Hydrogen Entry into Pure Iron Treated By Plasma Nitriding

Abstract

In order to prevent hydrogen embrittlement of steels, it is effective to decrease the amount of hydrogen entry into steel matrix under atmospheric corrosion environments. In this study, the nitrided layer was formed in the surface of pure iron by plasma nitriding to inhibit the hydrogen entry, and the hydrogen permeation tests were conducted to clarify the barrier effect of nitrided layer on the hydrogen entry into pure iron. Pure iron sheets of 1 mm thickness with 99.99 mass% purity were used as the specimens. The specimens were heat-treated at 1173 K for 18 ks and then furnace-cooled. The surfaces of the specimens were polished with diamond paste down to 1 μm. Plasma nitriding was performed in a nitrogen-hydrogen atmosphere composed of 75 % N2 and 25 % H2 for 3.6 ks at 673 K and at 600 Pa. The one side of the specimens was nitrided by masking another side. Figure 1a shows the cross-sectional image of the specimen etched with 3 % nital solution after the plasma nitriding. The layer with a different etching behavior from the matrix was observed in the surface region. The thickness of the surface layer was ca. 2.8 μm. Figure 1b shows the XRD patterns of the plasma-nitrided specimen. The peaks assigned to ε-Fe2-3N and γ ’-Fe4N were detected. The peaks of ε-Fe2-3N were larger than that of γ ’-Fe4N, suggesting that the surface layer was mainly composed of ε-Fe2-3N phase. In order to investigate the property of hydrogen entry in the surface of the nitrided specimen, the hydrogen permeation tests using the Devanathan-Stachurski cell1 were carried out. The hydrogen entry was conducted by galvanostatic polarization at −100 μA (i c) in deaerated boric-borate buffer solution with 0.01 M NaCl (pH 8.45) for 10.8 ks. The Pd-coated surface of the specimens in the hydrogen detection side was polarized at 0.2 V vs. SHE in deaerated 0.1 M NaOH solution. Figure 1c and 1d show time variation of the potential on the hydrogen entry side and the hydrogen permeation current density (i p), respectively. In the initial 30 s, the potential of both the untreated and the nitrided specimen drastically decreased to approximately the same value (ca. −0.75 V vs. SHE). In the case of the untreated specimen, the permeation current drastically increased to ca. 300 nAcm-2 in the period from 0.5 to 1.0 ks. After that, it gradually increased and reached to 450 nAcm-2 at 10.8 ks. On the other hand, the permeation current of the nitrided specimen slightly increased to 10 nAcm-2 in the period from 1.6 to 4.0 ks and stayed at ca. 10 nAcm-2 from 4.0 to 10.8 ks. The hydrogen permeation test revealed that the plasma nitriding treatment strongly inhibited the hydrogen entry into pure iron under the cathodic polarization. 1. M. A. V. Devanathan, and A. Stachurski, Proc. Roy. Soc. A, 270, 90 (1962). Figure 1