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Review of stress corrosion cracking of pipelinesteels in low and high pH solutions

B. Y. FANGState Key Laboratory for Corrosion and Protection of Metals,Institute of Metal Research, Chinese Academy of Sciences,62 Wencui Road, 110016, Shenyang, Peoples Republic of China

A. ATRENSDepartment of Mining, Minerals and Materials Engineering,The University of Queensland, Brisbane, QLD 4072, AustraliaE-mail: atrens@minmet.uq.oz.au

J. Q. WANG, E. H. HAN, Z. Y. ZHU, W. KEState Key Laboratory for Corrosion and Protection of Metals,Institute of Metal Research, Chinese Academy of Sciences,62 Wencui Road, 110016, Shenyang, Peoples Republic of China

This paper reviews the current understanding of the mechanisms of stress corrosioncracking of pipeline steels. The similarities, the differences and the influencing factors areconsidered for the high pH stress corrosion cracking caused by a concentratedbicarbonate-carbonate solution, and for the low pH stress corrosion cracking due to adiluter solution. For high pH stress corrosion cracking, it is well accepted that themechanism involves anodic dissolution for crack initiation and propagation. In contrast, ithas been suggested that the low pH stress corrosion cracking is associated with thedissolution of the crack tip and sides, accompanied by the ingress of hydrogen into thepipeline steel. But the precise influence of hydrogen on the mechanism needs to be furtherstudied. C 2003 Kluwer Academic Publishers

1. IntroductionStress corrosion cracking (SCC) from the external sur-face of a buried pipeline is a serious matter. There isalways the chance that a pipeline could leak or rup-ture. A pipeline failure can have serious consequencesfor people living or working close by and also for theenvironment. A pipeline failure may be catastrophicand may cause significant economic and environmen-tal loss. For Canada, there are typically 30 to 40 failureseach year on pipelines regulated by the National EnergyBoard (NEB) [1], some of which have caused fatalities.For example a death occurred in 1985 when a drainagetile plow ruptured a gas pipeline [1]. In 1965, the firstcase of SCC was reported [2]. Since that time, SCChas been recognized as a cause of pipeline failures incountries throughout the world. Pipelines in Australia[3], Canada [4], Iran, Iraq, Italy, Pakistan, Saudi Ara-bia, the former Soviet Union and the United States [5]have all suffered SCC. The Trans Canada pipeline hadthree SCC failures in the Northern Ontario portion be-tween March 1985 and March 1986, experienced itsfourth SCC-related pipeline rupture in December 1991near Cardinal, Ontario, and its fifth in July 1992 nearTunis, Ontario. In July 1995, SCC caused a rupture onthe Trans Canada pipeline near Rapid City, resulting ina major explosion. These SCC related failures resulted

in two public inquiries by the NEB of Canada. Thesecond inquiry resulted in a substantial publication [1].

It is now recognized that there are two forms ofSCC penetrating from the external surface of buriedpipelines. One is intergranular SCC (IGSCC) and isusually called the high pH SCC or classical SCC.The other is transgranular SCC (TGSCC), and is des-ignated near-neutral pH SCC or low pH SCC ornon-classical SCC.

At the present time in China, there are plans to builda 4,200-kilometre natural gas pipeline, the total cost ofwhich is expected to be more than 140 billion yuan (US$16.86 billion). It will be Chinas largest infrastructureproject. Accordingly, it is essential and imperative todesign the pipeline soundly and ensure that there are noSCC incidents. The present work is part of a researcheffort to understand pipeline SCC in order to minimizethe possibility of its occurrence.

