Journal of The Electrochemical Society, 161 (5) C261-C267 (2014) C2610013-4651/2014/161(5)/C261/7/$31.00 © The Electrochemical Society

Effect of Nitriding on the Hydrogen Diffusion Coefficient throughAISI 4340

J. J. M. Jebaraj,a,∗ D. J. Morrison,b J. B. McLaughlin,a and I. I. Sunia,c,∗∗,d,z

aDepartment of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, New York 13699-5705, USAbDepartment of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, New York 13699-5725, USAcMaterials Science and Engineering Program, Clarkson University, Potsdam, New York 13699-5705, USA

The effect of plasma nitriding on hydrogen permeation through AISI 4340 was studied both experimentally and theoretically.Effective hydrogen diffusion coefficients were measured for three different AISI 4340 membrane specimens: as-received, nitrided,and nitrided specimens with the compound layer removed by mechanical abrasion. The as-received specimen had a higher diffusioncoefficient (2.8 × 10−6 cm2/sec) than the nitrided specimens. The nitrided specimen with the compound layer intact had a muchlower hydrogen diffusion coefficient (0.65 × 10−6 cm2/sec) than the specimen without the compound layer (1.9 × 10−6 cm2/sec).These results demonstrate that the γ′-Fe4N rich compound surface layer and the N that diffuses more deeply into AISI 4340 duringplasma nitriding both reduce the effective hydrogen diffusion coefficient. Multiple permeation transients yield evidence for thepresence of only reversible trap sites in as-received specimens and the presence of both reversible and irreversible trap sites innitrided specimens, with and without the compound zone intact. In addition, density functional theory-based molecular dynamicssimulations yield hydrogen diffusion coefficients through γ′- Fe4N (1.5 × 10−5 cm2/sec) one order of magnitude lower than throughα-Fe (1.2 × 10−4 cm2/sec), which supports the experimental measurements of hydrogen permeation.© 2014 The Electrochemical Society. [DOI: 10.1149/2.077405jes] All rights reserved.

Manuscript submitted January 10, 2014; revised manuscript received February 26, 2014. Published April 1, 2014.

High strength alloys such as AISI 4340 are vulnerable to corro-sion when employed in sour environments where hydrogen sulfideand chlorides are present. Such corrosion can often be controlled bythe application of cathodic protection. However, this typically resultsin substantial hydrogen evolution at the protected surface, and sub-sequent adsorption and absorption of hydrogen into the metal latticemay lead to hydrogen embrittlement. Thus hydrogen uptake, perme-ation, diffusion, and trapping play a significant role in determiningmaterial susceptibility to hydrogen embrittlement.

Hydrogen embrittlement can be mitigated by reducing the hydro-gen uptake through surface modification by electrodeposition of Pb,1

Ni,2,3 and Bi;4 formation of aluminized layers;5 shot peening;6 lasersurface hardening;7 plasma deposition of an amorphous alloy;8 or ni-triding, carburizing, or sulfurizing.9–14 Plasma nitriding is an effectivesurface modification process due to the increased fatigue, wear, andcorrosion resistance; in addition to reduced hydrogen uptake withinthe nitrided layer.10,15–17 During plasma nitriding, low-pressure, hightemperature ammonia decomposition produces nitrogen atoms thatbombard the Fe surface and diffuse into the metal lattice. Nitrogenmay also form stable nitrides with nitride-forming elements such asMo, Cr, and Si, constituting the diffusion zone. This process alsoresults in the nucleation and growth of iron nitrides (ε-Fe3N, and γ′-Fe4N) on the surface (compound zone), which increases in thicknesswith time, gas composition, and temperature.18

Hydrogen diffusion through Fe alloys that are susceptible to hy-drogen embrittlement has been extensively studied using the electro-chemical hydrogen permeation method developed by Devanathan andStachurski.19,20 Here we report the effect of plasma nitriding on hydro-gen permeation through AISI 4340. As-received AISI 4340 specimensexhibit a higher hydrogen diffusion coefficient (2.8 × 10−6 cm2/sec)than nitrided specimens. In addition, nitrided specimens with the com-pound layer intact exhibit a much lower hydrogen diffusion coefficient(0.65 × 10−6 cm2/sec) than specimens where the compound layer isremoved by mechanical abrasion (1.9 × 10−6 cm2/sec). Multiple per-meation transients yield evidence for reversible and irreversible trapsin nitrided AISI 4340 specimens, with and without the compoundzone. The slow transport of hydrogen in all three specimens dur-ing decay may be due to the presence of reversible traps betweenwhich hydrogen exchanges before exiting the membrane. In addi-

∗Electrochemical Society Student Member.∗∗Electrochemical Society Active Member.

dPresent address: Materials Technology Center, Southern Illinois University,Carbondale, Illinois 62901,USA.

zE-mail: isuni@clarkson.edu

tion, density functional theory-based molecular dynamics simulationsyield hydrogen diffusion coefficients through γ′- Fe4N (1.5 × 10−5

cm2/sec) one order of magnitude lower than through α-Fe (1.2 × 10−4

cm2/sec).

