1-s2.0-s0360319906003636-main

International Journal of Hydrogen Energy 32 (2007) 12691276www.elsevier.com/locate/ijhydene

Analysis of electrochemical hydrogen permeation throughX-65 pipelinesteel and its implications on pipeline stress corrosion cracking

Y.F. Cheng

Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta, T2N 1N4 CanadaReceived 20 July 2006; accepted 20 July 2006

Available online 26 September 2006

Abstract

Electrochemical hydrogen permeation tests were performed to measure the hydrogen permeation current through the X-65 pipeline steel inthe electrolytes simulating the soil conditions to initiate near-neutral pH stress corrosion cracking (SCC) in pipelines. The hydrogen permeationcurrent was analyzed following the constant concentration model. It is shown that, AQDS, simulating the organic compound in the soil, inhibitshydrogen permeation by decreasing the sub-surface hydrogen concentration, while sulde promotes hydrogen permeation by inhibiting thehydrogen recombination and thus increasing the sub-surface hydrogen concentration. The steel specimen is more susceptible to stress corrosioncracking in the soil solution with a higher sub-surface hydrogen concentration, indicating that hydrogen is involved in near-neutral pH SCC inpipelines. It is suggested that hydrogen promotes the cracking of the steel, accompanying with the anodic dissolution on the crack sides andat the crack tip. 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Electrochemical hydrogen permeation; Slow strain rate tensile tests; Stress corrosion cracking; Pipelines; Sulde; Organic compound; Soil solutions

1. Introduction

Stress corrosion cracking has caused signicant failures innatural gas pipelines in Canada, resulting in extremely seriousconsequences for both the environment and the economy. Inthe global range, the great majority of pipeline failures haveinvolved high pH ( 9.0) SCC [1]. However, near-neutral pHSCC on pipelines, cracking at pH of about 6.5, was initiallyrecorded in Canada during the mid-1980s, and has been respon-sible for an increasing number of pipeline failures in recentyears [2].

There are major differences between the two forms ofpipeline SCC. High pH SCC, engendered by concentrated bi-carbonate or carbonate-bicarbonate solutions, has usually anintergranular morphology, and the cracks are sharp, with littlelateral corrosion. Near-neutral pH SCC, engendered by anaer-obic, dilute ground water, has a transgranular, quasi-cleavagecrack morphology with appreciable lateral corrosion of the

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0360-3199/$ - see front matter 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2006.07.018

crack sides [3]. There is almost universal agreement that crackinitiation and growth in the high pH environment occur byselective dissolution of the grain boundaries, while a passivelm forms on the remainder of the surface and on the cracksides to prevent corrosion at those locations [1,3]. A strongcorrelation has been found between the maximum rate of crackgrowth and the maximum corrosion rate that can be sustainedin that environment [4]. However, neither stage of the crackingprocess of near-neutral pH SCC is as well understood as is themechanism of high pH SCC.

Previous work showed [5] that hydrogen could play an im-portant role in near-neutral pH SCC in pipelines, altering thedissolution at and ahead of the crack-tip. Parkins [1] analyzedthe potential-pH range where high pH SCC and near-neutral pHSCC occurred, and found that in the near-neutral pH solution,hydrogen discharge is possible. He concluded that some syner-gistic effects between the hydrogen and the anodic dissolutionexist during the growth of near-neutral pH SCC of pipelines.Gu et al. [6] deduced an equation for the synergistic effect ofhydrogen and stress on the anodic dissolution rate. They sug-gested a local acidication generating during anodic dissolu-tion, and thus thought that near-neutral pH SCC is dominated by

1270 Y.F. Cheng / International Journal of Hydrogen Energy 32 (2007) 12691276

the mechanism of hydrogen-facilitated dissolution. Althoughthere is a considerable amount of evidence [7,8] that hydrogenplays a critical role in near-neutral pH SCC of pipelines, theprecise mechanism and predictive model have not been identi-ed/developed.

Hydrogen permeation through a metallic membrane by anelectrochemical technique is a widely used method for studyinghydrogen diffusivity and metallic embrittlement phenomenon[9,10]. In the course of electrochemical permeation, hydro-gen atoms are rst absorbed at the entry surface, then diffusethrough the metallic membrane, and are nally desorbed fromthe exit surface. On the entry surface, the production of hydro-gen can be controlled galvanostatically or potentiostatically, orunder the free corrosion status of the metal. On the exit surface,it is usual to apply a constant potential to ensure that all hydro-gen atoms can be ionized, assuring that the measured currentdensity is the hydrogen permeation ux.

