Galvanic Interactions and Cathodic Protection 



The possibility of galvanic corrosion of the more anodic components 

 must be considered wherever dissimilar metals are used in marine construc- 

 tion. If structural requirements preclude use of galvanically compatible 

 materials, electrical isolation of the members or cathodic protection must 

 be used to counteract these galvanic effects. Figure 5 ranks the corrosion 

 potentials of some commonly used alloys. These alloys listed higher up 

 (more negative) will corrode sacrif icially when electrically connected to 

 an alloy lower down (more positive) in this table. 



The relationship and countermeasures are illustrated by the use of 

 bronze propellers on steel ships. The steel hull (anode) is painted for 

 protection, and sacrificial or impressed current anodes are used to supply 

 current to the steel and the propeller (cathode). In this situation, 

 cathodic protection reduces corrosion of both the steel and bronze. If the 

 galvanic effect of the propeller were not counteracted cathodically , accel- 

 erated corrosion in the form of pitting would occur at paint holidays on 

 the steel. 



Cathodic Protection of Steels 



It will be necessary to cover steel members with protective coatings 

 and provide cathodic protection to prevent localized pitting and decrease 

 the rate of general corrosion. The required current capacity for cathodic 

 protection depends on the severity of environment, and the requirement 

 increases significantly with water resistivity, flow, and turbulence. 

 Table 3 illustrates the range in these parameters which can be encountered. 



Techniques for an OTEC Plant 



If replaceable steel modules are used, cathodic protection can be 

 maintained with sacrificial anodes of zinc or aluminum. It is now prac- 

 tical to design for lifetimes of 10 to 20 years with current technology. 

 Aluminum anodes are the most effective as the amount of time-weighted 

 protective current available per unit weight (ampere-hours per pound) is 

 3 1/2 times higher than that of zinc. 28 por weight critical applications 

 such as the floating OTEC platform, it is recommended that commercial alum- 

 inum anodes be used. These contain a nominal 0.045-percent mercury, which 

 means their environmental impact will also have to be assessed. 



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