Experimental

An AISI 4340 bar of 3.8 cm diameter was purchased from AEDMotorsport Products and cut into 1.3 mm thick disks. These speci-mens were mechanically abraded on both sides to 1.0 mm thicknessto remove the cold worked surface created during cutting. The spec-imens were further mechanically abraded on both sides using SiCpolishing pads to a final thickness of 0.88–0.92 mm, yielding a mirrorsmooth surface. The as-received bar was reported by the manufac-turer to be cold drawn, normalized, and tempered. The compositionof AISI 4340 bar is given in Table I. Nitrided specimens were pre-pared by plasma nitriding in the temperature range of 510–523◦C atAHT Corporation. For some experiments, the compound zone thatformed during nitriding was removed by mechanical abrasion with800 grit SiC for 5 min., followed by 1200 grit SiC for 5 min. AllAISI 4340 specimens were ultrasonically cleaned in methanol andwater and then coated with a ∼20 nm thick Pd layer on both sides byevaporation.

Microstructural characterizations were accomplished using anOlympus PME optical microscope, a JEOL 7400 scanning electronmicroscope (SEM) with energy dispersive X-ray spectrometer (EDX),Bruker Model D8-FOCUS - X-ray diffractometer, and a Leco M-400Knoop/Vickers Hardness Tester. The microstructures of nitrided spec-imens were analyzed by cutting and mounting cross sections in epoxy,then grinding with 600, 800, and 1200 grit SiC and polishing with 0.05μm alumina. The mirror surface finish was then etched with 2% Nital(2 mL HNO3 and 98 mL methanol) for 45 s. Hardness measurementswere performed along the cross section using a Knoop indenter witha 100 g load. For X-ray diffraction (XRD) studies, sample scans wereperformed over a 2θ range of 35–75◦.

Hydrogen permeation experiments were performed with an elec-trochemical cell made of Pyrex. The cell consists of hydrogen charg-ing and hydrogen measurement chambers separated by the AISI 4340specimen (Figure 1). The exposed membrane area on each side isabout 5.0 cm2. The electrolyte in the hydrogen charging chamberis 0.1 M H2SO4, while the electrolyte in the measurement chamberis 0.2 M NaOH. Both electrolytes were purged with high purity Arprior to experimentation. In addition, the measurement electrolytewas pre-electrolyzed at a constant potential of +2.0 V for 24 hr.

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C262 Journal of The Electrochemical Society, 161 (5) C261-C267 (2014)

Table I. Chemical composition of AISI 4340.

Element Composition (wt%)

C 0.39Cr 0.81Fe 95.6Mn 0.72Mo 0.23Ni 1.74Si 0.25Cu 0.18

Other 0.076

All electrochemical experiments were performed at room tempera-ture (22◦C) using a saturated calomel reference electrode (SCE) andPt counter electrode.

Before introducing the charging electrolyte, the measurement elec-trolyte was oxidized at a potential of +200 mV vs. SCE to re-move impurities, and the oxidation current was monitored. After 10–15 hr., the background current dropped to a steady state value of∼0.2 μA/cm2. At this time, the charging electrolyte was introduced,and the hydrogen permeation transient studies begun. In the hy-drogen charging chamber, a constant current density of 2 mA/cm2

was maintained using a Mastech HY 3005D power supply, while inthe hydrogen measurement chamber, a constant potential of +200mV vs. SCE was maintained by a Gamry G-750 floating groundpotentiostat which oxidized and quantified the hydrogen that trans-ported through the membrane specimens. During decay transients, thecharging electrolyte was removed to prevent corrosion and surfaceoxidation.

