- Email: [email protected]
Study of the electrochemical permeation of hydrogen in iron
Corrosion Science 51 (2009) 263–267
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Study of the electrochemical permeation of hydrogen in iron H. Addach a, P. Berçot b,*, M. Rezrazi b, J. Takadoum a a b
FEMTO-ST, Département MN2S, ENSMM, 26 Chemin de l’Epitaphe 25030 Besançon Cedex, France Institut Utinam CNRS UMR 6213, Université de Franche-Comté, 16 route de Gray, 25030 Besançon Cedex, France
a r t i c l e
i n f o
Article history: Received 4 June 2008 Accepted 30 October 2008 Available online 6 November 2008 Keywords: A. Hydrogen A. Iron B. Electrochemical permeation C. Diffusivity C. Hydrogen embrittlement
a b s t r a c t Hydrogen permeation through iron membranes of different thicknesses was studied by the electrochemical permeation technique. The membranes were charged with hydrogen by galvanostatic cathodic polarization in 0.1 M NaOH at 25 °C. The measured build-up and decay permeation current transient had been examined. The experimental results revealed that the diffusion apparently increases with decreasing membrane thickness. This result suggests that the hydrogen transport through membrane was mainly governed by hydrogen trapping at the trap sites present at the grain boundaries. The influence of the passive layer on the hydrogen permeation and its influence on the evaluation of diffusion and trapping characteristics were discussed. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The absorption of hydrogen by metals is a serious problem for many electrochemical processes – corrosion of metals, fuel cell of battery, electroplating of metals – leading to hydrogen embrittlement of metals, and therefore modifications of mechanical properties of the material with occasional stress corrosion cracking. The diffusion of hydrogen in metals, and more particularly in iron has been widely discussed in several reviews [1–4]. It is a significant problem encountered by the industry of the surface treatments. The study of this phenomenon must advance without question towards a better understanding of the mechanisms brought into play. In this respect, the electrochemical permeation made it possible to study the diffusion of hydrogen in iron, by electrolysis in basic medium. This technique developed by Devanathan and Stachurski [5], is the most commonly used method in measuring hydrogen diffusivity and metallic embrittlement phenomenon, has a great sensitivity and offers, moreover, the advantage of the continuous recording of diffusive flow according to time. A review published in the literature shows that it was a method that was vastly used, practical and its applications did not cease to widen [6–16]. The effects of hydrogen on the mechanical properties of iron and steels had been widely studied [17–20]. Hydrogen is known to degrade numerous metals especially iron-base alloys. Due to the presence of free interstitial in steel, hydrogen may diffuse easily in the metal and lead to its embrittlement [4].
* Corresponding author. Tel.: +33 3 81 66 20 30; fax: +33 3 81 66 30 33. E-mail address: [email protected] (P. Berçot). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.10.024
Many authors pointed out that both diffusivity and permeation rate of hydrogen are greatly affected by the surface properties [21,22]. This work aims to apply the standard electrochemical permeation method in order to calculate the hydrogen diffusivity, permeation rate and apparent solubility for the iron membrane at different thicknesses. The influence of the passive layer on the hydrogen permeation and its influence on the evaluation of diffusion and trapping characteristics are discussed. 2. Experimental 2.1. Materials and experimental procedure In this study, an electrochemical permeation technique was used to study the hydrogen transport in pure iron at different thickness. The materials chosen for this study were disks of rolled pure iron with purity of 99.99%. The diameter of the tested specimen was the same (25 mm), whereas different thicknesses had been chosen: 0.5, 1, 1.5, 2 mm. These specimens were ground with Carbimet-SiC grinding paper down to 1200 grit then rinsed with distilled water and ultrasonically cleaned in acetone. The preparation conditions of the samples were selected to reproduce the real use conditions in industrial applications where the material is not annealed. In the course of electrochemical permeation, hydrogen atoms were first absorbed at the entry surface, then diffused through the metallic membrane, and were finally desorbed from the exit surface. On the entry surface, the production of hydrogen could
264
H. Addach et al. / Corrosion Science 51 (2009) 263–267
be controlled galvanostatically or potentiostatically. On the exit surface, it is common to apply a constant potential to ensure that all hydrogen atoms could be ionized, ensuring that the measured current density was the hydrogen permeation flux. The instrumentation of electrochemical hydrogen permeation was composed of an electrolytic cell with two compartments (cathodic side and anodic side), a reference electrode (Hg/HgO/ NaOH 0.1 M), an auxiliary electrode (wire of Pt), and two potentiostat/galvanostat (Fig. 1). Water circulated in the double jacket in order to maintain a constant temperature in each chamber (T = ±0.5 °C). The specimen (working electrode) was clamped between the compartments. One side of the specimen acted as the cathodic side, or hydrogen entry side, of the cell. It was galvanostatically polarized at a constant charging current density (2 mA cm2) in NaOH 0.1 M. The anodic side, or hydrogen exit side, of the cell was potentiostatically maintained at a constant potential of 220 mV versus reference electrode (Hg/HgO/NaOH 0.1 M). This potential was sufficient to oxidize the hydrogen atoms emerging on the output face according to: Hads ! Hþ þ 1 e . The anodic current (exit side) gave a direct measure of the hydrogen flow rate J (permeation flux), is proportional to the anodic current (Ip) detected in this face. Solutions on both cells of the membrane were continuously deoxygenated by (Argon U) bubbling before and during the measurements.
