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Hydrogen Permeation Estimated by HMT in Carbon Steel

Exposed to Gaseous Hydrogen

Keitaro Horikawa1, Hiroaki Okada1;*, Hidetoshi Kobayashi1 and Wataru Urushihara2

1Department of Mechanical Science and Bioengineering, School of Engineering Science,Osaka University, Toyonaka 560-8531, Japan2Surface Design & Corrosion Research Section, Materials Research Laboratory, Kobe Steel, Ltd., Kobe 651-2271, Japan

Hydrogen permeation behavior in carbon steel exposed to gaseous hydrogen was visualized using a hydrogenmicroprint technique (HMT).Effects of hydrogen gas pressure and charging time on the hydrogen permeation were particularly examined. The amount of permeated hydrogenwas dependent on the charging time during the exposure to gaseous hydrogen. It was found that silver particles, which represented the evolutionsite of hydrogen atoms, were distributed almost uniformly in the matrix after hydrogen gas charging. These particles were arranged at theperiphery of the second phase particles such as Al2O3. Area density of the silver particles clearly increased when the time for hydrogen gascharging was increased. Preferential accumulation of silver particles around Al2O3 particles was clearly identified; however, no silver particleswere observed directly on the Al2O3 particles. This indicated that hydrogen atoms were diffused not through the inside of the second phaseparticles but through the interface between the second phase particles and the matrix phase. [doi:10.2320/matertrans.M2009121]

(Received April 7, 2009; Accepted June 5, 2009; Published July 23, 2009)

Keywords: carbon steel, hydrogen, diffusion, hydrogen microprint, inclusions

1. Introduction

It is well known that hydrogen has a deleterious effect onthe mechanical properties of many materials such as carbonsteels,1–5) stainless steels,6–9) and aluminum alloys.10–12)

Among these materials, ferritic and martensitic steels withhigh strength are more susceptible to hydrogen embrittlementbecause of the high diffusivity of hydrogen in the latticestructures. Recently, a number of systems using hydrogen gaswith high-pressure as a source of new energy have beendeveloped in the industries. In the case of the materials usedfor such industrial applications, it is important to clarify theinteraction between hydrogen gas and steels. When the steelsare exposed to a hydrogen gas atmosphere, hydrogen atomspermeate into the steels because of the dissociation frommolecular hydrogen to atomic hydrogen, and induce hydro-gen embrittlement. Thus far, few experiments have beenconducted to determine the local paths of hydrogen diffusionin steels affected by a hydrogen gas atmosphere. With respectto the hydrogen diffusion in steels, the authors have recentlyshown that a hydrogen microprint technique (HMT) isadaptable to visualize hydrogen diffusion at temperaturesbelow 120�C in a low alloy steel hydrogen charged cathodi-cally.13) The visualization method carried out using the HMTprovides intuitive views regarding the diffusion path ofhydrogen. In the present study, from a basic point of view,effects of hydrogen gas pressure and charging time on thehydrogen permeation were investigated by using the HMT incarbon steel, in which hydrogen gas charging was conducted.

2. Experimental Procedure

The material used in the present study was fully annealedcarbon steel whose chemical composition and tensile proper-ties at room temperature are given in Table 1 and Table 2.

The round specimens (diameter: 23mm; thickness: 1.6mm)were cut from the original plate. Hydrogen diffusion testswere performed along with carrying out the HMT. Prior tothe HMT, both sides of the specimen surfaces were polishedusing #240, #800, and #1200 SiC papers and buffed with asolution containing alumina powders (particle size: 0.3 mmand 0.05 mm). For the polished specimens, hydrogen gascharging was conducted by using the apparatus shown inFig. 1. Only one side of the specimen surfaces was exposeddirectly to gaseous hydrogen under a hydrogen pressure of0.25, 1.0, or 5.0MPa at room temperature. The hydrogen gas

Table 1 Chemical composition of carbon steel (mass%).

C Si Mn S P Fe

0.45 0.21 0.74 0.011 0.009 Bal.

Table 2 Tensile properties of carbon steel ( _"" ¼ 6:7� 10�3 s�1).

