Chemistry. — '^Tke Passivltij of Chromiwn' . (Third Communication). 

 By Dr. A. H. W. Atkn. (Comninnicated by Prof. A. F. Hollkman). 



(Communicated in the Meeting of March 23, 1918). 



When it is tried to exi)hiiii the results communicated in the foregoing 

 l)a|)ers ^) we should in the first [dace consider the possibilifj that 

 the hydrogen (or oxygen) present in the surface layer of the metal, 

 lias a certain influence on the phenomena. 



The phenomena, that render it probable that the hydrogen present 

 in the surface layer of the metal promotes the activity, are a»nong 

 others the following. 



When a piece of Goldschmidt chromium, which contains little 

 hydrogen, is placed in a feeble acid or in very dilute sulphuric 

 acid, it does not go spontaneously into solution with generation of 

 hydrogen. When the chromium is, however, cathodically polarized, 

 80 that hydrogen is generated in consequence of this, the chromium 

 goes also into solution as chromousion. When the current is broken, 

 also the dissolving as chromousion stops, when the acid is diluted 

 enough. In more concentrated acids, especially in hydrochloric acid, 

 and also at higher temperatures in diluted acids, the dissolving 

 accompanied by hydrogen generation, begins spontaneously after a 

 short time. 



From the fact, that the cathodic polarisation causes the solution 

 of the chromium, we may conclude that the hydrogen charge that 

 the metal acquires in this case, is the cause of the activity. At the 

 same time it follows from this that a hydrogen charge correspond- 

 ing to gaseous hydrogen of one atmosphere is not sufficient to 

 activate chromium, for in this case the activity would have to con- 

 tinue to exist when the current is broken. At that moment, and 

 also some time after, the metal has, namely, a hydrogen chai-ge 

 that is at least equal to one atmosphere, and yet the going into 

 solution ceases spontaneously. The activity disappears even when the 

 cathodic polarizing current is not broken, but only sufficiently 

 weakened '). Flade ''') denies this statement of Rathert, but describes 



1) These Proc. XX, p. 812, 1119. 



2) Rathert. Zeitschr. f. physik. Chemie 86. 567, (1914). 

 3j ibid. 88 569 (J914). 



