CORROSION SCIENCE

Hydrogen Embrittlement Interactionsin Cold-Worked Steel

H. Huang and W.J.D. Shaw*

ABSTRACT

Effects of cold work on hydrogen (H) transport, Hconcentration, and H embrittlement (HE) in type 1020 steel(UNS G10200) exposed in a sour environment wereexamined. Cold work decreased H diffusivity andincreased H concentration in the steel. The increase indislocations as a result of cold work was responsible forthe decrease in H diffusivity. The increase in H adsorptioncoverage on the surface and H trapping from cold workwere likely causes for increases in H absorption into thesteel. The fracture mode was dependent on specificsensitive locations within the steel that were affected bythe cold work. After a steady H damage state occurred inthe steel, the fracture toughness decreased with increasingcold work. Sufficiently low values of fracture toughness asa result of embrittlement occurred such that plane strainfracture occurred in relatively thin laboratory specimens.

KEY WORDS: cold work, hydrogen concentration,hydrogen diffusion, hydrogen embrittlement, hydrogensulfide, hydrogen transport, sour service, type 1020 steel

INTRODUCTION

Sulfide stress cracking (SSC) of steels is of particularconcern to oil and gas industries. Pipeline steels canfail when in contact with brine solutions containinghydrogen sulfide (H2S).1 Plastic deformation as aresult of bending, rolling, and handling often is

300010-9312/95/00000

© 1995, NACE I

Submitted for publication November 1993; in revised form, May 1994.* Department of Mechanical Engineering, University of Calgary, Calgary,

Alberta, T2N 1N4, Canada.

encountered in the manufacture and assembly ofpipeline components that later are exposed to serviceconditions.2 It is accepted generally that hydrogen(H) embrittlement (HE) can occur when the concen-tration of H in materials reaches a critical level.3

The H concentration in a material depends onenvironmental conditions, kinetics of H absorptioninto metals, and the presence of H traps.2-4 Trapsconsist of specific sites such as dislocations, voids,and grain boundaries. Cold work increasesdislocations, which, in turn, affects the HEinteraction.2

Many studies on the interactive behaviorbetween H and H trapping have been conducted.5-7

The electrochemical permeation method ofDevanathan and Stachurski often has been usedto determine H transport behavior.5 A barnacleelectrode has been developed to measure mobileH concentration of components exposed to serviceenvironments.6-7 The barnacle electrode techniqueuses only the extraction side of the permeation cell,based on the electrochemical permeationrelationships developed by Devanathan andStachurski. The barnacle electrode was used in thepresent study to measure H concentration and Htransport.

The objective of the present work was to obtainan understanding of the effects of cold work on Htransport, H concentration, and HE in pipeline steelexposed to a sour gas environment. The interactivebehavior between H transport, H concentration, andHE of cold-worked steel were correlated.

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EXPERIMENTAL AND DATA TREATMENT

The starting material used in this study was hot-rolled type 1020 steel (UNS G10200).(1)

Cold-work conditions were produced by coldrolling or reducing the hot-rolled material to a fixedfinished thickness of 6.35 mm (0.25 in.). The initialmachined thickness varied among specific valuesthat resulted in thickness reductions in steps of 10%ranging from 10% to 70% overall reduction. Thespecimens for determination of H transport and Hconcentration later were machined into squares30 mm by 30 mm (1.18 in. by 1.18 in.). The tensilespecimens were machined from the processedmaterials in the longitudinal transverse (L-T) direc-tion. The tensile specimens were subsized (200 mm[7.9 in.] in overall length) with threaded ends and areduced section of 3.2 mm (0.126 in.) diam. Thegauge length was 27 mm (1.06 in.). However, theelongation data were adjusted to a standard gaugelength of 50 mm (1.97 in.), according to Barba’s law.8

The compact fracture toughness specimens were25.4 mm (1.0 in.) in width and 6.35 mm (0.25 in.) inthickness and were taken from the L-T orientation.These specimens were precracked to a desired cracklength-to-width ratio (a:w) of 0.45.

