Magnesimn anodes are a convenient source of current for cathodic pro- 

 tection in sea water. Otherwise, current may be furnished from some 

 external source, such as a rectifier and applied .through more or less 

 permanent graphite on expendable steel anodes. 



Additional details concerning cathodic protection of steel in sea water 

 may be found in papers by H. A. Humble^ and a description of a practical 

 installation on an offshore drilling structure in a paper by E. Doremus 

 and a. Doremus^, 



>:n idea of the current required for protection in connection with organic 

 coatings may be obtained from the data in Table 1 which shows the current 

 density required to maintain piles covered with different coatings above 

 the protective potential after about 2 years at the Harbor Island Marine 

 Test Station. 



Zone 1^ - Tidal Zone 



Contrary to popular belief, corrosion does not reach a maximum in the 

 tidal zone. In fact, ininimum attack may occur here near the bottom of 

 the usual tidal range. 



An explanation for this has been found to lie in a powerful differential 

 aeration cell virhich becomes established each time the tide conies in. The 

 frequent and better access of air to the surfaces betvreen tides causes 

 the TJ'ater in contact vath them to be higher in dissolved oxygen than the 

 ¥irater that seeps through the layer of rust ai^d marine organisms on the 

 under v/ater surfaces. This makes the tidal surfaces more noble than the 

 undervTater surfaces and, therefore, the protected cathodes of the cell 

 that is 'developed. The measured distribution of potential in a i.articular 

 case showed the top of the tidal zone to be 90 mv, more noble than the 

 steel under low tide. 



The effect of these local action currents on corrosion in the tidal zone 

 was demonstrated-^ by the difference in corrosion of continuous as against 

 isolated specimens of steel disposed through the zones of interest. Iso- 

 lated specimens in the tidal zone were corroded ten times as much as com- 

 panion portions of a continuous plate which passed through this zone into 

 the water belovf lov; tide. 



1 



H. A, Humble, "Cathodic Prote'.-;tion of Steel in Sea 'ater with I/lagnesium 



Anodes," CORPOSIOIT, Vol. h, 19hQ, p. 358, 



H. A. Hi-inible, "The Cathodic Protection of Steel Piling in Sea ./ater," 



CORPOSIOi', Vol 5, 191^9, p. 292, 



^E. P. and G. L. Doremus, "Catiiodic Protection of J^^ourteen Offshore Drilling 



Platforms," C0riRO':i0n, Vol, 6, 19.'^0, p. 2l6. 

 3f. L. LaQue, laarburg Lecture "Corrosion Testing," Proceedings Am. Soc. 



Test. Ilat., Vol, ^1, 1951. 



E- 5 