2. High pH SCC and near-neutral pH SCCof pipelines

There are many similarities between the two forms ofpipeline SCC. Cracks of both forms usually occur onthe outside surface in colonies, mostly oriented lon-gitudinally along the pipe, primarily at the bottom of

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the pipeline [6]. These cracks coalescence to form longshallow flaws, that can lead to ruptures. The fracturesurfaces are usually covered with black magnetite film[7] or an iron carbonate film [7]. However, there aremany differences between the two forms of pipelineSCC. High pH SCC, engendered by concentrated bicar-bonate or carbonate-bicarbonate solutions associatedwith pH of 9, has usually an intergranular morphol-ogy, and the cracks are sharp, with little lateral corro-sion [1]. Near-neutral pH SCC, engendered by diluteground water with a relatively low pH of around 6.5,has a trangranular, quasi-cleavage crack morphologywith very little branching [1]. The transgranular cracksare generally wide with appreciable lateral corrosionof the crack sides. Moreover, the near-neutral pH SCCoccurs over a wider potential range [8] than high pHSCC which has only narrow width of no more than100 mV(SCE) [8].

SCC requires the simultaneous action of the follow-ing three factors: a potent environment at the pipe sur-face, a susceptible pipe material and a stress. All threefactors must be present together for SCC to occur. Ifany of them can be eliminated or reduced, then SCCcan be prevented. The following sections discuss thesethree factors in more detail.

3. Environment conditionsWhen new pipelines are built, they are coated for corro-sion protection and cathodic protection is applied oncethe pipeline is in the ground. After a period of pipelineoperation, the coating may deteriorate, leading to theformation of holidays and the diffusion of water, carbondioxide and other species through the coating. Alterna-tively the coating may disbond. Different types of coat-ing have different properties, and they have differentpropensities for deterioration or disbondment. The cor-relation of coating type to the incidence of SCC is mostmarked in relation to the near-neutral pH SCC where,with the exception of the involvement of asphalt in veryhigh soil resistivity locations, SCC failures have beenmostly associated with high electrical resistivity tapecoatings. The explanation may be that a high electricalresistivity tape coating can shield the cathodic protec-tion current from flowing to the pipeline surface, and ifthe coating is disbonded, then the near-neutral pH en-vironment forms and TGSCC can become possible. Ifthe coating has a high resistivity, then the near-neutralpH natural ground water is not changed by the cathodicprotection current. If the good coating is disbonded ordeteriorated, it can shield the cathodic protection cur-rent flowing at the pipeline surface, the natural groundwater can come into contact with the pipe material andthen TGSCC can become possible. This interpretationis supported by the NEB report [1].

For high pH SCC, SCC has been associated withpipelines with coal tar, asphalt and tape coatings, andno failures have been reported with modern thin filmcoatings. Most of the failures with coal tar coatingsmay be because this type of coating has been used onthe older pipelines, these pipelines have had longer pe-riods of service, and hence had more opportunity tocrack. This is supported by NEB [1]. The consequence

of coating defects is that they allow ground water accessto the pipeline surface. The area of the surface exposedto the ground water solution depends on the disbond-ment of the coating. Furthermore, the composition ofthe groundwater solution depends on the amount ofcathodic protection current reaching the pipe surface[1]. The ground water is not be changed if the coat-ing does not allow the cathodic protection current topass through, or if there is high electrical resistanceswithin the soil or the solution in the crevice betweenthe pipeline surface and the coating, or if there is nosignificant cathodic protection current reaching the ex-posed surfaces. The natural ground water solution has apH from 6 to 7 resulting from the equilibrium betweenHCO3 and CO

23 . This solution can cause TGSCC.

Laboratory tests often use NS1, NS2, NS3, NS4 solu-tions [9], the Nova solution [10] or 0.01 N NaHCO3[11] or N1 Trapped Water [11] to produce TGSCC.

However, a substantial cathodic current at thepipeline surface causes hydroxyl ions to be generatedand accumulated, and the pH increases according toreaction (1) or (2):

2H2O + 2e = H2 + 2OH (1)O2 + 2H2O + 4e = 4OH (2)

The solution chemistry also relates to the conversionof bicarbonate to carbonate ions. With time, the so-lution becomes concentrated and the concentration ofcarbonate is high, which leads to the tendency for thesolution to passivate the steel surface, and IGSCC canoccur. The extent that the pH increases depends on thepartial pressure of CO2. In the absence of CO2, the pHincreases to 1112. The solution consists of nearly allcarbonate ion, and the passive film characteristics andelectrochemistry are not conductive to SCC. However,if there is an appropriate partial pressure of CO2 thesolution becomes buffered because of the equilibriumbetween bicarbonate and carbonate ions, and the pHremains in the region of 9. IGSCC can occur when thepotential is in the appropriate range as defined by Fig. 1[12, 13].