Modeling of Hydrogen Diffusion

Ab initio molecular dynamic simulations based on density func-tional theory (DFT)21,22 were performed using the CASTEP modulein Materials Studio.23–25 A 2 × 2 × 2 supercell (Figure 2) was con-structed for both α – Fe (BCC) and γ′–Fe4N (antiperovskite) crystalstructures. The lattice parameters for the Fe and Fe4N unit cells were0.2866 and 0.3795 nm, respectively. The supercell consisted of eightunit cells with periodic boundary conditions which ensured that thecomputational cell was large enough to obtain reliable diffusion co-efficients. Four interstitial hydrogen atoms were introduced into eachsupercell. A generalized gradient approximation (GGA) was usedwith the Perdew, Burke, and Ernzerhof (PBE) functional.26 The simu-lations were carried out at 427◦C with a constant atoms, constant vol-ume, and constant temperature (NVT) ensemble. The temperature wasmaintained using a Nose-Hoover-Langevin (NHL) thermostat with anoise Q ratio of 1.0. Elevated temperature was used for the simulationsto reduce the computational time. Simulations were performed for atotal of 2 ps with a time step of 0.5 fs. The plane wave energy cut

Figure 1. Schematic of the experimental cell for hydrogen permeation exper-iments.

Figure 2. Crystal structure of a 2 × 2 × 2 supercell of α – Fe (a), and γ′ –Fe4N (b).

off was customized to 375 eV with a fine quality FFT grid. A coarse(10−5 eV/atom) threshold was chosen for determining self-consistentfield convergence. A customized 4 × 4 × 4 Monkhorst-Pack k-pointgrid27 was used along with ultrasoft reciprocal space pseudopotentials.The output of the molecular dynamics simulation was analyzed forthe mean square displacement of hydrogen atoms using the Forcitemodule. The diffusion coefficients of hydrogen were extrapolatedfrom the slope (a) of the mean square displacement curves based onequation 1,28:

D = a

6[1]

a = limt→∞

d

dt

Nα∑

i=1

⟨|ri (t) − ri (0)|2⟩

where ri is the position of atom i, Nα is the number of atoms inthe simulation, and the angular brackets represent the mean-squaredisplacement as a function of time.

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Journal of The Electrochemical Society, 161 (5) C261-C267 (2014) C263

Figure 3. Cross sectional analysis: Optical microscopy (a), SEM (b), andKnoop and EDX measurements (c) of the nitrided AISI 4340 specimen.

Results and Discussion

Microstructural analysis of AISI 4340 membrane.— Figure 3 il-lustrates optical microscopy, SEM, EDX, and hardness measurementsfor a nitrided AISI 4340 specimen with the compound layer intact.Figures 3a and 3b illustrate the 2.5 μm thick compound zone that isformed atop AISI 4340 during plasma nitridation. Figure 3c illustrates

Figure 4. Cross sectional analysis: Optical microscopy (a), and SEM (b) ofthe plasma nitrided AISI 4340 specimen with the compound layer removed.

a second effect of plasma nitridation, formation of a diffusion zone∼200 μm thick. EDX analysis yields a nitrogen composition of ∼25atom% near the surface, and gradually declines to near zero in thebulk sample. EDX analysis from the top of the specimen samples alarger area and yields a nitrogen composition of ∼20 atom%.

The Knoop hardness as a function of depth is also illustrated inFigure 3c, and this declines from a near-surface value of ∼530 HKto the bulk value of ∼300 HK at a depth of 200 μm. Interestingly,the nitrogen concentration declines to nearly zero at a depth of about50 μm, much more rapidly than the Knoop hardness reaches its bulkvalue. This might be due to variation in the solubility of nitrogen (0.1wt% to 0.01 wt%) in α–Fe with depth below the specimen surface.29

These values rapidly become less than the detection limit of quanti-tative EDX measurements, so the depth of the nitrogen compositiontail below the surface may not be measurable.

Figure 4 illustrates the optical microscope (a) and SEM (b) imagesof a plasma nitrided AISI 4340 specimen for which the compoundzone has been removed. Figure 5 illustrates the XRD results for allthree specimens. The as-received specimen (5a) yields α – Fe {110}and {200} peaks corresponding to the α-ferrite Fe phase.30 The ni-trided specimen with the compound zone intact (5b) shows intensepeaks corresponding to ε – Fe3N and γ′– Fe4N phases, with a muchlower intensity for the α-ferrite peaks. The nitrided specimen withthe compound zone removed (5c) shows a much higher intensityfor the α–Fe peaks, but peaks from both the ε – Fe3N and γ′-Fe4Nphases still appear, suggesting a small concentration of these phasesin the diffusion zone. Overall, these results confirm that our polishing

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C264 Journal of The Electrochemical Society, 161 (5) C261-C267 (2014)

Figure 5. X-ray diffraction spectrum of as-received (a), nitrided (b), and ni-trided AISI 4340 with the compound layer removed (c).

technique removes the compound layer, but retains much of the dif-fusion zone.

