Effect of dislocation cell walls on hydrogen adsorption, hydrogen trapping and hydrogen embrittlement resistance

Journal Pre-proof Effect of dislocation cell walls on hydrogen adsorption, hydrogen trapping and hydrogen embrittlement resistance Lin Chen (Conceptualization) (Methodology) (Investigation) (Validation)Writing- original draft), Xilin XiongDiscussion), Xuan TaoDiscussion), Lijie Qiao (Writing - review and editing) (Supervision), Yanjing Su (Conceptualization) (Methodology) (Writing - review and editing) (Supervision)

PII:

S0010-938X(19)32188-2

DOI:

https://doi.org/10.1016/j.corsci.2020.108428

Reference:

CS 108428

To appear in:

Corrosion Science

Received Date:

17 October 2019

Revised Date:

25 December 2019

Accepted Date:

2 January 2020

Please cite this article as: Chen L, Xiong X, Tao X, Qiao L, Su Y, Effect of dislocation cell walls on hydrogen adsorption, hydrogen trapping and hydrogen embrittlement resistance, Corrosion Science (2020), doi: https://doi.org/10.1016/j.corsci.2020.108428

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Effect of dislocation cell walls on hydrogen adsorption, hydrogen trapping and hydrogen embrittlement resistance

Lin Chen1, 2, Xilin Xiong1, 2, Xuan Tao1, 2, Lijie Qiao1, 2, Yanjing Su1, 2, * 1

Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing,

2

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Beijing 100083, PR China

Corrosion and Protection Center, University of Science and Technology Beijing,

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Beijing 100083, PR China

The hydrogen coverage first increases and then decreases with increasing

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

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Highlights

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[email protected])

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Correspondence and requests for materials should be addressed to Y.-J.S. (email:

deformation.



The hydrogen embrittlement resistance is improved when materials are experienced a severe deformation.

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

Dislocation cells are produced when the deformation is severe, and the dislocation cell walls with a sufficiently high dislocation density are proved to be an effective hydrogen trap.

Abstract

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This study investigated the effects of dislocation patterns introduced by cold

rolling deformation on the hydrogen absorption properties, hydrogen trap behaviors

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and hydrogen embrittlement (HE) resistance for Armco iron under electrochemical charging. The surface hydrogen coverage first increases and then decreases with

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increasing deformation, and reaches a maximum at 50% deformation. Dislocation cell

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walls appear when the deformation is greater than 50%. Furthermore, the HE resistance is improved when the deformation reaches 80%. Because the dislocation

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cell walls can be used as effective hydrogen traps when the dislocation density reaches a sufficiently high value.

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Keywords: Iron; Dislocation cell walls; Hydrogen traps; Hydrogen absorption;

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Hydrogen coverage; Hydrogen embrittlement resistance

1. Introduction Hydrogen embrittlement (HE) is a severe problem in industry because it often causes catastrophic failure of structural materials. Therefore, improving the HE 2

resistance of materials through the composition of materials and design of the structures is of vital importance. To increase the resistance to HE in steels, the dispersion of hydrogen traps, such as precipitates [1-5] and grain boundaries [6-10], have been proposed to hinder the movement of hydrogen and reduce the content of diffusible hydrogen [6, 11-13]. Dislocations occur in both machining and deformation

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processes, and dislocations are common kind of hydrogen trap [14-19], but the role of dislocations on HE resistance is controversial. For the relatively low binding energy (26.8 kJ/mol) of hydrogen atoms, dislocations are considered to be reversible

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hydrogen traps. [14, 17]. Li et al. [20] and Takasugi et al. [21] found that the HE

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susceptibilities increased when metals experienced tensile prestrain. However, Michler et al. [22] found that when the dislocation density of 316 austenitic stainless

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steel increases to a high value, the HE sensitivity is quite low, but further researches

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are needed.

With an increase in plastic deformation, the dislocation density increases [23],

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and the strength of a material improves [24]. A network structure of dislocations that cross each other forms when the plastic deformation level is low, subsequently,

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dislocation cells form as the deformation continues to increase [25]. Accompanied by the aggravation of the plastic deformation, the size of the dislocation cells and the thickness of the dislocation cell walls are reduced [26]. However, the dislocation density of the dislocation cell walls increases [27], which induces a substantial

3

amount of lattice strain (lattice distortion) [28, 29]. The interaction of the lattice strain with the strain field of the hydrogen atoms increases the binding energy of the hydrogen atoms in dislocation cell walls [30]. It was confirmed by thermal desorption spectroscopy (TDS) that the desorption peak corresponding to the dislocations was shifted to high temperatures with increasing deformation [31]. Thus, the binding

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energy between the dislocations with hydrogen atoms may be different, and the dislocation cell walls may play a beneficial role in the HE resistance.

The hydrogen evolution reaction in acid solutions and the hydrogen entry into

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metal are shown in the following reactions [32, 33]: k1 H   M  e-   M - H ads

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(1)

k2 M - H ads  M - H ads   H 2  2M

kdes

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kabs  M - H ads  H abs  M

(2) (3)

where 𝑀 is the metal electrode; 𝑀 − 𝐻𝑎𝑑𝑠 and 𝐻𝑎𝑏𝑠 represent the hydrogen atoms

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adsorbed on the metal surface and absorbed into the metal, respectively; 𝑘1 and 𝑘2

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are the discharge and recombination rate constants, respectively; 𝑘𝑎𝑏𝑠 is the rate constant for hydrogen absorption into the metal at the charging surface; and 𝑘des is

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the desorption rate constant for the hydrogen moving from the absorbed to adsorbed state.