The effective hydrogen diffusivity, the rate-limiting step Deff (m2 s1) is related to time lag by [23]
Deff ¼ L2 =6tL
where tL (s) is the lag time, defined as 0.63 times the steady-state value, and Deff determined by the transient tL. If the surface hydrogen is in thermodynamic equilibrium with subsurface hydrogen [23], the apparent hydrogen solubility Capp (mol m3) may be defined by
C app ¼ J 1 L=Deff
3.1. Typical permeation experiment/consecutive permeations Fig. 2 shows the complete transient of hydrogen permeation experiment: after the required time for passivation of the exit side, the process begins when the extraction current density anodic (Ja) was stabilized for approximately 10 nA cm2 (Fig. 3). On the entry side; with cathodic charging current applied (2 mA cm2), a first rise transient was obtained; hydrogen transported through the sample was detected in the exit side (anodic cell) by potentiostat.
For this study, the flux of hydrogen through the specimen was measured in terms of the steady-state current density, I1 p (A m 2 ), and was converted to the steady-state hydrogen permeation flux, J 1 (mol m2 s1), according to the following equation and results directly from the Fick’s first law:
@C Þ ¼ I1 p =nF @x x¼L
ð1Þ
The permeation flux (mol m1 s-1) was defined by
J 1 L ¼ I1 p L=nF
ð2Þ
where D is the hydrogen diffusion coefficient (m2 s1), indicates the steady-state permeation current density, n the number of electrons transferred, F the Faraday’s constant, L (m) the specimen thickness and J1 the steady-state flux.
ð4Þ
3. Results and discussion
2.2. Data analysis
J 1 ¼ ðD
ð3Þ
I1 p
Fig. 2. Permeation charging and discharging curves of membrane iron.
Fig. 1. Apparatus of electrochemical test.
265
H. Addach et al. / Corrosion Science 51 (2009) 263–267
meation revealed differences between these two consecutive permeations (Fig. 4). The significant difference observed over time in exit between the first and the second transient permeation was probably due to the difference of the cathodic surface quality between the two chargings. Indeed, the passive layer present on cathodic surface before the first charging could behave in a way different from that of cathodic surface before the second charging. This probably indicated an evolution entry side face between two transient consecutive permeations: in particular, the modifications of the surface quality or the solution in the vicinity of surface. 3.3. Evidence of the influence of the passive layer on the exit side
It was usually illustrated that the typical curves permeation had a clear incubation time in the initial stage, then a region of constant gradient, and then finally became constant (state flux current of hydrogen I1 p ). When the permeation rate achieved a steady-state level I1 p , the charging current was interrupted and the entry side of the membrane was polarized immediately at the same anodic potential as the exit side (220 mV) (Fig. 2). Once, the flow of decay transient (descharging) established the base line, a second cathodic charging was carried out while being placed under the same galvanostatic conditions described previously (Ic = 2 mA cm2). (Fig. 2 second transient permeation). It was noted that the second transient permeation was slower than the first one. It presented a weaker current of permeation and a higher exit time: the slope of the curve of discharging at the beginning of the second charging was weaker than that of the curve of passivation at the beginning of the first charging (Figs. 2 and 3). The difference observed was attributed to mechanical polishing which causes a modification of the microstructure of surface by strain hardening. This hindered hydrogen transport and made effective hydrogen traps. The permeation was weaker during the second charging since the microstructure of the material has been changed due to hydrogen trapped during the first charging. However, on certain curves, we observed that the exit time of the second transient permeation was shorter than that of the first; which resulted in a more important slope of discharging. This phenomenon could be explained by admitting that the second charging started before the exit of the totality of hydrogen introduced into the sample during the first charging. If the hydrogen totality did not have time to allow outgassing, this phenomenon could be attributed to terms of irreversible trapping. In addition, it was noted that by starting the charging after a sufficiently long time so that hydrogen could be degassed, the exit time of the second transient permeation lasted longer. The anodic current after the degassing of hydrogen was lower than the passivation current (current before the beginning of the first charging), which appeared logical since passivation continued during the permeation, from where the influence of the passivation time. 3.2. Evolution the surface cathodic side during consecutive permeations The evolution of the potential at the entry side face sample during the first transient permeations and the second transient per-
3.4. Influence of sample thickness In order to study the effect of thickness on the hydrogen permeation, different curves of permeation experiments were achieved. Fig. 6 shows the curves hydrogen permeations, with same charging condition (ic = 2.00 mA cm2) for samples at different thicknesses (0.5; 1; 1.5; 2 mm). The principal observation here was the reduction of steady-state level of current permeation with increasing the membrane thickness. This result suggested that the permeation and diffusion parameters were a function of sample thickness, as shown in Fig. 6. If thickness was increased, the permeation and diffusion values decreased, i.e., for 0.5 mm the maximum hydrogen permeation current reached was 1.84 lA cm2, while for samples of 1.5 mm the maximum permeation current achieved was 1.18 lA cm-2. Finally if the thickness of the sample was 2 mm, the maximum permeation current would be 0.79 lA cm2. -0.4 -0.6 -0.8
Ec (V) vs. SCE
Fig. 3. Curve of electrochemical passivation of membrane iron potentiostatically maintained at a constant potential of 220 mV/(Hg/HgO/NaOH 0.1 M).
