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Behaviour of hydrogen in gas nitrided iron studied by electrochemical permeation and desorption techniques
Corrosion Science 58 (2012) 260–266
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Behaviour of hydrogen in gas nitrided iron studied by electrochemical permeation and desorption techniques A. Gajek, Z. Wolarek, T. Zakroczymski ⇑ Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 18 November 2011 Accepted 30 January 2012 Available online 6 February 2012 Keywords: A. Iron B. Hydrogen permeation C. Hydrogen absorption
a b s t r a c t Hydrogen permeation through and desorption from cathodically charged iron membranes with nitrides c0 , (e + c0 ) and e layers have been studied. On the basis of the nonstationary permeation, the effective diffusion coefficients of hydrogen for the nitrided membranes, and hence the real diffusion coefficients in each nitride layer were evaluated. After cessation of charging, desorption rate of hydrogen was measured at both sides of the membrane. An analysis of the desorption rates enabled to distinguish the diffusible (mobile) hydrogen from the reversibly trapped one. Hydrogen trapping was enhanced by the c0 precipitates and especially by the nitrided compound layers. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nitriding is one of the important surface treatments predominantly used on steels to improve their hardness and wear resistance. On the other hand, nitriding, as a kind of modification of the chemical composition and structure of the surface/near surface region, can be also a potential way of increasing resistance of metals to the corrosive environments. This refers both to the corrosion consisting in the disintegration (oxidation) of a metal and to the corrosion caused by hydrogen (hydrogen embrittlement). As regards hydrogen embrittlement, the nitrided layers may influence both the entry of hydrogen beneath the metal surface and the transport of hydrogen inside the metal, and thereby they may reduce the absorption of hydrogen in the substrate metal. These effects were studied earlier with reference to nitrided iron [1,2] and carbon steels [3,4]. In the works [1,2], using the electrochemical permeation technique [5], the diffusivities and concentrations of the mobile (diffusible) hydrogen were evaluated. However, aside from the diffusible hydrogen, which occupies predominantly interstitial sites being the solid solution component, the majority of the absorbed hydrogen can be associated with structural defects such as dislocations, grain boundaries, non-metallic inclusions and other internal interfaces. These forms of hydrogen are commonly known as trapped hydrogen. As opposed to the diffusible hydrogen, the trapped hydrogen is not detectable in the permeating hydrogen flux but it can be determined by the electrochemical desorption technique [6,7],
⇑ Corresponding author. Fax: +48 22 343 33 33. E-mail address: [email protected] (T. Zakroczymski). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2012.01.031
applied to both sides of the previously hydrogen charged membrane. The interaction of hydrogen with the nitrided layer depends on its chemical composition and structure. In the case of iron and its alloys, depending on the method of nitriding (plasma or gaseous ammonia) and process conditions, the entire nitrided layer can include the interstitial solution of nitrogen in ferrite matrix, Fe4N nitride (phase c0 ), Fe2–3N nitride (phase e), and their mixtures. Therefore, studying the effect of nitriding on the entry and absorption of hydrogen, the nitrided layer should be well defined. In the present work an effort was made to investigate the entry and absorption of hydrogen in the nitrided iron with different layer of precise chemical composition and phase structure. It was achieved by using a highly controlled gas nitriding treatment NitregÒ.
2. Experimental 2.1. Materials The initial material was a commercial, hot-drawn, 20 mm diameter rod of Armco iron. Its chemical composition was as follows: C 0.020, Mn 0.097, Si 0.009, P 0.021, Cr 0.009, Ni 0.028, Cu 0.067, Al 0.041 wt.%, Fe balance. Plate specimens, 3 mm in thickness, were machined out from the rod perpendicularly to its axis. The specimens were one side nitrided using an advanced gaseous ammonia nitriding technology NitregÒ [8]. By controlling the furnace atmosphere, qualitatively described by the nitrogen potential N p ¼ pNH3 =ðpH2 Þ3=2 [9], it was possible to produce the nitrided layer with controlled chemical and phase composition and thickness. In this
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way, four different nitriding treatments were performed and, consequently, the following four modified surface layers were produced: I. A single, relatively thick, diffusion zone with the dispersed c0 nitride (F4N) precipitates in the metal matrix – designation: c0prec . II. A double layer consisting of a relatively thin outer, monophase layer of c0 nitride and an inner diffusion zone with c0 nitride precipitates – designation: (c0 ) + c0prec . III. A double layer consisting of an outer double-phase layer composed of e and c0 nitrides and the inner diffusion zone with c0 nitride precipitates – designation: (e + c0 ) + c0prec . IV. A triple layer consisting of an outer, porous compound layer of e nitride, below the sublayer of e and c0 nitrides, and the diffusion zone with c0 nitride precipitates – designation: (e) + (e + c0 ) + c0prec . The nitriding conditions, the phase compositions and thickness of the nitride layers (sublayers) and the diffusion zone of c0 nitride precipitates are given in Table 1, whereas their microstructure is shown in Fig. 1.
2.2. Electrochemical permeation and desorption measurements The nitrided plate specimens were abraded from the unnitrided side to the thickness of L = 2 mm and they were used as membranes in the electrochemical permeation and desorption measurements. In the permeation measurements, the nitrided side worked as the entry side (X = 0) and it was cathodically polarised in an aqueous 0.1 M NaOH solution at a constant current density ic = 10 mA/cm2. Hydrogen, which entered into the membrane, diffused through it and desorbed at its opposite exit side (X = L), was immediately electrochemically oxidized at a constant electrode potential of 0.15 VHg|HgO|0.1 M NaOH (0.32 VNHE). The recorded anodic current density was a sensitive measure of the permeation rate of hydrogen through the membrane (ip). After about 24 h, when the permeation rate achieved practically a steady-state, the partial build-up transient was measured as a result of sudden change of the cathodic current density ic from 10 mA/cm2 to 20 mA/cm2. As it was proved earlier [10], in contrast to the first build-up recorded during charging of the hydrogen-free membrane, during the partial build-up transient slow surface processes (involving hydrogen entry) and hydrogen trapping (involving hydrogen transport) are not revealed. Under these conditions
Table 1 Nitriding parameters and phase composition and thickness of the nitrided layers on iron.
