Thermometric behaviour of cobalt in HNO3 solutions

Cobalt corrodes in dilute HNO3 solutions according to an autocatalytic mechanism involving the formation of HNO2. The rate of metal dissolution increases with the acid concentration. Passivation of cobalt starts to take place in concentrated HNO3 solutions due to the formation of a passive oxide film on the metal surface. Addition of inorganic anions, in either neutral or acidic forms, affects the rate of cobalt dissolution and supports the proposed autocatalytic mechanism.