mechanistic aspect of non-steady electrochemical characteristic during stress corrosion cracking of...

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678 CORROSION—JULY 2014

Submitted for publication: October 7, 2013. Revised and accepted: January 21, 2014. Preprint available online: February 3, 2014, doi: http://dx.doi.org/10.5006/1153.

‡ Corresponding author. E-mail: [email protected]. * Corrosion and Protection Center, University of Science and Tech-

nology Beijing, Beijing, 100083, China. ** School of Materials Science and Engineering, Nanyang Technolog-

ical University, 50 Nanyang Avenue, 639798 Singapore. Depart-ment of Materials, University of Oxford, Oxfordshire, OX1 3PH, United Kingdom.

Mechanistic Aspect of Non-Steady Electrochemical Characteristic During Stress Corrosion Cracking of an X70 Pipeline Steel in Simulated Underground Water

Z.Y. Liu,* L. Lu,* Y.Z. Huang,** C.W. Du,* and X.G. Li‡,*

ABSTRACT

The stress corrosion cracking (SCC) and the electrochemical behavior of an API X70 pipeline steel in a simulated acidic soil solution were investigated using slow strain rate test-ing (SSRT), electrochemical polarization, and electrochemical impedance spectroscopy (EIS). The SCC mechanism of the X70 pipeline steel was found to be dependent on the range of applied potential, including anodic dissolution (AD), hy-drogen embrittlement (HE), and the combination of AD and HE (AD+HE). The potential range for each mechanism can be determined by comparing the current shift obtained from po-tentiodynamic curves at different scan rates. When both cur-rents of the fast scan (if) and the slow scan rate polarization curve (is) are positive, i.e., if > 0 and is > 0, the SCC mechanism follows AD. When if > 0 and is < 0, the SCC mechanism is controlled by both AD and HE. SCC obeys the HE mechanism when if < 0 and is < 0. In this study, we propose a methodol-ogy for calculating SCC susceptibility (Sψ). Sψ is related to the currents obtained by the fast and slow scan rate polarization curves and by the results of SSRT. This method facilitates the immediate identification of SCC susceptibility.

KEY WORDS: electrochemical character, mechanism, stress corrosion cracking, underground water, X70 pipeline steel

INTRODUCTION

Stress corrosion cracking (SCC) has been recognized as one of the most catastrophic failures in buried pipelines.1-3 In general, SCC comes in two types: high pH (more than about 9.0)4-5 and near-neutral pH (around 6.5).6-10 The high-pH SCC follows the anodic dissolution (AD) mechanism.4,7,11-12 Parkins, et al.,10 suggested that the near-neutral pH SCC depends on the cooperation of the AD and the effect of hydrogen in the steel. Beavers and Harle13 pointed out that SCC occurs because of AD and propagates in a mecha-nism of hydrogen-induced cracking (HIC). Wang and Atrens14 suggested that SCC is controlled by hydro-gen embrittlement (HE) below –800 mV vs. saturated calomel electrode (SCE) and by AD within the range of –700 mVSCE to –500 mVSCE. As demonstrated in previous studies,8-9 the low-pH SCC follows a trans-granular SCC (TGSCC) mechanism and involves the synergistic effect of AD and HE. However, these stud-ies have failed to connect this synergism with applied potential and to explain how the SCC mechanism varies within different potential ranges. These results indicate that hydrogen plays an important role in the SCC of pipeline steels in a soil environment. However, the relationship between the effect of hydrogen and AD has not been well understood. Cheng and his co-workers15-16 quantified the contributions of stress, hy-drogen, and their synergism to the dissolution of steel at the crack tip, i.e., hydrogen-induced dissolution (HID), in a near-neutral pH solution. However, Lu, et

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al.,1 reported that hydrogen has a limited effect on the active dissolution of pipeline steels in near-neutral pH groundwater.