In this work, electrochemical tests of hydrogen permeationthrough a X-65 pipeline steel membrane were conducted inthe dilute bicarbonate solution without and with the variousadditives simulating the conditions of the soil environment,where near-neutral pH SCC initiated in pipelines. By ttingthe experimental results with the theoretical models availablefor analysis of hydrogen permeation current data, the mod-eling method is determined to calculate the hydrogen diffu-sivity and the sub-surface hydrogen concentration. Combinedwith the slow strain rate tensile (SSRT) tests and scanningelectron microscopy (SEM), the electrochemical hydrogenpermeation technique will be severed as an effective methodto evaluate the role of hydrogen in near-neutral pH SCCin pipelines.

2. Experimental

2.1. Electrochemical hydrogen permeation tests

The working electrode for electrochemical hydrogen perme-ation tests is made of a sheet of X-65 pipeline steel with thechemical composition shown in Table 1. The metallographicobservation indicates that the microstructures of the steel con-tain ferrite and pearlite, with the latter randomly distributed inthe bulk material. There is no specic orientation of the mi-crostructures. Prior to the tests, all the specimens were groundto 600-grade emery paper, and cleaned with distilled water andmethanol. The nal thickness of the specimen was 0.10 cm, andthe working area was 6.50 cm2.

The electrochemical hydrogen permeation tests were car-ried out using a DevanathanStachurski two-component cell[9], separated by the steel membrane. The hydrogen charg-ing solution was 10mMNaHCO3. To investigate the inu-ences of various additives on hydrogen permeation, 10 ppmAQDS, 9,10-anthraquinone-2.6-disulfonic acid disodium salt,and 10 ppmNa2S were added in the basic solution to sim-ulate the effects of organic compound and sulfate reducingbacteria in the soil environment, respectively. The hydrogenexit cell was lled with 0.1M NaOH solution. All the solu-

Table 1Chemical composition of X-65 pipeline steel (wt%)

C Mn S P Si Cr Ni Cu Nb Al

0.11 1.50 0.008 0.013 0.26 0.006 < 0.02 0.04 0.04 0.05

tions were made from the analytic grade reagents and distilledwater.

The steel membrane facing the hydrogen charging side wasin an open circuit state. The detection cell included a satu-rated calomel reference electrode (SCE) and a platinum wire ascounter electrode. The hydrogen permeation current was mea-sured by anodically polarizing the detection side of the steelmembrane at +200mV through a SOLARTON1287 potentio-stat.

All the tests were performed at room temperature and thesolution was saturated with 10% CO2.

2.2. Cyclic voltammogram measurements

Cyclic voltammetry measurements were performed to inves-tigate the polarization behavior and electrochemical reactionsof X-65 steel under various potentials through the SOLAR-TON1287 potentiostat. The reference electrode was SCE andthe counter electrode was platinum wire. The potential wasscanned from 1.15V (SCE) towards positive direction un-til 0.70V (SCE), and then scanned backwards to 1.15V(SCE). The potential scanning rate was 0.33mV/s.

2.3. SSRT tests

The SSRT tests were performed through an Instron Materi-als Testing System on the cylindrical tensile specimens 24mmin gauge length and 2.5mm in gauge diameter, which were ma-chined in a transverse orientation from the X-65 pipeline steel.The specimens were ground to 600-grade emery paper, cleanedand masked with epoxy except the gauge section. The testingsolutions and the working conditions were identical to those forhydrogen charging. The strain rate was 5 107 s1. The sus-ceptibility of the steel to SCC was evaluated by the percentagechange in reduction-in-area (%).

3. Results

3.1. Model tting of hydrogen permeation current data

To analyze the hydrogen permeation current data properly,the rst step is to t the experimental results with the theoreticalmodel, determining the appropriate model (initial and boundaryconditions) and mathematical equations for calculation of suchparameters as hydrogen diffusivity and sub-surface hydrogenconcentration. It is established that the theoretical permeationtransients follow the dimensionless Eqs. (1) and (2) for con-stant concentration (CC) model and constant ux (CF) model,

Y.F. Cheng / International Journal of Hydrogen Energy 32 (2007) 12691276 1271

0

0.2

0.4

0.6

0.8

1

1.2

0.01 0.1 1 10

i t/i

CC ModelCF ModelExperimental data

==Dt/L2

Fig. 1. Fitting of the experimental data measured in 10mM NaHCO3 solutionpurged with 10% CO2 with the theoretical curves.

respectively [11,12].

