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BELL SYSTEM TECHNICAL JOURNAL 



process, ^'^ the granular oxide film having no retarding influence. This 

 is shown by the linear relationship in Fig. 1 which compares the cor- 

 rosion-time curves for zinc, copper, lead, and iron in the atmosphere. 

 In this case corrosion is expressed in terms of weight gain due to the 

 accumulation of corrosion products. It will be observed that the re- 

 lationship for copper is a parabolic one, indicating that the process is 

 controlled by the rate of difi"usion of oxygen through the increasingly 

 thicker oxide film. Up to a thickness of about 10 A the film is invisible 

 and when formed in pure air is impervious to volatile sulfur compounds 



Fig. 1— Corrosion-time relationships characteristic of certain metals exposed to the 



atmosphere. 



A third type of corrosion-time curve represented by lead becomes 

 parallel with the time axis after the initial stages. Evidently the film 

 in this case is impervious to the constituents of the environment. 

 The curve given for iron indicates that the film which forms in the 

 initial stages of the exposure exerts an accelerating influence upon 

 the subsequent rate of oxidation. 



It should be mentioned that this acceleration occurs only at 

 humidities above what has been called the "critical" humidity. By 

 this term is meant the relative humidity corresponding to the vapor 

 pressure of a saturated solution of the corrosion products, which de- 

 pends upon the composition and to some extent the structure of these 



