Electrochemical behaviour of nickel in nitric acid

The electrochemical investigations of nickel in a wide range of concentrations (1–14.6 M) of nitric acid were studied by potentiostatic, potentiodynamic and coulostatic method and by weight-loss measurements. The potentiostatic current-potential curves of nickel in nitric acid solutions can be classified into three categories, according to the proportion of the concentration of the acid. An increase of nitric acid concentration increases the influence of the cathodic reactions in the systems. The importance of cathodic reactions has been investigated by eliminating HNO2 by additions of p-nitro aniline in the electrolyte. The coulostatic polarization has given distinct repassivation curves, indicating an intermediate state of nickel electrode between active and passive state. In the first part of repassivating process the potential value is more positive than the stationary potential of spontaneous passivation of nickel electrode in concentrated nitric acid. The return of the stationary potential of the passivity is a slow, time-dependent process. According to the experimental results and data given in the literature, it may be possible that in the beginning of the passivation process on nickel Ni(NO3)2 × 4 H2O forms which changes to NiO2 and than transforms slowly to NiO.