1891.] 



The Passive State of Iron and Steel. 



487 



powdered iron cannot be fully overcome, even under strong magnetic 

 influence, until a temperature of about 51 C. is reached. 



Secondly. The author's experiments of Part I, Series II, at higher 

 temperatures confirm those of Part I, and further tend to demonstrate 

 the influence of magnetisation in somewhat lessening the passivity of 

 steel, showing that even previous to the critical temperature point of 

 transition from the passive to the active state, magnetised steel bars 

 were rather less passive in warm nitric acid than unmagnetised ones. 



Thirdly. The results in Part II, Series III, show that the passivity 

 of both unmagnetised wrought iron and unmagnetised steel in nitric 

 acid sp. gr. 1*42 is considerably and proportionately reduced as the 

 temperature of the acid increases, until the temperature point of 

 transition from the passive to the active state is reached at a tempera- 

 ture of about 195 F., and it was also found that the wrought iron 

 was less passive in the warm nitric acid than cast steel ; see also 

 remarks at foot of Diagram I, in Part II. 



Fourthly. The results of the observations of Part II, Series IV. 

 indicate that Scheurer-Kestner was to some extent in error in regard- 

 ing the passivity of iron as not dependent on the greater or less 

 degree of saturation of the acid. The author's experiments herein 

 recorded have shown that the passivity of the metals employed, viz., 

 wrought iron, soft cast steel, hard cast steel, soft Bessemer steel, and 

 tungsten steel, was very materially increased with the concentration 

 of the nitric acid, and it was also observed that wrought iron was much 

 less passive in the nitric acid of less concentration than most of the 

 steels, the soft Bessemer steel being found about -equal in passivity to 

 the wrought iron under the conditions of experimentation. A reference 

 to Table III shows that a considerable amount of E.M.F. was 

 developed between the different metals in every instance, which is a 

 circumstance of much interest in connexion with the passive state of 

 iron and steel. 



Fifthly. The results obtained in Part III, Series V and VI, on the 

 relative passivity of wrought iron and the various steels, soft cast 

 steel, hard cast steel, soft Bessemer steel, hard Bessemer steel, soft 

 Siemens steel, and hard Siemens steel, are of an important character, 

 showing, by the delicate electro-chemical method employed, the 

 powerful influence of difference in chemical composition and physical 

 structure, &c., on the passive state of the metals. Generally through- 

 out this series of experiments it will be observed that the wrought 

 iron was electro-positive to the steels with a considerable E.M.F., 

 amounting in some cases to as high as one-tenth to one-seventh of a 

 volt, the wrought iron being thus shown to be less passive than the 

 steels. In the experiments on the wrought-iron and various steel 

 bars on Table VI, which in course of their manufacture were drawn 

 cold through a wortle, and were hence in a different molecular condi- 