Figure 1 Potential-pH diagram showing the regimes for IGSCC andTGSCC at 24C in solutions containing different amounts of CO23 ,HCO3 and CO2 to achieve different pH values [12, 13].

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The great majority of pipelines use cathodic protec-tion so that the high pH solution is more likely to be gen-erated than the low pH environment. This is in agree-ment with field experience that the intergranular formof SCC is much more common than the transgranularform.

There is a difference of the polarization curve mea-sured in the high pH solution and the near-neutral so-lution. In the high pH solution, the curve exhibits anactive-passive transition over a certain potential rangeas illustrated in Fig. 2 from the work of Parkins [14].This transition has been shown to be associated withIGSCC of ferritic steels in various environments [15]. Incontrast, the near-neutral pH environments do not pro-mote passivation and do not exhibit an active-passivetransition (Fig. 3) [12]. The influence of potential andpH on IGSCC and TGSCC is demonstrated in Fig. 1[12, 13]. This shows the results from slow strain ratetests on a low-carbon steel in the solutions which con-tain bicarbonate, carbonate and carbon dioxide. ForTGSCC, the potential should be applied at or slightlycathodic (max. 50 mV) to the Ecorr [16] to reproduceTGSCC.

Figure 2 Potentiodynamic polarization curves showing the potentialrange for IGSCC in concentrated carbonate-bicarbonate solution at 90C[14].

Figure 3 Fast and slow sweep rate polarization curves at 24C for a linepipe steel in simulated ground water saturated with CO2, pH = 5.8 [12].

Researchers have learned that a range of chemicalspecies can promote near-neutral pH SCC. Near-neutralpH SCC has occurred in solutions with low concentra-tion of carbonic acid, bicarbonate ions and other speciesincluding chloride, sulphate and nitrate ions where highpH SCC has not been observed [1]. Rebak et al. [17]showed that TGSCC can be reproduced in aqueous sul-fate, bicarbonate and simulated soil solutions. So the en-vironment producing near-neutral pH SCC may vary. Inaddition, there is another factor for operating pipelinesthat can make solution concentrated. If the surroundingground is able to accept the evaporated water, which isinfluenced by the seasonal change, the water can slowlyevaporate from the solution due to heat transfer throughthe pipe wall. This can concentrate the solution, as hasbeen shown by Parkins [18].

IGSCC displays temperature sensitivity in the fieldand in laboratory tests. When the temperature increases,the intergranuar stress corrosion crack propagation rateincreases according to the Arrhenius Law [19]. In thefield, the temperature in the vicinity downstream of acompressor station is higher than in other places, andso the greatest risk of SCC is in this location. Whenthe temperature is high, the coating deterioration alsobecomes worse. In contrast, there is no systematic tem-perature dependence in the range 5C to 45C for near-neutral pH SCC [20].

The effect of oxygen on SCC of buried pipeline steelsneeds to be clarified. Delanry [4] cited a Canadian studythat showed SCC becomes more severe in the absenceof oxygen. In that study, cracks in the field were ob-served mainly in areas where oxygen access was re-stricted and the propensity to SCC decreased with thepresence of oxygen.

4. Metallurgical conditionsSo far there is no obvious evidence that provides a re-lationship between pipeline failure due to near-neutralpH SCC or high pH SCC and steel composition, grade,or microstructure. However, there is evidence from lab-oratory studies that shows certain batches of steel aremore resistant to SCC. Asahi showed that, for a rangeof pipeline steels from X52 to X80 grades, thermo-mechanical controlled processing or quenched andtempered steels with fine-grained bainitic structures,or acicular ferrite, uniform microstructures, were moreresistant to IGSCC than controlled rolled steels withferrite-pearlite structures [21]. The work by Kimuraet al. [22, 23] has shown that the low pH SCC suscep-tibility for the ferrite-pearlite structure is higher thanthat for uniform structure like a quench and tempersteel. The homogeneous structure has a high resistanceto SCC.