In addition, the lattice distortion introduced by plastic deformation and the dislocations on the surface could also affect the surface hydrogen adsorption

4

properties of the materials, such as the efficiency of hydrogen evolution [34] as well as the number of hydrogen adsorption sites (hydrogen coverage on surface) [35]. Therefore, surface hydrogen adsorption could affect the hydrogen concentration in the materials, thus eventually affecting the HE sensitivity of the materials. In this study, an electrochemical hydrogen permeation test, slow rate tensile test

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and TDS test [14, 16, 18, 36] combined with the Iyer-Pickering-Zamenzadeh (IPZ) model [33, 37-39] were used to investigate the effects of dislocations on the hydrogen

adsorption properties, hydrogen trap behaviors and HE resistances of Armco iron with

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various amounts of cold rolling (CR) deformation.

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2. Experimental

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2.1. Materials

Armco iron (99.9%) was selected as the test sample to prevent the effect of

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precipitates and inclusions on the test. The initial thickness of the as-received material

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was approximately 10 mm. CR was then performed on the plates, with reductions of

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30%, 50%, 70% and 80%. The final thicknesses are shown in Table 1.

2.2. Electrochemical measurements

Electrochemical measurements were carried out using an electrochemical

workstation (Ivium-n-Stat, Ivium Technologies BV, Holland). A platinum wire and saturated calomel electrode (SCE) were used as the counter electrode and the 5

reference electrode, respectively. For all electrochemical tests, the samples were polished with a 40 nm colloidal silica slurry for more than 1 h to remove the surface residual strain, then polished with deionized water for 0.5 h and ultrasonically cleaned in ethyl alcohol for 0.5 h to ensure the removal of the colloidal silica slurry. In addition, all of the electrochemical tests were conducted at approximately 298 K, and

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nitrogen was injected into a solution to remove oxygen throughout the experiment.

2.2.1. Potentiodynamic polarization tests

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The potentiodynamic polarization tests were performed in 0.05 M H2SO4 to

characterize the corrosion potential (𝐸𝑐𝑜𝑟𝑟 ). The potentiodynamic polarization curves

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were obtained at 0.33 mV/s in the potential range of open circuit potential ± 250

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mV.

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2.2.2. Electrochemical permeation tests

The modified “Devanathan–Stachurski cell” [40] shown in Fig. 1 was used to

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characterize the hydrogen diffusion and hydrogen adsorption behaviors. To protect the iron against anodic dissolution, a 100 nm Ni film was sprayed on the exit side. The

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surface exposed to the solution had an area of 0.785 cm2 (a circle with a diameter of 10 mm). The entry side and the exit side were immersed in 0.05 M H2SO4 and 0.2 M NaOH, respectively. To avoid the influence of the surface effects, the thickness of the samples for 6

detecting the effective hydrogen diffusion coefficient was greater than 1 mm [41]. Both sides of the membranes were investigated by the potentiostatic method, and the overpotentials were -800 mV and 280 mV for the entry side and the exit side, respectively. Unlike the previous experiments, the thickness of the membranes for the IPZ

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tests was approximately 0.5 mm to highlight the surface effects [39, 41]. Six different cathodic overpotentials (-620 mV, -600 mV, -580 mV, -560 mV, -540 mV and -520 mV) were applied to the entry side for each membrane with different amounts of

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deformation. Anodic overpotential (280 mV) was performed on the exit side.

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According to the IPZ model [33, 37-39], the Frumkin adsorption isotherm was

is as shown in Eq. (4): 𝛥𝐺𝜃0 = 𝛥𝐺00 + 𝑓𝑅𝑇𝜃

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introduced, and the free energy of the species adsorption correlated with the coverage

(4)

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where 𝜃 is the hydrogen coverage; 𝛥𝐺𝜃0 and 𝛥𝐺00 are the standard free energy of

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adsorption at certain coverage and zero coverage, respectively; 𝑅 is the gas constant; and 𝑇 is the absolute temperature; 𝑓 is a dimensionless factor that describes the

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deviation from the ideal Langmuir behavior. The change rate of the standard free energy of adsorption is represented by 𝑓𝑅𝑇. Three relationships were derived for the hydrogen evolution reaction (HER) and the hydrogen adsorption reaction (HAR), as shown in Eq. (5) - (11) based on the

7

original IPZ model and Frumkin adsorption isotherm as follows [37]: 𝑖𝑐 = 𝑖𝑟 + 𝑖∞

(5)

𝐹

𝑖∞ = 𝑘√𝑘 ⋅ √𝑖𝑟

(6)

2

1

𝑖𝑐 𝑒𝑥𝑝(𝑎𝛼𝜂) = 𝑖0′ (1 − 𝐹𝑘 ⋅ 𝑖∞ ) 𝐹𝐷𝐶𝑜

(8)

𝐿

𝑖0′ = 𝐹𝑘1 𝐶𝐻 + = 𝑖0 /(1 − 𝜃𝐻𝑒 )

