THE CORROSION OF IRON AND OTHER METALS 453 



powerful, the iron cannot displace hydrogen from the alkali 

 and form a corresponding salt. Action takes place readily, 

 however, when the assistance of oxygen is secured : though 

 the iron alone cannot displace the hydrogen, when this latter 

 has its attention engaged by oxygen and is attracted away, 

 the iron easily slips into its place. 



But the coating of hydroxide (or oxide) which is formed 

 is superficial and perhaps almost of molecular dimensions, 

 as the action comes to an end on account of the insolubility 

 of the product (ferrous hydroxide or oxide) in the alkali. 

 In the case of such a metal as zinc, the action of alkali is 

 continuous — if the solution be of sufficient strength — on 

 account of the solubility of the product. Again, no direct 

 immediate attack is possible in the case of salts such as 

 sodium chloride, as iron has not the power to displace the 

 metal ; it can only so act on salts of metals lower in the 

 scale of energy than itself — such as copper, for example. When 

 oxygen is included with the salt in the circuit, the salt is 

 easily electrolysed : it is to be supposed that in a case such 

 as that of sodium chloride, ferrous chloride is formed and 

 that the iron does not become " passive " as the salt is soluble. 

 Such action can only take place when there is no insulating 

 film on the surface of the metal. 



It should not be necessary to quote such elementary pro- 

 positions as these. They are based on fundamental principles 

 which should be the common property of all students of the 

 elements of chemistry ; our constant neglect to apply them 

 is clear evidence of the unscientific and illogical way in 

 which chemistry is taught. 



At the present day, there should be no difficulty in 

 interpreting the process of corrosion. What we most need 

 to know, to make our knowledge of steel practical, is the 

 relative corrodibility of the corrodible constituents — if there 

 be several ; also the manner in which the various electro- 

 negative constituents (not forgetting insulating impurities) 

 are distributed and influence corrosion. 



The metallographers do little to help us : they commemorate 

 their labours by christening " ites " innumerable and they talk 

 learnedly of phases but in the end give us very little more 

 than phrases. H. E. A. 



