THE CORROSION OF IRON AND OTHER METALS 321 



In other words, while iron dissolves as ferrous salt, ferrous 

 salt which has been produced previously is converted into 

 a basic ferric salt ; in the final stage this latter salt is acted 

 upon by water and converted into rust and acid, the which 

 acid is then available to dissolve a further quantity of iron. 



It is clear that when iron dissolves merely as ferrous salt 

 the rate of change (the value of C in the expression C = E/R) 

 must be less than when it is dissolved in a circuit in which 

 oxygen is included and operative. 



The heat of dissolution of iron (that given out on dis- 

 solving a gramme atomic proportion, i.e. 28 grammes), according 

 to Thomsen is 24,840 gramme-deg. C. heat units ; the corre- 

 sponding value for chlorhydric acid is 21,310 units; the 

 maximum electromotive force developed on dissolving iron 

 in these acids is therefore 



?4i840 = Q . volt and 2ij3io = . 46 vok 



46,000 46,000 



Probably the value given by carbonic acid does not 

 differ greatly from the second of these. 



It is to be supposed that when, in presence of oxygen, 

 the hydrogen displaced by iron in dissolving is oxidised 

 effectively in the primary circuit (entirely to water) a large 

 additional amount of energy is developed in the circuit, equal 

 to about 68,000 heat units, so that the electromotive force 

 is raised about one and a half volt beyond that of a single 

 fluid iron-acid cell. The part oxygen plays is therefore a 

 very important one in so far as it is included in the primary 

 circuit in which corrosion is effected ; at present, however, 

 we cannot say to what extent it is included in that circuit 

 and to what extent energy is wasted in secondary changes 

 which take place outside the circuit of corrosion. 



It is well known that when iron has once begun to rust 

 in air corrosion proceeds more rapidly, the rust appearing 

 to promote attack. There is little doubt that this is in some 

 measure due to the fact that the finely divided oxide serves 

 to attract and condense moisture. It has been supposed that 

 the rust also promotes corrosion by its electro-negative 

 character. It may be doubted whether ferric oxide itself has 

 much influence on this account but inasmuch as the electro- 

 negative impurities in steel are set free as corrosion proceeds 



21 