(9)

𝑎 = 𝐹/𝑅𝑇 𝑘=

𝑘𝑎𝑏𝑠 1+𝑘𝑑𝑒𝑠

(10) (11)

𝐿 𝐷

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𝑖∞ = 𝐹𝑘𝜃𝐻 =

(7)

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where 𝑖∞ is the steady-state hydrogen permeation current density; 𝑖𝑟 is hydrogen

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recombination current density; 𝑖𝑐 is the charging current density; 𝑖0 is the exchange current density of the HER; 𝜃𝐻𝑒 is he hydrogen coverage at equilibrium; 𝛼 is the

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electron transfer coefficient; 𝜂 is the overpotential of the HER; 𝑘 is the kinetic

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diffusion constant; 𝐿 is the thickness and 𝐷 is effective hydrogen diffusion coefficient of the samples.

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Finally, Eq. (8), (12) and (13) were developed for evaluating the kinetic parameters 𝑘1 , 𝑘2 , 𝑘 and 𝜃 [37]: √𝑖

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𝑙𝑛 ( 𝑖 𝑟 ) = 𝑙𝑛 ( ∞

√𝑖𝑟 𝑖∞

√𝑘2 𝐹

𝑖𝑐 𝑒𝑥𝑝(𝑎𝛼𝜂) =

1

𝛼𝑓

⋅ 𝑘) + 𝐹𝑘 ⋅ 𝑖∞ √𝑘2 𝐹

⋅

𝑖0′ 𝑘

(1 −

(12) 1 𝐹𝑘

⋅ 𝑖∞ )

(13)

2.3. Slow strain rate tensile (SSRT) tests

The flat dog-bone-type specimens used herein, which were machined along the 8

rolling direction, are shown in Fig. 2 (a). Prior to the test, the specimens were mechanically abraded with SiC grinding paper down to 2000 grit, rinsed with distilled water and cleaned with ethyl alcohol. The specimens were tested under dynamic hydrogen charging in 0.05 M H2SO4 at a current density of 0.5 mA/m2. A schematic of the equipment is shown in Fig. 2

mm/min-1, corresponding to a strain rate of 5×10-6 s-1.

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2.4. Dislocation density measurements

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(b). The SSRT was conducted at approximately 298 K with a cross-head rate of 0.006

The dislocation density (𝜌) was estimated by X-ray diffraction (XRD) using a D8

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advance Bruker instrument with Cu 𝐾𝛼 radiation (𝜆 =1.5418 Å). The specimens

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were mechanically polished, followed by electrolytic polishing with 20 ml perchloric acid and 80 ml ethanol at 30 V for 30 s. The XRD patterns of the samples were with a

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step size of 0.02 ° steps and 15 s counting time.

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A modified Williamson-Hall method [42] was used to estimate the dislocation density from the XRD profiles. The broadening of diffraction peaks is related to the

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average crystallite size (𝛿) and the lattice microstrain (𝜉) as follows: 𝛽 𝑐𝑜𝑠 𝜁 𝜆

=

0.9 𝛿

+

2𝜉 𝑠𝑖𝑛 𝜁

(14)

𝜆

where 𝜁 is the diffraction angle and 𝛽 is the full width at half-maximum (FWHM) of the diffraction peak at 𝜃. Williamson and Hall [42, 43] determined the relationship between dislocation 9

density (𝜌) and lattice microstrain (𝜉) as follows: 𝜌=

14.4𝜉 2 𝑏

(for bcc metals)

(15)

where 𝑏 is the Burgers vector.

2.5. Microstructural observations

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Optical microscopy (OM) images of the pure iron with different CR reductions were obtained with an OLTMPUS BX60M optical microscope. Mechanical grinding

and mechanical polishing were performed on the samples, and the microstructure was

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revealed by chemical etching with an alcohol solution containing 4% of nitric acid

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(vol.%).

Transmission electron microscopy (TEM) images were captured with a FEI

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Tecnai G20 at an accelerating voltage of 200 kV. The TEM samples were thinned to 40 μm by grinding with SiC paper and twin-jet electropolished using a mixture of 8%

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perchloric acid (vol.%) and 92% acetic acid (vol.%) at 243 K with a potential of 30 V.

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2.6. Thermal desorption spectroscopy (TDS)

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The hydrogen content in the specimens was calculated by the integral hydrogen desorption curve, which was obtained by TDS [44]. The specimens were charged in 0.05 M H2SO4 at a current density of 0.5 mA/cm2, and all specimens were hydrogenated long enough to ensure hydrogen saturation. Before the specimens were placed in the vacuum tube, they were cleaned and dried with deionized water and 10

ethanol (within 5 min). It is noteworthy that the equipment had been vacuumized before the end of hydrogen charging, and the time needed for vacuumizing after putting the sample into the equipment was less than 5 min. Meanwhile, in order to reduce the experimental error, the sample size needs to be as large as possible (length: 35 mm, width: 15 mm; thickness: 1.8 mm). The test was started when the vacuum

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level was sufficiently low and the heating rate was 100 K/h.

