The behaviour of magnesium electrodes has been investigated in 3.5% NaCl during free corrosion and potentiodynamic polarizations. The usual electrochemical measurements were complemented with real-time direct measurement of the amount of hydrogen evolved and real-time optical imaging of the electrode surface. It was found that the anodic reaction of magnesium oxidation results in the generation of dark regions on the surface of the electrode and such regions are efficient in supporting the cathodic reaction (hydrogen evolution). Further, compared to the behaviour observed at the free corrosion potential, the dark regions propagated significantly faster during anodic potentiodynamic polarization and did not propagate significantly during cathodic potentiodynamic polarization. The findings indicate that the negative difference effect is due to an increase in cathodic current during anodic polarization resulting from the modification of the electrode surface after the anodic reaction has occurred. Additionally, the real-time measurement of the amount of hydrogen evolved enabled a direct estimation of the corrosion current. Thus, the actual value of the corrosion current could be compared to the values of corrosion current estimated by potentiodynamic polarization. It was found that anodic polarization provides little information on the corrosion rate, whereas cathodic polarization provides a more accurate estimation. Finally, it is suggested that the corrosion rate on magnesium is controlled by the rate of the cathodic reaction. Such reaction is self-limiting on magnesium since it produces alkalinity and alkalinity promotes passivity. (C) 2013 The Author. Published by Elsevier Ltd. All rights reserved.