To study the influence of the passive layer on the exit side, a series of experiments were carried out with the same sample. Fig. 5 shows two permeation transients, obtained under the following conditions: the first one (curve A1) was obtained with a constant cathodic current density on the entrance side ic = 2 mA cm2, applied after a passivation time of 6 h. However the second permeation A2 was carried out after a passivation time of 48 h which is higher than the passivation time of the curve A1. We observed that the permeation of A2 was slower than that of A1 and a considerable difference on the curve of permeation was obtained. The passive layer formed on the surface of detection played a role of barrier to the permeation. It became more and more impervious, which explained the decrease of the characteristics (I1 p , Capp, etc.) of the second permeation.
-1.0
First transient
-1.2 -1.4 Second transient -1.6 -1.8 0
2
4
6
8
10
12
14
16
18
20
Time (h) Fig. 4. Potential evolution during consecutive permeations. Ec: entrance side potential evolution.
H. Addach et al. / Corrosion Science 51 (2009) 263–267
1.8 1.6
A1 A2
Ip (µA cm-2)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
2
4
6
8
10
12
14
16
18
Time (h) Fig. 5. Permeation charging curves of membrane iron during different time of passivation (exit side). A1: curve of permeation (6 h: time of passivation), A2: curve of permeation passivation time longer than that of A1(48 h).
The analysis of the results enabled us to calculate the coefficient of diffusion by the time lag method with mathematical expression derived from Fick’s second law [23]. In this investigation, the hydrogen diffusion coefficient, permeation rate, hydrogen trapping and apparent solubility of hydrogen measured in iron (0.1 M NaOH) defined by Eqs. (1)–(4) are presented in Table 1. As can be seen, most of the measured parameters had accuracy better than 4%. For the samples, the mean value of the hydrogen diffusivity increased gradually from 1.1 1011 to 3.49 1011 m2 s1, as the test thickness increased from 0.5 to
2 mm. The coefficients of diffusion values obtained were lower than those found for annealed iron (8 109 m2 s1) [24–26], but in agreement with the values found with mechanically polished iron without annealing [3]. Mechanical polishing induces a modification of the microstructure which leads to strain harden greatly the material surface provoking a decrease of hydrogen diffusivity. The ratio J1L/Deff or Capp was calculated from Eq. (4) and appeared to be highest for thickness 0.5 mm and lowest for the temperature 2 mm. The decrease of the steady-state hydrogen permeation flux J 1 with the increase of the thickness was an indication that the hydrogen trapping was dominant in the highest thickness. With these values it could be shown approximately that the hydrogen transported through membrane was mainly governed by hydrogen trapping at the trap sites present at the grain boundaries. Furthermore, as represented in Figs. 7 and 8, it is also shown that the curve of exit time versus thickness presented a linear regression which is attached to the phenomenon of diffusion. This curve does not pass by the origin which may be due to surface phenomena (Fig. 8).
2.2 2.0
L1
1.8 1.6
Ip (µA cm-2)
266
L2
1.4 1.2 1.0
L3
0.8 0.6
2.2
Ip (µA cm-2)
L4
0.4
2.0 1.8
L1
0.2
1.6
L2
0.0 0
2
1.4 Te1
1.2
Te3
Te2
Te4
4
Time (h)
L3 Fig. 7. Permeation charging curves of membrane iron at different thicknesses. Times exit (Tei) was: Te1 = 756 s, Te2 = 1152 s, Te3 = 1872 s, Te4 = 2556 s.
