electrochemical and stress corrosion cracking ...

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CORROSION97 ELECTROCHEMICAL AND STRESS COIULOSION CRACKING BEHAVIOUR OF MICRO-ALLOYED STEELS IN C02/HCO~ ENVIRONh4XNT

Emdo Sirdgaglia Snarn S.P.A. Viale De Gaaperi, 2- San Donate Milrmese Milan, Italy, 20100

Marim Cabrini Dipartimento di ChimiaaFMeaApplkata- Polknico dib4ilano Via Mancinelli, 7 Milan,holy,20133

ABSTRACT Tmnagmmdar Stress Corrosion Cracking @GSCC) is a relatively recent form of stress corrosion cracking of buried gas mrdoilpipelines. Aimof thepresent workis toevaluate theelectrochemical andSCCbehaviour of microalloyedsteels inC02/HCO~envimnmenta inorderto better datine the mechanisms involved in TGSCC. It baa been shown that in TGSCC environment all the te$ed steels remain active. It is the presence of the couple C@/HCOj and of a pH around seven to play a major rule in the TGSCC. There are strong evidences that TGSCC is caused by the entrance of atomic hydrogen into the metal, hindered by the presence of corrosion products due to the environment. Temperature seems to irrbibit TGSCC. No SCC was obtained by constant load and constant deformation testa. iNTRODUCTION The non cfmsical form of stress corrosion emcking (SCC), cafled transgrarudar SCC (TGSCC)IIJ, has been fomrd in these last decade on high pressure sss and oif pipeline system. me first instances of TGSCC with the presence of tmrragramdar crocks have been aaarniated with service and hydroteat failures in Canada pipeline network during the eigh~esl. Sinw then many studies have pointed out the main difference between this form of SCC and the classical one, called intergranular SCC (IGSCC)(2J, whose cracks are intergmnnlar mrd bomrd to a highly concentrated carbonrdebicarbonate solution (pH ss 9-10) and to a specific temperature range (50-70°C)z3. In those respects TGSCC is associated with a relatively dilute solutions with pH v81ues = 6.5 under disbanding coatings. TGSCC does not seem to display a particular temperature sensitivity.

(1)Even called Low pH SCC or Near Neutral SCC. (z) Even called High pH SCC or Carbomte-Bicarbomte

SCC (CBSCC).

Copyright @l997byNACEInternational. Requestsforpermission topublishthismanuscript inanyform, in part or in whole must be made in writing to NACE International, Conferences Division, PO. Box 21834o, Houston, Texas 77218-8340. The mateti?l presented and the views expressed in thw paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

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Paper No.

EXPERltvtllNTAL Materials Chemical compositions and meebmricaf properties of tested steels are repmted in table L The metallogmpbic examination of the alloy shows a ferritic-perlitic structure with the pearlitic colonies and manganese aufphur inclusions distributed along the rolling direction. The “@airrsize of X65b steel results smaller than the other. TABLE 1 CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES OF TESTED STEELS Steel

X65a X65b X52

Chemical composition (% weight)

c

Mn

0.095 0.081 0.073

1.75 1.50 1.77

Si 0.25 0.21 0.23

P 0,03 0.02 0.03

s 0.01 0.01 0.02

Mo 0.015 0.016 0.019

Mechanical properties (MPa) Ys (0.2) UTs 460 580 462 582 556 343

Groin sizs

ASTM 11 12,5 11

Environments Some electrochemical teata were carried out in a Na*SO, 0, lM anlution saturated with CO* reaching ~ercnt pH through the addition of H,SO, or NaOH. The ntber teats were carried out in NS43 solution saturated with CCh at 1 atrrr (pH 5.5) and at 0.05 atm @H 7.1), and in NS4 solution with the additinn of 12.2 @L of NaHCO, saturated with CO, at 1 atm (pH 6,8). Snme teata were carried out in NS4 selution saturated with Nz lowering the pH at 5.5 nr 6.8 through addition of H#.0~. The pH nf the wlutinns were m-cd before and after all the t@, it remained constant in a range of + 0,2 pH units in C@ saturated snhrtiona, arrd increased of about 0.5 pH units in Nz saturated aelutions Mnat of the tests was carried out at room temperature (25”C) with the exception of some test cnnducted at 5“, 50” and 70”C, Electroshernical tests Electrochemical testa were carried out in ASTM G5-82 standard cell on cylindrical specimens with an exposure area nf lcmz. All samples were grormd with emery paper (1200 SiC) and cleaned in acetnnc. The test solution w prcvionaly fluxed with nitrogen and than saturated with C02 for twelve houra. Two wan rates, 0.5 mV/s and 10 mV/s, were chosen for potentiodymarrrictests. Cyclic potentiodynamic tests were carried out with a scan rate of 0.24 mV/s mrd vertex potentia2 of 1 V vs SCE.