Liu, et al.,17-22 extensively studied the SCC mecha-nism of pipe steels, including X70, X80, and X100 steels, in soil environments. The research demon-strated that the non-steady electrochemical state in the crack tip of SCC may play a more important role in crack growth than the effect of HID described in other studies. When SCC expands in high velocity, crack tips in the steel always remain fresh, thus lead-ing to a non-steady electrochemical state. By contrast, crack walls are covered with corrosion product layers, resulting in an electrochemical reaction in a quasi-steady state. The difference of the electrochemical status in the crack tip and in the crack wall gives rise to a combined effect of hydrogen, possibly HID, HE, or HIC, and AD on SCC processes. Adequate evidence supports this mechanism.19,22 However, the relation-ship between the SCC mechanism and the electro-chemical reactions under cathodic condition has yet to be theoretically understood.

The present study aims to develop a quantitative model based on a non-steady electrochemical concept to understand the mechanism of pipeline SCC and to establish a method for assessing SCC susceptibility within various ranges of applied potential.

EXPERIMENTAL PROCEDURES

The specimens in this study were obtained from an X70 pipeline steel. The chemical composition of the steel consists (wt%) of 0.073 C, 0.22 Si, 1.52 Mn, 0.054 Nb, 0.18 Ni, 0.17 Cr, 0.20 Cu, 0.029 Al, 0.017 Ti, 0.026 V, 0.0022 S, 0.0035 P, and Fe balance. The me-chanical properties of the specimens are as follows: yield strength (σ0.5), 565 MPa; tensile strength (σb), 739 MPa; elongation (σ0), 15.5%; and reduction in area (RA), 58.4%.

A simulated solution of acidic soil with 4.0 pH was prepared as the test solution.23 The contents of the solution are as follows: 0.0020 mol/L calcium chloride (CaCl2), 0.016 mol/L sodium chloride (NaCl), 0.0020 mol/L sodium sulfate (Na2SO4), 0.0016 mol/L magnesium sulfate heptahydrate (MgSO4·7H2O), 0.0058 mol/L potassium nitrate (KNO3), and 0.0036 mol/L sodium bicarbonate (NaHCO3).

Slow strain rate test (SSRT) was performed at a strain rate of 0.5×10–6 s–1 both in air and in the solu-tion with different applied cathodic potentials. The goal was to investigate the relationship between SCC susceptibility and the cathodic potentials. These tests were conducted using a materials test system. To test the solution, the tensile specimen was immersed in a solution for 24 h before loading. Pure nitrogen gas (99.99% in mass) was utilized as the protective gas

during the whole test to remove the dissolved oxygen in the solution.

When electrochemical impedance spectroscopy (EIS) was conducted at an applied cathodic potential, the specimen was pre-polarized at the desired level for 30 min to ensure that the polarization current was stable before the EIS measurement. When EIS was performed at the open-circuit potential (OCP), OCP vs. time was monitored for 30 min prior to each test. Mea-surements were not started until OCP achieved an ap-proximately steady state. During the EIS measurement, a disturbing signal of 10 mV was applied with a mea-suring frequency ranging from 10,000 Hz to 0.01 Hz. Each test was conducted three times, and the given data point was the average of the three tests.

Potentiodynamic polarization curves were ob-tained by automatically changing the electrode po-tential from a cathodic branch to an anodic branch at various scan rates. The applied potential ranged from –700 mV to 1,200 mV vs. OCP for all tests. Before each test, the solution was pre-deoxidized using high-purity N2 gas for 2 h. The samples were pre-immersed in the solution for 30 min. A three-electrode system included X70 pipeline steel samples as the working electrode with a working area of 1 cm2, a SCE as the reference electrode, and a Pt plate as the counter elec-trode.

All specimens were sequentially ground to 800 grit using waterproof emery papers, with the polishing di-rection of SSRT samples parallel to the tensile direc-tion. All tests were performed at ambient temperature (about 22°C).

