for protecting steel. Cathodic protection is needed as backup in areas 

 where the continuity of the coating is affected, due to damage or applica- 

 tion problems. If no coatings were used, the cost of cathodic protection 

 would be greatly increased, both in terms of (.1). equipment needed and (2) 

 current required for protection of a bare (noncoated) structure. Examples 

 of structures where cathodic protection is used in conjunction with coatings 

 include: sheet piling, production platforms, piles, docks, and similar 

 structures continuously immersed in water. 



In these instances, the protective coating must possess: 



(a) Good dielectric strength, 



(h) good alkali resistance, 



(c) good adhesion characteristic, 



(d) low moisture absorbtion and transfer rates, 



(e) good coating thickness, and 



(f) resistance to the passage of ions. 



Carefully conducted tests and field use show that most coatings designed 

 for immersion in seawater which have the properties described above will 

 perform satisfactory at steel potentials ranging from -0.8 to -1.3 volts 

 with respect to a copper/copper sulfate reference cell. Above the 1.3-volt 

 potential many coatings will show degradation such as cathodic disbondment. 



If a good coating can be applied to both sides of the bulkhead pile 

 sheets before installation, cathodic protection current requirements would 

 be decreased drastically. The coating, if applied, should be as good as 

 the state of the art permits, such as white metal sandblast, inorganic zinc 

 primer, followed by two coats of coal-tar epoxy, for a dry film thickness 

 of at least 0.41 millimeter (16 mils). In any event, the piling, after 

 installation, should receive such a coating from the low water line upward 

 through the splash zone to the top of the bulkhead. Any cathodic protection 

 is marginal above the low water line and nonexistent in and above the high 

 water line. 



5. Marine Exposure . 



The characteristics of coating systems and structure material to be 

 protected, as well as the specific marine exposure, will determine which 

 coating systems can be effectively used. The specific marine exposures 

 must be carefully considered when selecting a coating system to achieve 

 good structure protection. Marine exposures are generally considered to be 

 marine atmosphere, splash and spray zone and submerged zone. More than one 

 of these exposures may occur on any single structure. 



For example, a marine atmosphere is one which carries airborne salt. 

 Since only pure water evaporates from a body of saltwater, this physical 

 process does not put salt into the air. Instead, salt becomes airborne 

 only under conditions in which finely divided saltwater droplets (spray and 



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