It

I= 2

()1/2

n=0

exp

[ (2n + 1)

2

4

], (1)

It

I= 1 4

n=0

(1)n(2n + 1) exp

[ (2n + 1)

22

4

], (2)

where It is the permeation current at instant time t and Iis the steady state value of the hydrogen permeation current.The dimensionless time is equal to Dt/L2 (D is the hydro-gen diffusivity and L is the membrane thickness). The reportedux continuity and CC [13] model and ux continuity model[14] were not considered due to too many unknown variablescontained in both models.

During the data tting, the experimental permeation curveswere corrected for the time of relaxation and then the normal-ized curves were plotted. The data in the normalized curveswere compared with the theoretical curves for CCmodel and CFmodel, identifying the initial and the boundary conditions pre-vailing during the electrochemical hydrogen permeation tests.Fig. 1 shows the normalized experimental data tting with themodel curves, which shows the typical case during the data t-ting for all the tests in this work. It is apparent that the measureddata points t very well with the theoretical curve plotted un-der the CC model, indicating that the present tests for elelctro-chemical hydrogen permeation follow the initial and boundaryconditions determined by the CC model. Thus, the analysis ofthe hydrogen permeation results in this work followed the CCmodel.

According to the CC model, the sub-surface hydrogen con-centration, C0H, is established instantaneously. In practice,the C0H value changes somewhat since it will take time forthe corrosion potential of the steel to reach the steady-state.This period is invariably short in relation to the measured

time-to-breakthrough for hydrogen permeation, and thus, anyinduced error is considered to be insignicant.

Several methods are commonly used to calculate the hydro-gen diffusivity under the CC model:

Time-to-breakthrough, tb [15]:

D = L2

15.3tb, (3)

where tb is found by extrapolating the linear portion of theinitial hydrogen permeation current transient to it = 0.

Time-lag method, tL [15]:

D = L2

6tL. (4)

The time of tL corresponds to the point on the permeationcurve at which it = 0.63i.

Fourier method [16]:

The equation of the hydrogen permeation transient is givenby

it

i= 1 2 exp

(2Dt

L

). (5)

The hydrogen diffusion coefcient is derived from the slope ofthe plot of ln(1 It /I) against the time, t.

Laplace method [17]:

This method makes use of the rst term of the summation(1) to give

It

I= 2

LDt

exp( L

2

4Dt

). (6)

Rearrangement of Eq. (6) gives

ln(It t1/2) = const. L2

4Dt. (7)

Thus D can be easily obtained from the slope of the plot ofln(It t1/2) vs t1.

For the determined hydrogen diffusivity, the sub-surface hy-drogen concentration is calculated by the following equation:

C0H =ILDF

, (8)

where F is Faradays constant.

3.2. Hydrogen permeation in 10mM NaHCO3 solution

Figs. 24 show the analysis of the hydrogen permeation datameasured in 10mM NaHCO3 solution by various methods.The analysis results of the hydrogen diffusivity and sub-surfacehydrogen concentration are summarized in Table 2.


0

0.2

0.4

0.6

0.8

1

1.2

0 2000 4000 10000

Curre

nt [

A]

tb tlag

Time [s]80006000

Fig. 2. Analysis of the hydrogen permeation current transient by time-to-breakthrough method and time-lag methods.

-2

-1.5

-1

-0.5

0

0.5

0 2000 5000

ln (1

-I t/I

)

Time [s]400030001000

Fig. 3. Analysis of the hydrogen permeation current transient by Fouriermethod.

The average values of hydrogen diffusivity and sub-surfacehydrogen concentration from the four methods were 9.49 107 cm2/s and 0.20molH/m3, respectively. The theoreticalvalue of the hydrogen diffusivity in the X-65 steel lattice isfound to be 1.50 106 cm2/s. It is seen that the differencebetween the calculated hydrogen diffusivity based on electro-chemical hydrogen permeation tests and the theoretical value isignorable, indicating the reliability of electrochemical methodin hydrogen permeation research.

