Effects of post-heat treatment on hydrogen ingress resistance and interfacial integrity of Al-based coatings formed on steels for hydrogen infrastructure

This study investigates the effectiveness of an aluminum powder-coating and subsequent heat treatment in achieving a balance between hydrogen ingress resistance and strength properties in various metallic substrates, specifically S45C, SCM435, FC200, and SUS304 steels. Specimens were prepared from four types of steel and uniformly coated with aluminum to a depth of 20mm with an Al concentration of 50%. The coated S45C, SCM435, and FC200 were subjected to either quenching or quenching followed by tempering, while SUS304 received solution heat treatment. Hydrogen permeation measurements were conducted using a thermal desorption analyzer TDA after exposing the coated specimens to hydrogen gas at 100MPa and 85°C for 24h. The hydrogen content was compared between coated specimens and uncoated base materials to determine the hydrogen cut ratio. The results show that while the coating initially provided 100% hydrogen ingress resistance before the additional heat treatment, this resistance significantly decreased after subsequent quenching and tempering. Secondary Electron Microscope (SEM) observation revealed the formation of cavities in the upper part of the coated layer. This phenomenon was accompanied by Al diffusion toward the substrate, leading to the disappearance of the high-Al content layer. We conclude that the interfacial conditions and the presence of a high-Al-concentration layer are the major factors governing the hydrogen ingress resistance, and their degradation due to thermal stress and diffusion during the heat treatments is responsible for the reduced protective performance. These findings are crucial for optimizing coating-and-heat-treatment processes to ensure long-term integrity of metallic materials in aggressive hydrogen environments.