Nitriding atmosphere Nitriding potential, Np (atm1/2) Temperature (oC) Time of nitriding (h) Diffusion zone – c0 precipitates (mm) layer (lm) (e + c0 ) layer (lm) (e) layer (lm) Entire layer of nitrides (lm)
Type of nitriding I
II
III
IV
20% NH, 80% NH3,dis 0.5 570 4 0.35–0.4 – – – –
25% NH3, 75% NH3,dis 0.9 560 8 0.2–0.3 9 – – 9
37% NH3, 63% NH3,dis 0.95 570 8 0.2–0.3 – 11 – 11
100% NH3 2.6 570 6 0.4–0.5 – 8 5 13
Fig. 1. Microstructure of the near-surface region of Armco iron samples after their gas nitriding under various, strictly controllable conditions.
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and with reference to the uniform membrane of thickness L, such as the unnitrided iron membrane, the measured permeation rate of hydrogen is fully controlled by its lattice diffusion inside the membrane, and the normalised permeation build-up can be described by the equation [11] o
1 2L X ð2n þ 1Þ2 L2 exp 1 o ¼ pffiffiffiffiffiffiffiffiffi 4Dt ip ip pDt n¼0
ip ip
! ð1Þ o
where ip is the measured permeation rate at time t, ip is the initial 1 steady-state permeation rate (t = 0), and ip is the new steady-state permeation rate (t ? 1). The fitting of the Eq. (1) to the experimental permeation transients (by means of the least square method) leads to the determination of the lattice diffusion coefficient of hydrogen D. The same procedure was applied to the nitrided membranes. In this case, however, the resulted value of D, should be considered as the effective diffusion coefficient Deff, characterizing the transport of hydrogen through the multilayer membrane taken as a whole. The presentation of changes in the permeation rate in the normalised form (1) enables to compare the permeation curves of hydrogen for various membranes. When the partial build-up transient was finished, the cathodic current was changed back to its previous density of 10 mA/cm2 and the hydrogen charging was continued up to 6 days. Then, the hydrogen charging was stopped and the membrane entry side was polarised immediately at the same potential of 0.15 VHg|HgO|0.1 M NaOH as the exit side. From this moment, the anodic currents were recorded at both the exit side (ia,L) and the entry side (ia,0). The current ia,L practically corresponded to the oxidation of hydrogen iH,L, whereas the current ia,0 comprised the partial current used to oxidise the desorbing hydrogen (iH,0) and the partial current used to oxidise the iron (iMe,0), i.e. the background current. The current iMe,0 was determined in the separate experiment on a hydrogen-free membrane. Integration of the desorption rates leads to the amount of hydrogen absorbed by the membrane during its charging. For comparison, measurements of permeation/desorption of hydrogen were also carried out for the unnitrided membranes. All experiments were carried out at 30 °C. 3. Results 3.1. Hydrogen permeation data Changes in hydrogen permeation during the first charging of the unnitrided and various nitrided membranes are shown in Fig. 2. The logarithmic scale of time was chosen to emphasize the effect of the nitrided layers during the early stage of charging., The so-called breakthrough time tb, after which a first portion of hydrogen reached the membrane exit side, was for the nitrided membranes longer than that for the unnitrided membrane. Moreover, the more complex nitrided layer the longer breakthrough time. Especially the large delay (about 2500 s) in detection of hydrogen was observed for the membrane comprising the outer, porous compound layer of e nitride. Similar remarks one can apply to the level of hydrogen permeation, especially after a long time of cathodic charging (Fig. 2). The rate of hydrogen permeation through the membrane, which had only precipitations of the c0 nitride, was similar to that observed for the unnitrided membrane (Fe). For these both membranes, the permeation rate systematically increased with time and after a few dozen hours it was much higher than that observed after a few hours (Fig. 2). Such behaviour as a result of a long-lasting cathodic polarization, observed earlier for iron membranes [10,12], reflects something of the kind of activation of the metal surface for hydrogen entry. On the contrary, this effect was not observed in the case of the others membranes with the continuous
Fig. 2. Changes in the hydrogen permeation rate through the unnitrided (Fe) and nitrided (different nitrides layers) iron membranes with charging time. Arrows indicate the breakthrough time tb".
outer layer of nitrides. For these membranes, the hydrogen permeation rate reached approximately constant and relatively low value after several hours of cathodic charging. Hydrogen permeated especially slowly through the membrane with the outer porous layer of e nitrides. In this case, the steady state permeation rate was about 3 orders of magnitude lower than the suitable permeation rate through the unnitrided membrane. Fig. 3 shows the normalised experimental partial build-up transients (scatter symbols) and the best fitted transients (lines), obtained by fitting the Eq. (1) to the experimental data. The more complex nitrided layer the longer time to reach the new steady state permeation, and hence the slower transport of hydrogen through the membrane. The values of diffusion coefficients of hydrogen in the unnitrided (D) and nitrided (Deff) membranes, resulted from the above fitting procedure and taking into account the geometrical membrane thickness L = 2 mm, are compared in Fig. 4. 3.2. Hydrogen desorption data The desorption rate of hydrogen from the unnitrided and nitrided membranes at their both sides, recorded after the finishing of hydrogen charging, are shown in Figs. 5 and 6, for the exit (X = L)
Fig. 3. The normalized, experimental partial permeation build-up transients (scatter symbols) and the best fitted transients (lines) for the unnitrided (Fe) and nitrided iron membranes. Arrows indicate the breakthrough time tb".
A. Gajek et al. / Corrosion Science 58 (2012) 260–266
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and entry (X = 0) side, respectively. It is noteworthy that the complete desorption of hydrogen required at least 100 ks, but no longer than about 500 ks. Thus, the applied charging time of about 6 days (520 ks) was sufficient to saturate the membranes with hydrogen. Comparing the hydrogen charging, represented by the permeation build-up curves (Fig. 2), with the desorption of hydrogen at the membrane exit side (Fig 5) one note, that these processes were not reversible. In particular, the breakthrough times tb" (arrows in Fig. 2) were distinctly longer than their equivalents tb;, i.e. the times after which the desorption rate begins to fall (arrows in Fig. 5). Moreover, the times tb; were close to the breakthrough times revealed on the partial build-up transients for the precharged membranes (Fig. 3). 4. Discussion 4.1. Effective and real hydrogen diffusivities
Fig. 4. The diffusion coefficient of hydrogen in the unnitrided (D) and in the nitrided (Deff) iron membranes.
Fig. 5. Desorption of hydrogen at the exit (X = L) side of the unnitrided (Fe) and nitrided (different nitrides layers) iron membranes. Arrows indicate the equivalents of the breakthrough time tb;.