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Though extensive research has been performed on the responsible medarrism of IGSCC, the conditions of TGSCC still rernainoot welldefiaod According to a geoeml clsssii%ation the SCC can happen in between or just through one nf two extreme medmnisrrrx snedic dissolution and hydrogen embrittlement. The former requires the snedic dissolution of the tip whereas the sidea ef the erwk would sat as a cathodic area being in paasivating condition or partially protected by screening mrreaion products. This complex balance between an active tip and the catfrcdic sides is anbatairred by the fihn rupture at the tip ef the crack due to the proaerrca efexterrral or internal atnaaea. They arc resparmble in keeping the velecity of film rupture at the same level of the flhrring velesity cd the sidea nf the mask. The hydrogen embrittlcment mechanism requires the dMuaion of atomic hydroge~ developed by the corrosinn reactions nr by the catfredic prelection, through the tip of the mask provoking a marked loss in ductility of the nrateria14. In the typical TGSCC environment it is not possible to observe any form ef paasivatinn of micm alloyed steels’>’. Nevertheless it is worth noticing that for the presence ef an anodic dissolution mechanism it is anfticient that, behvesrr the tip rmd the sides of the crack, the velneity ofdisaelution is remarkably dilferent. L&ewise the micro-alloyed steel have a tensile stress under 900 MPa and for the normal application of the hydrngen rnrdanianr they should be immune to mrrbrittlement phennmennn’. But published worka in the litemture do show that an hy&egenmechanism k involved duriogS1OW strainrateteatof rnicrnallnyedsteelundereathedic pelarizatinng. Ontheotherhandthempturehstancm reported by Canadiancemparries havebighl@ted thattheyhappenrdunder disbonded mmtirrg wherethecathcdic pmtectinn couldnetintervene. In factas aeonasthecathodic protection reaches theintxeated armsthelocalpH growsupatvaluessmnod9-10,similartnthoseefIGSC&’. Theaimof thepresent workk toassesstherelativepresence of thetwoabovementioned mechanisms fortheTGSCC tbreugh terraife teat (corratarrt load, slnw strain rate, constant deformation test) at diRerent pH in solutions purged with Co. Some electrochemiad teats have alae bemr carried out for a clearer detirrition of the environment.

