d. Corrosion Prevention . The corrosion process on a given structure 

 can be prevented or stopped if any one of the three conditions necessary 

 for corrosion can be eliminated. The principal methods for preventing or 

 mitigating corrosion are described below. 



(1) Coatings . A perfect coating will electrically insulate the 

 anode and cathode areas from contact with the electrolyte, preventing flow 

 of corrosion current. As is well known, a perfect coating is an impossi- 

 bility for anything other than a laboratory-scale project. Also, all 

 coatings disintegrate in time, imposing an ever- increasing area of poorly 

 coated or bare metal. Cathodic protection is an ideal way to deal with 

 coating discontinuities (holidays) and poor coating in general. Coatings 

 (with cathodic protection) are feasible for practically any subsurface 

 structure. An exception would be the underwater parts of offshore oil 

 production platforms where coating repair or replacement would not be 

 possible. 



(.2) Insulated Joints . Insulating joints between metal plates or 

 piping joints will minimize stray current or galvanic corrosion where it 

 interrupts corrosion current flow. In this application, the metallic 

 connection between widely separated anode and cathode areas is broken by 

 the insulating joint. Insulated joints also serve to separate dissimilar 

 metal areas, as well as to separate cathodically protected areas from 

 unprotected areas. 



(3) Cathodic Protection . The corrosion process on a given 

 subsurface structure can be prevented or stopped by supplying an excess of 

 electrons to all subsurface parts of the structure. The result is that the 

 structure becomes all cathode because of the electrons provided by forcing 

 direct current to flow through the contacting electrolyte (water or soil), 

 from a nearby subsurface source (anode) onto all subsurface parts of the 

 structure. Hence the name, cathodic protection. When the current is 

 adjusted properly, it will counteract corrosion current flowing into the 

 electrolyte from the substructure with an opposing and slightly more than 

 equal flow of current flowing from the electrolyte into the substructure. 

 Loss of metal has been transferred from the protected substructure to the" 

 external anode which will require occasional replacement. 



2. Coatings . 



a. Introduction . Environmental conditions affecting coastal structures 

 range from mild to severely corrosive. To provide suitable service life 

 for coastal structures, protective coatings are usually required, ranging 

 from little or none (other than decorative painting) to complex and extensive 

 multicoat systems. Specific coating demands depend upon type of substrate 

 to be coated and its environment. 



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