In Figure 21 the average of thermocouple readings for the important 

 temperatures measured by 1 , 2, and 3 junctions of the thermoelectric 

 units in a complete RTG are plotted against measured velocity, irrespective 

 of the direction of water flow. These are important data points because 

 they provide the design temperature for the cold junctions ia the RTG. 

 Point A is the average of the same readings for the August 1973 tank 

 experiments, without flow. The velocities indicated are those at the 

 beginning of a set of readings, but frequently they varied by 100Z during 

 the time required to take 18 thermocouple readings of the temperature. 

 The actual variability in average velocities for several minutes before 

 a series of readings would be responsible for the small variations shown. 

 Fluctuations in line voltage would also produce random variations; for 

 the harbor data shown, there was no control of power input, nor was it 

 monitored continuously. 



In surmary, low velocities of the order of 0.1 to 0.3 ft/sec enhance 

 the heat transfer from a convector in the ocean, but not significantly 

 in terms of overall electrical generation efficiency. The important point 

 is that any change, while transient and unpredictable, will always be 

 conservative in that cold junction temperatures will be depressed and 

 generator efficiency enhanced in the presence of ocean currents. 



NOTES ON CORROSION 



The experimental work discussed in this report is directed only to 

 the heat transfer, particularly to provide a minimum temperature at the 

 cold junctions of the RTG's thermoelectric units. The planned tests 

 required only short-term, periodic immersion during which no significant 

 corrosion would be expected. Surface treatment of the large steel shell, 

 Figure 13, was entirely cosmetic and did not include extensive precleaning 

 or priming. The ends of the heat pipes, which would have been capped 

 with pure copper Tor long-terra immersion, were capped with a more readily 

 available, less expensive, and more easily machined aluminum alloy. The 

 30-day immersion tests discussed in this Appendix provided a brief check 

 on the corrosion handbooks. Since aluminum alloys are electrically much 

 more active in seawater than either copper or steel (with E^* of -0.80, 

 -0.70, and -0.40 volt, respectively, for aluminum alloy, steel, and copper), 

 the aluminum caps corroded severely. There was some corrosion of the 

 steel adjacent to the copper fins and none of the copper (Figure 22). 



Several alternatives, none of which will probably afford complete 

 protection from corrosion for all parts of the RTG, are available in 

 the final RTG design. One solution might be to cover all the RTG surfaces 

 with heavy copper plating, either electrolytic or rolled. Welding would 

 introduce holidays in a rolled plating, so electrolytic plating might 

 be the more acceptable of these approaches. Electroplating the entire 



* Eu is the no-current voltage versus a saturated calomel electrode. 



45 