Stress Corrosion Cracking teat Constant load (NACE TMO1-77/90) and constant deformation (ASTM G30-90) ream temperature teata were performed in NS4 solution saturated with COZat pH 5.5 and 6.8 at the free corrosion potential (-760 mV w SCE) or polarizing at +50 mV vs I&. The cxpoaorc time was from 20 hours to 6 months. It was not cmraidercd poterdiafa more negative than -50 mV vs u because in the fiefd cnndition such petentiafs would give higher pH end consequently TGSCC environment could not be longer mantained under the disbonded mating (9). Slow Stmhr rate tests (ISO 7539-7) were carried out on cylindrical tensile specimens using a four position SSR testing machine with a maximum load nf 30 frN. The specimens were strained at conatit extension rate varying from 1.1X10”7 t. 6.7x10-5 S-l. Tlm samples were tested at free corrosion potential or pehwized at +50 mV vs I&. SCC susceptibility was evahrated through the area ratio (area reduction in the envirmmrent divided by the area reduction in air), the presence of brittfe area on fmcture surface and the presence of secondary cracks. RESULTS AND DISCfJSSION Electrochmnica3 tests The potentiodynmrric curves of X65a steel in NaZS04 O.lM anlutiorr, saturated with C@ at different pH and at two temperatures (25 and 70°C) are showed in figure 1. Active behaviour is observed with no particular dit7erenccs for pH between 6 and 8, this is cont3rmed by Pombaix diagrams that in ihese conditions dees not foresee any film formation. The highest ternfwaturea only seem to enhance corrosion rate. Onfy at H 9 end at temperature above 50”C a protective film of FeCO, ia formed on the metal surface in Fe’+ saturated solution.1 At pH >10 and at free corrosion potential, a carbonate film is formed but it is ordy at more noble potentials that a more stable and protective thin fihn is created. An increasing temperature and a slower potential acanaion enhance. the steel passivity at pH over 10. These cmrditiona are typical for the IGSCC environment. In NS4 solution, independently from the pH (5.5 or 7.1), all the steels show a fully active behaviour (figure 2). Increasing the bicarbonate concentmtion, the cyclic potentiodynamic curves show an hysteresis cycle whose amplitude is bermd to the bicarbonate cmrtenta. It is with high bicarbonate crarcentition that a localised attack occurs on the metal surface (figure 3). Cycfic vohanrrrreW teats show absence of peaks in low bicarbonate concentration solutions at pH between 5.5 and 7.1 (figure 4). The absence of peaka is interpreted by Wieckowski et al’” as the active ferrous dissolution on metal surface without intermediate compomrdr formation. In NS4 solution with 12.2 giL. of NsHCOS the presence of a peak on the voltammetry curves point nut the fnrnration of an intermediate oxided layer on the metrd surface. The exact determination of the oxide layer cnmpesition cannot be found with the usmd spectroscopy tecnique. This peak grows with the number nf potential acanaiona. The peak amplitude is lower after one week of immersion nf the sample at the free cerrosion petential and decreases with the immersion time until an equilibrium condition is reached (figure 5). In a potentiodyrramic teat carried out on the same density was nne order of magnitude lower than the current density measured in figure 2 SSmple, the anodic current (figure 6). From the electrochemical test ia evident that, in TGSCC environment, the chosen steels do not ahow any active-passive behaviour. Nevertheless for higher bicarbnmte concentration it is possible to get a severe pitting in short time. Even nther authors reported similar data”, The pitting attack seems due to a not fully protective naide layer film formed in particular potential range. Stress Corrnaion Cracking teats No SCC was observed in conatmrt Inad or constant defnrnration teat, whereas SSR teat have ahown the presence of SCC. Than, it can be reasonably drawn that the deformation mode playa a fundamental role in TGSCC, as alao reported in literature. In figure 7 the Slnw Strain Rate teat results in Na,SO, O.lM and NS4 solution saturated with CO, at several PH are showed. No SCC is detected at pH >7.5. At pH si7.5, in solution saturated with COZ SCC can occur. The lowest values of area ratio has been measured on the aamplca teatcd at pH around 7. No SCC is present in solution at the same pH but saturated with Nz (figure 8). In tigure 9 the temperature and potential effects in NS4 solution saturated with C02 at pH 6.8 are repnrted. At temperature of 5°C and 25°C the steel behaviour depends on the steel electrochemical petentiak no SCC is present when an rmodic polarization is applied, whereas SCC is always detected when the aamplcs arc. catholically polarized (figure 10). At free corrosion potential, there arc SCC and no SCC cases. In this respect it is very important the possibility of

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Cyclic voharnrnetry tests had 240 second ef condition time at -1 V, followed by 15 second of atabtisation time at free corrosion petart~ and the final run of 10 W* from -1 to +0.4 V vs SCE at acmr rate of 20mV/s.

environment at pH and potentials where the presmice ef earbonatb ‘kp6ciesis low. At free corrosion potential the hydrogeri evolntion due to the corrosion process is quite low, but it is enhanced by the preaenw of C@ and bicarbonate ions which also promote pitting attack. The role of pitting in the SCC crack initiation & Iowr pH ~~de the pi~ p~omoks hydrogen cvOh!tiOrr. The stdra.$tic m~e was emphsaked by Psrkins 3,13*W of pit initiation could explain the scattering of SCC resdtk at the free corrosion potential. CONCLUSIONS From this work some Dreliminmv results cmdd be dmwm ● ccmatmrtload or m“stsnt dcfo~tion tests do not produce TGSCC, ● it is the CMCOj couple to be responsible of this SCC form. They promote both hydrogen evolution end pitting, ●