3.3. Hydrogen permeation in 10mM NaHCO3 solutionadding 10ppm AQDS

After the steady state of the hydrogen permeation currentwas reached in 10mM NaHCO3 solution, 10 ppm AQDSwas added. The whole current transient recorded is shown in

2

3

4

5

0 0.0006

ln (t1

/2I t/

I)

0.0001 0.00021/t [s-1]0.0003 0.0004 0.0005

Fig. 4. Analysis of the hydrogen permeation current transient by Laplacemethod.

Fig. 5. It is seen that, upon the addition of AQDS, the hydrogenpermeation current decreased. The dropping current transientcould be analyzed by the Fourier method in Fig. 6:

ln(

It

I

)=

2D

L2t + ln 2. (9)

The hydrogen diffusivity in the AQDS-containing solution wascalculated to be 2.16108 cm2/s, an approximately 45 timesof reduction in hydrogen diffusivity compared with that (9.49107 cm2/s) measured in the solution without AQDS.

To identify the mechanism for decrease in hydrogen diffu-sivity by AQDS, the cyclic voltammogram was measured onX-65 steel in AQDS-containing solution, as shown in Fig. 7.During the potential sweeping negatively, the current peak atabout 0.90V (SCE) was attributed to the reduction of ironoxide present on the steel surface, which was formed duringthe positive potential scanning. Upon the addition of AQDS,there is no extra lm-forming reaction identied in the cyclicvoltammogram.

3.4. Hydrogen permeation in 10mM NaHCO3 solution with10ppm Na2S

The hydrogen permeation current transient measured inNa2S-containing solution is shown in Fig. 8. It is seen thatadding Na2S remarkably increased the hydrogen permeationcurrent. However, no steady state of hydrogen permeation cur-rent can be achieved. After a maximum value, the hydrogenpermeation current decreased with time.

The analysis of hydrogen permeation current measured inNa2S-containing solution was performed through the fourmethods under the CC model. There is little difference of thehydrogen diffusivity before (9.49 107 cm2/s) and after theaddition of 10 ppmNa2S (8.13 107 cm2/s) in the solution.However, the sub-surface hydrogen concentration increased


Table 2Summary of the analysis results of the hydrogen permeation current measured in 10mM NaHCO3 solution purged with 10% CO2

Methods Time-to-breakthrough Time-lag Fourier Laplace Average value

Hydrogen diffusivity (cm2/s) 1.06 106 9.24 107 8.64 107 9.46 107 9.49 107Sub-surface hydrogen concentration (molH/m3) 0.19 0.20 0.22 0.20 0.20

0.00

0.40

0.80

1.20

1.60

2.00

0

Hyd

roge

n pe

rmea

tion

curre

nt [

A]

Time [s]600005000040000300002000010000

Adding 10 ppm AQDS

Fig. 5. The hydrogen permeation current curve for X-65 steel in 10mMNaHCO3 solution with the addition of 10 ppm AQDS.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 30000

ln (I t

/I)

2500020000Time [s]15000100005000

Fig. 6. Analysis of the hydrogen permeation current transient in 10mMNaHCO3 + 10 ppm AQDS solution by Fourier method.

from 0.20 to 0.54molH/m3 upon the addition of suldeions.

3.5. Electrochemical hydrogen permeation tests and SSRTtests in various extracted soil solutions

Electrochemical hydrogen permeation tests were performedin the various extracted soil solutions to investigate the hydro-gen permeation through X-65 pipeline steel. The chemical com-

-0.003

-0.002

-0.001

0

0.001

-1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6

Curre

nt [A

]

Sweep1Sweep2Sweep3

0.002

Potential [mV, SCE]

Fig. 7. Cyclic voltammogram measured on X-65 steel in 10mMNaHCO3 + 10 ppm AQDS solution.

0.0

1.0

2.0

3.0

4.0

0 20000 40000 60000 80000 100000Time [s]

Hyd

roge

n pe

rmea

tion

curre

nt [

A]

Adding 10 ppm Na2S5.0

Fig. 8. The hydrogen permeation current curve for X-65 steel in 10mMNaHCO3 solution with the addition of 10 ppm Na2S.

positions of the extracted soil solutions are shown in Table 3.Fig. 9 shows the sub-surface hydrogen concentration inX-65 steel obtained in the various soil solutions. It is seenthat the sub-surface hydrogen concentration ranged from 0.2to 0.5molH/m3. The #2 soil solution produced the highesthydrogen concentration, while the #1 solution with the lowestsub-surface hydrogen concentration.