3. Results

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3.1. Hydrogen diffusion and hydrogen adsorption characteristic

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The grain sizes and morphologies of the samples with various amounts of CR deformation are shown in Fig. 3. The morphology of the grains gradually elongated,

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but the size of the grains showed no difference with an increase in the CR reduction. Fig. 4 shows the cathodic and anodic potentiodynamic polarization curves and

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the hydrogen permeation curves of 1 mm membranes with five different amounts of

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CR deformation in 0.05 M H2SO4. In the linear Tafel region, the relationship between the cathodic overpotential and current density are shown in Eq. (16). Eq. (17) is the

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formula for calculating the effective hydrogen diffusion coefficient (𝐷) [45]: 2.3𝑅𝑇

𝜂 = − (1−𝛼)𝐹 𝑙𝑜𝑔 𝑗 0 +

2.3𝑅𝑇 𝛼𝐹

𝑙𝑜𝑔 𝑗𝐶

(16)

𝑡0.63 = 𝐿2 /6𝐷

(17)

where 𝑗 0 is the exchange current density; 𝑗𝐶 is the cathodic current density; t 0.63 is

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the time where (𝑖 − 𝑖0 )/(𝑖∞ − 𝑖0 ) = 0.63; 𝑖0 is the initial current density of the hydrogen permeation; and 𝑖∞ is the steady-state permeation current density. The value of corrosion potential (𝐸𝑐𝑜𝑟𝑟 ), electron transfer coefficient (𝛼) and effective hydrogen diffusion coefficient (𝐷) are shown in Table 2, where it can be observed that 𝐸𝑐𝑜𝑟𝑟 and the polarization curves of the five different membranes have

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basically the same trends, and the values of 𝛼 are similar. 𝐷 decreased with increasing CR reduction.

The permeation curves for the 0.5 mm membranes produced with five different

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amounts of CR deformation under various applied overpotentials are shown in Fig. 5.

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The delay in the course of the permeation curves increases with increasing CR deformation, which is consistent with the previous hydrogen diffusion data for 1 mm

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membranes. Regarding the same membrane, 𝑖∞ decreased with a decrease in the overpotential, while the breakthrough time (𝑡𝑏 ) increased.

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According to Eq. (12) and (13), the relationships (𝑙𝑛( √𝑖𝑟 /𝑖∞ ) and 𝑖∞ ,

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（√𝑖𝑟 /𝑖∞ ) ⋅ 𝑖𝑐 ⋅ 𝑒𝑥𝑝( 𝛼𝜂𝐹/𝑅𝑇) and 𝑖∞ ) are shown in Fig. 6 (a) and (b), respectively. Table 3 illustrates the dynamics parameters (𝑘, 𝑘1 , and 𝑘2 ) derived from the slope

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and the intercept of the two graphs. The values of 𝑘 for different samples changed slightly; however, the values of 𝑘1 and 𝑘2 changed obviously. The trends of 𝑘1 and 𝑘2 are shown in Fig. 7 (a). The values of 𝑘1 first increased and then decreased with an increase in the CR reduction, and among the samples herein, the CR 70%

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sample had the highest value for 𝑘1 . Contrary to 𝑘1 , the value of 𝑘2 first decreased and then increased. Finally, using the value of 𝑘 in Table 2, the hydrogen coverage (𝜃) was obtained with Eq. (8). The relationship between 𝜃 and potential is shown in Fig. 6 (d). The values of 𝜃 first increased and then decreased following an increase in the CR reduction. The values of 𝑘𝑎𝑏𝑠 and 𝑘𝑑𝑒𝑠 , calculated by Eq. (5), (8), and

however, 𝑘𝑑𝑒𝑠 decreased with the CR reduction.

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3.2. Tensile tests under dynamic hydrogen charging

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(11), are shown in Table 3 and Fig. 7 (b). The value of 𝑘𝑎𝑏𝑠 remained the same;

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Fig. 8 shows the engineering stress–strain curves obtained from the in-situ hydrogen charging tensile test. With an increase in the CR deformation, the ultimate

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tensile strength (UTS) and the yield strength (YS) increased and the elongation decreased. The reduction in the elongation (RE), which is used to quantify HE

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susceptibility, is shown in Table 4. The HE resistance decreased before the

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deformation reached 70%, however, the HE resistance of the CR 80% samples was

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enhanced.

3.3. Dislocation density and dislocation structures

The XRD patterns of the (1 1 0), (2 0 0), and (2 1 1) planes for different CR

reductions are shown in Fig. 9. Because the grain sizes of the materials used in this paper are much larger than 100 nm, 0.9⁄δ could be ignored [46]. According to Eq. 13

(14) and Eq. (15), the total dislocation density (𝜌) increased with increasing CR reduction, as shown in Fig. 11 (b). The morphologies of the dislocations that varied with the degree of CR deformation are displayed in Fig. 10. The as-received sample had a small number of dislocations, and the dislocations were uniformly dispersed. When the deformation

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reached 30%, the number of dislocations increased and dislocation pile-ups occurred. With an increase in the deformation, dislocation cell walls appeared, moreover, the

size of the dislocation cells and the thickness of the dislocation cell walls decreased

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gradually. A schematic diagram is shown in Fig. 11 (a), and the thickness of the

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dislocation cell walls and the fraction of walls are shown in Table 5. Eq. (18) indicates the relationship between the wall thickness (𝑒) and the dislocation density in the

𝐾 = 𝑒√𝜌𝑊

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𝜌 = 𝑓𝑊 𝜌𝑊 + (1 − 𝑓𝑊 )𝜌𝐶

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dislocation cell walls (𝜌𝑊 ), where the value of K is 2.4 for iron [47, 48]. (18) (19)

The fraction of dislocation cell walls (𝑓𝑊 = 𝑆𝑊 ⁄𝑆) can be determined with TEM

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images. Finally, the dislocation density in the dislocation cell (𝜌𝐶 ) is calculated by Eq.