Hydrogen diffusion coefficients and solubility within AISI 4340membranes.— Figures 6–8 illustrate repeated hydrogen permeationtransients through as-received AISI 4340 membranes and throughplasma nitrided AISI 4340 membranes, with and without the com-

Figure 6. Repeated hydrogen permeation transients through an as-receivedAISI 4340 membrane.

Figure 7. Repeated hydrogen permeation transients through a nitrided AISI4340 membrane.

Figure 8. Repeated hydrogen permeation transients through a nitrided AISI4340 with the compound layer removed.

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Journal of The Electrochemical Society, 161 (5) C261-C267 (2014) C265

Table II. DH in as-received AISI 4340.

Diffusion coefficient (×10−6 cm2 sec−1)

Method 1st transient (rise) 1st transient (decay) 2nd transient (rise) 2nd transient (decay) 3rd transient (rise)

Time lag 2.8 2.2 1.8Breakthrough time 3.1 3.1 3.1

Fourier analysis 2.7 1.5 2.3 1.2 1.8Laplace analysis (slope) 2.9 1.1 2.2 1.5 1.8

Laplace analysis (intercept) 3.0 1.1 2.2 1.3 2.0Range 2.7–3.1 1.1–1.5 2.2–3.1 1.2–1.5 1.8–3.1

Table III. DH in nitrided AISI 4340.

Diffusion coefficient (×10−6 cm2 sec−1)

Method 1st transient (rise) 1st transient (decay) 2nd transient (rise) 2nd transient (decay) 3rd transient (rise)

Time lag 0.54 0.96 0.92Breakthrough time 0.99 1.6 1.7

Fourier analysis 0.55 0.77 0.97 0.78 0.99Laplace analysis (slope) 0.60 0.58 0.97 0.63 0.94

Laplace analysis (intercept) 0.58 0.59 0.96 0.53 0.87Range 0.54–0.99 0.58–0.77 0.96–1.6 0.53–0.78 0.87–1.7

Table IV. DH in nitrided AISI 4340 with compound layer removed.

Diffusion coefficient (×10−6 cm2 sec−1)

Method 1st transient (rise) 1st transient (decay) 2nd transient (rise) 2nd transient (decay) 3rd transient (rise)

Time lag 1.6 2.0 2.0Breakthrough time 2.1 2.8 2.8

Fourier analysis 1.7 0.84 2.1 0.87 2.0Laplace analysis (slope) 1.8 0.97 2.1 0.89 2.0

Laplace analysis (intercept) 1.7 0.82 2.1 0.82 1.9Range 1.6–2.1 0.82–0.97 2.0–2.8 0.82–0.89 1.9–2.0

pound zone. The results of electrochemical hydrogen permeationtransients were analyzed assuming constant hydrogen concentration atthe hydrogen charging surface as the boundary condition, as discussedelsewhere,31,32 to obtain effective hydrogen diffusion coefficients. Theresults of these analyzes are given in Tables II–IV. Table V summa-rizes the average hydrogen diffusion coefficient obtained for eachpermeation transient for all three specimen types.

For as-received AISI 4340, the average hydrogen diffusion coef-ficients for all decay transients (2.1–2.9 × 10−6 cm2/sec) are ∼ 2×lower than those obtained from all rise transients (1.2–1.4 × 10−6

cm2/sec). A similar trend is observed for the nitrided specimens, withand without the compound layer intact. Without the compound layer,the hydrogen diffusion coefficients are 1.8–2.1 × 10−6 cm2/sec and0.86–0.88 × 10−6 cm2/sec for the rise and decay transients, respec-tively. With the compound layer, the hydrogen diffusion coefficientsfor rise and decay transients are 0.65–1.1 × 10−6 cm2/sec and 0.65× 10−6 cm2/sec, respectively. The hydrogen that might be reversiblytrapped at high energy trap sites during the rise transients diffusesslowly along the lattice diffusion path during the decay transient, re-

sulting in a lower effective diffusion coefficient.33 The slow desorptionof hydrogen during the decay transient relative to the rise transienthas been observed by several authors in Fe alloys.34–36 The slightdecrease in effective diffusion coefficient values of subsequent risetransients for the as received specimens might be due to the formationof an oxide layer on the charging surface, which slows the transportof hydrogen.37 On the other hand, the first rise transient has a lowereffective diffusion coefficient compared to successive rise transientsin nitrided AISI 4340 specimens, with and without compound zone.This can be attributed to the filling of irreversible trap sites during thefirst rise transient thereby sampling primarily reversible trap sites insuccessive rise transients.32

