This work investigates the hydrogen evolution process on magnesium by real time imaging of the corroding surface at the free corrosion potential and during potentiodynamic polarization in alkaline and neutral environments, in the presence and absence of chloride ions. High-magnification imaging of the corrosion process from side view discloses that three different types of hydrogen bubbles are generated during corrosion: i) large and stable bubbles on the uncorroded regions, ii) a stream of fine bubbles at the corrosion front and iii) medium size bubbles behind the corrosion front. It is proposed that the generation of streams of bubbles at the corrosion front is due to the disruption of the oxide/hydroxide film that is possible when an anodic current is available and environmental conditions can induce depassivation. This results in direct exposure of the underlying magnesium to the electrolyte and, due to the large overpotential available, in hydrogen evolution. Thus, the corrosion front act as a current amplifier where the rate of magnesium oxidation is proportional to the sum of the current produced remotely, that maintains the depassivation, and the current produced at the corrosion front by hydrogen evolution. (C) 2016 The Authors. Published by Elsevier Ltd.