The aqueous and gaseous environment usedwas based upon that of some problem wells inAlberta.2 The aqueous components were:48,500 mg/L sodium cations (Na+), 14,250 mg/Lcalcium cations (Ca2+), 1,045 mg/L magnesiumcations (Mg2+), 91,500 mg/L chloride anions (Cl–),180 mg/L bicarbonate anions (HCO3

–), and 150 mg/Lsulfate anions (SO4

2–). The gaseous componentsconsisted of 34 vol% H2S, 10 vol% carbon dioxide(CO2), and the balance methane (CH4). Tests wereperformed in the brine environment saturated with theH2S-CO2-CH4 mixture, which was bubbled through ata flow rate of 0.4 L/min, at 1 atm (101.3 kPa)pressure, and at 25°C (77°F).

The barnacle electrode was applied to measureH transport and the H concentration. Material thatcontained diffusible H after exposure to the sour gasenvironment was clamped against a small port at oneend of a polytetrafluoroethylene (PTFE) cell. The cellwas filled with a 0.2 M sodium hydroxide (NaOH)solution, and a charged nickel (Ni)-nickel oxide (NiO)electrode was placed into the cell. A majorsimplification of the barnacle electrode over theelectrochemical permeation technique is the use ofthe Ni-NiO electrode (cathode) to replace thepotentiostat and to act as a stable nonpolarizing

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(1) UNS numbers are listed in Metals and Alloys in the Unified NumberingSystem, published by the Society of Automotive Engineers (SAE) andcosponsored by ASTM.

power electrode that maintains a zero H concentra-tion at the exit surface of the steel (anode) byoxidizing the emerging H atoms to water. The anodiccurrent was determined by monitoring the voltagedrop across a 100.01-Ω precision resister with theuse of a time-based chart recorder.

The H extraction curves of the current vs timewere fitted using the theoretical curves plotted from:7

Jt

ZF= C0

Deff

π t

12

(1)

if

tmax = L2

4Deff (2)

where J is the flux of current density at a specifictime, t is the time, Z is the transfer number ofelectrons, F is the Faraday constant, C0 is the Hconcentration in the steel, Deff is the effective Hdiffusion coefficient, L is the thickness of thespecimen, and tmax is the maximum measurementperiod.

If H traps are heavily occupied, H trap densitycan be estimated by Equation (1) based on theanalysis of McNabb and Foster:9

NT =C0

3DL

Deff– 1 (3)

where NT is the number of H trapping sites per unitvolume and DL is the lattice H diffusion coefficient.

If DL/Deff was much larger than 1, the descriptionfor the H trap density was simplified to:

N ≈

C0DL

3Deff (4)

The mercury (Hg)-filled eudiometer method is astandard method recommended for determining thediffusible H content of martensitic, bainitic, and ferriticweld metal.10 This method was associated withEquation (1) to determine effective H diffusivity. Priorto the test, Hg was placed in a bath and brought up tooperating temperature. The test specimen wasinserted into the eudiometer tube. A vacuum wasdrawn through the stopcock, and Hg was allowed tofill the tube completely and just pass through thestopcock opening. H evolved from the best specimenand displaced the Hg. This cell was operated at atemperature of 45°C (113°F) for ≥ 72 h.

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TABLE 1H Concentration after 48 h of Unloaded Immersion

(Standard H Measurement Method)

Cold Work H Concentration(%) (µmol/cm 3)

0 9.01 ± 0.2310 10.13 ± 1.4820 13.38 ± 1.8730 17.50 ± 2.8940 21.80 ± 0.7060 24.01 ± 1.9570 27.60 ± 0.2

FIGURE 1. Relationship between H diffusivity and cold work.

