344 



BELL SYSTEM TECHNICAL JOURNAL 



The corrosion of iron can be predicted by proper interpretation of 

 such time-potential curves. The fact that the iron is painted should 

 not alter this conclusion, and therefore the method has been applied to 

 a study of painted iron and primer pigments. 



The customary procedure for determining time-potential curves was 

 followed with the exception that the potentiometer ordinarily employed 

 was replaced by a vacuum tube electrometer when measurements were 

 made of the potential of painted iron or of iron in other media of high 

 electrical resistance. This instrument, slightly modified to take 

 advantage of certain improvements which have been made in com- 

 pensated single tube circuits, is described elsewhere.^ 



Common usage has defined iron which is corroding as "active" and 

 iron which is not corroding as "passive." In order to obtain a back- 

 ground of information which might serve as a guide in the study of 

 painted iron and paint pigments, a series of time-potential curves 

 depicting iron and steel in the active and passive states was de- 

 termined. The results are shown in Fig. 1. 



uj^ 0.1 

 If) 



5 

 > 



z 



~ -0.1 



_i 



< 



Z -0.2 



LU 



I- 



o 



0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 



TIME IN HOURS 



Fig. 1 — Time-potential curves for active and passive iron. 



Active (Corrosion) 



1. Iron in tap- water. 



2. Iron inO.OlN NaCl. 



3. Uncleaned iron in tap-water. 



Passive (No Corrosion) 



4. Stainless steel in tap-water. 



5. Iron in 0.01x\ KjCrjOy. 



6. Uncleaned stainless steel in tap-water. 



^ Compton and Ilaring, Trans. Electrochem. Soc, 62, 345 (1932); D. B. Penicl:, 

 Rev. Set. Inst., 6, 115 (1935) and Bell Laboratories Record, 14, 74 (1935). 



I I 