RESULTS

Since the electrochemical process involved in SCC is non-steady, it is necessary to know how long it takes to stabilize in a solution. EIS technique is an effective method that enables the monitoring of this process during SCC. Figure 1(a) shows the EIS plots obtained at various applied cathodic potentials. Two time constants are observed in all curves under cathodic potentials. The capacitive semicircle in the high-frequency range is caused by an electric double-layer process or by electrode reactions at the inter-face. In the low-frequency range, the inclined line is attributable to the diffusion processes. At OCP, the high-frequency semicircle similarly reflects the elec-trode reactions at the interface and the electric double layer processes. The secondary semicircle is related to the absorption processes. At anodic potentials such as –650 mVSCE, two time constants are also found: the high-frequency semicircle reflects the growth of the corrosion product layer, and the low-frequency capac-itive semicircle characterizes the electric double-layer processes.24

The time constant for the electric double layer is the parameter that describes the recovery rate of an † Trade name.

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electrochemical reaction from the non-steady to the steady state. Figure 1(b) provides the statistic result for the recovery time against the various applied po-tentials. The recovery time decreases if the applied potential becomes extremely negative. Recovery time generally ranges from approximately 0.03 s to 0.096 s. This range indicates that a short period of non-steady state in an electric double layer is possible. This time must, at least, not be less than the recovery time.

Figure 2 shows the polarization curves obtained at various scan rates. Three groups can be identifi ed according to the corrosion potentials from each curve. Those curves with corrosion potentials at about –700 mVSCE belong to the fi rst group. They were col-lected at a low scan rate that ranged between 0.5 mV/s

and 5 mV/s. Therefore, polarization reactions take place in a quasi-steady state, under which the effect of non-Faraday processes in the electric double layer can be ignored during the potentiodynamic polarization measurement. The second group of polarization curves with a scan rate ranging from 10 mV/s to 25 mV/s has a more negative null-current potential than the fi rst group. The third group of curves obtained at high scan rates, i.e., 50, 100, and 150 mV/s, exhibits null-current potentials at around –1,000 mVSCE with simi-lar curve shapes. These results reveal that when the corrosion potential is above –1,000 mVSCE, the in-crease of the scan rate leads to the transformation of the electrochemical state. As a result, the anodic reac-tion occurs within a common cathodic potential range. However, the variation of the scan rate does not change the electrochemical reactions when the corrosion potential is below –1,000 mVSCE.

To investigate the effect of applied potential on the SCC susceptibility of the X70 pipeline steel in the testing solution, RA loss ratio (Sψ) is defi ned as fol-lows:18,25

S 1 – 100%E

0

S 1=S 1Ψ

Ψ×ΨS 1ΨS 1

(1)

where Ψ0 and ΨE are the RA measured in air and in solution, respectively, at applied polarization poten-tials.

Figure 3(a) shows that Sψ increases as the po-tential drops to about –950 mVSCE. Therefore, SCC susceptibility increases with a decrease in applied potential. However, a sharp fall of Sψ can be observed between –950 mVSCE and –1,050 mVSCE and is followed

(a) (b)FIGURE 1. Nyquist curves of EIS (a) and the statistic of time constants related to charge-transfer processes at varying potentials (b). An equivalent circuit shown in Figure 1(b) was used to fit the EIS data to quantify the electrochemical parameters, in which Rsol represents the solution resistance, Qf represents the capacitance of the corrosion product film, Rf represents the resistance of the corrosion product film, Qdl represents the double-layer capacitance, and Rt represents the charge-transfer resistance. Time constant is calculated according to the characteristic frequency of Rt, semicircle in Figure 1(a).

FIGURE 2. Polarization curves obtained at different potential scan rates.