A batch of tensile and fracture toughnessspecimens were prepared for testing by beingimmersed unloaded in a reaction kettle containing thesour environment. Selected immersion times of 0 h,24 h, 72 h, and 216 h were chosen to determine thetime required for a steady condition of detrimental Hto occur in the 0% and 30% cold-worked tensilespecimens. Based on these results, an immersiontime of 72 h was chosen as sufficient for anequilibrium damage condition to occur. At the end ofthe preconditioning immersion time, the specimenswere removed quickly and tested by loadingaccording to ASTM standards for tensile and fracturemechanics testing methods.11-13

RESULTS

H Diffusion in SteelThe H concentrations in cold-worked steels after

48 h of unstressed immersion were measured usingthe standard Hg-filled eudiometer (Table 1). Resultsindicated cold work caused a steady increase in Hconcentration in this steel.

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Based on the extraction curve information ofbarnacle measurements and H concentrations ofvarious cold-worked steels after 48 h ofpreconditioning immersion, the H diffusioncoefficients of the cold-worked steels (Figure 1)were determined using Equation (1). The result forthe H diffusion coefficient of 0% cold-worked steelwas in good agreement with previous work.14-15 Hay’sresults showed 2 x 10–6 cm2/s (3.1 x 10–5 in.2/s) at25°C is typical of H diffusion coefficients for pipelinesteels containing standard amounts of carbon andmanganese.15 Cold work has been shown todecrease H diffusion coefficients. This behavior hasbeen accepted widely by other researchers and isattributed to an increase in H trapping in cold-workedsteel.14,16-18 Figure 1 indicates that the H diffusioncoefficients decreased with increasing cold work.This decrease leveled out when cold work reached30% to 40%. This change in diffusivity withdeformation was in agreement with Kumnick andJohnson’s work on deformed high-purity iron (Fe).18

Oriani suggested the value for the latticediffusivity of pure Fe is ~ 3 x 10–5 cm2/s (4.65 x10–4 in.2/s) at 25°C.16 Deff was measured to be 1.22 x10–6 cm2/s (1.89 x 10–5 in.2/s) for the 0% cold-workedsteel in this study. Therefore, the calculation for the Htrap density could be made according to Equation (4).The calculated H trap density data for cold-workedsteels are shown in Figure 2. The H trap densityincreased with cold work up to 30% and thensaturated.

H ConcentrationUsing the diffusion coefficients of H in the cold-

worked steels, C0 was calculated after variousexposure times from the measured extraction currentcurve obtained from the barnacle electrode usingEquation (1).

The H concentrations correlated with cold workare shown in Figures 3 and 4. The H concentrationswere influenced by cold work and exposure time inthe sour gas environment. Both cold work andexposure time caused a steady increase in Hconcentration in the steel (Figure 3). H concentrationin cold-worked steel increased with exposure time(Figure 4).

EmbrittlementPreconditioning immersion did not change the

yield or ultimate stress significantly for tensilespecimens after various preimmersion times. Thestrain to fracture was affected most, decreasing withincreasing exposure time and decreasing withincreasing amounts of cold work. The change inplastic deformation properties often is used as acriterion for the susceptibility of steel to HE.19

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FIGURE 2. Relationship between H trap density and cold work. FIGURE 3. Relationship between H concentration and coldwork.

FIGURE 4. Preconditioning immersion effects on Hconcentration in cold-worked steel.

Figure 5 indicates 30% cold-worked steel wasconsiderably more sensitive to HE than hot-rolledsteel. Results in Figure 5 suggested the immersiontime for a steady H damage condition was ≈ 70 h forthe hot-rolled condition, decreasing significantly forthe cold-worked materials.

