CHROMIUM, iron, cobalt, nickel, copper, and bismuth are said to exhibit the phenomenon of passivity. These metals have electrode potentials varying from about Fe = + 0.06 to Bi = −0.67 volt—that is, they are not very electro-positive. They also all exhibit dual valencies. This suggests that passivity may be due to an electrical double layer on the surface of the metal, especially when it is remembered that an anode of iron becomes passive in nitric acid of sufficient concentration.