To characterize the transport (diffusion) of hydrogen in a given material, in the first place the diffusion paths and the mobility of hydrogen on these paths must be determined. For the uniform, unnitrided iron membranes, the experimental partial build-up curves overlapped with the fitted ones (Fig. 3). It indicates that the conditions: D = const and L = const, assumed in Eq. (1), were fulfilled. The obtained value of diffusion coefficient D = 8.2 105 cm2/s (Fig. 4) was in accordance with reliable literature data [13] and it can be regarded as a real value of the lattice diffusion coefficient of hydrogen in iron (ferrite). Moreover, one may conclude that all diffusion paths through the unnitrided membrane had the same length equal to the membrane thickness L = 2 mm. In the case of nitrided, multilayer membranes with complex structure (Fig. 1), the obtained values of the hydrogen diffusion coefficient (Fig. 4) should be considered as the effective diffusion coefficient Deff, characterizing the transport of hydrogen through the membrane taken as a whole. However, assuming the continuity of hydrogen flux, i.e. that there were no barriers for hydrogen transport between the sublayers, it is possible to evaluate the real diffusion coefficient of hydrogen in the individual sublayers of the nitrided membranes. 1 Under the steady state, the same flux of hydrogen ip flows through the whole membrane as well as through each of sublayers 1
ip ¼
Deff DCF Di DC i F ¼ L Li
where DC is the total concentration gradient of hydrogen in the whole membrane, DCi is a gradient of hydrogen in a sublayer ‘‘i’’ with thickness Li and the diffusion coefficient Di, and F is Faraday constant. Since DC is the sum of DCi, the expression (2) can be transformed into the equation
X Li L ¼ Deff Di
ð2Þ
(a) Membrane I – with c0prec zone
Fig. 6. Desorption of hydrogen at the previously entry (X = 0) side of the unnitrided (Fe) and nitrided (different nitrides layers) iron membranes.
This two-layer membrane included the unnitrided iron zone and the diffusion zone of c0 nitride precipitations. Substituting L = 2 mm, LFe = 1.6 mm, and Lc0prec = 0.4 mm (Table 1) and Deff = 2.2 105 cm2/s and DFe = 8.2 105 cm2/s mm (Fig. 4) to Eq. (3), the diffusion coefficient of hydrogen in the c0prec zone Dc0prec = 5.6 106 cm2/s was obtained. The structure of the diffusion zone was heterogeneous – the concentration of c0prec was the highest near the surface and it gradually decreased until zero at the end of the zone. Therefore, the above value of Dc0prec should be still treated as the average value characterizing the transport of hydrogen through the whole c0 zone.
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The analysis of hydrogen transport in the c0 precipitations zone was described in details in the previous work [14]. This analysis was based on the original approach proposed earlier to characterize hydrogen transport in a duplex stainless steel [15]. It was proved that hydrogen bypasses the precipitations and moves only through the ferrite matrix with the same velocity as observed in the iron. In consequence, the real diffusion paths have various lengths and they are longer than the geometrical thickness of the diffusion zone. Undoubtedly in this work, various length of the diffusion paths was the reason for rather poor fitting of the model equation (1) to the experimental build-up transient (Fig. 3, curve designated as c0prec ). One can see that at first the experimental build-up transient (Fig. 3, curve c0prec ) was ahead of the fitted curve, whereas at the end the experimental curve lagged behind the fitted one. Using the suitable fitting procedure [14,15], the shortest diffusion path of hydrogen through the whole membrane Lmin = 3.2 mm and the longest one Lmax = 5.0 mm were evaluated. In turn, taking into account that the real path of diffusion through the unnitrided iron substrate was equal the thickness of this layer (LFe = 1.6 mm), the elongation of the diffusion path must have occurred only in the c0 precipitation zone. Thus, in this approximately 0.4 mm thick zone (Table 1) the shortest diffusion path had 1.6 mm and the longest one had 3.4 mm.
side of the membrane, C0. In the case of unnitrided iron membrane, the lattice diffusion coefficient of hydrogen D is constant and the concentration of hydrogen C0 is
(b) Membrane II – with (c0 ) + c0prec
C0 ¼
In this case the membrane was composed of three layers. It can be considered as the previous membrane I with an added continuous layer of the c0 nitride layer (Fig. 1). By substitution of suitable values of L, Li, Deff and Di, including Dc0prec = 5.6 106 cm2/s, determined for the membrane I, the value of the diffusion coefficient of hydrogen in the c0 nitride layer D(c0 ) = 9.7 108 cm2/s was obtained.
This equation may be adapted for the nitrided membranes by replacing the lattice diffusion coefficient D with the effective diffusion coefficient Deff
Fig. 7. The real diffusion coefficient of hydrogen in the unnitrided iron (Fe) and in the layers of different iron nitrides.
1
(c) Membrane III – with (e + c0 ) + c0prec
Using the same procedure as that used for the three-layer membrane II, the diffusion coefficient of hydrogen in the (e + c0 ) nitride layer D(e+c0 ) = 1.0 107 cm2/s was evaluated. (d) Membrane IV – with (e) + (e + c0 ) + c0prec
If this membrane is treated as three-layer membrane, the average diffusion coefficient of hydrogen in the coupled nitride layer D(e)+(e+c0 ) = 1.3 108 cm2/s was obtained. However, considering four layers in the membrane and assuming that the diffusion coefficient of hydrogen in the inner (e + c0 ) sublayer is the same as that determined for the outer (e + c0 ) sublayer in the membrane III, the diffusion coefficient of hydrogen in the outer e nitride layer D(e) = 5.4 109 cm2/s was evaluated. The comparison of the evaluated diffusion coefficients, characterizing the transport of hydrogen in a given sublayers (zones) of the nitrided iron is shown in Fig. 7.
