Magnesium (Mg) is widely used as a large transportation and structural materials for their weight savings. However, Mg has large corrosion rate, and the hydrogen evolution occurs during the anodic dissolution of Mg. It was reported that the hydrogen evolution rate on Mg increases with noble potential from corrosion potential, which is called the Negative Difference Effect (NDE) ^1-8). In order to analysis of the NDE phenomenon, Rossrucker et al.⁴⁾ applied an electrochemical flow cell coupled to downstream analysis for investigation of the influence of pH alteration in the near surface region of dissolving Mg. In this method, the polarization measurement and simultaneous determination of the amount of dissolved Mg ions via inductively coupled plasma - mass spectroscopy. In the present study, we developed the pH sensing channel flow double electrode (CFDE) to monitor the pH changes on the Mg electrode due to the anodic dissolution. Schematic of the pH sensing CFDE is shown in Fig. 1. In the CFDE cell, the working electrode (WE) and detecting electrode (DE) were placed upstream and downstream in the channel. An Mg (1×4 mm²) and a W (0.1×4 mm²) were used for the WE and DE. The counter electrode (CE) was a Pt wire, which was set at channel outlet. The reference electrode was KCl-saturated silver/silver chloride electrode (SSE). The rest potential of W (DE) was measured for estimation of pH during the anodic dissolution of Mg (WE). The 0.1 mol dm^-3 sodium sulfate solution (pH = 5.9) was used for the measurement. The degassing of the electrolyte solution was carried out for 1 h under an N₂stream. A laminar flow condition was established in this measurement. Furthermore, the surface of WE was recorded by a USB digital microscope.

The anodic polarization curve measurement of Mg and pH measurement was carried out by developed system. It was confirmed that the increase of pH is corresponded to the increase of anodic current of Mg with hydrogen generation.

Reference

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