Hydrogen Video Imaging System for Visualizing Microstructure-Dependent Hydrogen Diffusion Behaviors in Polycrystalline Metals
Authors: Kakinuma Hiroshi , Ajito Saya , Hojo Tomohiko , Koyama Motomichi , Akiyama Eiji 
Published: February 2024

Hydrogen diffusion in polycrystalline metals is known to depend on microstructure. The hydrogen-microstructure interactions have been extensively studied last decades, but the microstructure-dependent hydrogen diffusion behavior is still unclear. To clarify the hydrogen diffusion behavior, it is necessary to visualize the time variations of the microscopic hydrogen distribution in polycrystalline metals. Thus, we developed a hydrogen video imaging system (HVIS) in this study to analyze the hydrogen distribution in polycrystalline metals with sub-micrometer spatial and video-rate time resolutions. The inexpensive and atmospheric-control-free system is expected to be widely used as a versatile approach for analyzing the hydrogen-microstructure interaction in detail.
Pure polycrystalline Ni (thickness: 125 µm) was used as a specimen. The specimen was electrochemically polished at a constant voltage of 50 V in a mixed solution of perchloric acid and acetic acid. The thickness was approximately 110 µm after polishing. Subsequently, a polyaniline layer (PANI) was polymerized on one side of pure Ni. The specimen was set to the acrylic cell, which is used to preserve a 0.1 M NaOH aqueous solution. So, only the bare Ni side of the specimen was exposed to the 0.1 M NaOH solution. Electrochemical hydrogen charging was conducted at the bare Ni side of the specimen at a constant current density of -10 A / m2. An inverted optical microscope was used to observe the color distribution of the PANI side. In our previous studies,1-3) it has been clarified that PANI changes its color due to the reaction with hydrogen in metals. Thus, the distribution of hydrogen permeated through pure polycrystalline Ni can be observed as a color distribution of PANI during hydrogen charging.
The color of PANI was purple before the hydrogen charging. After 20 h of hydrogen charging, the color of PANI partially turned white. If PANI reacts with hydrogen, the color changes from purple or dark blue to yellow or transparent.1,2) Therefore, the local whitening of PANI is thought to be caused by the reaction with atomic state hydrogen permeated through pure polycrystalline Ni. The number of color changed areas increased with time during hydrogen charging. After 120 h of hydrogen charging, the specimen was immersed in a 1 M NaOH aqueous solution to remove PANI. Subsequently, the microstructure of pure polycrystalline Ni was analyzed using FE-SEM and EBSD. It was found that the color changed area of PANI corresponded to the grain boundaries (GBs) of pure Ni. This suggests that the GBs in pure Ni are preferential hydrogen diffusion paths. Furthermore, the clear color change of PANI was observed at the random GBs, while the whitening scarcely occurred at the Σ3 GBs, indicating that the preferential hydrogen diffusion mainly occurred at the random GBs. The results showed that HVIS can clarify the microstructure-dependent hydrogen diffusion in polycrystalline metals.
References:
1) H. Kakinuma, S. Ajito, T. Hojo, M. Koyama, E. Akiyama, Real-time Visualization of Hydrogen Distribution in Metals Using Polyaniline: An Ultrasensitive Hydrogenochromic Sensor, Adv. Mater. Inter., 9, 2101984 (2022).
2) H. Kakinuma, S. Ajito, T. Hojo, M. Koyama, S. Hiromoto, E. Akiyama, Simultaneous Observations of the Corrosion Behavior of an Fe Sheet and the Associated Hydrogen Distribution therein Employing a Hydrogenochromic Sensor, Corros. Sci., 206, 110534 (2022).
3) H. Kakinuma, S. Ajito, T. Hojo, M. Koyama, S. Hiromoto, E. Akiyama, In situ 2D Mapping of Hydrogen Entry into an Fe Sheet under a Droplet of NaCl Solution Using a Hydrogenochromic Sensor, Int. J. Hydrog. Energy, 47, 38468 (2022).