Yield Stress

(MPa)

Tensile Strength

(MPa)

Fracture Strain

(%)

331 586 50.7

Fig. 1 Experimental device for hydrogen gas charging.

*Graduate Student, Osaka University

Materials Transactions, Vol. 50, No. 9 (2009) pp. 2201 to 2206#2009 The Japan Institute of Metals

charging time was 5 or 30min. Before hydrogen gascharging, the rear side of the surface exposed to gaseoushydrogen was covered with a liquid nuclear emulsion (IlfordL-4, diluted by pure water, 2 times) composed of gelatin andsilver bromide crystals using a wire loop method. Within5min after hydrogen gas charging, the specimens weredipped into formalin (40mass% HCHO water solution) for3 s to harden the gelatin layer and then immersed in a fixingsolution (15mass% Na2S2O3 water solution) for 6min toremove the silver bromide particles that had not reacted withhydrogen. During the tests, 10mass% water solution ofNaNO2 was used to dilute the emulsion and to make thefixing solution in order to prevent any corrosion in aqueoussolutions. Arrangement of the silver particles was observedwith a scanning electron microscope (SEM) equipped with anenergy dispersive X-ray spectrometer (EDXS).

3. Results

HMT images in the specimens with and without hydrogengas charging are shown in Figs. 2 and 3. No particles werevisible in the uncharged specimen; however, the distributionof white particles was clearly identified in the matrix phase inthe specimens that were hydrogen charged even for 5minunder hydrogen pressures of 0.25, 1.0, and 5.0MPa. In thespecimens that were hydrogen charged for 30min (Fig. 3),white particles having a large diameter were partiallyobserved as compared to in the specimens that were hydrogencharged for 5min. All of the white particles observed wereidentified as silver by an EDXS analysis, as shown in Fig. 4.

Magnified images adjacent to Al2O3 inclusions are shownin Figs. 5 and 6. It was shown that silver particles werearranged at the interface between the Al2O3 inclusion and thematrix phase after hydrogen gas charging for 5min. Theaccumulation of the silver particles around the inclusionsclearly increased when the charging time was prolonged from5min to 30min as shown in Fig. 6. However, in this case,no silver particles were identified directly on the Al2O3

inclusions.In order to estimate the difference in hydrogen diffusion

behavior under different conditions, the area density of silverparticles with respect to the entire observed area wasmeasured in the SEM micrographs. The area density ofsilver particles was determined on the basis of four areas,which were randomly selected in order to ensure thereliability of the results. The relationship between the areadensity of silver particles and the hydrogen gas chargingconditions is summarized in Fig. 7. It is clear that the areadensity of the silver particles increased when the hydrogengas charging time was increased. On the basis of thedistribution of the observed silver particles, we roughlyestimated the number of hydrogen atoms that evolved fromthe specimen surface by assuming that all of the silverparticles observed represented the evolution of atomichydrogen. Suppose that total area of the silver particlesdivided by the atomic diameter of silver represents thenumber of silver atoms, the number of hydrogen atoms isthen the same as the number of silver atoms, according tothe principle chemical reaction of the HMT, Agþ þ H ¼Agþ Hþ. The relationship between the molar quantity of

0 MPa 0.25 MPa

5 MPa1 MPa

25 µm 25 µm

25 µm25 µm

(a) (b)

(c) (d)

Fig. 2 HMT images after hydrogen gas charging in carbon steel exposed to gaseous hydrogen for 5min at room temperature.

2202 K. Horikawa, H. Okada, H. Kobayashi and W. Urushihara

evolved hydrogen and the hydrogen gas pressure is summa-rized in Fig. 8. When the effects of hydrogen pressure andcharging time are compared, it is obvious that hydrogenpermeation is highly affected by the charging time from5min to 30min rather than by the hydrogen pressure from 0to 5MPa at room temperature.