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(19). The dislocation parameters are shown in Table 5, and the relationships of the CR reductions and the dislocation parameters are shown in Fig. 11 (b). With an increase in the deformation, 𝜌𝑊 increased drastically, and 𝜌𝐶 decreased.

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3.4. TDS results

Fig. 12 illustrates the effect of cold deformations on the TDS results for the pure iron. The temperature range for peak 1 was from 300 K to 500 K, which is consistent with the results by Choo [16]. The hydrogen trap corresponding to peak 1 was due to dislocations. Moreover, 𝜌 increased with increasing deformation, and the area of

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peak 1 increased obviously. The corresponding peak at 650 K (peak 3) remained for the as-received and CR 30% samples, while the number of grain boundaries was

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found to be basically the same for the 0% and 30% deformations in Fig. 3.

Furthermore, peak 3 had a high hydrogen trap binding energy in the high temperature

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range of 550 ~750 K, which further proved that it corresponded to high-angle grain

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boundaries [14]. Hydrogen traps in pure iron usually include dislocations, vacancies and grain boundaries. The temperature range of peak 2 is similar to that for the

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vacancy results from Choo [16], and there was no change in peak 2 with the various amounts of deformation. In addition, Fig. 12 (c-e) shows that new peaks (peak 4)

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appeared for the CR 30%, CR 50% and CR 70% samples at 450 K. As the

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deformation continued to increase, the area of peak 4 decreased, and the area of peak 3 increased. Considering the results in Fig. 10 and Fig. 11 (b), dislocation pile-ups and dislocation cells appeared at 30% and 50% deformations respectively. Due to the 𝜌𝑊 and 𝜌𝐶 trends for the 50% to 80% deformation samples, the number of dislocation pile-ups inside dislocation cells decreased after 50% deformation; therefore, peak 4 15

represented dislocation pile-up. By comparing the data for the CR 50%, CR 70%, and CR 80% samples in Fig 10 and Fig. 11 (b), it can be seen that 𝜌𝑊 increased continuously with an increase in the deformation; conversely, 𝜌𝐶 decreased. Simultaneously, Fig. 12 (d-f) shows an increase in the area of peak 3, however, the number of grain boundaries of three deformed materials (CR 50%, CR 70%, and CR

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80%) did not change. As a result, the broadening of the peak 3 was due to the appearance of dislocation cell walls. It should be noted that peak 1 obviously shifted to the high temperature region, however, C decreased after dislocation cells

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appeared. These phenomena indicate that the dislocation cell wall was composed of a

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central structure with a high trap binding capacity and an external structure with a low

4. Discussion

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trap binding capacity.

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4.1. Hydrogen permeation behavior

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Fig. 3 illustrates that the grain size for the five different CR reductions was basically the same; therefore, the effect of CR on the grain size and grain boundaries

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can be neglected. According to Fig. 4, the CR can delay the 𝑡𝑏 and reduce the 𝐷 under the same gain size condition because of the trapping effect of dislocations [14, 49, 50]. The relationship of the permeation behavior and the overpotentials for 0.5 mm

16

membranes is illustrated in Fig. 5. For the same samples, as the overpotential increased, 𝑡𝑏 decreased and 𝑖∞ increased. These phenomena are consistent with literature reports [51-53] due to the surface effect [39, 41]. In addition, 𝐷 was positively correlated with the amount of hydrogen ion adsorption on the surface [54]. Hence, for the same electrochemical system, 𝐷 decreased and 𝑡𝑏 was extended with

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an increase in the overpotential.

4.2. Effects of the dislocation patterns on the IPZ kinetic parameters

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The results of the kinetic parameters (𝑘, 𝑘1 , 𝑘2 , and 𝜃) of the IPZ are shown in

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Fig. 6, Fig. 7 and Table 3. The value of 𝑘1 (the kinetic parameter of Volmer adsorption action), which is related to the rate of the HER, is shown in Fig. 7 (a) The

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value of 𝑘1 first increased and then decreased after the CR 70% case. In our experiments, 𝜌𝑊 increased with an increase in the CR reductions, however, 𝜌𝐶

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increased first and decreased after the CR 50% case. The conclusions of Alami [35,

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55] should be considered because it was observed here that although the number of additional adsorption sites increased with an increase in the dislocation density, the

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value of 𝜌𝑊 was very high when the deformation reached 80%, which changes the results. The reason is that the dislocations that were concentrated in the wall led to a disorder in the atomic arrangement, which had geometric and energy effects. Simultaneously, due to the stress concentration, the electronic structure of the surface may be changed by the new geometric structure and energy disturbance [35]. 17

Ultimately, these results made additional adsorption sites inaccessible or inactivated them. In addition, the dissolution energy of metal to hydrogen atoms first decreased and then increased with an increase of electron cloud density [56], which indicated that the interaction between the metal atoms and hydrogen atoms was affected by the density of the electron cloud. In summary, the value of 𝑘1 increased first and then

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decreased when the deformation reached 80%. The kinetic parameter of Tafel chemical desorption is 𝑘2 , as shown in reaction

(2). The difficulty of this process is related to the Gibbs free energy (𝛥𝐺), as shown in

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Eq. (20): 𝛥𝐺 = 𝐸𝐻2 + 𝐸𝑀 − 𝐸𝑀−𝐻𝑎𝑑𝑠

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(20)

where 𝐸𝐻2 is the binding energy between hydrogen atoms; 𝐸𝑀 is the binding energy

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between metal atoms; and 𝐸𝑀−𝐻𝑎𝑑𝑠 is the binding energy between adsorbed hydrogen atoms and metal atoms.

