THE CORROSION OF IRON AND OTHER METALS 325 



expression C = E/R — must also affect the rate and manner in 

 which corrosion takes place. 



This subject is dealt with by Friend in his book at consider- 

 able length but in a somewhat unsatisfactory and unconvincing 

 manner. A useful paper on the subject is that read at the May 

 meeting of the Iron and Steel Institute by Mr. Cobb. In this it 

 is pointed out that iron is definitely electro-positive to most of 

 its impurities; manganese and 80 per cent, ferro-manganese, 

 however, are positive to iron. Mr. Cobb recognises that the 

 impurity acts as the corrosion centre and that its influence 

 depends more on quality and distribution than on quantity. 



That common sense is beginning to prevail in such work is 

 clear also from a passage such as the following : 



" When a weak solution of picric acid is applied to an 

 ordinary steel, the carbon eutectic is all etched out quickly, 

 while the ferrite is unattacked. This is a misleading result, 

 unless it is remembered that the characteristic pearlite structure 

 of the carbon eutectic is that of alternating layers of iron and 

 carbide of iron, which, when covered with such an electrolyti- 

 cally conducting film as dilute picric acid, become so many 

 active electro-chemical couples. Thus the pure iron layers of 

 the eutectic pass into solution, while the iron of the ferrite has 

 no such opportunity, except along its surface contact with the 

 eutectic. In a refined iron, which contains the eutectic in a very 

 coarse form in a matrix of carbide, it can be seen plainly that 

 the carbide lines in the eutectic itself, which penetrate from the 

 matrix, are unaffected by etching." 



Mr. Cobb then proceeds : 



"From what has been said, it would follow that, if other 

 conditions could be made exactly the same, the iron dissolved 

 in a given time, on etching two irons containing no impurity 

 but carbon, would be in the ratio of the amounts of the pearlite 

 (eutectic) present. The all-pearlite iron containing 0*9 per cent, 

 carbon would show the greatest rate of dissolution ; any excess 

 of iron, or of carbon, would lessen it." 



In view of this pronouncement, the following statement is of 

 interest in the Report on the influence of carbon and other 

 elements on the corrosion of steel by Prof. Arnold and others 

 presented to the Chemical Section at the recent meeting 

 of the British Association. The Committee have examined a 

 series of six iron-carbon alloys containing from o'io to 0*96 per 

 cent, of carbon, prepared from Swedish bar iron and charcoal. 