4. Discussion

4.1 Hydrogen permeation estimated by HMTThe amount of evolved hydrogen on the surface opposite

from the surface exposed to gaseous hydrogen was theoret-ically calculated. According to Sievert’s law,14) the amountof hydrogen absorption, C0

H is dependent on the appliedhydrogen gas pressure as follows:

C0H

CFe¼

ffiffiffiffiffiffiP

P0

rexp �

�Hs

RTþ

�Ss

R

� �ð1Þ

where CFe represents the mass concentration of the solvent ina unit volume; Ss and Hs represent the entropy and enthalpyof the hydrogen dissolved in the specimen; P and T representhydrogen gas pressure and temperature during the testing;and P0 and R represent ambient air pressure (0.1MPa) andthe gas constant, respectively. On the basis of eq. (1), wecan determine the amount of hydrogen absorbed on thesurface exposed to gaseous hydrogen. In contrast, hydrogendiffusion in the specimen is generally expressed by usingthe following Fick’s law:14)

J ¼ �D@CH

@x; ð2Þ

where J is the mass flux per unit area and unit time; D, thediffusion coefficient; CH, the hydrogen concentration; and x,the specimen thickness. When the specimen thickness is d

and the amount of hydrogen absorbed on the surface is C0H,

eq. (2) can be expressed as follows:

J ¼ DC0H

d; ð3Þ

The diffusion coefficient, D, is expressed as follows:

D ¼ D0 exp �Q

RT

� �; ð4Þ

where D0 and Q represent the frequency term and theactivation energy, respectively. On the basis of eqs. (1), (3),and (4), we can determine the following equation:

25 µm25 µm

0 MPa 0.25 MPa

5 MPa1 MPa

25 µm 25 µm

(a) (b)

(c) (d)

Fig. 3 HMT images after hydrogen gas charging in carbon steel exposed to gaseous hydrogen for 30min at room temperature.

Ag

Fe

Energy, E / keV

Fig. 4 Example of EDX analysis of the white particles observed in Fig. 3.

Hydrogen Permeation Estimated by HMT in Carbon Steel Exposed to Gaseous Hydrogen 2203

5 µm5 µm

5 µm5 µm

0 MPa 0.25 MPa

5 MPa1 MPa

(a) (b)

(c) (d)

Fig. 5 Magnified images of Fig. 2, area adjacent to the Al2O3 particle.

5 µm5 µm

5 µm5 µm

0 MPa 0.25 MPa

5 MPa1 MPa

(a) (b)

(c) (d)

Fig. 6 Magnified images of Fig. 3, area adjacent to the Al2O3 particle.

2204 K. Horikawa, H. Okada, H. Kobayashi and W. Urushihara

J ¼D0CFe

d

ffiffiffiffiffiffiP

P0

rexp �

1

RTðQþ�HsÞ þ

�Ss

R

� �; ð5Þ

Thus, the number of evolved hydrogen atoms, Na, in thesurface opposite from the surface exposed to gaseoushydrogen can be represented as follows:

Na ¼D0CFe

d

ffiffiffiffiffiffiP

P0

sexp �

1

RTðQþ�HsÞ þ

�Ss

R

� �At; ð6Þ

where A and t represent the area exposed to gaseous hydrogenand the time for hydrogen gas charging. Using eq. (6), we canestimate the number of hydrogen atoms diffused to the rearside of the surface that is exposed to gaseous hydrogen byusing the reported parameters14,15) given in Table 3. Therelationship between the amount of evolved hydrogen and theapplied hydrogen gas pressure as a function of hydrogencharging time is summarized in Fig. 9, together with theexperimental data of the HMT, shown in Fig. 8. Thecalculated data suggests that hydrogen atoms can diffuse tothe rear side of the surface that is exposed gaseous hydrogenunder the present testing conditions. However, the calculatedvalue of the hydrogen evolution was more than 10 times ashigh as the experimental value determined from the HMTexperiments. One reason for this difference is that, in thecalculation, the effects of hydrogen trapping in the specimenwere completely ignored and the test specimen was assumednot to be carbon steel but to be pure iron. Moreover, in theHMT, the total number of evolved hydrogen atoms was

estimated from the area density of silver particles in limitedareas, and the silver particles on the surfaces were actuallythree-dimensional and not two-dimensional. Ichitani16,17) hasalso pointed out, on the basis of an electrochemical hydrogenpermeation method, that the detection efficiency of conven-tional HMT was approximately as low as 1% and that a highratio of the evolved hydrogen atoms did not react with silverbromide. In accordance with the earlier report,16) the presentexperimental data of hydrogen permeability obtained byusing the HMT was approximately 2 orders of magnitudelower than the comparable calculated values. Thus, theestimation of hydrogen permeability using the HMT wouldbe valid even in the case of the hydrogen gas charging.

