Chemical Considerations on Passivity *

--It can be shown tha t the value o f the charge/ radius ratio o f metallic ions determines whether the metal on dissolut ion tends to become passive or not. Us ing this criterion it is possible to clarify the influence o f p H and electrode potential on the occurrence and breakdown of passivity. I N T R O D U C T I O N IN MANY cases a base metal is less reactive than would be expected from its standard electrode potential. This important phenomenon is called passivity and when a metal is passive it shows a much lower corrosive rate than otherwise. 1,2 Well-known examples are aluminium, chromium and chromium alloys, titanium, etc. In aqueous solutions the explanation usually given for this phenomenon is that the surface of the metal is covered by a layer of oxide or adsorbed oxygen which isolates the metal from its environment. From thermodynamic considerations it can be deduced under which circumstances passivity may be expected. The Pourbaix diagrams s are extremely valuable in this respect. On the other hand, however, thermodynamics is not able to furnish an explanation why passivity occurs. In the present paper the chemical background of passivity is considered on the basis of the chemical reactions and the properties of the metal ions involved in passivation. The discussion is limited to cases of "non-specific" passivity and cases such as Pb in H2SO4, which is protected by an insoluble film of PbSO4, are not considered, and it is assumed that no complexing agents are present in the solution. Finally we shall limit ourselves to metals with inert-gas configurations and to the transition metals. Basic reactions The basic corrosion reaction is usually written as the anodic half-reaction of a galvanic couple.