FDip L
ð3Þ
1
C0 ¼
FDeff ip L
ð4Þ
Using Eqs. (4) and (5) the concentration of hydrogen beneath the entry side of the unnitrided and nitrided membranes, respectively, were calculated (Table 2). The concentration C0 can be considered as a measure of the intensity of hydrogen entry into the metal under given charging condition. In the case of the nitrided membranes, depending on the type of nitriding, the concentration C0 characterizes the entry of hydrogen into the outer nitrided layer. It can be concluded from Table 2 that the presence of c0 precipitates in iron virtually does not affect the entry of hydrogen, whereas the entry of hydrogen into the layers of nitrides, especially into the e nitride, was significantly less intensive. 4.3. Different forms of absorbed hydrogen and their amounts The results of the non-stationary permeation measurements (Fig. 3) indicate that times needed to achieve the steady state permeation rate of hydrogen ranged from about 200 s for the unnitrided membrane to about 10 ks for the nitrided membrane with the Table 2 Characteristics of the diffusible hydrogen in the unnitrided iron membrane (Fe) and in the nitrided membranes with different nitrides layer (I–IV). Hydrogen charging conditions: 0.1 M NaOH, ic = 10 mA/cm2. Fe
4.2. Intensity of hydrogen entry
Structure of the nitrided layer I
In a steady-state (before the desorption), the permeation rate of 1 hydrogen (ip ) through the membrane is proportional to the concentration gradient (DC) and the diffusion coefficient of hydrogen in the membrane (D or Deff), and inversely proportional to the membrane thickness (L). Since using the electrochemical technique for the detection of hydrogen CL = 0, the concentration gradient of hydrogen can be replaced by its concentration beneath the entry
II (c0 )
III (e + c0 )
IV (e + c0 )
c0 prec
c0 prec
c0 prec
c0 prec
c0 prec
2 i1 p (lA/cm )
21.0
6.1
0.56
0.46
0.033
D (cm2/s) Deff (cm2/s)
8.2 105 –
– 2.2 10-5
– 1.2 10-
– 1.1 10-
– 1.8 10-
5
5
6
C0 (mol H/ cm3)
5.3 107
5.7 107
9.7 10-
8.7 10-
3.8 10-
8
8
8
A. Gajek et al. / Corrosion Science 58 (2012) 260–266
Fig. 8. Amounts of the diffusible hydrogen (qHd), the trapped hydrogen (qHt), and their sum (qH) in the unnitrided (Fe) and nitrided (I–IV) iron membranes.
(e + (e + c0 ) + c0prec layers. However, the time needed for the complete desorption of hydrogen from the membranes was much longer – at least 100 ks (Figs. 5 and 6). Therefore, it is obvious that the measured desorption rate related not only to the diffusible hydrogen, which left the membrane much faster, but also to the hydrogen originating from reversible traps. Integration of the individual desorption rate curve in Fig. 5 gives the amount of hydrogen qH,L that left the membrane its exit side, whereas integration of the suitable curve in Fig. 6 gives the amount of hydrogen qH,0, which left the membranes its entry side. The sum qH,L + qH,0 corresponds to the total amounts of absorbed hydrogen in the chosen membrane before the desorption, qH. The amount of the diffusible (lattice) hydrogen in the unnitrided membrane can be determined from permeation data using the following expression [6] 1
qHd ¼
ip L2 2DF
ð5Þ
and in the nitrided membrane 1
qHd ¼
ip L2 2Deff F
ð6Þ
Subtraction qHd from qH gives the amount of the reversible trapped hydrogen, qHt.
Fig. 9. Amounts of the trapped hydrogen desorbed from the unnitrided (Fe) and nitrided (I–IV) iron membranes at their exit (qHt,L) and entry (qHt,0) side.
265
Fig. 10. The contribution of the membrane exit and entry side to the desorption of the trapped hydrogen from the unnitrided (Fe) and nitrided (I–IV) iron membranes.
Amounts of the diffusible and trapped hydrogen in the studied membranes are compared in Fig. 8. First of all, the dominant participation of the trapped hydrogen attracts notice. The participation of the diffusible hydrogen was much lower, and for the membrane with the e nitride layer almost negligible. Another noticeable fact is that the amount of the trapped hydrogen for the membrane with the diffusion zone of c0 nitride precipitates (I) was higher than that in the unnitrided iron (Fe). Undoubtedly, this enhanced trapping of hydrogen was produced by the nitride precipitates–ferrite matrix interfaces. As it was shown earlier [14], these interfaces, apart from dislocations and grain boundaries, are additional effective traps for hydrogen. In turn, the amounts of the trapped hydrogen in the membranes with the phase nitrides layers (II–IV) were lower than that for the unnitrided membrane, but they were still significant. The above balance of the hydrogen absorption (Fig. 8) does not inform about distribution of the trapped hydrogen in the cross-section of the membrane. Relevant information may be obtained by comparison of the amounts of hydrogen escaping from the membranes their opposite sides. The suitable data shown in Fig. 9 indicate that in each case much more hydrogen escaped from the membrane its entry side (qHt,0) than from its exit side (qHt,L). For the nitrided membranes, this effect was the greater the more complex structure of the nitrided layer. For the nitrided membrane with the porous e phase, virtually the entire amount of hydrogen originated from traps desorbed from the membrane its entry side. The involvement of the individual membrane sides in the desorption of trapped hydrogen is even more depicted in Fig. 10, showing values of the ratios of the amount of trapped hydrogen escaping from the exit (qHt,L) and entry (qHt,0) side to the total amount of trapped hydrogen (qHt). As it is known, for the linear gradient of hydrogen concentration in the membrane (from C0 at the entry side to CL = 0 at the exit side), 1/3 of the total amount of hydrogen should escape at the exit side, and 2/3 at the entry side [6]. Generally, these predicted ratios were not valid with regard to the trapped hydrogen – the ratio qHt,L/qHt was lower than 1/3, while the ratio qHt,0/qHt adequately higher than 2/3 (Fig. 10). This means that hydrogen identified as the trapped hydrogen was distributed in the membrane near its entry side. Moreover, with regard to the nitrided membranes with the phase nitride layers (II–IV), the trapped hydrogen was practically accumulated only in those layers. It is worthy to note that relatively small amounts of hydrogen escaping from the nitrided membranes their exit sides indicate that the amounts of hydrogen absorbed in the unnitrided iron substrate were small, almost negligible in the case of the
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membrane IV with the layer of e nitride. Therefore, the nitrided layers on iron, despite their strong tendency for hydrogen trapping, protect the metal substrate from hydrogen. 5. Conclusions 1. Nitrided layers on iron affect the entry, and thus the subsurface concentration of hydrogen, as well as the transport (diffusion) and trapping of hydrogen. These effects strongly depended on the type (structure) of nitrided layers. 2. The outer, porous e nitride layer was the most effective in hindrance of hydrogen entry and hydrogen transport, but at the same time it was very capable of hydrogen trapping. For the above reasons, the c0 and (e + c0 ) nitride layers were slightly less effective. 3. The least effects were exerted by the layer containing only c0 nitride precipitates. Intensity of hydrogen entry into this layer was practically the same as into the unnitrided iron. Although the effective diffusion coefficient of hydrogen in this layer was lower than that in iron, but this effect was caused by longer diffusion paths. Hydrogen trapping was somewhat higher owing to the c0 nitride–ferrite interfaces. 4. Thus, nitriding of iron may be an effective method to prevent hydrogen corrosion (embrittlement) of iron (steel), provided that a continuous layer of iron nitrides, preferably with an outer layer of e nitride, is formed on the metal surface. Acknowledgments The authors are grateful for the financial support of this work by the Ministry of Science and Higher Education, Republic of Poland, Research Project No. 3 T08C 050 30.
