Effect of Surface Microstructure on the Hydrogen Permeation Behavior of Plasma-Nitrided AISI 4135 Steel

Abstract

Hydrogen embrittlement is a serious problem for high strength steels. The reduction of hydrogen uptake by the formation of surface layers with low hydrogen diffusivity is thought to be effective in preventing hydrogen embrittlement. We reported that the plasma-nitrided surface layer composed of ε-Fe2-3N and γ’-Fe4N phases strongly inhibited hydrogen diffusion into the pure iron matrix.1) In this study, we controlled the microstructure of surface nitrided layers of an AISI 4135 steel by plasma nitriding, and investigated the hydrogen permeation behavior.AISI 4135 steel sheets of 1 mm in thickness were used as the specimens. Prior to plasma nitriding, the specimen surfaces were polished using a diamond past down to 1 μm. The plasma nitriding treatments were conducted in nitrogen-hydrogen atmospheres at 600 Pa for 3.6 ks. The temperature of the specimen surface was kept at 450 °C. Two types of nitrogen compound layers were formed on the specimen surface by changing the mixing ratio of nitrogen and hydrogen gases. One was composed of ε-Fe2-3N and γ’-Fe4N phases (specimen A), and the other was done of only γ’-Fe4N phase (specimen B). The compound layers of specimen A and B were 10 μm and 1 μm in thickness, respectively. In addition, the thicknesses of the nitrogen solid solution layers of specimen A and B were estimated to be ca. 270 μm and 210 μm, respectively.To investigate the effect of the surface microstructure on the hydrogen permeation through the specimens, the hydrogen permeation test using a conventional Devanathan-Stachurski cell was carried out. The nitrided surface was arranged in the hydrogen detection side. A Pd thin film was deposited on the specimen surfaces in the hydrogen detection side. The hydrogen detection side was polarized at 0.18 V vs. SHE in 0.1 M NaOH fully deaerated by pure Ar gas. The hydrogen entry side was polarized at −0.4 V vs. SHE in 0.1 M H2SO4. The hydrogen permeation test of specimen A reveals that the rise transient of the hydrogen permeation current was delayed and the steady state permeation current was significantly reduced. On the other hand, the hydrogen permeation behavior of specimen B was almost similar to that of the as-polished specimen. These results indicate that the nitrogen compound layer composed of ε-Fe2-3N and γ’-Fe4N phases inhibits the hydrogen permeation. Particularly, the existence of ε-Fe2-3N phase with the hcp structure is thought to be effective in preventing the hydrogen permeation.1) Y. Sugawara, I. Muto, and N. Hara, ECS Trans, 75(29), 43 (2017).