Real-Time 2D Mapping of Hydrogen Entry and pH Distribution inside a NaCl Droplet on an Fe Sheet Using a Hydrogenochromic Sensor
Authors: Kakinuma Hiroshi , Hiromoto Sachiko , Hojo Tomohiko , Ajito Saya , Koyama Motomichi , Akiyama Eiji 
Published: December 2024

Atmospheric corrosion on a steel is known to promote hydrogen entry into the steel. However, the mechanism of corrosion-induced hydrogen entry is still unclear, because it is difficult to detect the small amount of hydrogen entered steels due to corrosion. In previous literature, electrochemical hydrogen permeation tests with Devanathan-Stachurski cells have been conducted to detect corrosion-induced hydrogen entry. While the electrochemical hydrogen permeation tests can measure the corrosion-induced hydrogen permeation current with high sensitivity and high time resolution, the hydrogen distribution in the steels cannot be measured. It has been challenging to analyze the two-dimensional information of corrosion-induced hydrogen entry. In this study, a hydrogenochromic sensor consisting of the polyaniline (PANI) and Ni layers was used to visualize the hydrogen distribution in an Fe sheet under corrosion conditions.
A PANI layer polymerized on a metal surface reacts with atomic state hydrogen in the metal, resulting in the color change from blue or purple to yellow or transparent.1,2 The intermediate Ni layer between the metal surface and the PANI layer works as a catalysis of the hydrogenation reaction of the PANI layer.1 Therefore, the small amounts of hydrogen that entered the metals can be visualized using the hydrogenochromic sensor consisting of the Ni and PANI layers.3
–6 In this study, the Ni layer was plated on one side of an Fe sheet by current-static polarization at –30 A m–2 for 180 s in the Watts bath at 333 K. In addition, the PANI layer was electrically polymerized on the Ni layer at a constant voltage of 1 V for 200 s in a 0.5 M H2SO4-0.5 M aniline solution. A droplet of 0.1 M NaCl containing thymol blue was placed on the other side of the specimen (bare Fe surface). The color of thymol blue is known to be changed by the increase in pH from neutral to alkaline. Thus, the hydrogen entry behavior and pH change in the droplet were analyzed simultaneously by observing the droplet and hydrogenochromic sensor with digital cameras.
Large crystallographic pits occurred in the initial stage of corrosion, but hydrogen entry was hardly promoted by the crystallographic pits. Subsequently, a rust-formed area was observed. Because the hydrogen entry proceeded under the rust-formed area, acidification was thought to occur under the rust-formed area, causing the hydrogen evolution reaction. The pH on the non-corroded area was alkaline, suggesting that alkalization proceeded on the non-corroded area due to the oxygen reduction reaction. The edges of the rust-formed area were close to the alkalized area, resulting in the thickening of rust at the edges. After the disappearance of the droplet due to drying, hydrogen entry almost ceased. However, hydrogen entry was confirmed under the thick rust layer formed at the edge of the rust-formed area. It was found that hydrogen entry is likely to proceed under the thick rust layer, which contains concentrated NaCl solution, even after the disappearance of the droplet.

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 Assosciated 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).
4) H. Kakinuma, S. Ajito, T. Hojo, M. Koyama, E. Akiyama, In situ visualization of misorientation-dependent hydrogen diffusion at grain boundaries of pure polycrystalline Ni using a hydrogen video imaging system, Acta Mater., 263, 119536 (2024).
5) H. Kakinuma, S. Ajito, M. Koyama, E. Akiyama, Microscopic visualization of hydrogen diffusion in UNS S32750 super duplex stainless steel: Roles of crystal structure and grain size, Int. J. Hydrog. Energy, 59, 866-873 (2024).
6) H. Kakinuma, S. Hiromoto, T. Hojo, S. Ajito, M. Koyama, E. Akiyama, Mechanism of Local Hydrogen Entry into Fe sheets Induced by Atmospheric Corrosion: Significance of Potential, pH, and Rust Layer Thickness, Corros. Sci., 112043 (2024).