76 president's address — section b. 



solution containing sodium carbonate and a small quantity of sodium, 

 bicarbonate is less than the concentration in pure water, so that the 

 fact of rusting going on in sodium carbonate solutions appears to the 

 author to dispose of the carbonic acid theory altogether. 



The action of oxygen and water on iron is affected by the presence 

 of dissolved salts, but the effect is usually not remarkable except in 

 the case of special salts like bichromates, Avhich render the iron passive. 

 Until the passivity of iron is more thoroughly worked out, the corrosion 

 problem is not likely to advance much. It has recently been suggested 

 that there are several kinds of passivity of iron, as " passive " iron 

 does not always exhibit the same properties, but nothing definite as 

 to the cause of any kind of passivity in iron has been so far brought 

 to light. 



The effect of mechanical treatment in altering the solution pressure 

 of metals has been already investigated by the author (1906) for silver, 

 platinum, and gold, and by W. H. Walker with his "ferroxyl" indicator 

 for steel. These results went to show that strained parts of a metal 

 tend to dissolve more readily in solutions than unstrained. It does 

 not seem, however, that iron in this respect is likely to be made more 

 specially corrodible than other metals which are subjected to a great 

 deal of mechanical treatment. 



Apart from the passivity of iron which clearly marks iron out 

 from most of the commonly used metals, the complex composition of 

 commercial iron is to be regarded as a contributing course to corrosion. 

 There has been a great deal of labour given to the system iron-carbon 

 by metallographers, and the modern iron-carbon diagram illustrating the 

 effect of composition on melting point and transition temperature is not 

 likely to be altered much by future work. The system iron-manganese 

 and iron-silicon are also well worked out, but we can say very little about 

 the combination of iron with manganese, silicon, sulphur, phosphorus, 

 and carbon which exist in all usual steels. The quantities of all other 

 elements than iron in steel are small, but they have an important effect 

 on all its properties. The amounts of oxygen, nitrogen, and hydrogen 

 present in steel are very hard to determine, and yet very small amounts 

 of these elements have likely a great effect on the properties of steel. 



Iron is not alone in being sensitive to traces of impurities. The 

 presence of almost undetected traces of iron or arsenic in copper 

 absolutely prohibits its use for certain purposes in engineering works.. 

 The full explanation of the effects of small quantities of impurity on 

 the physical and chemical properties of a metal is still wanting. The 

 quantity of impurity present may be too small to effect the properties 

 of the molten metal, but when the metal solidifies the effect is noticeable 

 enough. One has to bear in mind that when steel begins to solidify 

 the metal which first sets solid is much purer than the molten liquor 

 which is left in its immediate neighbourhood, and the later stages of 



