134 TRANSMISSIVITY IN PASSIVE IRON WIRES 



it is an oxidation product and is usually regarded as a higher oxide 

 of iron.'^ 



The question remains why the film persists for an indefinite period 

 with unaltered properties, even in acid of a strength lower than that 

 required to passivate. Why should not the film undergo still further 

 solution and eventually lose its continuity and break down? In point 

 of fact, the passivating film is unstable except in solutions possessing 

 considerable oxidizing power;* its preservation thus appears to depend 

 upon a more or less continuous process of slow oxidation; this process 

 exercises what may be described as a regulative control over its local 

 variations of composition or thickness. That this is the case is 

 indicated by the general fact that passivity disappears spontaneously 

 in pure water and in solutions of salts and other substances, except 

 those having strong oxidizing properties. In the latter case there is 

 evidence that local interruptions of the film (unless too extensive) are 

 automatically repaired by local oxidative action. This may occur 

 even in solutions whose oxidative properties are insufficiently intense 

 to repassivate a completely activated wire. The case of nitric acid 

 of specific gravity 1.20 or less will illustrate; in this solution an active 

 wire continues to react until it is completely dissolved, but a passive 

 wire remains unaltered indefinitely. I have already described how 

 a fihn partially destroyed by brief immersion in m/1,200 NaCl is 

 restored to its original state by brief immersion in 1.20 HNOs.^ A 

 similar restoration after partial mechanical removal of the film is 

 seen when the passive wire, while immersed in 1.20 HNO3, is scraped 

 locally with a piece of glass; in order to secure activation by this 

 means several scrapes in rapid succession are as a rule required; if 

 the interval between successive scrapes is lengthened this character- 

 istic summation effect is less readily obtained. When a wire which is 

 thus treated is connected through a voltmeter with another wire 

 {e.g. of passive iron or platinum) serving as an indifferent electrode, 

 there is seen at each scrape a slight temporary excursion of the needle 



^Bennett and Burnham.^ Bennett, C. W., and Burnham, W. S., Tr. Am. 

 Eleclrochem. Soc, 1916, xxix, 217. Langmuir (Langmuir, I., Tr. Am. Electro- 

 chem. Soc, 1916, xxix, 260), however, has a somewhat different conception. 



* For examples of this behavior cf. Lillie, R. S., Science, 1919, 1, 259, 416. 



9 LilUe, R. S., Science, 1919, 1, 259. 



