348 BELL SYSTEM TECHNICAL JOURNAL 



soap is formed, bul ihis theory becomes untenable in view of evi- 

 dence that red lead pigment alone passivates iron in much the same 

 manner as red lead primer, and that even water solutions of the pigment 

 have an effect. Other theories are that the iron is rendered passive by 

 the alkalinity of the red lead, or because it is an oxidizing agent. In all 

 probability, both of these factors are involved. 



The ease with which it has been found possible to make potential 

 measurements on painted iron with the aid of the vacuum tube elec- 

 trometer, suggests the application of the time-potential method of 

 study to the determination of the corrosion behavior of iron encased in 

 concrete or buried underground or immersed in oil or other highly 

 resistant media. Field study would be facilitated by substitution of a 

 vacuum tube voltmeter for the electrometer. 



The fact that there is a potential difference of at least 0.5 volt 

 between the active and passive states of iron suggests a rapid po- 

 tentiometric method for the determination of the permeability of all 

 types of organic coatings. The time-potential curve for an iron 

 electrode coated wdth the organic material and immersed in a salt 

 solution, for example, would break sharply at the moment penetration 

 was attained. 



