346 



BELL SYSTEM TECHNICAL JOURNAL 



-0.1 -r 



t-0.2 



-0.4 

 -0.5 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 



TIME IN HOURS 



Fig. 2 — Time-potential curves for painted iron in water. 



Red Oxide Primer 



1. Linseed oil vehicle. 



2. Linseed oil plus varnish. 



Red Lead Primer 



3. No visible pores. 



4. Visible pores. 



Reference Curves 



5. Dry linseed oil film. 



6. Iron in tap- water. 



7. Iron in O.OIN KjCrzOr. 



In other words, the metal was passive initially and, judging from the 

 behavior of the curve, it tended to remain so for a period of about 

 twenty minutes. By the end of this time, water appears to have per- 

 meated the paint film in sufficient quantity to induce the active corro- 

 sion indicated by the sharp drop in potential which followed. From 

 this time on, the curve is similar to that for the linseed oil coated 

 electrode. Ultimate equilibrium required a much longer time, however. 

 After several days' immersion, the paint film was removed and the iron 

 was found to be corroded. The very gradual slope of the curves for 

 active painted electrodes, after their sharp breaks from the passive 

 region, may be attributed to the action of the films as partial barriers 

 to moisture. 



The impermeability of primers and their adherence to iron are known 

 to be increased by the addition of varnish. Accordingly, red oxide 