The lowest hydrogen diffusion coefficients in Table V are obtainedfor the plasma nitrided AISI 4340 specimens with the compound zoneintact. This illustrates the reduction in hydrogen permeation due toformation of γ′-Fe4N, which acts as an effective hydrogen diffusionbarrier.9,11,14 This observation is supported by the permeation curves inFigure 9, where the steady state permeation current density is signifi-cantly reduced by the compound layer. The reduction in hydrogen

Table V. Average DH for each hydrogen permeation transient in each membrane.

Diffusion coefficient (×10−6 cm2 sec−1)

Specimen 1st transient (rise) 1st transient (decay) 2nd transient (rise) 2nd transient (decay) 3rd transient (rise)

As-received 2.9 1.2 2.4 1.4 2.1Nitrided 0.65 0.65 1.1 0.65 1.1

Nitrided without compound layer 1.8 0.88 2.2 0.86 2.1

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C266 Journal of The Electrochemical Society, 161 (5) C261-C267 (2014)

Figure 9. Repeated hydrogen permeation transients through an as-received(- - - -), nitrided (- - - - - -), and nitrided AISI 4340 with the compound zoneremoved (- .. - .. -).

permeation attributable to the presence of the compound zone isgreater than that attributable to the diffusion zone, as previouslyreported.14 The hydrogen diffusion coefficient values reported inTable III for the nitrided specimens with compound layer removedcomplement this understanding, wherein the diffusion coefficient val-ues obtained for both the rise and decay transients are slightly lowerthan for as-received AISI 4340 (Table II). Nevertheless, since the dif-fusion zone is much thicker than the compound zone, hydrogen trans-port is still impeded.14 These results are generally consistent with theeffective diffusion coefficients of hydrogen within Armco iron cal-culated by electrochemical hydrogen permeation studies.14 However,their results in nitrided specimens with and without compound zonelacked information regarding the effective diffusivity of hydrogen inmultiple permeation transients. Modeling studies will be describedbelow to estimate the hydrogen diffusion coefficient values throughthe compound zone, which is composed mainly of γ′-Fe4N.

The effective hydrogen solubility, which is averaged over the dif-ferent microstructures present, can be determined from:

Capp = i∞L

nF Def f

where i∞ is the steady state permeation current density, L is themembrane thickness, n is the number of electrons transferred dur-ing hydrogen oxidation/reduction, and F is Faraday’s constant.Table VI summarizes the hydrogen solubilities obtained from thepermeation transients in Figures 6–9. The overall solubility of the ni-trided membrane, which includes the compound zone, diffusion zone,and ferrite layer, is lower than the as-received ferritic structure asreflected by the reduction in the permeation current, in agreementwith the literature.10,34 However, the nitrided specimens with com-

Table VI. Apparent hydrogen solubility (Capp) in each membrane.

Hydrogen solubility (×10−6 mol cm−3)

1st transient 2nd transient 3rd transientSpecimen (rise) (rise) (rise)

As-received 9.5 7.8 6.1Nitrided 6 5.3 3.4

Nitrided withoutcompound layer

4.5 3.5 3.3

pound layer removed show a decrease in solubility compared to thenitrided specimens with compound layer due to the increased mobilityof hydrogen in the absence of the compound layer. In addition, thesolubility decreases for subsequent rise transients in as-received spec-imens, as reflected by the reduction in steady state permeation currentdensity. This may be due to surface oxide film formation in the ab-sence of charging, which would hinder the entry of hydrogen.37,38

On the other hand, the steady state permeation current densities arealmost equal for the nitrided AISI 4340 specimens with and withoutthe compound zone, where a significant oxide film formation wasnot observed. Nevertheless, the solubility decreases with successivetransients in these specimens due to the increased hydrogen transportrate.

