THE CORROSION OF IRON AND OTHER METALS 451 



" This (the removal of passivity on heating passive iron 

 in hydrogen at 250 ) is strong evidence in favour of the 

 film being composed of an oxide of iron and this explanation 

 is supported by the fact that many dilute acids which dissolve 

 the oxides of iron also destroy passivity. It is more difficult, 

 however, to explain the action of the alkalies on this basis. 

 It is known, however, that under certain conditions iron can 

 be oxidised by alkalies with the production of a visible film 

 of oxide. Presumably such oxidation can only take place 

 with the liberation of hydrogen and it has not so far been 

 recognised that iron can react with cold dilute alkalies." 



They also appear to have difficulty in understanding the 

 process by which hydrogen peroxide is formed as they remark : 



" In the case of iron a small quantity of hydrogen peroxide 

 is formed during oxidation, just as with the other metals. The 

 precise significance of the formation of hydrogen peroxide dur- 

 ing the aerial oxidation of metals has not yet been discovered." 



There is, however, no difficulty whatever in representing 

 either process by the aid of the principles laid down in the 

 previous articles. Traube many years ago explained the 

 significance of the formation of hydrogen peroxide. 



Zinc which carries the necessary electro-negative impurity 

 is at once attacked by caustic soda, because zinc hydroxide 

 has acidic properties and the formation of sodium zincate 



Zn + 2NaOH = Zn (ONa), + H 2 



is an exothermic process — that is to say, one involving the 

 liberation of energy, which takes the form either of electricity 

 or of heat according to circumstances. 



Ferrous hydroxide, Fe(OH) 2 , has no acidic properties ; 

 presumably its formation would involve an endothermic change 

 and therefore it cannot take place without assistance — unless 

 energy be supplied from some external source. In presence 

 of oxygen acting as depolariser, action takes place, as in 

 the case of zinc, probably in the way represented by the 

 equation 



HONa HOH O /OH NaOH HO 



HONa + HOH + 6 = lOH + NaOH + HO 



It may be supposed that hydrogen peroxide is produced, as 

 in all cases in which oxygen is active as depolariser and that in 

 the case of iron it escapes notice probably because it at once 

 undergoes decomposition at the instance of the ferrous hydroxide. 



