The fraction of pit dissolution charge that contributes to hydrogen evolution (HE) occurring within metastable pits (analyzed in real-time) was determined to evaluate the error it introduces in the pit size estimations derived merely from the analysis of dissolution current. With in situ optical time-lapse imaging, the charge consumed by HE (C<sub>HE</sub>) during the current transients was determined from the size of the hydrogen bubbles evolving at real-time metastable pit locations in an aged Al-Mg alloy. The ratio C<sub>HE</sub>/C<sub>pit</sub> was observed to range between 0.019 and 0.052 depending on the charge of the transient. This difference was attributed to the catalytic nature of the pit surface that develops as a consequence of the pertinent pitting mechanism. Therefore, it was concluded that the HE within the metastable pits might not play a significant role in the underestimation of metastable pit sizes determined from the current transients recorded at anodic potentials below E<sub>pit</sub>.

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