Fig. 10 shows the stress-strain curves of X-65 steel mea-sured in the different soil solutions by SSRT tests. The SCC


Table 3Chemical compositions of the extracted soil solutions

Soils pH Ca2+ (mg/L) Mg2+ (mg/L) Na+ (mg/L) K+ (mg/L) SO24 (mg/L) Cl (mg/L) HCO3 (mg/L)

#1 7.0 60 32 51 2 2 112 343#2 6.7 20 15 17 3 45 26 104#3 6.6 20 7 8 4 5 7 110#4 6.6 480 507 668 4 1530 50 63

0

0.1

0.2

0.3

0.4

0.5

Soil #2 Soil #3 Soil #4

Sub-

surfa

ce h

ydro

gen

conc

entra

tion

[mol/

m3]

Soil #1

Fig. 9. The sub-surface hydrogen concentration measured in various soilsolutions.

0

100

200

300

400

500

600

700

0 3 6 9 12 15 18 21

Stre

ss [M

Pa]

#1

#2 #3 #4

Strain [%]

Air

Fig. 10. Stressstrain curves of X-65 steel specimens in air and in the varioussoil solutions.

Table 4Summary of the reduction-in-area measured on X-65 steel in air and thevarious soil solutions by SSRT tests

Soil solutions Air #1 #2 #3 #4

Reduction-in-area (%) 1.00 0.71 0.56 0.58 0.69

susceptibility is evaluated by measuring the percentage changeof reduction-in-area. The results, as shown in Table 4, showedthat the SCC susceptibility of steel in the various soil solutions

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.2 0.25 0.3

Red

uctio

n-in

-are

a [%

]

0.75

0.50.45Sub-surface H concentration [mol/m3]

0.35 0.4

Fig. 11. The correlation of the SCC susceptibility of steel with the sub-surfacehydrogen concentration.

Fig. 12. SEM photos of the fracture surface of the X-65 steel in #2 soilsolution.

has the same order as the sub-surface hydrogen concentration,that is, there are the highest and the lowest susceptibilities ofthe steel to SCC in the #2 and the #1 soil solutions, respectively.

Fig. 11 shows the correlation of SCC susceptibility of X-65steel with the sub-surface concentration measured in the vari-ous soil solutions. With the increase in sub-surface hydrogenconcentration, the reduction-in-area decreases, indicating theincreasing susceptibility of the steel to SCC.

Fig. 12 shows the SEM pictures of the fracture surface ofthe steel in the #2 soil solution, where the highest sub-surfacehydrogen concentration and SCC susceptibility occurred. It isseen that there is a brittle fracture feature, accompanying with


the apparent dissolution and corrosion product on both the cracksides and the crack tip.

4. Discussion

4.1. Effects of AQDS and sulde additives on hydrogenpermeation

The hydrogen permeation current measurements showed thataddingAQDS in the bicarbonate solution could decrease the hy-drogen diffusivity in the steel. Since it has been established thatsurface lm would inhibit the hydrogen permeation [18,19],the cyclic voltammetry technique is used to identify if thereis lm-forming reaction accompanying with the addition ofAQDS. The measured cyclic voltammogram in Fig. 7 indicatedthat there is no extra lm formed on the steel surface upon theaddition of AQDS in the solution. Thus, it is concludedthat the major reduction of hydrogen diffusivity is not due tothe formation of new lm on the steel surface.

According to Eq. (8), the sub-surface hydrogen concentra-tion in the AQDS-containing solution (D=2.16108 cm2/s)is calculated to be 0.11mol/m3, which is only a half of thatmeasured in the bicarbonate solution withoutAQDS. Therefore,AQDS may either inhibit the discharge of hydrogen ions and/orwater molecules, or accelerate the hydrogen recombination re-action. Both of these effects would decrease the concentrationof hydrogen atoms adsorbed/absorbed on the steel surface, andthus, the sub-surface hydrogen concentration. From this workit is seen that the organic compounds in the soil, simulated byAQDS, would serve as the inhibitor for hydrogen permeationinto the steel. A similar effect of organic compounds on theinhibition of hydrogen permeation has been also reported byDuarte et al. [20].

