Effect of Fluctuation of Electrolyte Flow Rate in Hydrogen Entry Cell on Hydrogen Permeation into Steel Sheet

The sensitive measurement of hydrogen permeated into the steel is the key of issue not only to investigate the kinetics of hydrogen embrittlement of the steel but also to develop the prevention surface from the hydrogen embrittlement. Conventionally, a Devanathan-Stachurski (DS) double electrochemical cell [1] was used to measure the hydrogen permeation into the sheet specimen. However, it was a relatively difficult to measure the localized permeation of the specimen. It has been expected to develop the more sensitive method to detect the local hydrogen permeation into the steel specimen.

The authors have modified the DS cell for hydrogen permeation measurement into a steel sheet to flow an electrolyte solution in the hydrogen entry side [2]. Sinusoidal fluctuation of the electrolyte flow rate resulted in the conduction of fluctuations of not only hydrogen evolution current in the entry cell but also hydrogen detection current in the exit cell. Amplitude of the detection current wave was dependent on a frequency of the fluctuation and decreased with increase in the frequency. Furthermore, a phase shift of the detection current wave from the evolution current wave was observed and it was strongly depended on the frequency. The phase shift of the detection current wave was significantly sensitive to the thickness and diffusion coefficient of the sheet [3].

The DS cell with a micro-capillary cell and the flow rate fluctuation was also applied to measure the local hydrogen permeation behavior into the steel sheet. The phase shift on a single grain of the steel sheet was two times larger than that on two grains, indicating that the hydrogen permeation was accelerated due to existence of the grain boundary of the sheet. From the comparison with theoretical diffusion equation of hydrogen into the steel [4], it was revealed that the diffusion coefficient on the grain boundary was at least two times larger than that on the single grain. These results suggested that the metallurgical structure of steel substrate strongly influenced to the permeation. However, it was difficult to the solve analytically the diffusion problem for the two- or three-dimension permeation.

The authors have applied a finite element method (FEM) for the two- or three-dimension diffusion problem. A numerical calculation of FEM revealed successfully that the electrolyte fluctuation appeared the phase shift of current waves depending on the thickness and diffusivity of the substrate. The difference in phase shift due to local diffusivity was also simulated as well as the size effect of local substrate where hydrogen was permeated. The results will be reported in the paper.

References

[1] M.A.V. Devanathan, Z. Stachurski, Proc. R. Soc. Lond. A 270 90-102 (1962).

[2] K. Fushimi, M. Jin, T. Nakanishi, Y. Hasegawa, T. Kawano, M. Kimura, ECS Electrochem. Lett., 3(6) C21-C23 (2014).

[3] K. Fushimi, M. Jin, Y. Kitagawa, T. Nakanishi, Y. Hasegawa, ISIJ Int., accepted.

[4] K. Sekine, Chem. Lett., 4(8) 841-846 (1975).