Grit blasting pipe surfaces before applying a coatingcan provide better coating bonding, and can be bene-ficial in avoiding the potential region for intergranularcracking. Moreover, if appropriately applied, grit blast-ing leaves the pipe surface in a state of compression thatis beneficial in at least delaying, if not preventing, theincidence of SCC in a variety of systems [12].

It is well established [24, 25] that the mill scaledsurfaces on pipeline steels are more susceptible to SCCthan polished surfaces. The cause is not understood, but

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it has been accepted that the mill scale plays a detrimen-tal role. Most cracks initiate at the base of pits wherethere are cracks in the mill scale [25]. The mill scale canhold potential within the cracking range [24, 25]. Fur-thermore, the potential for pitting on a pre-pitted sur-face is much below that for non-pitted surfaces, and thismay facilitate changes in solution composition withinnew pit enclaves and also the discharge of hydrogen.Wang and Atrens [26] carried out observations of SCCinitiation of X65 pipeline steel exposed to the high pHsolution. They found for the pipeline steel tested withthe original surface as was present in service, SCC initi-ation correlated with the fracture of the surface productfilm. This SCC initiation from the original surface oc-curred at a stress lower than for a polished surface ofthe same steel.

For near-neutral pH SCC, the following five factorshave been suggested [11] to influence crack initiation:inclusions, aligned surface defects, persistent slip bandsproduced by mechanical pre-treatment of the steel, pre-existing defects on the pipeline surface and coating dis-bondment. High pH SCC cracks are often found in thearea containing pits in field failures [27] and in labora-tory tests [25]. Pits cause a stress concentration, partic-ularly when relatively deep. High pH crack nucleationappears to increase with increasing pit density.

Generally, it is common to use a resistant material inengineering structures for a particular environment toavoiding SCC. This may be feasible for new pipelines.However, the new steels must have high strength andfracture toughness, which is essential for the structures.

When a specimen is cold worked or notched, the initi-ation time is shortened and so cracking becomes easierthan on a polished specimen. The reason is the stressconcentration in the area of the notch or the cold work.Similarly the coarse grain heat affected zone is highlysusceptible to SCC. Harle [28] showed that the coarsegrain heat affected zone microstructure adjacent to theweld in an X65 steel, is significantly more susceptibleto cracking than the base material in the near-neutralpH environment.

When pipeline steel is exposed to high pH solution,grain boundaries undergo preferential dissolution [29].If a stress of an appropriate magnitude is not present, thedissolution does not penetrate far. The propensity forsuch intergranular attack is thought to be related to thepresence of carbon, sulfur or phosphorus segregatedto the grain boundaries in ferritic steels [30], as wellas the properties of the solution that foster selectiveattack on the grain boundaries. Recently Wang et al.[31] measured the grain boundary compositions forX65 and X52 pipeline steels using analytical electronmicroscopy (AEM). The results showed that there wasno segregation at proeutectoid ferrite grain boundaries,which indicated that species P and S are not respon-sible for preferential dissolution of grain boundariesduring intergranular SCC [3133]. Due to the ubiqui-tous hydro-carbon contamination of specimen surfaces,the distribution of carbon could not be measured usingAEM [34]. For X42 steel, there was also no measurableS and P segregation at ferrite : ferrite grain boundaries[35].

5. Mechanical conditionsIn order to propagate for SCC cracks, there must bean appropriate stress at the crack tip. The stress canbe applied or residual in nature. SCC can initiate andpropagate only if there is sufficient stress. Below somevalue of stress, the crack may not initiate. This stressis the threshold stress. Many cases of longitudinal SCCin the body of pipe were attributed to the hoop stressgenerated by the internal pressure. It was found thatthe threshold stress was markedly reduced for an actualpipe surface in the presence of mill scale or pits [36].The number of cracks depends on the maximum stress[12]. For a higher stress, there are more cracks and theyare closer spaced.

