This work illustrates how side-view optical imaging, anodic potentiodynamic polarization, and real-time hydrogen evolution measurements can be applied and complementarily used to study the processes occurring on a corroding magnesium surface. Side-view imaging reveals that hydrogen evolution takes three different forms: (i) large and stable bubbles on the uncorroded regions, (ii) a stream of fine hydrogen bubbles at the corrosion front, and (iii) medium-sized bubbles behind the corrosion front. The observation suggests that the relatively large bubbles ahead and behind the corrosion front are associated with a purely cathodic reaction that provides the “remote current.” This current, combined with the depassivating role of chloride ions, maintains the corrosion front active, generating regions where either the metal is locally directly exposed to the electrolyte or is only covered by a poorly protective chloride-rich film. Regardless of the precise nature or morphology of the poorly protective (or absent) surface film at the corrosion front, additional hydrogen evolution occurs as a result of the large overpotential available. Overall, the anodic current associated with magnesium oxidation is the sum of the “remote current,” which is manifested with hydrogen evolution ahead and behind the corrosion front, and the “local” current associated with hydrogen evolution at the corrosion front. From the electrical viewpoint, the corrosion front acts as a current amplifier, where the current amplification effect can be measured directly by real-time gravimetric hydrogen measurement performed simultaneously with anodic potentiodynamic polarization.

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