An elucidation of magnesium dissolution mechanisms is one of the key issues for wider usage of magnesium and its alloys in industry because the magnesium has high reactivity in aqueous solutions. The hydrogen evolution rate is increased with positive polarization from the open circuit potential, which is the so called negative difference effect (NDE).^1,2 The real-time hydrogen measurement methods have been developed to investigate the NDE.^3-6

Authors⁷ developed a pH sensing channel flow double electrode (CFDE) in order to investigate the pH changes due to the magnesium dissolution. Our group^8-10 have been applied the CFDE for analysis of dissolution mechanisms of copper and copper alloys because the oxidation state of the metal ions dissolved from the metal working electrode (WE) can be detected by arranging a detecting electrode (DE) downstream of the WE. In the pH sensing CFDE, the DE was used as the pH sensor, namely, the hydroxide ions generated during the anodic polarization of magnesium WE can be detected at the pH sensor. It indicates that this method allows for the estimation of the hydrogen evolution rate from pH values without the hydrogen gas collection. In addition, a video recording of the WE surface was performed during the measurement in this system.

In the present study, the magnesium dissolution during anodic polarization was examined by the pH sensing CFDE. The relationship between the breakdown of the oxide film formed on the magnesium surface and increase of pH was discussed, and the hydrogen evolution rate was estimated from the pH values.

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