1.0 0.8
L4
0.6 0.4
60
0.2
y = 16.41x + 17.52
50
0.0 0
2
4
6
8
10
12
14
16
Time (h)
Table 1 Data of steady-state hydrogen permeation flux J 1 , effective diffusivity Deff, solubility hydrogen Capp versus different thickness iron membrane during permeation experiment. L (mm)
J1 (mol m
0.5 1 1.5 2
9.48 1008 8.13 1008 6.16 1008 4.04 1008
s
1
)
2
1
Deff (m s
)
1.10 1011 1.82 1011 2.66 1011 3.49 1011
Te 1/2 (s1/2)
40
Fig. 6. Permeation charging curves of membrane iron at different thicknesses (Li): L1 = 0.5 mm, L2 = 1 mm, L3 = 1.5 mm, L4 = 2 mm.
2
R2 = 0.99
18
30
20
10
3
Capp (mol m ) 4.30 4.46 3.46 2.31
0 0.0
0.5
1.0
1.5
2.0
Thickness (mm) Fig. 8. Square root of the time exit versus thickness.
2.5
H. Addach et al. / Corrosion Science 51 (2009) 263–267
4. Conclusion Hydrogen permeation through a metallic iron membrane of different thicknesses was measured by the electrochemical permeation technique. The principal experimental results showed that hydrogen had easy mobility when the material was a thin film, than in materials that had a considerable thickness where its mobility was reduced, and the time from release of trapping was increased. The significant reduction in time to reach the steady state and considerable increase of breakthrough time, affected the diffusion parameter. It was also verified that, the evolution of the passive layer formed on the detection side during permeation acted as a barrier for hydrogen permeation, depending on time and on the electrochemical conditions of the surface. References [1] A.M. Brass, J.R. Collet-Lacoste, Acta Mater. 46 (1998) 869. [2] B.G. Pound, in: J. O’M Bockris et al. (Eds.), Modern Aspects of Electrochemistry, vol. 25, Plenum Press, New York, 1993. [3] H. Addach, P. Berçot, M. Rezrazi, M. Wery, Mater. Lett. 59 (2005) 1347–1351. [4] P.W. Liu, J.K. Wu, Mater. Lett. 57 (2003) 1224. [5] M. Devanathan, Z. Stachurski, Proc. R. Soc 270A (1962) 90. [6] I. Flis-Kabulska, J. Flis, T. Zakroczymski, Electrochim. Acta 53 (2008) 3094– 3101.
267
[7] U. Prakash, N. Parvathavarthini, R.K. Dayal, Intermetallics 15 (2007) 17–19. [8] N. Amokrane, C. Gabrielli, G. Maurin, L. Mirkova, Electrochim. Acta 53 (2007) 1962–1971. [9] I. Flis-Kabulska, T. Zakroczymski, J. Flis, Electrochim. Acta 52 (2007) 2966– 2977. [10] L.W. Tsay, M.Y. Chi, Y.F. Wu, J.K. Wu, D.-Y. Lin, Corros. Sci. 48 (2006) 1926– 1938. [11] E.M.K. Hillier, M.J. Robinson, Corros. Sci. 48 (2006) 1019–1035. [12] C. Gabrielli, G. Maurin, L. Mirkova, H. Perrot, B. Tribollet, J. Electroanal. Chem. 590 (2006) 1–14. [13] W.C. Luu, P.W. Liu, J.K. Wu, Corros. Sci. 44 (2002) 1783–1791. [14] T. Zakroczymski, E. Owczarek, Acta Mater. 50 (2002) 2701–2713. [15] J. Mike, Danielson, Corros. Sci. 44 (2002) 829–840. [16] Kumkum Banerjee, U.K. Chatterjee, Scripta Mater. 44 (2001) 213–216. [17] J.P. Hirth, Metall. Trans. 11A (1980) 861. [18] Y. Fukai, The metal–hydrogen system: basic bulk properties, in: U. Gonser (Ed.), Springer Series in Material Science, Vol. 21, Second rev. and updated ed., 2005, XII, 497 p. 291, Springer-Verlag, ISBN: 978-3-540-00494-3. [19] R. Kirchheim, Mater. Sci. 32 (1988) 262. [20] Yukitaka Murakami, Hisao Matsunaga, Int. J. Fatigue 28 (2006) 1509–1520. [21] M. Nagano, Y. Hayashi, N. Othani, M. Isshiki, K. Igaki, Trans. JIM 22 (1981) 423. [22] M. Nagano, Y. Hayashi, N. Othani, M. Isshiki, K. Igaki, Scripta Met. 16 (1982). [23] J. Crank, The Mathematics of Diffusion, second ed., Oxford University Press, Oxford, 1979. p. 51. [24] J.-L. Dillard, S. Talbot-Besnard, Determination of the diffusion of hydrogen by measuring the permeability in high purity iron, Compt. Rend. Acad. Sci. (Paris). Ser. C 269 (1969) 1173. [25] E.W. Johnson, M.L. Hill, Trans. Met. Soc. Aime 218 (1960) 1104. [26] B. Laveissiere, J.L. Philippart, J. Pagetti, Electrochim. Acta 36 (3=4 ) (1991) 615– 620.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Study of the electrochemical permeation of hydrogen in iron H. Addach a, P. Berçot b,*, M. Rezrazi b, J. Takadoum a a b
FEMTO-ST, Département MN2S, ENSMM, 26 Chemin de l’Epitaphe 25030 Besançon Cedex, France Institut Utinam CNRS UMR 6213, Université de Franche-Comté, 16 route de Gray, 25030 Besançon Cedex, France
a r t i c l e
i n f o
Article history: Received 4 June 2008 Accepted 30 October 2008 Available online 6 November 2008 Keywords: A. Hydrogen A. Iron B. Electrochemical permeation C. Diffusivity C. Hydrogen embrittlement