Beavers [37] has indicated that no cracks have beenfound under constant displacement conditions. Thispoints out the importance of pressure fluctuations. Pres-sure fluctuations may be necessary for cracks to occurnot only with near-neutral pH SCC but also with highpH SCC.

Parkins [38] correlated the reduction in thresholdstress with the strain-hardening behavior of the steelwhen subjected to cyclic stresses superimposed on amonotonically increasing stress. He found that the ini-tiation becomes easy and the propensities for inter-granular cracking of steels have a close relationshipwith cyclic stress-strain behavior as shown in Fig. 4.The superimposed cyclic stress (on the order of 15 to80 M N/m2) made plastic deformation start at muchlower levels of mean stress, and the slope of the plas-tic portion of the curve changed abruptly at a certainstress, which corresponded to the threshold stress forSCC. Parkins showed that cyclic loading significantlydecreased the threshold stress for IGSCC below that as-sociated with a static load. While it has not been demon-strated by appropriate measurements, it seems likelythat cyclic loading may also facilitate transgranularcracking in near-neutral pH solution and Parkins has re-marked that cyclic loading of pipelines appears to play amajor role on the propagation of both types of SCC [12].

Figure 4 Comparison of a typical stress-strain curves produced withmonotonic loading and with cyclic loads superimposed on the steadyloads. Arrow indicates inflection point where the stress corresponds tothe threshold stress for SCC [38].

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There are also dormant cracks in pipelines. Theseare attributed to the decreasing micro-plastic strain ratewith time. The phenomenon of dormant cracks mayexplain why relatively few failures occur in the fieldcompared with the number of small cracks which arediscovered in inspections. However, crack may grow inother cases, especially if adjacent cracks coalesce [39].When a film is ruptured or an active crack is closer to thedormant one, the reactivation of a dormant crack maybe possible and then coalescence may occur. This isconsistant with Parkins [40]. Coalescence may possiblyinfluence crack growth, but there is no predictive modelwhich has been developed that successfully defines theeffect of coalescence on crack propagation smoothlyand the crack growth rate exactly. So it is incompletefrom this point, and needs further to be studied.

To compare different steels, slow strain rate test(SSRT) has been used, which places emphasis on elec-trochemical effects (dissolution and passivation kinet-ics). In SSRT, the deformation of the specimen is im-posed by the test at a given rate, which may override thecreep rate and strain-hardening properties of the steels[41]. A better approach to comparing these steels maybe using cyclic loading tests. Cyclic loading tests aremore similar to the field conditions than SSRT.

6. Mechanistic understandingSince the cases of high pH SCC have about 40 yearsof history, researchers have studied IGSCC extensively,and its mechanism, caused by preferential dissolutionat the grain boundaries, is well accepted. For IGSCC,i.e., high pH SCC, Parkins showed that the crack rate fora variety of alloy in various environment can be evalu-ated from the dissolution current densities measured onbulk specimens under the same environmental condi-tions [40]. The crack propagation rate can be calculatedaccording to Faradays law:

CPR = (ia M)/(z F d) (3)

where CPR is the crack propagation rate, ia is the an-odic current density, M is the atomic weight, z is thevalency of solvated species, F is the faraday and d is thedensity of metal. It has been found that the calculatedpropagation rate is in conformity with the estimatedone [12, 29]. So the mechanism of high pH SCC is at-tributed to anodic dissolution resulting from selectivedissolution at the grain boundaries and repeated ruptureof passivating films that form over the crack tip.

Fig. 1 shows that there is not a gradual transition fromIGSCC to TGSCC. There is a discontinuity between thetwo forms, and the potential range for TGSCC is lowerthan for IGSCC. This implies that they have differentmechanisms. If the mechanism of near-neutral pH SCCwere anodic dissolution, then SCC in the pH 5.8 solu-tion extends to lower potential from about 0.67 V(SCE), the highest potential for transgranular crackingin Fig. 1. Fig. 3 shows that the current density is about1 104 A/cm2 at 0.67 V (SCE). Then the CPR isabout 4 108 mm/s according to Equation 3, whichcorresponds to the observed stress corrosion crack ve-