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The values of 𝐸𝐻2 and 𝐸𝑀 were assumed to remain the same in the overall

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electrochemical reactions, and the changes in 𝛥𝐺 were considered to be related to the 𝐸𝑀−𝐻𝑎𝑑𝑠 trend. The bond between adsorbed hydrogen atoms and metal atoms was

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mainly formed by the electrons in the hydrogen atom and the d-band electrons in the metal that are not paired [57]. The surface electronic structure (the d-band center) and the activity for the oxygen-reduction reaction exhibited ‘volcano-type’ behavior [58]. Similarly, a decrease in the CO adsorption occurred because of the increase in the

18

distance caused by tensile strain [59-61]. It should be noted that the existence of dislocations can change the distance between metal atoms [62]. Moreover, due to the disorder in the arrangement of the metal atoms created by the dislocations concentrated in the dislocation cell walls, some geometric and energetic effects were simultaneously aroused, and the electronic structure was modified [35]. Therefore, the

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variation in the kinetic parameters (𝑘2 ) was due to the variation in the dislocation patterns.

The relationship of the trap activation energy (𝐸𝑎 ), interaction energy between

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trapping site and hydrogen (𝐸𝑏 ), and saddle point energy around the trapping site (𝐸𝑠 )

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is shown in Eq. (21) [16]. 𝐸𝑎 = 𝐸𝑠 + 𝐸𝑏

(21)

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Three kinetic parameters (𝑘, 𝑘𝑎𝑏𝑠 , and 𝑘des ) are shown in reaction (3). Because

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the 𝐸𝑠 of the grain boundaries and dislocations are similar [16], the energy of hydrogen entry into dislocations with different structures was basically the same, and

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a schematic of the binding energy for different traps is shown in Fig. 13 (a). Therefore, 𝑘𝑎𝑏𝑠 remains the same.

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Per the TSD data in Fig. 12, the increase in the binding energy of the hydrogen

traps was consistent with the increase in the deformation. As seen in the Eq. (22), 𝑝 represents the escape rate [63]. −(𝑄𝐷 +𝐸𝑏 )

𝑝 = 𝑝0 𝑒𝑥𝑝 {

𝑅𝑇

}

(22)

19

where 𝑝0 is a constant, and 𝑄𝐷 is the activation energy for diffusion. Therefore, the dislocations generated during CR process made it very difficult for hydrogen to escape; furthermore, 𝑘des decreased as the dislocation density increased. The value of the hydrogen coverage (𝜃) is shown in Fig. 6 (d). The 𝜃 value

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increased first and then decreased, and the maximum value appeared when the deformation reached 50%. These results indicate that the appearance of dislocation cells affected the amount of hydrogen adsorbed on the surface, and dislocation cell

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walls with a high dislocation density can reduce the value of 𝜃.

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4.3. Effects of the dislocation patterns on the HE susceptibility

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The SSRT data with and without hydrogen are shown in Fig. 8 and Table 4. To further investigate the effect of the amount of CR deformation on the HE, three

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dislocation densities were considered. The relation between the RE and total

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dislocation density is shown in Fig. 14. The HE sensitivity is aggravated with an increase in the CR deformation, however, the RE decreased for the CR 80% case. The

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RE decreased because there was a reduction in the 𝜌𝐶 and an increase in the 𝜌𝑊 . This trend revealed that a high value of 𝜌𝑊 could lead to an improved HE resistance [22]. In Fig. 12, the peak area of the dislocations (peak 1) increased, and the peak shifted to the high temperature range when the deformation increased. A new peak 20

(peak 4) appeared in the TDS data, the peak at 450 K appeared when dislocation pileups were formed. However, peak 4 disappeared for the CR 80% sample. Simultaneously, the peak area of the grain boundaries (peak 3) increased. From the XRD data and TEM images, new dislocation patterns were formed for CR 80% sample. These phenomena illustrate that the dislocation cell walls, similar to the grain

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boundaries, were formed when the deformation reached 80%; in addition, these structures can hinder the movement of hydrogen. At the same time, among the 𝜌𝐶

values for the different samples, it can be seen that CR 80% sample had the lowest

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value, which is similar to the value of the dislocation density for the sample without

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CR. Additionally, the reduction in the number of dislocations in the dislocation cells can also be observed in the TEM images (Fig. 10). Therefore, it was learned that most

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hydrogen was located at the dislocation cell walls and grain boundaries for the CR

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80% sample. Consequentially, the amount of diffusible hydrogen decreased, and a schematic is shown in Fig. 13 (b).