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by an incline of Sψ when the applied potential is fur-ther lowered down. The result suggests a decrease in SCC susceptibility at about –1,000 mVSCE instead of a continuous increase that was previously reported.26 Three different potential ranges can be defi ned ac-cording to Figure 3(b). At above –700 mVSCE, the curves of both the slow scan rate and the fast scan rate fall within the anodic range, thereby indicating that both the crack tip and the non-cracking area are controlled by anodic reactions, i.e., the SCC mecha-nism is AD.22 At below –1,000 mVSCE, the two curves are both in the cathodic region, indicating that both the crack tip and the non-crack tip zones are under cathodic polarization. Therefore, the SCC mechanism in this potential range is HE, which is believed to be a key factor in the occurrence of SCC in pipeline steels.27-28 The slow scan rate polarization curve is located in the cathodic region, whereas the fast scan rate curve is located in the anodic region. These lo-cations indicate that AD may initiate the crack or sharpen the crack tip to accelerate crack propagation while hydrogen revolution takes place in the non-cracking surface. Both effects accelerate the initia-tion and propagation of SCC.29-31 Therefore, the SCC processes are controlled by the combination of AD and HE. The results in Figure 3 indicate that the SCC mechanism is consistent with the comparison of the slow and fast scan rate polarization curves.

Fractographs of the SSRT samples at different ap-plied potentials are shown in Figure 4. At OCP (Figure 4[a]), a shearing fracture area with dimple fracture morphology is observed at an edge of the fracture sur-face, and no cleavage fracture surface is found. This result indicates low SCC susceptibility. When the ap-plied potential is reduced to –750 mVSCE, a small brit-tle surface area becomes visible (Figure 4[b]), although the fracture surface is dominated by dimples indicat-

ing that SCC start to initiate but have not grown up. This result suggests that low applied potential results in high SCC susceptibility. With the further decrease of the applied potential to –980 mVSCE, a wide zone of SCC wall with a distinct brittle pattern is observed on the right lower side of the fractography (Figure 4[c]), demonstrating the continuous increase of SCC sus-ceptibility. However, when the potential is reduced to –1,050 mVSCE, the area of the SCC fracture surface signifi cantly decreases again, implying the decrease in SCC susceptibility (Figure 4[d]). When the potential is as low as –1,350 mVSCE (Figure 4[e]), a wide SCC plat-form becomes visible, and a hydrogen blister crack can be seen in the middle of the platform. This result reveals that the increasing SCC susceptibility and the HIC play a key role during SCC. In summary, the fractography verifi es the same tendency of SCC char-acterization as manifested by Sψ.

DISCUSSION

Analysis of Electrochemical Reactions of X70 Steel in an Acidic Soil Environment

In the present study, the solution contains HCO3–,

carbonic acid (H2CO3), and H+ ions with a pH of 4.0; H2CO3 in this case is produced from the hydrolysis of HCO3

–. These ions signifi cantly contribute to the occurrence of SCC in pipeline steels.23,32-34 Under a steady electrochemical condition, the primary ca-thodic reactions involved in hydrogen reduction in acidic soil solution are listed below.21

2H 2e H2+ ↔2e+ ↔2e+ (2)

2H CO 2e H 2HCO2 3CO2 3CO 2 3H 22 3H 2HC2 3HCO2 3O–+ ↔2e+ ↔2e H 2+H 2H 22 3H 2+H 22 3H 2 (3)

2H O 2e H 2OH2 2O 22 2O 2e H2 2e H –+ ↔O 2+ ↔O 2e H+ ↔e H2 2+ ↔2 2O 22 2O 2+ ↔O 22 2O 2e H2 2e H+ ↔e H2 2e H + (4)

(a) (b)FIGURE 3. SCC mechanism alternation along with the potential (a) and the corresponding fast and slow scanning rate polarization curves relative position (b).

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FIGURE 4. Failure surface SEM morphology of SSRT sample under (a) OCP (about –700 mVSCE), (b) –750 mVSCE, (c) –980 mVSCE, (d) –1,050 mVSCE, and (e) –1,350 mVSCE.

