THE CORROSION OF IRON AND OTHER METALS 4 49 



explanation, however, is provided by the fact that all these 

 agents, without exception, render iron passive and that this 

 passivity persists after removal from the solution which caused 

 it. Iron, so long as it is passive, does not rust in presence 

 of oxygen and water. If exposed to atmospheric air, passive 

 iron very gradually rusts, owing to the removal of the passivity 

 of the iron by the carbon dioxide of the air. Passivity of 

 metals may vary in amount and the amount of rusting subse- 

 quently induced will depend on the extent to which the passivity 

 is removed. 



" The mechanism of the process of the rusting of iron does 

 not appear to differ essentially from the aerial oxidation of 

 other metals. Similar reagents inhibit in all cases and traces 

 of hydrogen peroxide can be detected when iron rusts, just 

 as in the case of the other metals. All the metals concerned 

 will, however, readily oxidise in solutions of potassium iodide 

 and in certain other reducing agents which render the existence 

 of hydrogen peroxide impossible. Consequently the hydrogen 

 peroxide is probably not an intermediate product of the main 

 reaction but may be the result of a secondary reaction, since 

 the process of oxidation can apparently continue without its 

 formation. 



" We consider, therefore, that metals undergo aerial oxida- 

 tion by direct action of oxygen dissolved in water. In the case 

 of iron, ferrous hydroxide is first produced and is afterwards 

 further oxidised to the ferric state, forming rust. Iron does 

 not pass into solution before rust can be formed. The action 

 of carbonic acid or of any other acid is separate and inde- 

 pendent of the main process of rusting which proceeds in their 

 absence. 



" Several experiments which have been recently recorded 

 as affording support to the old view that carbonic acid is 

 necessary for the rusting of iron are explained by the results 

 now recorded. The fact that iron which has been treated with 

 dilute chromic acid or other inhibiting agents and subsequently 

 washed does not rust in air free from carbon dioxide but begins 

 to rust after carbon dioxide has been admitted is due to the 

 destruction of the passivity of the iron through the action of 

 the carbon dioxide. 



"A similar explanation can be given of an experiment 

 recorded by Friend {Proceedings of the Chemical Society, 1910, 

 26, 179) and intended as a lecture experiment to demonstrate 

 the necessity of carbonic acid for the rusting of iron. In this 

 apparatus a hollow iron cylinder previously immersed in 

 potassium hydroxide solution is shown afterwards not to rust 

 in presence of water and air free from carbon dioxide. This 

 result is due to the passivity of iron induced by the alkali and 

 does not prove, as is assumed, that iron cannot rust unless 

 carbon dioxide is present. If carbon dioxide is admitted, the 



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