A novel approach to on-line measurement of gas evolution kinetics: Application to the negative difference effect of Mg in chloride solution

• On-line, time resolved volumetric detection of hydrogen during electrochemical experiments reported for the first time.
• Simultaneous measurement of partial elementary dissolution rates by AESEC.
• Excellent agreement between the quantity of dissolved zinc and evolved hydrogen for Zn in HCl solution.
• Clear evidence for negative difference effect (NDE) for anodized Mg in dilute NaCl solution.
• Results indicate an electrochemical dissolution of Mg with n = 2 occurs independently from the NDE mechanism which involves rapid spontaneous dissolution.

Gas evolution at the surface of a dissolving material plays an important role in the overall dissolution process. In order to measure the kinetics of the gas evolution as a function of time (time resolved volumetry, TRV), a microfluidic system was constructed combined with high speed camera and a dedicated image analysis algorithm to measure the volume of gas as a function of time. This instrumental set up was added to an existing coupling between an electrochemical cell and a time resolved atomic emission spectroscopic detection. This article presents the instrumental development and first results concerning the spontaneous dissolution of zinc in a dilute HCl solution, the spontaneous dissolution of Mg in 0.01 M NaCl at open circuit potential and under an applied anodic potential. For Zn dissolution, an excellent correlation was obtained between the quantity of hydrogen measured and the quantity of zinc dissolved, with average accuracy of 5%. For Mg however, the anomalous production of hydrogen was observed attributed to the negative difference effect. A steady Mg dissolution was observed correlating with the electrochemical current for an n = 2 dissolution mechanism with superimposed bursts of Mg dissolution and hydrogen release corresponding to the anomalous dissolution mechanism.

Figure optionsDownload as PowerPoint slide