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The occurrence of these reactions is affected by applied cathodic potentials. When the potential is positive to the upper reduction potential of H2O, H+ and H2CO3 are the main reactants of the cathodic re-actions. If the supply of reactive species, such as H+ and H2CO3, is not as fast as their consumption under an applied potential, the mass-transport step be-comes the rate-limiting step of the cathodic processes. However, water reduction, which is not affected by the mass-transport step, becomes thermodynamically possible at potentials as negative as –1,200 mVSCE.

As demonstrated in previous studies, the anodic reaction of X70 steel in a deoxygenated low-pH solu-tion is dominated by the oxidation of iron.35 A layer of porous iron(II) hydroxide (Fe[OH]2) might form on the electrode surface.

Fe 2OH Fe(OH) 2eH F–H F 2+ ↔2O+ ↔2OH F+ ↔H F + (5)

When the cathodic reactions are restricted, the anodic reactions accelerate accordingly. In a steady electrochemical state under cathodic potentials, cathodic reactions, such as those in Equations (2) through (4), occur if the applied potential is lower than the corresponding electrode potential of these reactions. However, under a non-steady state, the ca-thodic reactions (i.e., the reduction of H+ and H2CO3) becomes inhibited within a recovery time (Figure 1) because of the effect of mass-transport processes; an anodic reaction (Equation [5]) also occurs accordingly.

When polarization is implemented at a slow scan rate, such as at 0.5 mV/s with a step height of 1 mV, the time between two potential steps is thus longer than the recovery time of the electric double layer (Figure 1). Therefore, the concentrations of the reac-tants at the metal/solution interface are almost the same as those in the bulk solution, thereby leading to a quasi-steady condition. When the potential scan rate is suffi ciently quick, no suffi cient reagents at the metal/solution interface react because the transport rate of the reactants is slower than their reaction rate. If the scan rate is suffi ciently high, the reactant concentration on the metal surface is close to zero because the potential shift within a unit time is ex-tremely large so that it almost consumes all reactive ions. In this case, anodic reactions will take place be-cause of the mass-transport effect.

Based on the above derivation, we can hypoth-esize that the potential of –1,000 mVSCE should be the critical value for different dominated mechanisms, as manifested by the potentiodynamic polarization curves shown in Figure 2. Below –1,000 mVSCE, wa-ter reduction is the dominant electrochemical reac-tion, whereas above this potential, the mass diffusion taking over the water reduction becomes the pre-dominant step in electrode reactions. Consequently, cathodic reactions are sustained when they are polar-ized at high potential scan rates. Therefore, anodic

reactions can occur at a much lower potential than OCP.

Relationship of Electrochemical Reactions Under Non-Steady Electrochemical State and the Stress Corrosion Cracking Mechanism

As indicated by the slow and fast potential scan-ning rates and the development of the steady status and non-steady status of the steel electrode, three zones can be observed in the polarization curves, as illustrated in Figure 2. When the applied potential is more positive than the null-current potential of about –700 mVSCE, the polarization curves measured at both slow and fast scanning rates fall within the anodic polarization range, indicating that the cracking is controlled by an anodic reaction. Therefore, the SCC mechanism is AD-based. When the applied potential is more negative than the null-current potential mea-sured at a fast scanning rate, i.e., more negative than –1,000 mVSCE, the steel is in the cathodic polarization region, and the cathodic reaction charges hydrogen to the steel. So, the SCC follows the HE mechanism. When the polarized potential is between the two null-current potentials, the steel is in cathodic polarization if the potential scanning rate is slow, such as at 0.5 mV/s, and in anodic polarization if the scanning rate is fast, such as at 100 mV/s. The steel is in a non-steady state, and AD could occur under a cathodic polarization potential to contribute to the cracking process. Both the anodic polarization and the hydro-gen effect contribute to the acceleration of SCC.18 Therefore, the SCC of the steel is a combination of the AD and HE mechanisms within the potential range of –700 mVSCE and −1,000 mVSCE. Apparently, the SCC mechanism of pipelines in an acidic soil environment depends on the potential applied on the steel. Fur-thermore, the critical potential range is determined by the measurement of polarization curves at both fast and slow potential scanning rates.