Effect of nitriding on mechanical properties and hydrogenembrittlement.— The ratio of ε/γ′ phase determines the mechanicalproperties of the compound layer. A thin γ′-Fe4N phase is extremelyductile and exhibits superior fatigue properties,39 whereas a mixedphase of ε/γ′ is hard and brittle.40,41 In addition, the compound zonehas an open, porous structure.42 This can be mechanically abradedto remove the compound zone or oxidized to form a passive film,43

which enhances the corrosion resistance. Alternatively, the thicknessof the compound zone can be controlled by manipulating the plasmanitriding process parameters.18 Regardless, mechanical abrasion inpractice may cause removal of the compound zone, so the effect thishas on hydrogen transport is of interest.

The determination of hydrogen diffusivity, solubility and trappingis important for understanding susceptibility to hydrogen embrittle-ment. However, other environmental factors such as cathodic pro-tection potential, temperature, pressure, stress, strain, and strain ratealso impact the materials susceptibility to hydrogen-induced stresscorrosion cracking. Because the overall effect depends on complexrelationships between the hydrogen mobility, formation of calcare-ous deposits, metal hydride formation, hydrogen dissolution, andhydrogen trapping at defects; the material response depends on allof these variables, as well as their inter-relationships.44 To the bestof the authors’ knowledge, only the research group of Pound hassuggested a general formalism for relating hydrogen transport pa-rameters extracted from electrochemical measurements to predic-tion of hydrogen embrittlement lifetimes.45,46 This approach employspotentiostatic pulse measurements to determine an apparent trap-ping rate constant for irreversible traps in the presence of reversibletraps.

However, that approach has not been widely employed given thecomplex inter-relationships between the experimental variables de-scribed above. Indeed, given the complexity of the problem, a moredirect approach of reducing hydrogen transport by a surface coat-ing method might be considered more appealing. Other researchgroups have followed a similar approach and suggested electrode-posited coatings of Pb, Ni, or Bi;1–4 surface nitriding, carburizing,or sulfurizing;9–14 or other surface modification by aluminizing, shotpeening, laser treatment, or plasma thin film deposition.5–8 In thosecases, as in the current case, the reduction of hydrogen ingress intometal alloys susceptible to hydrogen embrittlement can be demon-strated.

Simulations of hydrogen diffusivity within α-Fe and γ′- Fe4N.—The hydrogen diffusion coefficients in bcc α-Fe were found to be∼1.2 × 10−4 cm2/sec at 427◦C (Figure 10), in good agreement withboth modeling47 and experimental studies of pure Fe48 at elevated tem-perature. The hydrogen diffusivity within the antiperovskite γ′ – Fe4Ncrystal structure (1.5 × 10−5 cm2/sec) at 427◦C is about one order ofmagnitude lower than within the bcc α-Fe phase. These results are gen-erally consistent with our experimental results, which yield reducedhydrogen diffusion coefficient values for plasma nitrided specimenswith the compound layer intact. Since the compound layer is only∼2.5 μm thick, and the entire membrane is ∼900 μm thick, the

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Journal of The Electrochemical Society, 161 (5) C261-C267 (2014) C267

Figure 10. MSD fits to calculate diffusion coefficient of hydrogen in α – Fe,and γ′ – Fe4N.

overall effect illustrated by the hydrogen diffusion coefficients inTable V is considerably less than one order of magnitude.

Conclusions

The effect of nitriding in determining hydrogen permeation proper-ties through AISI 4340 was studied both experimentally and theoreti-cally. Hydrogen diffusion coefficients were analyzed on three differentspecimens: as-received, nitrided, and nitrided with compound layerremoved. The as-received specimen had a higher effective diffusioncoefficient (2.8 × 10−6 cm2/sec) compared to the nitrided withoutcompound layer (1.9 × 10−6 cm2/sec) and nitrided specimens (0.65× 10−6 cm2/sec). Multiple permeation transients revealed the natureof trap sites to be reversible based on the lower diffusion coefficientvalues for decay transients compared to rise transients in all the threeconditions. Moreover, the presence of irreversible trap sites in nitridedspecimens with and without compound zone can be evidenced basedon the lower effective diffusion coefficient for the first rise transientcompared to the subsequent rise transient. In addition, first principlesmolecular- dynamics simulations showed that the diffusion coefficientof hydrogen through γ′- Fe4N (1.52 × 10−5 cm2/sec) is an order ofmagnitude lower than the α-Fe (1.23 × 10−4 cm2/sec) which supportsthe fact that the compound zone acts as an effective barrier for theentry of hydrogen.

Acknowledgments

The authors gratefully acknowledge the support of this researchby General Electric Oil and Gas.

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