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Hydrogen enhanced decohesion (HEDE) [64-66] and hydrogen enhanced local plasticity (HELP) [67-69] are the two major mechanisms in hydrogen embrittlement

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phenomenon. HEDE considers a reduction in the cohesive bond strength between the metal atoms in the presence of hydrogen. HELP proposes an increase in dislocation mobility in the presence of hydrogen, which results into highly localized plastic deformation and faster failure. Both of the mechanisms revealed that the local

21

hydrogen concentration is vital to the occurrence of HE. Many researchers have discussed the effect of grain size on HE resistance. As the number of grain boundaries increases, the hydrogen concentration per unit grain boundaries decreases; therefore, metals with fine grains have excellent HE resistance [6, 8-11]. For the materials in this experiment, the dislocation density increased with increasing deformation,

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leading to a change in the dislocation structure. When the deformation reached 50%, dislocation cell walls formed, and with a continuously increase in the deformation, the thickness of the dislocation cell walls and the size of dislocation cells decreased. The

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dislocation cell walls in the CR 80% sample restricted the movement of hydrogen and

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dispersed the hydrogen uniformly (a sketch is shown in Fig. 13 (b)). In summary, the

value.

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5. Conclusions

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HE resistance of metals can be improved when the dislocation density reaches a high

During CR, severe plastic deformation and dislocations are introduced in metals,

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and the dislocation morphology also changes. These different dislocation

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morphologies affect the hydrogen absorption properties and HE resistance of the metals.

1. The total dislocation density increases as the deformation increases. Dislocation pile-ups occur in CR 30% samples. Also, when the deformation reaches 50%, dislocation cell walls are formed. When the deformation continues to increase, the 22

size of the dislocation cell walls and the thickness of the dislocation cell walls decrease, concurrently, the dislocation density of the dislocation cell walls increases, and the dislocation density of dislocation cell walls decreases. 2. With increasing deformation, the dislocation density increases, resulting in an increased number of hydrogen traps, which consequently leads to a decrease in the

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effective diffusion coefficient. Moreover, for the same membrane, the effective diffusion coefficient is decreased and the hydrogen breakthrough time is extended with a decrease in the overpotential.

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3. The increase in deformation leads to a change in the dislocation structure, which

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affects the hydrogen adsorption process. With a change in the dislocation density and dislocation morphology, 𝑘1 first increases and then decreases, 𝑘2 has the opposite

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trend, 𝑘𝑎𝑑𝑠 basically remains the same, and 𝑘𝑑𝑒𝑠 continues to decrease. The value

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of hydrogen coverage (𝜃) increases first and then decreases, and the maximum value appears when the deformation reaches 50%. Most importantly, the appearance of

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dislocation cells affects the amount of hydrogen adsorbed on the surface, thus affecting the process of hydrogen entering the material.

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4. When the deformation increases to 80%, due to the high dislocation density of dislocation cell walls, the movement of hydrogen can be restricted. Such dislocation cell walls are similar to grain boundaries and can disperse hydrogen in materials, therefore, the hydrogen embrittlement resistance can be improved.

23

In summary, the dislocation patterns vary with an increase in the deformation, the effective hydrogen diffusion coefficient decreases, and the kinetic parameters for the hydrogen adsorption and desorption processes are changed. Most importantly, the hydrogen embrittlement resistance is improved when the dislocation density of

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dislocation cell walls reaches a high value.

Declaration of interests

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The authors declare that they have no known competing financial interests or

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personal relationships that could have appeared to influence the work reported in this

Author statement

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paper.

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Lin Chen: Conceptualization, Methodology, Investigation, Validation, WritingOriginal draft preparation.

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Xilin Xiong: Discussion

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Xuan Tao: Discussion Lijie Qiao: Writing - Review & Editing, Supervision. Yanjing Su: Conceptualization, Methodology, Writing - Review & Editing,

Supervision.

24

Acknowledgements This work is supported by the National Key Research and Development Program National (2016YFB0300204) and the National Natural Science Foundation of China

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(Grant No. 51571028).

25

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Fig. 1. The modified Devanathan–Stachurski cell used in the experiment.

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Fig. 2. (a) Tensile sample geometry in mm (thickness = 1.6  mm). (b) Schematic

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diagram of the equipment of in situ tensile test under ongoing hydrogen charging.

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Fig. 3. The grain size and morphology of the samples with CR deformation amounts of 0%, 30%, 50%, 70% and 80%.

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Fig. 4. (a) Potentiodynamic polarization curves and (b) the normalized hydrogen

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permeation curves of various membranes in 0.05 M H2SO4.

Fig. 5. Permeation curves for different overpotential and membranes deformed by

33

various CR (0.2 mol/L NaOH in the oxidation cell and 0.05 M H2SO4 in the hydrogen charging cell at 298 K). (a), (b), (c), (d), (e) are as-received, CR 30%, CR 50%, CR

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70%, CR 80% membranes, respectively.

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Fig. 6. (a) Relation between the 𝑙𝑛( √𝑖𝑟 /𝑖∞ ) and 𝑖∞ , (b) relation between the

（√𝑖𝑟 /𝑖∞ ) ⋅ 𝑖𝑐 ⋅ 𝑒𝑥𝑝( 𝛼𝜂𝐹/𝑅𝑇) and 𝑖∞ , (c) relation between 𝑖∞ and the

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overpotentials for five different CR reduction samples, (d) relations among the

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hydrogen coverage (𝜃), five different CR reductions and the applied potential.

