Hiroshi Kakinuma

Hydrogen, the key element for solving the major environmental issues, exhibits rapid diffusion and localization in lattice defects of metals, which leads to embrittlement. However, the microstructure-hydrogen interactions in metallic materials are poorly understood. Therefore, a technique for capturing the microstructure-dependent hydrogen diffusion in real-time has long been targeted by researchers. Here, we successfully visualized the preferential hydrogen diffusion at grain boundaries (GBs) o

The relationship between the change in the open-circuit potential (OCP) and the morphology of metastable pitting at Al–Fe–Si particles of AA1050 aluminum in 0.1 M NaCl was clarified. First, a metastable pit grew in the depth direction as the OCP decreased suddenly. Second, the pit started to grow on the Al surface, and the OCP remained low. Finally, the pit was repassivated with the reincrease of the OCP. In the early stage of OCP measurements of the electrode area of 1 cm 2 , the potential osci

The mechanism of trenching and pitting at intermetallic particles in AA1050 aluminum was investigated by open circuit potential measurements and microscopic polarization measurements, and the role played by the two types of intermetallic particles, Al-Fe and Al-Fe-Si, in this process was determined. Trenching was observed only around the Al-Fe-Si particles in the naturally aerated 0.1 M NaCl solution. Under the anodic polarization of AA1050 in naturally aerated 0.1 M NaCl, a crystallographic pit

Abstract Hydrogen is one of the key elements for the economy based on renewable and sustainable resources. Because a very low concentration of hydrogen in metallic structural materials often causes hydrogen embrittlement and decreases their mechanical property, there is a great interest in the development of an inexpensive and versatile sensor for hydrogen in metals with high sensitivity and high spatial resolution. Here, real‐time visualization of atomic state hydrogen in a metal has been succe

The microscopic hydrogen diffusion in UNS S32750 super duplex stainless steel was analyzed using a polyaniline-nickel-based hydrogen video imaging system. The hydrogen flux in the body-centered-cubic ferrite (α) phases was larger than in the face-centered-cubic austenite (γ) phases, causing hydrogen diffusion from the α to γ phases. The order of the hydrogen flux was as follows: (high) the center of the α phases > near the α-γ boundaries in the α phases > near the α-γ boundaries in the γ phases

Local hydrogen entry under a NaCl droplet on Fe sheets was analysed by employing a hydrogenochromic sensor and electrochemical hydrogen permeation tests. Large crystallographic pits barely promoted hydrogen entry, because crystallographic pitting proceeded at potentials higher than the potential of the hydrogen evolution reaction. Hydrogen entry was accelerated because of acidification under an island-like rust layer. The rust layer became thick in its outer regions, mainly owing to severe corro

Measurements of the open circuit potential of AA1050 were performed in 10 mM CrO3, and the Al-matrix around the intermetallic particles was found to dissolve locally. Pre-immersion in 1 M NaOH promoted the dissolution of the Al-matrix around intermetallic particles, resulting in the improvement of pitting corrosion resistance in NaCl solutions. The pitting corrosion resistance provided by molybdate treatment is effectively improved by pre-immersion in NaOH. The maximum pit depth of the specimen

Trenching around intermetallic particles on AA1050 aluminum in 0.1 M NaCl at pH 6.0 was analyzed by in situ observations. Deep trenches, which become the initiation site for pitting, were formed around the Al–Fe–Si particles but not around the Al–Fe particles. The open-circuit potentials of the bulk intermetallic compounds and Al-matrix of AA1050 without intermetallic particles were measured. It was determined that the Al–Fe–Si and Al–Fe particles were cathodically polarized by the Al-matrix of

Hydrogen diffusion behaviour at the coherent and incoherent Σ3 grain boundaries (GBs) of pure polycrystalline Ni was observed in situ using a polyaniline-based hydrogen video imaging system. The hydrogen flux at the coherent Σ3 GBs, also referred to as twin boundaries, was found to be the same as that in the grain interior. In contrast, enhanced hydrogen diffusion was observed at the incoherent portions of the Σ3 GBs. Hydrogen diffusion was notably enhanced at the micro/nanoscale incoherent port

Structural metallic materials used in hydrogen gas or corrosive environments may suffer from loss of ductility owing to hydrogen atoms (hydrogen embrittlement). To design hydrogen-resistant metallic materials, it is crucial to elucidate the mechanism of hydrogen entry and diffusion. However, visualization of corrosion-induced hydrogen entry and microstructure-dependent hydrogen diffusion requires a highly sensitive hydrogen detection technique with high spatial and temporal resolutions. Hydrogen

Commercial purity aluminum alloys show high formability and high strength, and its application is expanded to infrastructure and battery materials. The aluminum alloys contain various types of intermetallic particles, and it is well known that the intermetallic particles have an important role in improving the mechanical properties. However, some of the particles can be a pit initiation site under corrosion environments. The intermetallic particles, such as Al 5 Fe 2 and Al 8 Fe 2 Si, show highe

Because of the high formability and high strength, Al-Fe alloys are used for ships, battery materials, and so on. The mechanical properties of Al-Fe alloys can be improved by controlling the shape and hardness of the intermetallic particles. However, it has been reported that some of the intermetallic particles become pit initiation sites. Because the electrode potential of the intermetallic particles, such as Al 3 Fe and Al 8 Fe 2 Si, is higher than that of Al matrix, oxygen reduction reaction