THE ALUMINUM ELECTROLYTIC CONDENSER 



53 



metals is slightly lower than for metals containing small quantities 

 of alloying materials, but this difference is of inconsequential im- 

 portance. The difference in the rate of film formation under similar 

 conditions between 99.1 per cent aluminum and 99.6 per cent aluminum 

 is shown in Fig. 8. After 24 hours in these particular cases the 



20 30 40 



TIME OF FORMATION-MINUTES 



50 



60 



Fig. 8 — Rate of film formation as influenced by composition of metal 



.10 20 .30 



SILICON CONTENT OF ALUMINUM 



.40 

 PERCENT 



Fig. 9 — ^The relation between silicon contents of anodes and failures due to corrosion 

 in groups of aluminum condensers classified with respect to anode composition 



leakage current of the 99.1 per cent aluminum was about 3 micro- 

 amperes per sq. cm., or nearly six times that of the 99.6 per cent 

 material at the formation potential of 60 volts. 



In the matter of life, however, the purer metals seem to be more 

 readily attacked by agencies capable of causing electrolytic corrosion. 



