Chemistry. — ''On the Passivity of Chromium''. (First communi- 

 cation). By Dr. A. H. W. Aten. (Communicated by Prof. 

 A. F. Holleman). 



(Communicated in the meeting of Oct. 27, 1917). 



1. Introduction. 



The passivity of chromium has been extensively investigated by 

 HiTTORF ^). The results of these investigations are briefly the following: 



On anodic polarisation, depending on circumstances, chromium can 

 dissolve bivalently (as chromousion), trivalently (as chromicion) and 

 six valently (as chromate). 



When dissolving as chromousion the metal is active. It has a potential 

 which is negative with respect to the hydrogen electrode, and which 

 lies between that of Zn and Cd. In this state the metal is also 

 chemically active, it generates hydrogen from acids etc. If on the 

 other hand the metal dissolves as chromate, it is passive. It then 

 presents a strongly positive potential, and does not act on acids. 



Solution as chromicion has only been found in a few cases. The cir- 

 cumstances under which this takes place, are not accurately known. 



Chromium is activated by bringing it in contact with strong acids, 

 especially with hydrochloric acid. The activity is fui-ther promoted by 

 rise of temperature, presence of chlorine ions, cathodic polarisation 

 and especially by bringing the chromium in contact with molten 

 chlorides (KCl + NaCl, ZnCl,). 



The passivity is promoted by anodic polarisation and by contact 

 with oxidizing agents (bromine water, nitric acid). 



HiTTORF exclusively examined chromium that had been prepared 

 by aluminothermical way according to Goldschmidt. 



Hittorf's results and those of other investigators will be discussed 

 more in particulars in connection with the results obtained by me. 



2. Summary of the different theories of pas.s-ivity. ^) 



The earliest theory of passivity, which we owe to Faraday "), 



1) Zeitschr. f. physik. Chemie 25, 729 (1898); 30, 481, (1899); 34, 91 (1902). 



3) For a full account cf. among others Fbedenhagen. Zeitschr. f. physik. 

 Chemie 63, 1 (1908), Allen, Le Blanc, Schoch, Reichenstein, Sentee, Ghem. 

 News, 109, 25, 63, 75, 87, 115, (1915); 108, 249, (1913). Bennetts and Burnham, 

 Zeitschr. f. Elektrochemie 22, 377, (1916). 



») Phil. Mag. 9, 61 (1836). 