a b s t r a c t Hydrogen permeation through iron membranes of different thicknesses was studied by the electrochemical permeation technique. The membranes were charged with hydrogen by galvanostatic cathodic polarization in 0.1 M NaOH at 25 °C. The measured build-up and decay permeation current transient had been examined. The experimental results revealed that the diffusion apparently increases with decreasing membrane thickness. This result suggests that the hydrogen transport through membrane was mainly governed by hydrogen trapping at the trap sites present at the grain boundaries. The influence of the passive layer on the hydrogen permeation and its influence on the evaluation of diffusion and trapping characteristics were discussed. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The absorption of hydrogen by metals is a serious problem for many electrochemical processes – corrosion of metals, fuel cell of battery, electroplating of metals – leading to hydrogen embrittlement of metals, and therefore modifications of mechanical properties of the material with occasional stress corrosion cracking. The diffusion of hydrogen in metals, and more particularly in iron has been widely discussed in several reviews [1–4]. It is a significant problem encountered by the industry of the surface treatments. The study of this phenomenon must advance without question towards a better understanding of the mechanisms brought into play. In this respect, the electrochemical permeation made it possible to study the diffusion of hydrogen in iron, by electrolysis in basic medium. This technique developed by Devanathan and Stachurski [5], is the most commonly used method in measuring hydrogen diffusivity and metallic embrittlement phenomenon, has a great sensitivity and offers, moreover, the advantage of the continuous recording of diffusive flow according to time. A review published in the literature shows that it was a method that was vastly used, practical and its applications did not cease to widen [6–16]. The effects of hydrogen on the mechanical properties of iron and steels had been widely studied [17–20]. Hydrogen is known to degrade numerous metals especially iron-base alloys. Due to the presence of free interstitial in steel, hydrogen may diffuse easily in the metal and lead to its embrittlement [4].
* Corresponding author. Tel.: +33 3 81 66 20 30; fax: +33 3 81 66 30 33. E-mail address: [email protected] (P. Berçot). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.10.024
Many authors pointed out that both diffusivity and permeation rate of hydrogen are greatly affected by the surface properties [21,22]. This work aims to apply the standard electrochemical permeation method in order to calculate the hydrogen diffusivity, permeation rate and apparent solubility for the iron membrane at different thicknesses. The influence of the passive layer on the hydrogen permeation and its influence on the evaluation of diffusion and trapping characteristics are discussed. 2. Experimental 2.1. Materials and experimental procedure In this study, an electrochemical permeation technique was used to study the hydrogen transport in pure iron at different thickness. The materials chosen for this study were disks of rolled pure iron with purity of 99.99%. The diameter of the tested specimen was the same (25 mm), whereas different thicknesses had been chosen: 0.5, 1, 1.5, 2 mm. These specimens were ground with Carbimet-SiC grinding paper down to 1200 grit then rinsed with distilled water and ultrasonically cleaned in acetone. The preparation conditions of the samples were selected to reproduce the real use conditions in industrial applications where the material is not annealed. In the course of electrochemical permeation, hydrogen atoms were first absorbed at the entry surface, then diffused through the metallic membrane, and were finally desorbed from the exit surface. On the entry surface, the production of hydrogen could
264
H. Addach et al. / Corrosion Science 51 (2009) 263–267
be controlled galvanostatically or potentiostatically. On the exit surface, it is common to apply a constant potential to ensure that all hydrogen atoms could be ionized, ensuring that the measured current density was the hydrogen permeation flux. The instrumentation of electrochemical hydrogen permeation was composed of an electrolytic cell with two compartments (cathodic side and anodic side), a reference electrode (Hg/HgO/ NaOH 0.1 M), an auxiliary electrode (wire of Pt), and two potentiostat/galvanostat (Fig. 1). Water circulated in the double jacket in order to maintain a constant temperature in each chamber (T = ±0.5 °C). The specimen (working electrode) was clamped between the compartments. One side of the specimen acted as the cathodic side, or hydrogen entry side, of the cell. It was galvanostatically polarized at a constant charging current density (2 mA cm2) in NaOH 0.1 M. The anodic side, or hydrogen exit side, of the cell was potentiostatically maintained at a constant potential of 220 mV versus reference electrode (Hg/HgO/NaOH 0.1 M). This potential was sufficient to oxidize the hydrogen atoms emerging on the output face according to: Hads ! Hþ þ 1 e . The anodic current (exit side) gave a direct measure of the hydrogen flow rate J (permeation flux), is proportional to the anodic current (Ip) detected in this face. Solutions on both cells of the membrane were continuously deoxygenated by (Argon U) bubbling before and during the measurements.