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Behaviour of hydrogen in gas nitrided iron studied by electrochemical permeation and desorption techniques A. Gajek, Z. Wolarek, T. Zakroczymski ⇑ Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 18 November 2011 Accepted 30 January 2012 Available online 6 February 2012 Keywords: A. Iron B. Hydrogen permeation C. Hydrogen absorption
a b s t r a c t Hydrogen permeation through and desorption from cathodically charged iron membranes with nitrides c0 , (e + c0 ) and e layers have been studied. On the basis of the nonstationary permeation, the effective diffusion coefficients of hydrogen for the nitrided membranes, and hence the real diffusion coefficients in each nitride layer were evaluated. After cessation of charging, desorption rate of hydrogen was measured at both sides of the membrane. An analysis of the desorption rates enabled to distinguish the diffusible (mobile) hydrogen from the reversibly trapped one. Hydrogen trapping was enhanced by the c0 precipitates and especially by the nitrided compound layers. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nitriding is one of the important surface treatments predominantly used on steels to improve their hardness and wear resistance. On the other hand, nitriding, as a kind of modification of the chemical composition and structure of the surface/near surface region, can be also a potential way of increasing resistance of metals to the corrosive environments. This refers both to the corrosion consisting in the disintegration (oxidation) of a metal and to the corrosion caused by hydrogen (hydrogen embrittlement). As regards hydrogen embrittlement, the nitrided layers may influence both the entry of hydrogen beneath the metal surface and the transport of hydrogen inside the metal, and thereby they may reduce the absorption of hydrogen in the substrate metal. These effects were studied earlier with reference to nitrided iron [1,2] and carbon steels [3,4]. In the works [1,2], using the electrochemical permeation technique [5], the diffusivities and concentrations of the mobile (diffusible) hydrogen were evaluated. However, aside from the diffusible hydrogen, which occupies predominantly interstitial sites being the solid solution component, the majority of the absorbed hydrogen can be associated with structural defects such as dislocations, grain boundaries, non-metallic inclusions and other internal interfaces. These forms of hydrogen are commonly known as trapped hydrogen. As opposed to the diffusible hydrogen, the trapped hydrogen is not detectable in the permeating hydrogen flux but it can be determined by the electrochemical desorption technique [6,7],
⇑ Corresponding author. Fax: +48 22 343 33 33. E-mail address: [email protected] (T. Zakroczymski). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2012.01.031
applied to both sides of the previously hydrogen charged membrane. The interaction of hydrogen with the nitrided layer depends on its chemical composition and structure. In the case of iron and its alloys, depending on the method of nitriding (plasma or gaseous ammonia) and process conditions, the entire nitrided layer can include the interstitial solution of nitrogen in ferrite matrix, Fe4N nitride (phase c0 ), Fe2–3N nitride (phase e), and their mixtures. Therefore, studying the effect of nitriding on the entry and absorption of hydrogen, the nitrided layer should be well defined. In the present work an effort was made to investigate the entry and absorption of hydrogen in the nitrided iron with different layer of precise chemical composition and phase structure. It was achieved by using a highly controlled gas nitriding treatment NitregÒ.
2. Experimental 2.1. Materials The initial material was a commercial, hot-drawn, 20 mm diameter rod of Armco iron. Its chemical composition was as follows: C 0.020, Mn 0.097, Si 0.009, P 0.021, Cr 0.009, Ni 0.028, Cu 0.067, Al 0.041 wt.%, Fe balance. Plate specimens, 3 mm in thickness, were machined out from the rod perpendicularly to its axis. The specimens were one side nitrided using an advanced gaseous ammonia nitriding technology NitregÒ [8]. By controlling the furnace atmosphere, qualitatively described by the nitrogen potential N p ¼ pNH3 =ðpH2 Þ3=2 [9], it was possible to produce the nitrided layer with controlled chemical and phase composition and thickness. In this
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way, four different nitriding treatments were performed and, consequently, the following four modified surface layers were produced: I. A single, relatively thick, diffusion zone with the dispersed c0 nitride (F4N) precipitates in the metal matrix – designation: c0prec . II. A double layer consisting of a relatively thin outer, monophase layer of c0 nitride and an inner diffusion zone with c0 nitride precipitates – designation: (c0 ) + c0prec . III. A double layer consisting of an outer double-phase layer composed of e and c0 nitrides and the inner diffusion zone with c0 nitride precipitates – designation: (e + c0 ) + c0prec . IV. A triple layer consisting of an outer, porous compound layer of e nitride, below the sublayer of e and c0 nitrides, and the diffusion zone with c0 nitride precipitates – designation: (e) + (e + c0 ) + c0prec . The nitriding conditions, the phase compositions and thickness of the nitride layers (sublayers) and the diffusion zone of c0 nitride precipitates are given in Table 1, whereas their microstructure is shown in Fig. 1.
2.2. Electrochemical permeation and desorption measurements The nitrided plate specimens were abraded from the unnitrided side to the thickness of L = 2 mm and they were used as membranes in the electrochemical permeation and desorption measurements. In the permeation measurements, the nitrided side worked as the entry side (X = 0) and it was cathodically polarised in an aqueous 0.1 M NaOH solution at a constant current density ic = 10 mA/cm2. Hydrogen, which entered into the membrane, diffused through it and desorbed at its opposite exit side (X = L), was immediately electrochemically oxidized at a constant electrode potential of 0.15 VHg|HgO|0.1 M NaOH (0.32 VNHE). The recorded anodic current density was a sensitive measure of the permeation rate of hydrogen through the membrane (ip). After about 24 h, when the permeation rate achieved practically a steady-state, the partial build-up transient was measured as a result of sudden change of the cathodic current density ic from 10 mA/cm2 to 20 mA/cm2. As it was proved earlier [10], in contrast to the first build-up recorded during charging of the hydrogen-free membrane, during the partial build-up transient slow surface processes (involving hydrogen entry) and hydrogen trapping (involving hydrogen transport) are not revealed. Under these conditions
Table 1 Nitriding parameters and phase composition and thickness of the nitrided layers on iron.