The fracture toughness of cold-worked steel after72 h of preconditioning immersion is shown in Figure6. When comparing the fracture toughness of the 0%cold-worked material tested after 72 h of precondi-tioning to that of the 0% cold-worked steel tested inambient air, a drop in fracture toughness occurred onthe order of 23%. This condition was not particularlyserious because the fracture toughness value wasstill well above 200 MPa√m (182 ksi√in.) and exhibi-ted a plane stress condition. Generally, structuralsteels with a fracture toughness value > 150 MPa√m(136.5 ksi√in.) are considered adequate with respectto energy absorbing properties. However, all of theother cold-worked steels changed to a plane strainfracture toughness mode. They exhibited engineeringbrittle fracture once exposed to the immersionenvironment and, hence, reached a steady Hdamage state. Along with the brittle fracture failure,drastic reductions in fracture toughness values wereseen, leveling out at 20% cold work (Figure 6). Thevalue of 96 MPa√m (87.4 ksi√in.) was measured tobe the stress intensity of 10% cold-worked steel forplane strain as affected by 72 h of preconditioning.This value represented the initial position at whichvalid plane strain conditions occurred.

DISCUSSION

H diffusivity decreased with increasing cold workand leveled out when cold work reached 30% to40%. The change in effective diffusivity with

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deformation (Figure 1) may have been related to thechange in dislocation density with deformationreported by Keh.20 The dislocation density increasedwith cold work and leveled out when cold workreached 30% to 40%.20 The above relation also wasfound on pure Fe18 and suggested dislocations insteel were the source of the H trapping behavior.Results in Figure 2 showed the H trap densityincreased with cold work.

Darken and Smith first suggested that thedelayed transport of H in cold-worked steels wascaused by attractive interactions between dissolvedH and microstructural imperfections.21 Orianianalyzed the then-available data for steels obtainedfrom various absorption and permeation experiments,in terms of trapping concepts.16 His calculationssuggested a trap binding energy near 27.2 kJ/mol

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FIGURE 5. Relationship between the ductility of tensilespecimens vs the unstressed preconditioning immersion time.

FIGURE 6. Effects of cold work on fracture toughness with andwithout 72-h preconditioning.

(2 x 104 ft-lb/mol) H. The corresponding trap densityfor annealed steel was estimated to be on the orderof 1025 m–3. Results of H trapping density obtained inthe present study were in good agreement withOriani’s analysis.

Pressouyre suggested H traps are reversibleif their binding energy is < 60 kJ/mol (4.4 x104 ft-lb/mol).22 Therefore, dislocations generally areconsidered to be reversible traps. Thus, cold workcauses a decrease in H diffusivity, which isunderstandable in terms of dislocations acting as Htraps. Results in the present study showed that theamount of H absorption increased proportionally withincreasing cold work. These results were identicalwith previous results.14,17-18,21 The previous workinterpreted this phenomenon simply in terms ofincreases in the number of H traps.17,21

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A semi-infinite slab as a H absorption model candemonstrate the effect of H diffusivity and trapdensity factors on the amount of H absorption perunit area. The initial H content in the steel is assumedto be zero, and the H concentration just below theinput surface (CS

H) is held constant and in equilibriumwith the environment from t = 0 onwards. Under ahigh-fugacity situation, Oriani16 solved the McNabband Foster diffusion equations9 by introducing a Htrap term:

qa(t) ≈ 2NTCSH

12 DL

12 (5)

where qa(t) is the amount of H absorption per unitarea and CS

H is the H concentration just below thesubsurface.

Equation (5) shows that the amount of absorbedH is proportional to the square root of the trapdensity. CS

H was considered to be independent ofcold work, according to the traditional model. Thetendency of the absorbed H amount to increase withthe degree of cold work from Equation (5) did not fitthe present experimental results if CS

H wasconsidered to be independent of cold work.2 Themeasured H concentrations in cold-worked steel afterexposure in the sour gas environment are shown inFigure 3, which indicated that the increase of Hconcentration was almost linearly proportional to thedegree of cold work. If CS

H was considered to berelated to cold work, the difference between thetheoretical analysis and the actual measurementvalues might be compensated for to some extent.

Nanis’ analysis of the chemical and electro-chemical factors affecting H absorption concludedthat CS

H was dependent on H surface coverage.23 Ata steady state of H diffusion through the membrane,the H concentration just below the surface of thecomponent could be expressed as a function of Hadsorption coverage on the surface by:

CS

H =Kabsθ

ZFDeff

L+ Kdes

(6)

where Kabs and Kdes are the H absorption anddesorption constants, respectively, and u is the Hadsorption coverage on the surface.

