In Situ Investigation of the Role of Hydrogen Evolution on the Estimated Metastable Pit Sizes in an Al-Mg Alloy

The fraction of pit dissolution charge (Cpit) that contributes to hydrogen evolution (HE) occurring within the metastable pits (analyzed in real-time) was determined to evaluate the error it introduces in the metastable pit size estimations derived merely from the analysis of dissolution current. With in situ optical time-lapse imaging, the charge consumed by HE (CHE) during the current transients was determined from the size of the hydrogen bubbles evolving at the real-time metastable pit locations in an aged Al-Mg alloy. The ratio CHE/Cpit 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 Epit.