Analysis on Anodic Partial Current of Magnesium by In-situ GC-EC Cell
Authors: Hirayama Yusuke , Hoshi Yoshinao , Shitanda Isao , Itagaki Masayuki 
Published: February 2020

The anodic dissolution mechanisms of magnesium have been investigated by various electrochemical methods. It is well known that the hydrogen evolution was occurred at magnesium surface during anodic polarization of magnesium1, which is called the negative difference effect, NDE. Because the measured current includes the anodic current related to the dissolving magnesium and the cathodic current attributed to the hydrogen evolution at magnesium surface during anodic polarization, it is difficult to explore the dissolution mechanisms of magnesium based on the polarization curve of Mg 2, 3. In the present study, the anodic partial current of Mg was estimated by using in-situ GC-EC cell4. The in-situ GC-EC cell4 allows for the determination of hydrogen evolution rate during the electrochemical measurement of magnesium. Because the hydrogen gas evolved from dissolving magnesium was delivered to the gas chromatograph with the carrier gas during electrochemical measurement of magnesium, the anodic partial current associated with the magnesium dissolution can be estimated.
The electrochemical measurement was performed by a three electrode system. The working electrode was magnesium and the counter electrode was Pt. A KCl-saturated Ag/AgCl electrode (SSE) was used as the reference electrode. The electrolyte solution was 1.0 M NaCl. The potential scan rate was 100 mV / min. The procedure to estimate anodic partial current of magnesium dissolution during electrochemical measurement was described as follows. The polarization curve measurement of magnesium was carried out by using in-situ GC-EC cell4. The potential scan was performed to the noble direction. The cathodic current was calculated from the volume of hydrogen gas. The hydrogen gas was collected every 164 seconds after starting the electrochemical measurement and it takes 150 seconds to analyze the gas at each gas collection. The flow rate of carrier gas was 100 mL min-1.
The cathodic current i
H2 (A) was estimated for the volume of hydrogen gas. The hydrogen evolution rate was calculated by equation (1).

v = V
H2 × φ / (V
m × 22400 ) (1)
In this equation, the v is the hydrogen evolution rate (mol s−1), the V
m is the volume of the sample loop (ml), the V
H2 is the volume of the hydrogen gas (ml) and the φ is the flow rate of carrier gas (mL s−1). The cathode current i
H2 (A) was calculated by equation (2).

i
H2 = nFv (2)
In this equation, the n is the number of reactive electrons and the F is the Faraday constant. The partial anodic current i
Mg was calculated by adding the measured anodic current i
m and the cathodic current i
H2.

i
Mg = i
m + | i
H2 | (3)
In the present study, the magnesium dissolution during anodic polarization was investigated by the diagram composed of the i
Mg and the electrode potential of magnesium. On the basis of the tafel slope determined from this diagram, the dissolution mechanisms of magnesium were discussed.
References:
1) G. Song, A. Atrens, D. Stjohn, J. Nairn, Y. Li, Corros. Sci, 39, 855 (1997).
2) G. Song, A. Atrens, ADV. FUNCT. MATER, 5, 837 (2003).
3) Z. Shi, M. Liu, A. Atrens, Corros. Sci, 52, 579 (2010).
4) Y. Hoshi, R. Takemiya, I. Shitanda, and M. Itagaki, J. Electrochem. Soc, 163, C303 (2016).