(Contribution from The International Nickel Company, Inc.) 



COIffiOSION AND PROTECTION OF STEEL IN SHORE 

 LINE AND On^SHORE STRUCTURES 



by F. L. LaQue 



This discussion of corrosion of steel is offered as a supplement to the 

 many other papers presented at this Conference w?iich are concerned pri- 

 marily with the action of marine organisms on tirbers. For many purposes, 

 steel has structural advantages over timber and, for som^e of these, it is 

 the only pood choice. Its principal disadvantage is its susceptibility 

 to corrosion which, unless arrested by appropriate m.eans, can occur at 

 rates that will destroy the structure vrell before the desired life has 

 been reached. It is appropriate, therefore, that some attention be given 

 this subject at this Conference, 



From measurements on test piles installed at Kure Beach, N. C. over a 

 per'od 01 several vears a tvioical orofile of corrosion in clean sea water 

 has been established-'-, Maxijnum attack occurs in the wave splash region 

 just above high tide level. Here it is' about five times as great as under- 

 water and about twice that in the atmosphere above the splash zone. Mini- 

 mum attack occurs towards the bottom of the tidal zone where it is from one- 

 half to three-quarters that well belo^/ lovr tide. This distribution of 

 attack has been confirmed by similar measurements on piles withdrawn from 

 actual installations. 



The greatest variation in rates of attack are bbserved at the mud line, 

 in the splash zone and above it and, to a lesser extent, vfithin the tidal 

 ran,?e , 



It will facilitate discussion to consider each of the five zones of attack 

 separately: 



Zone 1 - Below the Uud Line 



As compared with attack in some of the other sones, that below the mud 

 line is not likely to occur at a seriouc rate. Ordinarily, the rate of 

 corrosion .fill not e:x.ead th.at in the vKiter belovir Ioyj' tide. In some in- 

 stances, as vdiere sulfate reducing bacteria are present to stimulate 

 corrosion, there may be accelerated attaclc at and just below the mud line. 

 The steel in this region may become anodic to the steel in the water and, 

 thereby suffer extra damage to the e::tent bf two to three times that which 

 occurs in the waterand elsewhere in the mud. The area affected is some- 

 what uncertain, mit ordinarily v/ill not extend more thcji a couple of feet 

 below the mud line Yfhere t]ie location of this remains substantially fijr.ed. 

 '.'/here there is cyclic building up aid v/ashing away of mud from around the 

 bottoms of yjiles, the area af^-'ected will vary accordingly. here no other 



■^H. A. Humble, "The Gathodic Protection of Steel Piling in Sea Water," 

 Corrosion, Vol. 5, ^o, 9, September I9I49, 