Sulde ions have been recognized as the promoter for hy-drogen entry into the steel [16,21]. The measured results inthis work conrmed that adding sulde signicantly increasedthe hydrogen permeation current. Furthermore, upon the ad-dition of sulde ions, there is an apparent increase in sub-surface hydrogen concentration, while the hydrogen diffusivityis almost constant. Apparently, sulde ions promote hydrogenpermeation mainly by inhibiting hydrogen recombination andincreasing the sub-surface hydrogen concentration.

The maximum value phenomenon for hydrogen permeationcurrent measured in the sulde-containing solution has beenobserved by other work [22], and results from the hydrogenpermeation-promoting capability of sulde ions. With the risein hydrogen concentration within the steel membrane above acritical value, the hydrogen trapping sites are activated and mi-crocracks form at the steel surface and inside the membrane.When the surface microcracks ssure, the number of hydrogendiffusion paths is reduced and the hydrogen permeation cur-rent is dropped, resulting in a hump in the permeation currentcurve. Only when the charging solution contains the hydrogenpermeation promoter, such as sulde, can the hydrogen con-centration inside the steel reach and exceed the critical value,and the maximum value is reached.

4.2. Role of hydrogen in near-neutral pH SCC of pipelines

There have been extensive evidences [17] indicating thathydrogen plays an important role in near-neutral pH SCC inpipelines, and that pre-charging hydrogen enhances the suscep-tibility of steel to SCC [68]. The present work conrmed thatthere is a higher susceptibility of X-65 pipeline steel to SCC inthe soil solution with a higher sub-surface hydrogen concentra-tion. Combined with the SEM observation, it is believed thathydrogen is involved in the cracking process of the steel, alter-ing the anodic dissolution rate on the crack sides and at cracktip.

The electrochemical reactions of a steel specimen withoutapplied stress in deoxygenated, near-neutral pH solution can bedescribed as

anodic reaction: Fe Fe2+ + 2e; (10)cathodic reaction: H2O + e Hads + OH. (11)

Considering the electrochemical and/or chemical recombina-tion of hydrogen atoms, if it is assumed that x is the percentageof hydrogen atoms permeating into the steel, the whole elec-trode reaction for the steel in deoxygenated, near-neutral pHsolution can be described as

Fe(xH) + 2H2O Fe2+ + 2OH + 2(1 x)2 x H2+ 2x

2 xHabs. (12)Deformation and the presence of cracks could attract more

hydrogen atoms penetrating into the steel due to the existenceof a high, tri-axial stress concentration at crack tip [23]. Fora stressed steel specimen, the whole electrochemical reactioncan be written as

Fe(, yH) + 2H2O Fe2+ + 2OH + 2(1 y)2 y H2

+ 2y2 yHabs, (13)

where y is more than x.In a recent work [24], a comprehensive analysis of the free-

energy change of the stressed pipeline steel showed that thesynergistic effects of hydrogen and stress on the anodic disso-lution rate at crack tip could be described as

i2 = i1 exp[(2(y x)/(2 y)(2 x))(GH2 2GHabs)

RT

]

exp[U T S

RT

] exp

[W(21 + 22 + 23)

2ERT

]

exp[hVHRT

], (14)

where i1 and i2 are anodic dissolution current density at cracktip for stressed and unstressed steels, respectively, GH2 andGHabs are the Gibbs free energy of formation of hydrogen gasand absorbed hydrogen atoms, respectively, R is gas constant,T is temperature, is charge transfer coefcient, U is the


internal energy change, S is the entropy change, W is atomicweight, E is Youngs modulus, is the density of the steel, 1,2 and 3 are the principal stresses, and h is a volume stress.