The calculation based on eq. (6) also suggests that theamount of hydrogen evolution increases linearly with thecharging time. However, in the case of its dependence on thehydrogen gas pressure, the amount of hydrogen evolutionincreases in proportion to the square root of this pressure.These theoretical expectations are partial in agreement withthe qualitative result of the HMT experiments, that is, theamount of evolved hydrogen increased particularly when thecharging time was increased, while the amount of evolvedhydrogen remained almost unchanged in spite of an increasein the hydrogen pressures up to 5MPa, as previously shownin Figs. 8 and 9. It is assumed that hydrogen permeation inthe present testing condition does not completely representthe diffusion under steady state, since there is inconsistency

Fig. 7 Area density of silver particles vs. hydrogen gas charging

conditions.

Fig. 8 Relationship between the molar quantity of evolved hydrogen and

the applied hydrogen gas pressure in specimens that were hydrogen

charged for 5 or 30min.

Table 3 Data used for the calculation of hydrogen permeation.14;15Þ

Parameters Values

CFe (mol�m�3) 1:41� 105

�Hs (J�mol�1) 2:9� 104

�Ss (J�(K�mol)�1) 49.9

T(K) 298

R (J�(K�mol)�1) 8.31

P0 (Pa) 0:1� 106

D0 (m2�s�1) 4:2� 10�8

Q (J�mol�1) 3850

d (m) 1:6� 10�3

A (m2) 1:0� 10�4

t (s) 300, 1800

Fig. 9 Comparison of the amount of hydrogen evolved from the surface

estimated from the theoretical calculation and the HMT result.

Hydrogen Permeation Estimated by HMT in Carbon Steel Exposed to Gaseous Hydrogen 2205

between the theoretical calculation and the HMT result, as forthe effect of gas pressure. It is probable that the hydrogen gascharging times up to 30min is too short to reach the steadystate diffusion since the dissociation of molecular hydrogeninto atomic hydrogen on the specimen surface dependsstrongly on charging time and surface condition, as reportedelsewhere.16)

4.2 Interaction of hydrogen with inclusionsIn the present testing conditions, hydrogen was released

mainly from the matrix phase and not from the Al2O3

inclusions, as shown in Figs. 5 and 6. It is reported that thediffusion coefficient of hydrogen in the carbon steels rangesfrom 4:2� 10�9�1:0� 10�8 m2/s,14,18) while that in theAl2O3 ranges from 3:4� 10�46�2:6� 10�29 m2/s.19) Thus,it is assumed that the low diffusivity of hydrogen in the Al2O3

is closely related to the HMT result obtained in this study, inwhich silver particles were not visible directly on the Al2O3

inclusions. Otsuka reported20) on the basis of tritium auto-radiography (TAR) that tritium was trapped on the interfacebetween matrix phases and Al2O3 inclusions in steel. Inprinciple, TAR can detect trapped tritium that does notdiffuse out to the surface; in contrast, HMT can detectdiffused hydrogen. On the basis of the results of both thereported TAR20) and the HMT obtained in the present study,we concluded that the interface between Al2O3 and thematrix phase worked as the diffusion path of hydrogen aswell as the reversible weak trapping site of hydrogen at roomtemperature.

5. Summary

Hydrogen permeation behavior in carbon steel exposed togaseous hydrogen was investigated by using an HMT. Theresults obtained were summarized as follows: (1) The areadensity of the visualized hydrogen atoms increased partic-ularly when the hydrogen gas charging time was increased.(2) The HMT result in the present testing conditions wasin accord qualitatively with the theoretical calculation. (3)

Preferential accumulation of the visualized hydrogen atomswas observed at the periphery of the second phase particles,Al2O3, while not observed directly on the particles.

Acknowledgments

The work was supported by the Ministry of Education,Culture, Sports, Science and Technology of JapaneseGovernment, Grant-in-Aid for Scientific Research (C),20560652, 2008.

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