The effective hydrogen diffusivity, the rate-limiting step Deff (m2 s1) is related to time lag by [23]
Deff ¼ L2 =6tL
where tL (s) is the lag time, defined as 0.63 times the steady-state value, and Deff determined by the transient tL. If the surface hydrogen is in thermodynamic equilibrium with subsurface hydrogen [23], the apparent hydrogen solubility Capp (mol m3) may be defined by
C app ¼ J 1 L=Deff
3.1. Typical permeation experiment/consecutive permeations Fig. 2 shows the complete transient of hydrogen permeation experiment: after the required time for passivation of the exit side, the process begins when the extraction current density anodic (Ja) was stabilized for approximately 10 nA cm2 (Fig. 3). On the entry side; with cathodic charging current applied (2 mA cm2), a first rise transient was obtained; hydrogen transported through the sample was detected in the exit side (anodic cell) by potentiostat.
For this study, the flux of hydrogen through the specimen was measured in terms of the steady-state current density, I1 p (A m 2 ), and was converted to the steady-state hydrogen permeation flux, J 1 (mol m2 s1), according to the following equation and results directly from the Fick’s first law:
@C Þ ¼ I1 p =nF @x x¼L
ð1Þ
The permeation flux (mol m1 s-1) was defined by
J 1 L ¼ I1 p L=nF
ð2Þ
where D is the hydrogen diffusion coefficient (m2 s1), indicates the steady-state permeation current density, n the number of electrons transferred, F the Faraday’s constant, L (m) the specimen thickness and J1 the steady-state flux.
ð4Þ
3. Results and discussion
2.2. Data analysis
J 1 ¼ ðD
ð3Þ
I1 p
Fig. 2. Permeation charging and discharging curves of membrane iron.
Fig. 1. Apparatus of electrochemical test.
265
H. Addach et al. / Corrosion Science 51 (2009) 263–267
meation revealed differences between these two consecutive permeations (Fig. 4). The significant difference observed over time in exit between the first and the second transient permeation was probably due to the difference of the cathodic surface quality between the two chargings. Indeed, the passive layer present on cathodic surface before the first charging could behave in a way different from that of cathodic surface before the second charging. This probably indicated an evolution entry side face between two transient consecutive permeations: in particular, the modifications of the surface quality or the solution in the vicinity of surface. 3.3. Evidence of the influence of the passive layer on the exit side
It was usually illustrated that the typical curves permeation had a clear incubation time in the initial stage, then a region of constant gradient, and then finally became constant (state flux current of hydrogen I1 p ). When the permeation rate achieved a steady-state level I1 p , the charging current was interrupted and the entry side of the membrane was polarized immediately at the same anodic potential as the exit side (220 mV) (Fig. 2). Once, the flow of decay transient (descharging) established the base line, a second cathodic charging was carried out while being placed under the same galvanostatic conditions described previously (Ic = 2 mA cm2). (Fig. 2 second transient permeation). It was noted that the second transient permeation was slower than the first one. It presented a weaker current of permeation and a higher exit time: the slope of the curve of discharging at the beginning of the second charging was weaker than that of the curve of passivation at the beginning of the first charging (Figs. 2 and 3). The difference observed was attributed to mechanical polishing which causes a modification of the microstructure of surface by strain hardening. This hindered hydrogen transport and made effective hydrogen traps. The permeation was weaker during the second charging since the microstructure of the material has been changed due to hydrogen trapped during the first charging. However, on certain curves, we observed that the exit time of the second transient permeation was shorter than that of the first; which resulted in a more important slope of discharging. This phenomenon could be explained by admitting that the second charging started before the exit of the totality of hydrogen introduced into the sample during the first charging. If the hydrogen totality did not have time to allow outgassing, this phenomenon could be attributed to terms of irreversible trapping. In addition, it was noted that by starting the charging after a sufficiently long time so that hydrogen could be degassed, the exit time of the second transient permeation lasted longer. The anodic current after the degassing of hydrogen was lower than the passivation current (current before the beginning of the first charging), which appeared logical since passivation continued during the permeation, from where the influence of the passivation time. 3.2. Evolution the surface cathodic side during consecutive permeations The evolution of the potential at the entry side face sample during the first transient permeations and the second transient per-
3.4. Influence of sample thickness In order to study the effect of thickness on the hydrogen permeation, different curves of permeation experiments were achieved. Fig. 6 shows the curves hydrogen permeations, with same charging condition (ic = 2.00 mA cm2) for samples at different thicknesses (0.5; 1; 1.5; 2 mm). The principal observation here was the reduction of steady-state level of current permeation with increasing the membrane thickness. This result suggested that the permeation and diffusion parameters were a function of sample thickness, as shown in Fig. 6. If thickness was increased, the permeation and diffusion values decreased, i.e., for 0.5 mm the maximum hydrogen permeation current reached was 1.84 lA cm2, while for samples of 1.5 mm the maximum permeation current achieved was 1.18 lA cm-2. Finally if the thickness of the sample was 2 mm, the maximum permeation current would be 0.79 lA cm2. -0.4 -0.6 -0.8
Ec (V) vs. SCE
Fig. 3. Curve of electrochemical passivation of membrane iron potentiostatically maintained at a constant potential of 220 mV/(Hg/HgO/NaOH 0.1 M).