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Fig. 7. The trends for 𝑘1 𝑘2 , 𝑘𝑎𝑑𝑠 and 𝑘𝑑𝑒𝑠 as a function of CR reduction. (𝑘1 : the

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kinetic parameter of Volmer adsorption action; 𝑘2 : the kinetic parameter of Tafel

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chemical desorption; 𝑘𝑎𝑑𝑠 : the rate constant for hydrogen absorption into the metal at the charging surface; and, 𝑘𝑑𝑒𝑠 : the desorption rate constant for the hydrogen moving

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from the absorbed to adsorbed state).

Fig. 8. The Slow strain rate tensile (SSRT) curves by electrochemical charging in a 0.05 M H2SO4 at a current density of 0.5 mA/m2, where the SSRT testing was performed at 298 K with a crosshead rate of 0.006 mm/min-1, corresponding to a

35

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strain rate of 5×10-6 s-1. (b) is the magnified image in (a).

Fig. 9. The XRD patterns of the (1 1 0), (2 0 0), and (2 1 1) planes for different CR

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reductions: (a) CR 0%, (b) CR 30%, (c) CR 50%, (d) CR 70%, and (e) CR 80%.

Fig. 10. TEM images of iron with various CR reduction values. (a) and (a’): asreceived, (b) and (b’): CR 30%, (c) and (c’): CR 50%, (d) and (d’): CR 70%, and (e) 36

ro of

and (e’): CR 80%.

-p

Fig. 11. (a) Schematic of the thickness of a dislocation cell. (b) Three types of

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dislocation density (𝜌: total dislocation density; 𝜌𝑊 : dislocation density in the

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dislocation cell walls; and 𝜌𝐶 : dislocation density inside the dislocation cells).

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Fig. 12. Effect of cold deformation on the TDS spectrum of the purity iron. The heat rate was 100 K h-1. (a): TDS spectra for all the samples. TDS of (b) as-received, (c) CR 30%, (d): CR 50%, (e): CR 70%, and (f): CR 80% samples.

37

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distribution of hydrogen in materials.

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Fig. 13. (a) Schematic of the binding energy of different traps. (b) Schematic of the

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Fig. 14. The relation between the RE and total dislocation density.

38

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-p

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Table 1. The final thicknesses of the five plates. CR 30%

CR 50%

CR 70%

CR 80%

10.16 mm

6.85 mm

4.68 mm

2.94 mm

1.86 mm

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Thickness

as-received

40

Table 2. The corrosion potential (𝐸𝑐𝑜𝑟𝑟 ), electron transfer coefficient (𝛼) and effective hydrogen diffusion coefficient (𝐷) of five different CR deformed membranes. As -

CR 30%

CR 50%

CR 70%

CR 80%

-0.57

-0.57

-0.57

-0.57

-0.56

0.23

0.21

0.22

0.22

0.22

2.37×1

1.35×1

9.51×1

4.50×1

3.98×1

𝐸𝑐𝑜𝑟𝑟 (VSCE) 𝛼 𝐷 0-5

0-6

0-7

0-7

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(cm2/s)

41

0-7

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received

Table 3. Values of five rate constants obtained by the IPZ analyses for five different samples. 𝑘,

𝑘1 ,

(mol m-

(m s-1)

2 -1

s )

𝑘2 ,

𝑘𝑎𝑏𝑠 ,

(mol m-

(mol

2 -1

𝑘𝑑𝑒𝑠 , (cm s-1)

cm-2 s-1)

s )

As -

5.18×1

receive

0-6

5.03×1 0-7

6.75×1 0-4

1.04×1 0-3

4.75×1 0-4

d

C R 50%

5.43×1 0-6

C R 70%

0-4 3.81×1

0-6 5.48×1

0-6

C

5.60×1

5.13×1 0-6

3.49×1 0-5

0-6

1.09×1

1.27×1

6.87×1

1.10×1

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8.99×1

0-6

0-3

0-6

4.50×1

42

1.90×1

0-5

0-3

0-4

2.70×1

0-5

0-3

0-4

0-7

1.08×1

0-3

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R 80%

0-6

1.65×1

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0-6

1.31×1

-p

R 30%

5.39×1

1.03×1

re

C

8.99×1

Table 4. The elongation loss in 0.05 M H2SO4 with 0.5 mA/cm2 hydrogen charging. As received 13±3

CR 50%

CR 70%

CR 80%

35±2

52±2

60±3

48±3

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RE (%)

CR 30%

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Table 5. Values of pure iron dislocation parameters. 𝑒

Asreceived CR 30% CR 50%

70%

0.49

0.49

0.49

-

-

1.16

1.16

1.16

13

15.9

1.80

3.34

1.27

2.16

3

9.72

21.3 8

90. 37

26.2 6

2.24

4.79

1.14

7.05

0.52

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80%

m-2)

-

10

CR

m-2)

𝜌𝐶 (1014

-

1.35 CR

m-2)

(%)

𝜌𝑊 (1014

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(nm)

𝜌 (1014

𝑓𝑊

44