarized 

 potential of the steel -./ill be held close to the desired 0,8 volt vs. 

 calomel and not in excess of 0,9 volt. 



In some instances it has been recommended that the cathodic protection 

 of stfeel in sea water should be made completely effective immediately by 

 the short time (one to tvo weeks) application c£ current at a high current 

 density, e.g. 50 to 100 milliamperes per square foot2. This will serve 

 to lay do.TO a protective calcareous coating and, when this has been formed, 

 protection may be maintained with the much lovrer current density of 3 

 milliamperes per gquare foot or less previously mentioned. In vievv of 

 the possibility of damaging organic coatings by excessive current, it would 

 seem that the initial use of high current densities should be restricted 

 to bare steel surfaces. here coatings are used, also, the current should 

 be controlled to the amount required to polarize the steel to the desired 

 level of potential, 



A. J. Eickhoff and D. L. Hawke, "Some Factors Affecting faint Performance 



on Gathodically Protected Steel," CORROSION, Vol. 7, 1951, p. 70. 



L. P. Sudrabin, F. J. Lefebvre, D. L. Hav^ke and A. J. Pickhoff, "Some 



Effects of Cathodic Protection on Conventional Paints," CO ^^OSION, Vol. 8, 



1952, p. 109. 



R. P, Devoluy, "Coating Experiences with Cathodic Protection Undervirater," 



Paper presented at NACF iJeeting, Galveston, llarch, 1952. 



2see C, C. Cox, U. S. Patents 2,200,ii69 (I9ii0) and 2,1^17,061; (I9h7) and 

 British Patent No. 5U0,Ii87 (l9iil). 



E - U 