Nitriding atmosphere Nitriding potential, Np (atm1/2) Temperature (oC) Time of nitriding (h) Diffusion zone – c0 precipitates (mm) layer (lm) (e + c0 ) layer (lm) (e) layer (lm) Entire layer of nitrides (lm)
Type of nitriding I
II
III
IV
20% NH, 80% NH3,dis 0.5 570 4 0.35–0.4 – – – –
25% NH3, 75% NH3,dis 0.9 560 8 0.2–0.3 9 – – 9
37% NH3, 63% NH3,dis 0.95 570 8 0.2–0.3 – 11 – 11
100% NH3 2.6 570 6 0.4–0.5 – 8 5 13
Fig. 1. Microstructure of the near-surface region of Armco iron samples after their gas nitriding under various, strictly controllable conditions.
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and with reference to the uniform membrane of thickness L, such as the unnitrided iron membrane, the measured permeation rate of hydrogen is fully controlled by its lattice diffusion inside the membrane, and the normalised permeation build-up can be described by the equation [11] o
1 2L X ð2n þ 1Þ2 L2 exp 1 o ¼ pffiffiffiffiffiffiffiffiffi 4Dt ip ip pDt n¼0
ip ip
! ð1Þ o
where ip is the measured permeation rate at time t, ip is the initial 1 steady-state permeation rate (t = 0), and ip is the new steady-state permeation rate (t ? 1). The fitting of the Eq. (1) to the experimental permeation transients (by means of the least square method) leads to the determination of the lattice diffusion coefficient of hydrogen D. The same procedure was applied to the nitrided membranes. In this case, however, the resulted value of D, should be considered as the effective diffusion coefficient Deff, characterizing the transport of hydrogen through the multilayer membrane taken as a whole. The presentation of changes in the permeation rate in the normalised form (1) enables to compare the permeation curves of hydrogen for various membranes. When the partial build-up transient was finished, the cathodic current was changed back to its previous density of 10 mA/cm2 and the hydrogen charging was continued up to 6 days. Then, the hydrogen charging was stopped and the membrane entry side was polarised immediately at the same potential of 0.15 VHg|HgO|0.1 M NaOH as the exit side. From this moment, the anodic currents were recorded at both the exit side (ia,L) and the entry side (ia,0). The current ia,L practically corresponded to the oxidation of hydrogen iH,L, whereas the current ia,0 comprised the partial current used to oxidise the desorbing hydrogen (iH,0) and the partial current used to oxidise the iron (iMe,0), i.e. the background current. The current iMe,0 was determined in the separate experiment on a hydrogen-free membrane. Integration of the desorption rates leads to the amount of hydrogen absorbed by the membrane during its charging. For comparison, measurements of permeation/desorption of hydrogen were also carried out for the unnitrided membranes. All experiments were carried out at 30 °C. 3. Results 3.1. Hydrogen permeation data Changes in hydrogen permeation during the first charging of the unnitrided and various nitrided membranes are shown in Fig. 2. The logarithmic scale of time was chosen to emphasize the effect of the nitrided layers during the early stage of charging., The so-called breakthrough time tb, after which a first portion of hydrogen reached the membrane exit side, was for the nitrided membranes longer than that for the unnitrided membrane. Moreover, the more complex nitrided layer the longer breakthrough time. Especially the large delay (about 2500 s) in detection of hydrogen was observed for the membrane comprising the outer, porous compound layer of e nitride. Similar remarks one can apply to the level of hydrogen permeation, especially after a long time of cathodic charging (Fig. 2). The rate of hydrogen permeation through the membrane, which had only precipitations of the c0 nitride, was similar to that observed for the unnitrided membrane (Fe). For these both membranes, the permeation rate systematically increased with time and after a few dozen hours it was much higher than that observed after a few hours (Fig. 2). Such behaviour as a result of a long-lasting cathodic polarization, observed earlier for iron membranes [10,12], reflects something of the kind of activation of the metal surface for hydrogen entry. On the contrary, this effect was not observed in the case of the others membranes with the continuous
Fig. 2. Changes in the hydrogen permeation rate through the unnitrided (Fe) and nitrided (different nitrides layers) iron membranes with charging time. Arrows indicate the breakthrough time tb".
outer layer of nitrides. For these membranes, the hydrogen permeation rate reached approximately constant and relatively low value after several hours of cathodic charging. Hydrogen permeated especially slowly through the membrane with the outer porous layer of e nitrides. In this case, the steady state permeation rate was about 3 orders of magnitude lower than the suitable permeation rate through the unnitrided membrane. Fig. 3 shows the normalised experimental partial build-up transients (scatter symbols) and the best fitted transients (lines), obtained by fitting the Eq. (1) to the experimental data. The more complex nitrided layer the longer time to reach the new steady state permeation, and hence the slower transport of hydrogen through the membrane. The values of diffusion coefficients of hydrogen in the unnitrided (D) and nitrided (Deff) membranes, resulted from the above fitting procedure and taking into account the geometrical membrane thickness L = 2 mm, are compared in Fig. 4. 3.2. Hydrogen desorption data The desorption rate of hydrogen from the unnitrided and nitrided membranes at their both sides, recorded after the finishing of hydrogen charging, are shown in Figs. 5 and 6, for the exit (X = L)
Fig. 3. The normalized, experimental partial permeation build-up transients (scatter symbols) and the best fitted transients (lines) for the unnitrided (Fe) and nitrided iron membranes. Arrows indicate the breakthrough time tb".
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and entry (X = 0) side, respectively. It is noteworthy that the complete desorption of hydrogen required at least 100 ks, but no longer than about 500 ks. Thus, the applied charging time of about 6 days (520 ks) was sufficient to saturate the membranes with hydrogen. Comparing the hydrogen charging, represented by the permeation build-up curves (Fig. 2), with the desorption of hydrogen at the membrane exit side (Fig 5) one note, that these processes were not reversible. In particular, the breakthrough times tb" (arrows in Fig. 2) were distinctly longer than their equivalents tb;, i.e. the times after which the desorption rate begins to fall (arrows in Fig. 5). Moreover, the times tb; were close to the breakthrough times revealed on the partial build-up transients for the precharged membranes (Fig. 3). 4. Discussion 4.1. Effective and real hydrogen diffusivities
Fig. 4. The diffusion coefficient of hydrogen in the unnitrided (D) and in the nitrided (Deff) iron membranes.
Fig. 5. Desorption of hydrogen at the exit (X = L) side of the unnitrided (Fe) and nitrided (different nitrides layers) iron membranes. Arrows indicate the equivalents of the breakthrough time tb;.
