681 



it became desirable to get acquainted willi llie iiniuenselj extensive 

 literature on this phenomenon. 



The most important hypotheses that luive been proposed as an 

 explanation of this phenomenon, and which have been collected by 

 Fredenhagen ^) in an interesting summary, are the Ibllowing: 



1. Tlie oxide theory of Faraday, who assumes that j)assive metals 

 are covered by a coat of oxide "). 



2. IvRtJGER FiNKEi.sTEhv's ') valeuce tlieory which slightly modified is 

 also adopted by Muller '). In this it is assumed that the passivity 

 consists in a change of the proportion between tiie components of 

 different valence. , 



3. Le Blanc's velocity theory ^), which supposes the phenomena 

 of passivity to be due to the slight velocity with wiiich the formation 

 of metal ions would take place. 



4. The velocity theory of Fredenhagen ''), Muthmann, and Fraüen- 

 berger ^), who start from the supposition that the passivity is caused 

 by the slight reaction velocity between the anodically separated 

 oxygen and the passi liable metal, whicii causes oxjgen charges or 

 solutions of ox3gen in metal to originate. 



5. The theory of Grave *"), who assumes a retarded heterogeneous 

 equilibrium metal-electrolyte which is under catalytic influences. 



If we now consider the passivity which has arisen by an electrolytic 

 way, it seems- to us that too little attention has been paid to the 

 prbnary character of the phenomenon. 



First of all we should inquire to what it is owing that in case of 

 anodic polarisation of base metals the potential ditference is modified 

 in such a way that the tension of separation for the 0^ is reached, 

 and we should also question why in case of cathodic polarisation 

 of base metals the potential difference changes in such a way that 

 the tension of separation for the H^ is reached. 



1) Z. f. phys. Chem. 63, 1 (1908). 



-) Faraday has not expressed himself so positively as is generally thought. In 

 a letter to P.. Taylor Faraday writes explicitly [Phil Mag X175. Jan. 21 (1837)]: 

 "I have said (Phil. Mag. IX. 61 1837) liiat my impression is, thai the surface of 

 the metal is oxidized, or else, tliat the superficial particles of the metal are in 

 such a relation to the oxygen of the electrolyte as to be equivalent to an oxidation, 

 meaning by that not an actual oxidation but a relation... 



3) Z. f. phys. Ghem. 39, 104, ' 11J02). 



*) „ „ „ „ 48, 577, (1904). Z. f. Electr. Chem. 11, 755, 823, (1905) 



5) Chem. News 109, 63 (1914). 



6) Z. f. phys. chem. 43, 1 (1903). 



7) Sitzber. K. Bayr. Akad. 34, 201 (1904). 



8) Z f. phys. Chem. 77, 513 (191 Ij. 



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