THE BEHAVIOR OF COBALT IN CONCENTRATED NITRIC-ACID SOLUTIONS

In our previous investigation it has been established that cobalt, unl ike iron or nickel, does not passivate in concentrated or even fumic n itric acid. The dissolution of cobalt in this aggressive medium is, ho wever, significantly reduced by the formation of a hydrated nitrate sa lt layer adhering tightly to the metal surface. On the basis of the op en-circuit corrosion potential measurements of cobalt electrode, in a wide range of nitric acid concentrations, and polarization curves, obt ained in 11 mol dm-3 nitric acid using a regulating device with negati ve output resistance, along with electrode impedance results, it was p roved that the hydration number of nitrate salt plays the crucial role in the electrochemical behaviour of cobalt in concentrated nitric aci d solutions. The importance of the free water released through a decre ase in the hydration number was clearly observed.