To study the influence of the passive layer on the exit side, a series of experiments were carried out with the same sample. Fig. 5 shows two permeation transients, obtained under the following conditions: the first one (curve A1) was obtained with a constant cathodic current density on the entrance side ic = 2 mA cm2, applied after a passivation time of 6 h. However the second permeation A2 was carried out after a passivation time of 48 h which is higher than the passivation time of the curve A1. We observed that the permeation of A2 was slower than that of A1 and a considerable difference on the curve of permeation was obtained. The passive layer formed on the surface of detection played a role of barrier to the permeation. It became more and more impervious, which explained the decrease of the characteristics (I1 p , Capp, etc.) of the second permeation.
-1.0
First transient
-1.2 -1.4 Second transient -1.6 -1.8 0
2
4
6
8
10
12
14
16
18
20
Time (h) Fig. 4. Potential evolution during consecutive permeations. Ec: entrance side potential evolution.
H. Addach et al. / Corrosion Science 51 (2009) 263–267
1.8 1.6
A1 A2
Ip (µA cm-2)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
2
4
6
8
10
12
14
16
18
Time (h) Fig. 5. Permeation charging curves of membrane iron during different time of passivation (exit side). A1: curve of permeation (6 h: time of passivation), A2: curve of permeation passivation time longer than that of A1(48 h).
The analysis of the results enabled us to calculate the coefficient of diffusion by the time lag method with mathematical expression derived from Fick’s second law [23]. In this investigation, the hydrogen diffusion coefficient, permeation rate, hydrogen trapping and apparent solubility of hydrogen measured in iron (0.1 M NaOH) defined by Eqs. (1)–(4) are presented in Table 1. As can be seen, most of the measured parameters had accuracy better than 4%. For the samples, the mean value of the hydrogen diffusivity increased gradually from 1.1 1011 to 3.49 1011 m2 s1, as the test thickness increased from 0.5 to
2 mm. The coefficients of diffusion values obtained were lower than those found for annealed iron (8 109 m2 s1) [24–26], but in agreement with the values found with mechanically polished iron without annealing [3]. Mechanical polishing induces a modification of the microstructure which leads to strain harden greatly the material surface provoking a decrease of hydrogen diffusivity. The ratio J1L/Deff or Capp was calculated from Eq. (4) and appeared to be highest for thickness 0.5 mm and lowest for the temperature 2 mm. The decrease of the steady-state hydrogen permeation flux J 1 with the increase of the thickness was an indication that the hydrogen trapping was dominant in the highest thickness. With these values it could be shown approximately that the hydrogen transported through membrane was mainly governed by hydrogen trapping at the trap sites present at the grain boundaries. Furthermore, as represented in Figs. 7 and 8, it is also shown that the curve of exit time versus thickness presented a linear regression which is attached to the phenomenon of diffusion. This curve does not pass by the origin which may be due to surface phenomena (Fig. 8).
2.2 2.0
L1
1.8 1.6
Ip (µA cm-2)
266
L2
1.4 1.2 1.0
L3
0.8 0.6
2.2
Ip (µA cm-2)
L4
0.4
2.0 1.8
L1
0.2
1.6
L2
0.0 0
2
1.4 Te1
1.2
Te3
Te2
Te4
4
Time (h)
L3 Fig. 7. Permeation charging curves of membrane iron at different thicknesses. Times exit (Tei) was: Te1 = 756 s, Te2 = 1152 s, Te3 = 1872 s, Te4 = 2556 s.