To characterize the transport (diffusion) of hydrogen in a given material, in the first place the diffusion paths and the mobility of hydrogen on these paths must be determined. For the uniform, unnitrided iron membranes, the experimental partial build-up curves overlapped with the fitted ones (Fig. 3). It indicates that the conditions: D = const and L = const, assumed in Eq. (1), were fulfilled. The obtained value of diffusion coefficient D = 8.2 105 cm2/s (Fig. 4) was in accordance with reliable literature data [13] and it can be regarded as a real value of the lattice diffusion coefficient of hydrogen in iron (ferrite). Moreover, one may conclude that all diffusion paths through the unnitrided membrane had the same length equal to the membrane thickness L = 2 mm. In the case of nitrided, multilayer membranes with complex structure (Fig. 1), the obtained values of the hydrogen diffusion coefficient (Fig. 4) should be considered as the effective diffusion coefficient Deff, characterizing the transport of hydrogen through the membrane taken as a whole. However, assuming the continuity of hydrogen flux, i.e. that there were no barriers for hydrogen transport between the sublayers, it is possible to evaluate the real diffusion coefficient of hydrogen in the individual sublayers of the nitrided membranes. 1 Under the steady state, the same flux of hydrogen ip flows through the whole membrane as well as through each of sublayers 1
ip ¼
Deff DCF Di DC i F ¼ L Li
where DC is the total concentration gradient of hydrogen in the whole membrane, DCi is a gradient of hydrogen in a sublayer ‘‘i’’ with thickness Li and the diffusion coefficient Di, and F is Faraday constant. Since DC is the sum of DCi, the expression (2) can be transformed into the equation
X Li L ¼ Deff Di
ð2Þ
(a) Membrane I – with c0prec zone
Fig. 6. Desorption of hydrogen at the previously entry (X = 0) side of the unnitrided (Fe) and nitrided (different nitrides layers) iron membranes.
This two-layer membrane included the unnitrided iron zone and the diffusion zone of c0 nitride precipitations. Substituting L = 2 mm, LFe = 1.6 mm, and Lc0prec = 0.4 mm (Table 1) and Deff = 2.2 105 cm2/s and DFe = 8.2 105 cm2/s mm (Fig. 4) to Eq. (3), the diffusion coefficient of hydrogen in the c0prec zone Dc0prec = 5.6 106 cm2/s was obtained. The structure of the diffusion zone was heterogeneous – the concentration of c0prec was the highest near the surface and it gradually decreased until zero at the end of the zone. Therefore, the above value of Dc0prec should be still treated as the average value characterizing the transport of hydrogen through the whole c0 zone.
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The analysis of hydrogen transport in the c0 precipitations zone was described in details in the previous work [14]. This analysis was based on the original approach proposed earlier to characterize hydrogen transport in a duplex stainless steel [15]. It was proved that hydrogen bypasses the precipitations and moves only through the ferrite matrix with the same velocity as observed in the iron. In consequence, the real diffusion paths have various lengths and they are longer than the geometrical thickness of the diffusion zone. Undoubtedly in this work, various length of the diffusion paths was the reason for rather poor fitting of the model equation (1) to the experimental build-up transient (Fig. 3, curve designated as c0prec ). One can see that at first the experimental build-up transient (Fig. 3, curve c0prec ) was ahead of the fitted curve, whereas at the end the experimental curve lagged behind the fitted one. Using the suitable fitting procedure [14,15], the shortest diffusion path of hydrogen through the whole membrane Lmin = 3.2 mm and the longest one Lmax = 5.0 mm were evaluated. In turn, taking into account that the real path of diffusion through the unnitrided iron substrate was equal the thickness of this layer (LFe = 1.6 mm), the elongation of the diffusion path must have occurred only in the c0 precipitation zone. Thus, in this approximately 0.4 mm thick zone (Table 1) the shortest diffusion path had 1.6 mm and the longest one had 3.4 mm.
side of the membrane, C0. In the case of unnitrided iron membrane, the lattice diffusion coefficient of hydrogen D is constant and the concentration of hydrogen C0 is
(b) Membrane II – with (c0 ) + c0prec
C0 ¼
In this case the membrane was composed of three layers. It can be considered as the previous membrane I with an added continuous layer of the c0 nitride layer (Fig. 1). By substitution of suitable values of L, Li, Deff and Di, including Dc0prec = 5.6 106 cm2/s, determined for the membrane I, the value of the diffusion coefficient of hydrogen in the c0 nitride layer D(c0 ) = 9.7 108 cm2/s was obtained.
This equation may be adapted for the nitrided membranes by replacing the lattice diffusion coefficient D with the effective diffusion coefficient Deff
Fig. 7. The real diffusion coefficient of hydrogen in the unnitrided iron (Fe) and in the layers of different iron nitrides.
1
(c) Membrane III – with (e + c0 ) + c0prec
Using the same procedure as that used for the three-layer membrane II, the diffusion coefficient of hydrogen in the (e + c0 ) nitride layer D(e+c0 ) = 1.0 107 cm2/s was evaluated. (d) Membrane IV – with (e) + (e + c0 ) + c0prec
If this membrane is treated as three-layer membrane, the average diffusion coefficient of hydrogen in the coupled nitride layer D(e)+(e+c0 ) = 1.3 108 cm2/s was obtained. However, considering four layers in the membrane and assuming that the diffusion coefficient of hydrogen in the inner (e + c0 ) sublayer is the same as that determined for the outer (e + c0 ) sublayer in the membrane III, the diffusion coefficient of hydrogen in the outer e nitride layer D(e) = 5.4 109 cm2/s was evaluated. The comparison of the evaluated diffusion coefficients, characterizing the transport of hydrogen in a given sublayers (zones) of the nitrided iron is shown in Fig. 7.
