and also during the first 10-day period. The greatest of such changes are 

 shown by System 1 19 (Figure 18). The changes in electrical properties of 

 System 123 (Figure 19) were quite rapid in the initial 6-day period but showed 

 very little change after that time. 



The initial water uptake, as reflected in the electrical measurements, 

 could be a factor in the performance of the coating. For this reason strong 

 changes in initial AC resistance, which may be equated with strong water uptake, 

 are shown for each system in Table 5. For some systems strong water uptake 

 occurred rapidly, during the first 6 hours. For others the strong water uptake 

 occurred more slowly, during the period between the 6-hour and 10-day mea- 

 surements. These changes were considered strong when the resistances dropped 

 to about one half the initial values during the first 6 hours, that is, log (R|/R) > 

 0.3, or when they dropped to about half the value of the 6-hour measurements 

 at the time of the 1 0-day measurement. By this criterion five of the 1 3 systems 

 have fast strong water uptake and eight systems (including two of the five just 

 mentioned) have slow strong water uptake, as shown in Table 5. 



Also shown in Table 5 are the low final resistances which are shown by 

 some of the coatings after 1 year. A low final AC resistance was considered to 

 be a panel resistance of less than 1 0^ ohms for any of the three panels of a 

 system. This is a value which had been considered a minimum value for good 

 performance on the basis of earlier experiments.''^ A low final DC resistance 

 value was considered to be a value below lO'^ ohms (log R = 7.0) for any panel 

 of a system. This was the minimum value proposed as a qualification require- 

 ment. ''■' On this basis, four systems showed low final AC resistance after 1 

 year of exposure, and five systems showed low DC resistance values after expo- 

 sure for 1 year. It is of interest that all systems which had low AC resistance 

 after 1 year also had low DC resistances at that time. This was true in spite of 

 the fact that the DC resistance values are a combination of low resistance and 

 of the effects of polarization. However, the effect of polarization may be 

 proportionately less at lower resistance values. 



For the calculation of the DC resistances, the open-cell potentials were 

 measured. The initial values varied from -I-IOO mv to -195 mv. The final values 

 varied from -75 mv to -850 mv. As in previous experiments, there were consid- 

 erable variations in the open-cell potentials. Except that in general the potentials 

 became more negative, no specific trends could be established and the individual 

 values are not reported. 



Relationships Between Electrical Properties and Performance 



It might be expected that the electrical properties of coatings immersed 

 in seawater would correlate most closely with the performance of the coating 

 systems exposed in seawater. Bacon and coworkers^ had suggested such a 



41 