The grain boundaries and deformation slip bandshave been found to be the areas favored energeti-cally by H adsorption. Cold work may cause anincrease in H adsorption coverage on the surface.24 Itis reasonable that an increase in H adsorptioncoverage on the surface corresponds to an increase

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CORROSION SCIENCE

FIGURE 7. Comparison between ductility and H concentrationfor 0% and 30% cold-worked steel during exposure to the sourenvironment.

FIGURE 8. Fractography of 20% cold-worked steel after 72 hof immersion in sour gas environment.

FIGURE 9. Fractography of 70% cold-worked steel after 72 hof immersion in sour gas environment.

in H concentration just below the surface. Thediffusion boundary conditions employed in Equation(6) are not the same as those in the present study. Adetailed relation of cold work to H surface coveragecannot be given in the present study. However, it iscertain that the H concentration just below thesurface was dependent on H surface coverage andincreased with an increase in H surface coverage.The almost linear increase in absorbed H withincreasing cold work during exposure to the sourenvironment (Figure 3) may be interpreted from theabove discussion. Increases in the trap density andH concentration just below the surface due to coldwork may have increased H concentration in thecold-worked steel.

The test results indicated cold work caused anincrease in H concentration, which, in turn, causedthe steel to become more sensitive to HE. Thedeformed structure itself may be an importantparameter governing sensitivity to HE. A comparisonof the loss of ductile properties of cold-worked steelwith the corresponding H concentration is made inFigure 7. At a H concentration level of 10 µmol/cm3

(163.9 µmol/in.3) in the steel after preconditioningexposure, the ductility loss of the 0% cold-workedsteel was ~ 37%, while the ductility loss of 30% cold-worked steel was ~ 70%. These results indicatedthat, except for the increase in H concentration, thedeformed structure itself had a pronounced influenceon HE damage. A continued increase in H concen-tration beyond a mechanical damage amount had nofurther detrimental damage effects.

With increasing deformation and strong traps, therate of escape of H in H traps was smaller than thecapture rate. These traps may have served as Hcollectors during the production of embrittlement

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sites. If their saturability was small, they may haveprevented H from diffusing to potential embrittlementsites. Therefore, the fracture mode was dependentupon the sensitive locations as affected by cold work.The fractography of 20% cold-worked materialshowed predominantly river-pattern cleavage fracture(Figure 8). In the case of 20% cold-worked material,H induced brittle fracture. With further increases incold work, more dislocation pileup occurred alonggrain boundaries.2 Secondary cracks induced by H(Figure 9), increased with the degree of cold workalong the grain boundaries.25-26

CONCLUSIONS

Cold work decreased H diffusivity and increased Habsorption.

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The increase in dislocations as a result of coldwork was responsible for the decrease in Hdiffusivity. The increase in H adsorption coverage on thesurface and H trapping as a result of cold work werelikely causes for an increase in H absorption into thesteel. The H concentration increase and distortedstructure dominated the sensitivity of cold-workedsteel to HE. After the steady H damage level occurred in thesteel, the fracture toughness decreased withincreasing cold work. Sufficiently low values offracture toughness due to embrittlement occurredsuch that a promotion of plane strain fractureoccurred even in relatively thin laboratory specimens. A continued increase in H concentration beyond amechanical damaging amount had no furtherdetrimental damage effects.

ACKNOWLEDGMENTS

The authors acknowledge financial support of thePetroleum Graduate Research Program and theAlberta Oil Sands Technology and ResearchAuthority and the assistance of R. Konzuk of theUniversity of Alberta in cold rolling the material.

REFERENCES

1. H. Huang, W.J.D. Shaw, Corros. Sci. 34 (1993): p. 61.2. H. Huang, “Investigation into the Mechanism of Mechanical

Degradation of Deformed Pipeline Steel Exposed to a Sour GasEnvironment” (Ph.D. diss., University of Calgary, 1993).

