36 



BELL SYSTEM TECHNICAL JOURNAL 



Studies have shown that the points of failure of air-formed films 

 on iron and steel surfaces indicated by the initial appearance of anodic 

 or rust spots depends upon the previous history of the specimen and 

 upon the medium in which the test is conducted.^'' For example, an 

 increase in time of pre-exposure to oxygen or exposure to higher tem- 

 peratures decreases the number of initial anodes, while increasing the 

 chloride content of the medium or the presence of sulfide on the metal 

 surface ^^ increases them. Whether corrosion continues at the points 

 of initial attack often depends upon the self-healing ability of the film, 

 that is, upon plugging the fissures or pores in the film with corrosion 

 products. 



Various methods have been considered for the determination of the 

 quality of protective films. In the case of aluminum and its alloys 

 the amount of leakage current which may pass through anodically 

 formed films throws some light upon resistance to corrosion. Another 

 promising method applied to iron steel and alloy steels has been to 

 determine by potential measurement the amount of chloride required 

 to destroy passive films formed in water or chromate solutions.^^ 



The rate of film formation is in a sense a measure of the activity of 

 a metal surface, that is, a measure of the rate at which a metal might 

 corrode in a homogeneous environment in the absence of film forming 

 constituents. For example, it will be seen in Fig. 7 that the potential 



<0 0.3 



O 0.1 



300 400 



TIME IN MINUTES 



Fig. 7 — Kffect of traces of lopper and platinuin on (lie potential of lead in 0.1 N H2SO4. 