locity in field or in laboratory tests. However, when thepotential is below 0.67 V (SCE), the current densityis reduced according to Fig. 3, and so should the CPR.When the potential is lowered below the open circuit po-tential, the CPR would be very small, even not detected,cease. However, this is not a case. TGSCC can occurnear the open circuit potential and especially below thefree corrosion potential and the CPR calculated fromEquation 3 is lower than the measured CPR [42]. So adissolution mechanism is not supported. Nevertheless,dissolution does occur within the crack for near-neutralpH SCC as evidenced by the heavy lateral corrosion onthe crack sides. It is difficult to imagine that such dis-solution occurs only on the crack sides and not at thecrack tip.

Nevertheless, it is concluded that there is some pro-cess other than dissolution involved in the crack prop-agation in the low pH solution. Fig. 1 includes the linefor equilibrium discharge of hydrogen. The potentialrange for TGSCC is below this line. This indicates thatin the low pH solution, hydrogen discharge is possi-ble, so the mechanism for near-neutral pH SCC maybe related to hydrogen. Gu [10, 43] deduced an equa-tion for the synergistic effect of hydrogen and stresson the anodic dissolution rate. He suggested that themechanism of SCC for pipeline steels in near-neutralpH solutions might be as follows. As the anodic po-tential comes close to Ecorr., local dissolution or pittingoccurs, generating H+, which result in local acidifi-cation within individual pits. That acidification couldfacilitate the crack initiation and propagation process.So Gu suggested that near-neutral pH SCC is domi-nated by the mechanism of hydrogen-facilitated AD.At cathodic potentials, it is suggested that, when thehydrogen concentration reaches a critical value, hydro-gen induced cracking controls the cracking process.

There is a considerable amount of evidence [20, 44,45] that hydrogen plays a critical role in the near-neutralpH solution. However the precise mechanism has notbeen identified. It is well known that the hydrogen canreduce the ductility of the steel without producing mul-tiple cracks, but the same amount of hydrogen in thesteel held below the yield stress may not have any no-ticeable effect. Normally, when severe SCC occurs, thereduction area (RA) value is low, but the embrittlingeffect of hydrogen can confuse the issue. Parkins [20]showed that in some specimens held near Ecorr in thenear-neutral pH environment, the RA decreased fromabout 70 percent to 30 percent even in the absence ofsecondary cracking. Apparently, enough hydrogen maybe introduced into the steels during the test that theductility is reduced to 30 percent elongation. However,Fessler pointed out that an operating pipeline is neverexpected to undergo anywhere near 30 percent elon-gation, so the results from laboratory slow strain ratetests may not necessarily be applicable to an operatingpipeline, and the extrapolating the laboratory results tothe field can be dangerous [46]. This is reasonable.

Some investigators think that the magnitude of theRA correlates with the rate of hydrogen absorption [10].And some think that hydrogen can reduce the yieldstrength of the steel, then promote the onset of plastic

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deformation, or can lower the total deformation whichcan be tolerated by the steel before fracture [47].

Whilst the mechanism of the high pH SCC is al-most universally accepted, there is less agreement onthe mechanism of the near-neutral pH SCC. It needs tobe further studied.

7. Summary1. When the coating deteriorates and disbonds, theground water can be in contact with the pipeline surface.In the absence of a cathodic protection current throughthe crevice between the coating and the surface, TGSCCcan occur. If the current flows at the surface, then theenvironment is changed into the high pH concentratedsolution and that give rise to IGSCC.

2. For high pH SCC, the mechanism for cracking isbelieved to be anodic dissolution caused by preferentialdissolution of some elements or active phase at the grainboundary.

3. In case of near-neutral pH SCC, there is less agree-ment on the mechanism. It appears that both corrosionand hydrogen may be important to the mechanism,but the precise role of hydrogen needs to be furtherclarified.

AcknowledgmentsThis work is supported by The Hundred Talents Pro-gram and The Special Funds For the Major State BasicResearch Projects G19990650. The authors acknowl-edge the assistance.

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Received 6 June 2001and accepted 27 March 2002

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