These data not otilv account for the peculiar distribution of corrosion but 

 point to the necessity of using specimens large enough to extend through 

 the zones of interest in studying corrosion and protection of steel piling. 

 The exposure of small specimens below low tide and between tides vdll not 

 give the proper answers. 



In some harbors, also, corrosion in the tidal zone may be reduced by 

 the protective effect of oil and grease films deposited on the metal by 

 falling tides where the surface of the water is regularly covered with oil. 



From the standpoint of protection, the tidal zone presents a difficult 

 problem. Cathodic protection cannot be depended upon to extend much above 

 half tide level except where the tidal range is so small relative to the 

 common height of waves that the whole tidal range is kept submerged most 

 of the time. 



Organic coatings, such as coal tar enamels and vinyl systems are effective 

 for a tiiTie, but it seems unlikely that they would survive for the desired 

 life of the structure and their replacement involves many practical dif- 

 ficulties. Chief among these is the proper preparation of the surfaces 

 to receive ne.v coatings during the short time between tides that these 

 surfaces can be kept dry - especially if there is any wave action or 

 spray to contend Tith. The soft coal tar enamels are also subject to 

 penetration by barnacles i7hich can embed themselves in the enamel and 

 eventually expose the underlying steSl, Developments are -underway to 

 reinforce such enamels against barnacle penetration by the incorporation 

 of sand or the application of cement slurries. These steps may suffice 

 to avoid this form of deterioration of enamels. 



The inadequacy of organic coatings and cathodic protection for long time 

 prevention of corrosion in the tidal zone has led to interest in metallic 

 coatings for these re^^ions. 



Protective metal sheathing may be applied in the. form of rolled sheet 

 metal made to conform to the shape of the piling, Thj.s is obviously 

 easiest "dth tubular piles, more difficult livith structural shapes, such 

 as 'H' pile sections, and very difficult with interlocking sheet piles 

 where covering the knuckles would requii-e butt straps and a complicated 

 method of sealing the lower end of the sheathing to keep v/ater from rising 

 and falling along the knuckles and under the butt straps. 'H' piles may 

 be sheathed with conforming shapes or the sheathing may be icrapped around. 

 The latter practice requires that the lov/er ends of the sheathing be sup- 

 plemented by a previously formed metal box vfhich can be inserted to fill 

 the space between the sheathing and the web. V.Tien this has been accomplished, 

 the space between the sheathing and the web can be f JAled v/ith sea vrater 

 to which caustic soda has been added to raise the pH to 11, 5« Ibis requires 

 about Ij. ounces of NaOH per cubic foot of sea water. The air free alkaline 

 sea water prepared in this \raj is not harmfully corrosive to steel under 

 the conditions that exist inside the sheathing; the rate of corrosion 

 observed in a particular test was only 0,0002 ipy. 



] . 7 