1.0 0.8
L4
0.6 0.4
60
0.2
y = 16.41x + 17.52
50
0.0 0
2
4
6
8
10
12
14
16
Time (h)
Table 1 Data of steady-state hydrogen permeation flux J 1 , effective diffusivity Deff, solubility hydrogen Capp versus different thickness iron membrane during permeation experiment. L (mm)
J1 (mol m
0.5 1 1.5 2
9.48 1008 8.13 1008 6.16 1008 4.04 1008
s
1
)
2
1
Deff (m s
)
1.10 1011 1.82 1011 2.66 1011 3.49 1011
Te 1/2 (s1/2)
40
Fig. 6. Permeation charging curves of membrane iron at different thicknesses (Li): L1 = 0.5 mm, L2 = 1 mm, L3 = 1.5 mm, L4 = 2 mm.
2
R2 = 0.99
18
30
20
10
3
Capp (mol m ) 4.30 4.46 3.46 2.31
0 0.0
0.5
1.0
1.5
2.0
Thickness (mm) Fig. 8. Square root of the time exit versus thickness.
2.5
H. Addach et al. / Corrosion Science 51 (2009) 263–267
4. Conclusion Hydrogen permeation through a metallic iron membrane of different thicknesses was measured by the electrochemical permeation technique. The principal experimental results showed that hydrogen had easy mobility when the material was a thin film, than in materials that had a considerable thickness where its mobility was reduced, and the time from release of trapping was increased. The significant reduction in time to reach the steady state and considerable increase of breakthrough time, affected the diffusion parameter. It was also verified that, the evolution of the passive layer formed on the detection side during permeation acted as a barrier for hydrogen permeation, depending on time and on the electrochemical conditions of the surface. References [1] A.M. Brass, J.R. Collet-Lacoste, Acta Mater. 46 (1998) 869. [2] B.G. Pound, in: J. O’M Bockris et al. (Eds.), Modern Aspects of Electrochemistry, vol. 25, Plenum Press, New York, 1993. [3] H. Addach, P. Berçot, M. Rezrazi, M. Wery, Mater. Lett. 59 (2005) 1347–1351. [4] P.W. Liu, J.K. Wu, Mater. Lett. 57 (2003) 1224. [5] M. Devanathan, Z. Stachurski, Proc. R. Soc 270A (1962) 90. [6] I. Flis-Kabulska, J. Flis, T. Zakroczymski, Electrochim. Acta 53 (2008) 3094– 3101.
267
[7] U. Prakash, N. Parvathavarthini, R.K. Dayal, Intermetallics 15 (2007) 17–19. [8] N. Amokrane, C. Gabrielli, G. Maurin, L. Mirkova, Electrochim. Acta 53 (2007) 1962–1971. [9] I. Flis-Kabulska, T. Zakroczymski, J. Flis, Electrochim. Acta 52 (2007) 2966– 2977. [10] L.W. Tsay, M.Y. Chi, Y.F. Wu, J.K. Wu, D.-Y. Lin, Corros. Sci. 48 (2006) 1926– 1938. [11] E.M.K. Hillier, M.J. Robinson, Corros. Sci. 48 (2006) 1019–1035. [12] C. Gabrielli, G. Maurin, L. Mirkova, H. Perrot, B. Tribollet, J. Electroanal. Chem. 590 (2006) 1–14. [13] W.C. Luu, P.W. Liu, J.K. Wu, Corros. Sci. 44 (2002) 1783–1791. [14] T. Zakroczymski, E. Owczarek, Acta Mater. 50 (2002) 2701–2713. [15] J. Mike, Danielson, Corros. Sci. 44 (2002) 829–840. [16] Kumkum Banerjee, U.K. Chatterjee, Scripta Mater. 44 (2001) 213–216. [17] J.P. Hirth, Metall. Trans. 11A (1980) 861. [18] Y. Fukai, The metal–hydrogen system: basic bulk properties, in: U. Gonser (Ed.), Springer Series in Material Science, Vol. 21, Second rev. and updated ed., 2005, XII, 497 p. 291, Springer-Verlag, ISBN: 978-3-540-00494-3. [19] R. Kirchheim, Mater. Sci. 32 (1988) 262. [20] Yukitaka Murakami, Hisao Matsunaga, Int. J. Fatigue 28 (2006) 1509–1520. [21] M. Nagano, Y. Hayashi, N. Othani, M. Isshiki, K. Igaki, Trans. JIM 22 (1981) 423. [22] M. Nagano, Y. Hayashi, N. Othani, M. Isshiki, K. Igaki, Scripta Met. 16 (1982). [23] J. Crank, The Mathematics of Diffusion, second ed., Oxford University Press, Oxford, 1979. p. 51. [24] J.-L. Dillard, S. Talbot-Besnard, Determination of the diffusion of hydrogen by measuring the permeability in high purity iron, Compt. Rend. Acad. Sci. (Paris). Ser. C 269 (1969) 1173. [25] E.W. Johnson, M.L. Hill, Trans. Met. Soc. Aime 218 (1960) 1104. [26] B. Laveissiere, J.L. Philippart, J. Pagetti, Electrochim. Acta 36 (3=4 ) (1991) 615– 620.