CORROSION OF IRON AND STEEL 59 



The corrosion of iron by its transformation into rust is a more 

 important phase of its corrosion and a more usual phase than its 

 corrosion by the direct action of acids. This paper deals with the 

 corrosion of iron when it forms rust under the influence of oxygen 

 and water and small amounts of other substances which accelerate 

 or diminish the rate of corrosion. 



Although opinion with regard to the rusting process is not 

 unanimous, the author considers that certain facts connected with 

 the rusting of iron have been established experimentally.^ 



(1) It is not possible for iron to rust {i.e., form ferric hydrate) 

 by the action of oxygen or water alone on iron. 



(2) Both of these acting together are necessary for rusting to 

 take place. 



(3) Presence of Carbon dioxide is not necessar3' for rusting to 

 take place. 



It has been suggested in the past by some chemists that 

 rusting cannot take place by water and oxygen acting on iron 

 unless carbonic acid or some other acid is present. It is not 

 supposed that the ion HCO's has any specific action, but that the 

 carbon dioxide acts directly according to the equation 



Fe+H20+2C02+0=Fe H2(C03)2. 



This view is considered by its adherents to receive support from 

 the fact that iron will not rust in water containing a sufficient 

 amount of free alkali. 



The author considers that carbonic acid (carbon dioxide in 

 water) need not. on the evidence, be considered as anything more 

 than an added substance increasing the concentration of hydrogen 

 ion. Water and oxygen alone have been shown by Cushman, 

 Dunstan, and others- to be sufficient for the rusting of iron or 

 steel. It is found also that a small concentration of free alkali or 

 any concentration of sodium carbonate does not prevent corrosion 

 as it should do if carbon dioxide is necessary for corrosion, as 

 free carbon dioxide is removed from acting on iron by such 

 solutions. 



The following theory seems to be the one which will account 

 most readily for the observed facts : — 



If a piece of pure iron is placed in pure water an immeasurably 

 small amount of the iron dissolves. It dissolves as Fe++ ion, and 

 the iron rod becomes charged negatively to the same extent. In 

 behaving like this iron acts like other metals. Every metal has 

 a solution pressure or driving force which tends to make the metal 

 go into the ionic condition. 



Hydrogen behaves also in some respects like a metal, but it 

 has a smaller solution pressure than iron ; iron therefore displaces 

 hydrogen from solutions. 



There is a small concentration of hydrogen ion in water — one 

 gram of hydrogen ion in 10^ litres. When the ion forms 



1. Cushman and Gardner, " Corrosion of Iron and Steel," 1910. 



2. See Cushman and Gardner ; " Corrosion of Iron and Steel." 