With the negative shift of cathodic potential shown in the AD + HE zone (Figure 3[b]), the cathodic current density at a slow scanning rate increases, whereas the anodic current density at a fast scanning rate decreases. The maximum SCC susceptibility is observed, as indicated by the increasing Sψ in Figure 3(a). As the hydrogen evolution is enhanced by the cathodic current density, the HE effect is believed to dominate SCC growth, and dissolution would ac-celerate the cracking process. When the potential is more negative than –1,000 mVSCE, SCC susceptibility decreases. This decrease is primarily caused by the signifi cant reduction of the dissolution current at suf-fi ciently cathodic potentials. Consequently, the AD effect could not be ignored because it could remark-ably affect SCC susceptibility at a cathodic potential. The potential dependence of SCC susceptibility is consistent with the fracture surface characterization (Figure 4).

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Quantitative Model of the Relationship Between Stress Corrosion Cracking Mechanisms and Non-Steady Electrochemical Reactions

In the crack tip, the continuous SCC growth un-der tensile stress causes the fresh metal surface to be subjected to a non-steady polarization state. When a new crack tip is generated, the electrolyte fl ows into the crack immediately, and the applied potential starts to charge the electric double layer with the introduc-tion of a cathodic potential. Thus, the condition in the crack tip can be simulated with an open fresh sample in a steady/non-steady electrochemical environment.

A defi ned function i –

in a steady/non-steady electrochemical environment.i –

in a steady/non-steady electrochemical environment.i

in a steady/non-steady electrochemical environment.i

in a steady/non-steady electrochemical environment.

if si –f si – if si

s, where if and is are the

currents determined from the fast and slow scan rate polarization curves, respectively, is related to the SCC susceptibility of a pipeline steel in an anodic condi-tion.36 Existing theories could explain these results. In the present study, a quantitative model based on non-steady electrochemical theory in the crack tip was de-veloped to assess the effects of AD and HE accurately during the occurrence of SCC in the pipeline steel in a soil environment. The detailed information of the model is given below.

Within the AD potential range (if > 0, is > 0), SCC can be illustrated by the difference between if and is through

i –can be illustrated by the difference between i

i –can be illustrated by the difference between i

ican be illustrated by the difference between i

ican be illustrated by the difference between i


s. Here, only if if > is, i.e.,

i – ii

f si –f si – if si

s > 0, could

localized corrosion such as SCC initiate or propagate. At the same time, if plays an important role in SCC growth processes because it is one of the key factors that cause the sharpening and propagating of the crack tip. Considering these points, the relationship between SCC susceptibility and polarization current can be defi ned as follows:

S k i

i – ii

Sa fia fif si –f si – if si

s0= ⋅S k= ⋅S ka f= ⋅a f ⋅ +⋅ +ΨS kΨS k

(6)

where SΨ is the SCC susceptibility factor (%), which is the RA loss ratio in Equation (1); ka is a constant that is related to material, medium, and current density; and S0 is the nominal SCC susceptibility factor when if = is. According to Equation (6), a high if indicates the quick growth of the crack tip. Consequently, a large

i –the quick growth of the crack tip. Consequently, a

i –the quick growth of the crack tip. Consequently, a

ithe quick growth of the crack tip. Consequently, a

ithe quick growth of the crack tip. Consequently, a


s indicates a great corrosion rate difference

between the non-cracking surface and the crack tip that results in the fast growth of SCC. If if >> is, then SΨ ≈ ka ·

iif2

that results in the fast growth of SCC. If i2

that results in the fast growth of SCC. If i

s + S0. This condition is in agreement with

the literature.7

In the potential range of the mixed mechanism of AD and HE (if > 0, is < 0), SCC susceptibility can be assumed as a function of if (the role of AD) and is (the role of HE), which can be expressed as:

S k i ki k i

ii

–1 She s as ai ks ai ki ks ai k d fid fif

sac= ⋅S k= ⋅S khe= ⋅he + ⋅i k+ ⋅i ks a+ ⋅s ai ks ai k+ ⋅i ks ai k d f+ ⋅d f ⋅ +⋅ +⋅ +⋅ +–1⋅ +–1ΨS kΨS k

(7)

where khe is the hydrogen factor, which is a constant that depends on materials, media, and current den-

sity; kad is the AD factor similar to khe; Sac is a factor that describes the relationship of the roles of AD and HE; | is | represents the role of cathodic hydrogen evo-lution. A great | is | indicates the signifi cant role of HE. This relationship shows great SCC susceptibility. if · This relationship shows great SCC susceptibility. iThis relationship shows great SCC susceptibility. iii

–1f

s defi nes the AD effect on the crack tip and is as-

sociated with the high propagating rate of SCC. If is drops down to icorr, the fi rst part in Equation (7) can be approximated to zero. Equation (7) then becomes Equation (6). When the potential is suffi ciently low, i.e., if is close to 0, Equation (7) is converted to Equa-tion (8), which is only dependent on the effect of HE.

If the electrochemical status is controlled by ca-thodic reactions (if < 0, is < 0), SCC depends on the effect of HE. Therefore, the relationship between SCC susceptibility and polarization current is:

S k i Si Sc dc di Sc di Sc= ⋅S k= ⋅S kc d= ⋅c di S+i SΨS kΨS k (8)

where kc is the HE factor, which is a constant that is related to materials, media, and current density; Sc is a factor that describes the role of HE.

In summary, Equations (6) through (8) can be written together as follows:

SΨ =

ka ⋅ if ⋅if – isis

⎛

⎝⎜⎛⎜⎛

⎝⎜⎝

⎞

⎠⎟⎞⎟⎞

⎠⎟⎠+S0 (if > is > 0)

khe ⋅ is + kad ⋅ if ⋅ifis

– 1 +Sc (if > 0,is < 0)

kc ⋅ is +Sc (if < 0,is < 0)

⎧

⎨

⎪⎧⎪⎧

⎪⎪⎪⎪

⎪⎨⎪⎨

⎪⎪⎪

⎩

⎪⎨⎪⎨

⎪⎪⎪⎪

⎪⎩⎪⎩

⎪⎪⎪

(9)

SΨ at various potentials calculated through the above equation against the applied potential is plotted in Figure 3(a), which is consistent with the experimen-tal results. In this fi gure, points 1 through 8 are the experiment data obtained by SSRT and can be clas-sifi ed into three groups, i.e., 1-2, 2-5, and 6-8. Based on these equations, SCC susceptibility at any chosen potential can be identifi ed according to Equation (9). This condition provides a standard and quick method for determining the SCC susceptibility mechanism.

CONCLUSIONS

❖ This study developed a methodology to calculate SCC susceptibility (SΨ), in which SΨ was related to the currents obtained by the fast and slow scan rate po-larization curves and by the results of SSRT.❖ The SCC mechanism of X70 pipeline steel varies according to the change in applied potential, including AD, HE, and a combination of AD and HE. For each mechanism, the potential range can be determined according to the comparison of the fast and slow scan rate polarization curves. With the potential above –700 mVSCE (if > 0 and is > 0) in the present study, the SCC mechanism is AD. With the potential ranging

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from –700 mVSCE to –1,000 mVSCE (if > 0 and is < 0), the SCC mechanism is AD + HE. With the potential below –1,000 mVSCE (if < 0 and is < 0), SCC follows the HE mechanism. The model produced in this study should facilitate the immediate identification of SCC suscep-tibility.

ACKNOWLEDGMENTS

This study was supported by the National High-Tech R&D Program of China (no. 2012AA040105).

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