FDip L
ð3Þ
1
C0 ¼
FDeff ip L
ð4Þ
Using Eqs. (4) and (5) the concentration of hydrogen beneath the entry side of the unnitrided and nitrided membranes, respectively, were calculated (Table 2). The concentration C0 can be considered as a measure of the intensity of hydrogen entry into the metal under given charging condition. In the case of the nitrided membranes, depending on the type of nitriding, the concentration C0 characterizes the entry of hydrogen into the outer nitrided layer. It can be concluded from Table 2 that the presence of c0 precipitates in iron virtually does not affect the entry of hydrogen, whereas the entry of hydrogen into the layers of nitrides, especially into the e nitride, was significantly less intensive. 4.3. Different forms of absorbed hydrogen and their amounts The results of the non-stationary permeation measurements (Fig. 3) indicate that times needed to achieve the steady state permeation rate of hydrogen ranged from about 200 s for the unnitrided membrane to about 10 ks for the nitrided membrane with the Table 2 Characteristics of the diffusible hydrogen in the unnitrided iron membrane (Fe) and in the nitrided membranes with different nitrides layer (I–IV). Hydrogen charging conditions: 0.1 M NaOH, ic = 10 mA/cm2. Fe
4.2. Intensity of hydrogen entry
Structure of the nitrided layer I
In a steady-state (before the desorption), the permeation rate of 1 hydrogen (ip ) through the membrane is proportional to the concentration gradient (DC) and the diffusion coefficient of hydrogen in the membrane (D or Deff), and inversely proportional to the membrane thickness (L). Since using the electrochemical technique for the detection of hydrogen CL = 0, the concentration gradient of hydrogen can be replaced by its concentration beneath the entry
II (c0 )
III (e + c0 )
IV (e + c0 )
c0 prec
c0 prec
c0 prec
c0 prec
c0 prec
2 i1 p (lA/cm )
21.0
6.1
0.56
0.46
0.033
D (cm2/s) Deff (cm2/s)
8.2 105 –
– 2.2 10-5
– 1.2 10-
– 1.1 10-
– 1.8 10-
5
5
6
C0 (mol H/ cm3)
5.3 107
5.7 107
9.7 10-
8.7 10-
3.8 10-
8
8
8
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Fig. 8. Amounts of the diffusible hydrogen (qHd), the trapped hydrogen (qHt), and their sum (qH) in the unnitrided (Fe) and nitrided (I–IV) iron membranes.
(e + (e + c0 ) + c0prec layers. However, the time needed for the complete desorption of hydrogen from the membranes was much longer – at least 100 ks (Figs. 5 and 6). Therefore, it is obvious that the measured desorption rate related not only to the diffusible hydrogen, which left the membrane much faster, but also to the hydrogen originating from reversible traps. Integration of the individual desorption rate curve in Fig. 5 gives the amount of hydrogen qH,L that left the membrane its exit side, whereas integration of the suitable curve in Fig. 6 gives the amount of hydrogen qH,0, which left the membranes its entry side. The sum qH,L + qH,0 corresponds to the total amounts of absorbed hydrogen in the chosen membrane before the desorption, qH. The amount of the diffusible (lattice) hydrogen in the unnitrided membrane can be determined from permeation data using the following expression [6] 1
qHd ¼
ip L2 2DF
ð5Þ
and in the nitrided membrane 1
qHd ¼
ip L2 2Deff F
ð6Þ
Subtraction qHd from qH gives the amount of the reversible trapped hydrogen, qHt.
Fig. 9. Amounts of the trapped hydrogen desorbed from the unnitrided (Fe) and nitrided (I–IV) iron membranes at their exit (qHt,L) and entry (qHt,0) side.
265
Fig. 10. The contribution of the membrane exit and entry side to the desorption of the trapped hydrogen from the unnitrided (Fe) and nitrided (I–IV) iron membranes.
Amounts of the diffusible and trapped hydrogen in the studied membranes are compared in Fig. 8. First of all, the dominant participation of the trapped hydrogen attracts notice. The participation of the diffusible hydrogen was much lower, and for the membrane with the e nitride layer almost negligible. Another noticeable fact is that the amount of the trapped hydrogen for the membrane with the diffusion zone of c0 nitride precipitates (I) was higher than that in the unnitrided iron (Fe). Undoubtedly, this enhanced trapping of hydrogen was produced by the nitride precipitates–ferrite matrix interfaces. As it was shown earlier [14], these interfaces, apart from dislocations and grain boundaries, are additional effective traps for hydrogen. In turn, the amounts of the trapped hydrogen in the membranes with the phase nitrides layers (II–IV) were lower than that for the unnitrided membrane, but they were still significant. The above balance of the hydrogen absorption (Fig. 8) does not inform about distribution of the trapped hydrogen in the cross-section of the membrane. Relevant information may be obtained by comparison of the amounts of hydrogen escaping from the membranes their opposite sides. The suitable data shown in Fig. 9 indicate that in each case much more hydrogen escaped from the membrane its entry side (qHt,0) than from its exit side (qHt,L). For the nitrided membranes, this effect was the greater the more complex structure of the nitrided layer. For the nitrided membrane with the porous e phase, virtually the entire amount of hydrogen originated from traps desorbed from the membrane its entry side. The involvement of the individual membrane sides in the desorption of trapped hydrogen is even more depicted in Fig. 10, showing values of the ratios of the amount of trapped hydrogen escaping from the exit (qHt,L) and entry (qHt,0) side to the total amount of trapped hydrogen (qHt). As it is known, for the linear gradient of hydrogen concentration in the membrane (from C0 at the entry side to CL = 0 at the exit side), 1/3 of the total amount of hydrogen should escape at the exit side, and 2/3 at the entry side [6]. Generally, these predicted ratios were not valid with regard to the trapped hydrogen – the ratio qHt,L/qHt was lower than 1/3, while the ratio qHt,0/qHt adequately higher than 2/3 (Fig. 10). This means that hydrogen identified as the trapped hydrogen was distributed in the membrane near its entry side. Moreover, with regard to the nitrided membranes with the phase nitride layers (II–IV), the trapped hydrogen was practically accumulated only in those layers. It is worthy to note that relatively small amounts of hydrogen escaping from the nitrided membranes their exit sides indicate that the amounts of hydrogen absorbed in the unnitrided iron substrate were small, almost negligible in the case of the
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membrane IV with the layer of e nitride. Therefore, the nitrided layers on iron, despite their strong tendency for hydrogen trapping, protect the metal substrate from hydrogen. 5. Conclusions 1. Nitrided layers on iron affect the entry, and thus the subsurface concentration of hydrogen, as well as the transport (diffusion) and trapping of hydrogen. These effects strongly depended on the type (structure) of nitrided layers. 2. The outer, porous e nitride layer was the most effective in hindrance of hydrogen entry and hydrogen transport, but at the same time it was very capable of hydrogen trapping. For the above reasons, the c0 and (e + c0 ) nitride layers were slightly less effective. 3. The least effects were exerted by the layer containing only c0 nitride precipitates. Intensity of hydrogen entry into this layer was practically the same as into the unnitrided iron. Although the effective diffusion coefficient of hydrogen in this layer was lower than that in iron, but this effect was caused by longer diffusion paths. Hydrogen trapping was somewhat higher owing to the c0 nitride–ferrite interfaces. 4. Thus, nitriding of iron may be an effective method to prevent hydrogen corrosion (embrittlement) of iron (steel), provided that a continuous layer of iron nitrides, preferably with an outer layer of e nitride, is formed on the metal surface. Acknowledgments The authors are grateful for the financial support of this work by the Ministry of Science and Higher Education, Republic of Poland, Research Project No. 3 T08C 050 30.
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