462 Dr Hollis, On Metallic " Passivity " in 



Metallic "Passivity" in relation to Time and Temperature. 

 By Dr W. A. Hollts, Trinity College. 



[Received 8 April 1904.] 



In a previous paper* certain peculiarities in the behaviour 

 of the metal nickel when placed in contact with fuming nitric 

 acid have been noted. It was there suggested that the passivity 

 shown by certain samples of that metal depended to some extent 

 upon their molecular arrangement ; those which excelled in the 

 qualities of tenacity and ductility, such as wire and sheet nickel, 

 were observed to be more resistant to the action of the acid than 

 others possessing a crystalline structure. There were however 

 anomalous phenomena in this connection that could not be so 

 accounted for. Further observations led to the conclusion, so far 

 as regards the metals iron, nickel, and cobalt, that passivity is 

 only a relative state. In other words, the variety of these metals 

 which mainly exhibits the phenomenon, and undergoes no ap- 

 parent change when immersed for a short while in the acid at 

 a given temperature, shows unmistakable signs of solution if we 

 either raise the temperature or prolong the immersion sufficiently. 

 A series of experiments were undertaken with a view to determine 

 more precisely than heretofore this functional relationship between 

 metallic passivity on the one hand, and time and temperature on 

 the other. 



For various reasons samples of sheet metal were selected for 

 examination. This type of metal, besides being in the case of 

 iron and nickel highly resistant to the action of the acid at the 

 ordinary temperature of the laboratory, lent itself more readily to 

 accurate linear measurement than another. The iron was the 

 ordinary ferrotype plate of commerce, cleaned and roughly polished. 

 Two varieties of nickel were used; sheet nickel about 01 mm. in 

 thickness, obtained from Messrs Johnson, Matthey, and Company, 

 and the Mond tubular nickel. The latter is about the same 

 thickness as the sheet. The same Hatton Garden firm supplied 

 me with a sample of rolled cobalt, a type of this metal rarely seen 

 owing to the difficulty of manufacture. Some copper sheeting was 

 used for control purposes. The copper and the cobalt sheeting 

 were about one millimetre thick. 



Three units have been employed throughout the investigation. 

 A time-unit, the second ; a degree (F.) as the temperature unit ; 



* Proc. Cavib. Phil. Soc. Vol. xn. p. 253. 



