Among the ferrous metals (Fe, Ni and Co), cobalt has certainly been the least investigated. Furthermore, its passivating ability in acid medium is fairly weak compared with the other two metals. The electrochemical behaviour of cobalt in nitric acid was examined in a wide range of concentrations. The dissolution of cobalt in these media was also studied by the thermometric method. Electrochemical measurements showed three different states under open circuit corrosion conditions according to the nitric acid concentration. This difference was related to the hydration number of the cobalt salt covering the electrode surface. The effect of the cathodic process, i.e., the reduction of the nitrate anion, was checked by eliminating the nitrous anion from the nitric acid with the addition of para-nitroaniline to the electrolyte.