Plasma-Nitrided Barrier Layers against Hydrogen Permeation in Pure Iron

Abstract

Plasma nitriding was performed to suppress hydrogen uptake and permeation in pure iron. The influence of the phase structure in the nitrogen compound layer on hydrogen uptake and permeation behaviors was examined. The phase structure in the nitrogen compound layer could be controlled by changing the gas composition in N₂-H₂ plasma. The nitrogen compound layers consisting of ε-Fe_2-3N and γ'-Fe₄N with a thickness of 4.9 µm, and of γ'-Fe₄N with a thickness of 2.0 µm were formed by plasma nitriding in this study. During the hydrogen permeation tests, plasma nitriding resulted in small permeation current values compared to the as-polished specimen when it reached the steady state. Especially, hydrogen uptake and permeation for the specimen with the nitrogen compound layer consisting of ε-Fe_2-3N and γ'-Fe₄N were extremely suppressed. Hydrogen uptake and permeation behaviors for the plasma-nitrided pure iron specimens are discussed from the perspective of the catalytic activity for hydrogen evolution and hydrogen diffusivity in the nitrogen compound layers.