Neutron imaging quantifying hydrogen diffusion and distribution in metallic materials - a review

Neutron imaging (NI) has emerged as a pivotal non-destructive characterization technique, leveraging its exceptional penetration through heavy metals and high sensitivity to light elements such as hydrogen and lithium. These unique properties render NI indispensable for the quantitative assessment of hydrogen in metallic materials, where hydrogen accumulation can significantly degrade mechanical performance. In this context, in-situ experimental setups capable of precise control over temperature, gas environment, and mechanical stress enable real-time monitoring of hydrogen absorption, diffusion, and spatial distribution. Recent advancements in NI have achieved hydrogen detection with concentrations as low as 5-10 wppm and spatial resolutions on the order of ~10 μm. To overcome challenges associated with ultra-low hydrogen quantification, such as the relatively low neutron flux, optimized imaging approaches, including the black body grid method, have been developed, enhancing measurement precision and enabling hydrogen concentration evolution to be resolved at the micrometer scale. This review highlights the latest developments in NI for hydrogen quantification, focusing on applications in structural metallic alloys and solid-state hydrogen storage materials, and discusses strategies to further improve spatial resolution, sensitivity, and experimental accuracy.