3. B.J. Berkowitz, F.H. Heubaum, Corrosion 40 (1980): p. 240.

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4. A.J. Kumnick, H.H. Johnson, Acta Metall. 28 (1980): p. 33.5. M.A.V. Devanathan, Z. Stachurski, Proc. Royal Soc. 270 (1962): p. 90.6. D.A. Berman, W. Beck, J.J. DeLuccia, “The Determination of Hydrogen

in High-Strength Steel Structures by an Electrochemical Technique,” inHydrogen in Metals, eds. I.M. Bernstein, A.W. Thompson (Metals Park,OH: ASM, 1974), p. 595.

7. J.J. DeLuccia, D.A. Berman, “An Electrochemical Technique toMeasure Diffusible Hydrogen in Metals (Barnacle Electrode),” inElectrochemical Corrosion Testing, eds. F. Mansfeld, U. Bertocci(Philadelphia, PA: ASTM, 1981), p. 256.

8. G.E. Dieter, Mechanical Metallurgy (New York, NY: McGraw-Hill BookCo., 1976), p. 346.

9. A. McNabb, P.K. Foster, Trans. AIME 227 (1963): p. 618.10. American Welding Society A4.3-86, “Standard Methods for

Determination of the Diffusible Hydrogen Content of Martensitic,Bainitic, and Ferritic Weld Metal Produced by Arc Welding” (Miami, FL:AWS, 1986).

11. ASTM Standard E 8-90a, “Standard Test Methods of Tension Testingof Metallic Materials” (Philadelphia, PA: ASTM, 1991), p. 130.

12. ASTM Standard E 399-90, “Standard Test Method for Plane StrainFracture Toughness of Metallic Materials” (Philadelphia, PA: ASTM,1991), p. 485.

13. ASTM Standard E 561-86, “Standard Practice for R-CurveDetermination” (Philadelphia, PA: ASTM, 1991), p. 577.

14. S.X. Xie, J.P. Hirth, Corrosion 38 (1982): p. 486.15. M.G. Hay, “An Electrochemical Device for Monitoring Hydrogen

Diffusing Through Steel,” 22nd Conf. Metallurgists (Edmonton,Canada: ClM, 1983).

16. R.A. Oriani, Acta Metall. 18 (1970): p. 147.17. M.L. Hill, E. Johnson, Trans. AIME 215 (1959): p. 717.18. A.J. Kumnick, H.H. Johnson, Met. Trans. 5 (1974): p. 1,199.19. J.C. Turn Jr., B.E. Wilde, C.A. Troiano, Corrosion 39 (1983): p. 364.20. A.S. Keh, “Dislocation Arrangement in Alpha Iron During Deformation

and Recovery,” in Direct Observation of Imperfection in Crystal, eds.J.B. Newkirk, J.H. Wernick (New York, NY: Interscience Publishers,1962), p. 213.

21. L.S. Darken, R.P. Smith, Corrosion 5 (1949): p. 1.22. G.M. Pressouyre, Met. Trans. 10A (1979): p. 1,571.23. L. Nanis, “Chemical and Electrochemical Factors Affecting Hydrogen

Adsorption in Metals,” in Environment-Sensitive Fracture ofEngineering Materials, ed. Z.A. Foroulis (Warrendale, PA: MetallurgicalSociety of AIME, 1979), p. 361.

24. H. Huang, W.J.D. Shaw, Corrosion 48 (1992): p. 931.25. H. Huang, W.J.D. Shaw, Can. Metall. Qtly. 32 (1993): p. 341.26. W.J.D. Shaw, K. Szklarz, H. Huang, D. Diakow, “Required

Environmental Representative Materials Service Evaluation,” inMaterials Performance: Sulphur and Energy, eds., P.R. Roberge, K.Szklarz, V.S. Sastri (Edmonton, Canada: CIM, 1992), p. 195.

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,