corrode sacrif icially. The methods for precluding galvanic attack in both 

 cases are the same - decouple the galvanically incompatible materials by 

 "electrical isolation techniques" or use cathodic protection. "Isolation 

 techniques" include coating cathodic components to minimize their galvanic 

 effects on less noble components and isolation of incompatible materials 

 (viz, using a nonconductive pipe section between steel piping and the heat 

 exchangers) . 



Cathodic protection is attained by shifting the portential of the most 

 cathodic element to a point near the potential of the most anodic metal in 

 the couple. Under these conditions there will be no active galvanic couple. 

 If all parts are protected at the protective potential for the most anodic 

 element, local galvanic cells are stifled, and even general corrosion 

 virtually is eliminated. The protective potential of a metal is usually 

 100 to 200 mV more negative than its freely corroding potential. 



If reinforced concrete is used for the cold water pipe, the feasibility 

 of applying cathodic protection to steel rebars should be investigated. 

 While cathodic protection has been applied in some cases to rebars in bridge 

 applications, it is common practice to permit the bars to corrode freely. 

 The alkaline conditions in concrete reduce steel corrosion rates and conse- 

 quently the current requirements for cathodic protection.-"-^ 



Metal strainers used to prevent particulate or foreign object intrusion 

 into the piping system probably will require impressed current protection. 

 The protective current requirements will be high due to large surface areas 

 and inherent turbulence. 



In general, it is difficult to design a cathodic protection system for 

 a structure before its final configuration and location are decided. How- 

 ever, the design considerations listed above can be used to determine 

 current requirements and the type of cathodic protection system necessary. 

 It is important that the need for cathodic protection be recognized, and 

 that weight and cost considerations be included early in overall OTEC 

 design criteria. 



Coatings, Cathodic Protection, and Corrosion 



Although coatings are discussed separately, two factors effecting 

 materials compatibility should be addressed. First, while painting 



17 