According to the slip-oxidation model [3], the crack growthrate is proportional to the dissolution rate of the steel at cracktip. For near-neutral pH SCC, the crack growth rate of the steelunder the synergism of hydrogen and stress, CGR(,H), is cor-related with the dissolution-based cracking rate, CGR(, H =0), by

CGR(, H) = kCHkH kkHi1 WnF

= kCHkH kkHCGR(, H = 0), (15)where kH is the effect of hydrogen on the anodic dissolutionrate in the absence of stress, i.e., the free corrosion rate of Fein deoxygenated, near-neutral pH solution. k is the effect ofstress on the anodic dissolution in the absence of hydrogen,i.e., a pure anodic dissolution-based cracking mechanism. kHreects the synergistic effect of hydrogen and stress on the an-odic dissolution at crack-tip, and kCH is the effect of the con-centration difference of hydrogen atoms between the stressedsteel and unstressed steel on the anodic dissolution reaction.The synergism of hydrogen and stress on crack growth rate,CGR(, H), can be quantitatively predicted after these affect-ing factors are determined.

5. Conclusions

Electrochemical hydrogen permeation tests were performedto measure the hydrogen permeation current through the X-65pipeline steel in the various solutions simulating the soil con-ditions to initiate near-neutral pH stress corrosion cracking inpipelines. The hydrogen permeation current was analyzed basedon the constant concentration model. The typical hydrogendiffusivity and sub-surface hydrogen concentration in 10mMNaHCO3 solution are calculated to be 9.49 107 cm2/s and0.20molH/m3, respectively. AQDS, a chemical simulating theorganic compound in the soil, inhibits the hydrogen perme-ation by decreasing the sub-surface hydrogen concentration,while sulde promotes hydrogen permeation by inhibiting thehydrogen recombination and thus increasing the sub-surfacehydrogen concentration. Slow strain rate tensile testing resultsshow that the steel is more susceptible to SCC in the soil

solution with a higher sub-surface hydrogen concentration, in-dicating that hydrogen is involved in near-neutral pH SCC inpipelines. Combined with the SEM observations, it is suggestedthat hydrogen promotes the cracking of the steel, accompa-nying with the anodic dissolution on the crack sides and atcrack tip.

Acknowledgements

This work was supported by Canada Research Chairs pro-gram and Natural Science and Engineering Research Councilof Canada (NSERC).References

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[3] Fang BY, Atrens A, Wang JQ, Han EH, Zhu ZY, Ke WJ. Mater Sci2003;38:127.

[4] Parkins RN. Corrosion 1987;43:130.[5] Cheng YF, Yang L, King F. International pipeline conference. Calgary:

ASME; 2002. p. 342.[6] Gu B, Luo JL, Mao X. Corrosion 1999;55:96.[7] King F, Jack TR, Chen W, Wang SH, Elboujdaini M, Revie W, et al.

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[10] Oriani RA. Acta Metall 1970;18:147.[11] Nanis L, Namboodhiri TKG. J Electrochem Soc 1972;119:691.[12] Boes N, Zuchner H. J Less-common Metals 1976;49:223.[13] Pumphrey PH. Scr Metall 1980;14:695.[14] Zhang TY, Zheng YP. Acta Mater 1998;46:5023.[15] Devanathan MAV, Stachurski Z. Proc R Soc 1962;A270:90.[16] Turnbull A, Saenz de Santa Maria M, Thomas ND. Corros Sci

1989;29:89.[17] Nanis L, Namboodhiri TKG. J Electrochem Soc 1972;49:223.[18] Zheng G, Popov BN, White RE. J Electrochem Soc 1993;140:3153.[19] Song RH, Pyun SI, Oriani RA. J Electrochem Soc 1990;137:1703.[20] Duarte HA, See DM, Popov BN, White RE. J Electrochem Soc

1997;144:2313.[21] Iyer RN, Takeuchi I, Zamanzadeh M, Pickering HW. Corrosion

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Analysis of electrochemical hydrogen permeation through X-65 pipeline steel and its implications on pipeline stress corrosion crackingIntroductionExperimentalElectrochemical hydrogen permeation testsCyclic voltammogram measurementsSSRT tests

ResultsModel fitting of hydrogen permeation current dataHydrogen permeation in 10 mM =NaHCO3 =solutionHydrogen permeation in 10 mM =NaHCO3 solution adding 10 ppm AQDSHydrogen permeation in 10 mM =NaHCO3 solution with 10 ppm Na2SElectrochemical hydrogen permeation tests and SSRT tests in various extracted soil solutions

DiscussionEffects of AQDS and sulfide additives on hydrogen permeationRole of hydrogen in near-neutral pH SCC of pipelines

ConclusionsAcknowledgementsReferences

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