The electrochemical behaviour of cobalt in KOH solutions of different concentrations was studied. The effects of applied potential, temperature and the presence of aggressive Cl- ions were investigated. Different electrochemical methods such as open-circuit potential measurements, polarisation techniques and electrochemical impedance spectroscopy (EIS) were used. The electrochemical behaviour of cobalt in naturally aerated KOH solutions is characterized by three different regions according to the alkali concentration. Corrosion behaviour was observed at high concentrations (0.3-1.0 M); passivation at lower concentrations (0.01-0.05 M), and at intermediate concentrations (0.1-0.2 M) corrosion followed by passivation was recorded. The corrosion parameters (i(corr), E-corr, and R-corr) under various conditions were calculated. Equivalent-circuit models for the electrode-electrolyte interface under different conditions were proposed. The experimental impedance data were fitted to theoretical data according to the proposed models. The relevance of the proposed models to the corrosion-passivation phenomena occurring at the electrode-solution interface was discussed. The electrochemical experimental results and discussions were supported by surface analytical techniques.