✎

at fire corresion potential, depending on the solution competition (nded by C02~C0 ~ muple) the phenOmenOn shows a stochastic behaviow, temperature over 50”C seems to inhibite the TGSCC in the CO#HCO~ environment. REFERENCES

1, B.Dekinty, J.O’Beinre, 00 and Gas Journal, June 15 (1992). 2, J.A.Bsavera. “On the mechanism of stress corrosion cracking of natural ma rri!xlines”. Proc. EpRGING, 8th Symposium on Line Pipe Research, Houstorr, AGA, pp. 17.1-17.11.3. RN.Parkins, W.K.Blancluud Jr., B. S.Dehmty, Corrosion 50, 5(1994): p.394. 4. D. Sinigaglia, G.Ro, P.Pedcfemi, Cedimento per fatica cd ambierrtsle dei materiali metallic”, Ed. CLUP, Milano. 5. M.Pourbsix, Atfss of electrechenricsJ eq~libria in aqueous solutions, Pergamorr Press, London, 1966. 6. A.Ikeds, M.Ueda e S.Mukai, “CO* Corrosion bmhviour and mechanism of carbon steel and SI1OYsteel”, CORROSION/83, paper no. 45, (Houston, TXNACE International, 1983).

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pitting forrnedoq since cracks proferentiaflystartedfrom pits. Iftfre samples are tested under very slow rate deformation (10-’ s-’), io the NS4 solution they rmdago to a &rrorrdcorrosion problems, while in high bicarbonate solution can develop a protective film hindering the presenceofSCC(figure11). SCC becomes less evident rdso in cathedic pelarir.ed specimens increasing the temperature. At 70”C no SCC was detected in all the cmrditiona It is evident tba~ unlike IGSCC, the temperature inhibits TGSCC. This is probrddly due to the decrease of CO, srdubtity with an incrmse of temperature. The effect of temperature and applied petentkd on SSR tests lets guess a timdsmentaf ro)e cd hydrogen evolution on TGSCC. Nevertheless the classical behaviour of a alrenger SCC c@cct for slower strain rates, ~ical in the case of an hydrogen embrittlenront medrmrism, it is only observed in the NS4 selution saturated with 100% of COI (pH=5.5)( figure 1la). Instead there are no particularly differences in all the samples trated at free cmmaion potential. No changes were also detected for the samples catiodicslly protected in high bicarbonate eoncentmted solution (figure llb) at diffwent ti rstea. This latter efkct could be explained by the fhct that in such condition the cathodic current decmses during the test from about -0.4 MA to values one er two order of magnitude lower. The cathodic current dscreaae muld be due to a thicker hindering corrosion deposit that has time to form at the slower strain rata. In order to have a constant hydrogen evolution on the ste@ surface seine slow strain rate tests have been moducted with the samples grdvanestatitally cmrtreUcd at -0.4 MA. To maintain this current vahrc in NS4 sehrtion a cathodic potential of -1000 mV vs SCE is requiti while in the high bicarbonate mntent solution the specimen cathodic potential was -800 mV vs SCE. This latter is very close to the cathodic petcntird imposed in the potentioatatic SSRtest (-790 mV vs SCE). The results of the galvmroststic SSR tests are summarised in figure 12. The reduction of area ratio versus strain rate curves is very siniilaq for the two environments (NS4 and high bicarbonate solution). Thelowest ratio values weremeasured for a * rate of 3.10< S-l. No SCC was observed for a strain rate of 10-’ S-l. These roaults, sJong with the elcctrecherrrkd results, seem to confirpr the possibility of occurrence of a corrosion product film formed high HCO~ selution or at high cathodic polarization (during the gatvrmostatic tests). This film must be broken with an appropriate rate to permit the hydrogen entrsncd into the metrd lattice. It is pessible that hydrogen evolution shordd be enhanced on tlria film, taking @count the lower hydrogen over-pixentkd in high bicarbonate cmrcentrsted solution. By the way some authors subatain that Ca arid bicarbemte species have en electrochstalist role ~ hytigen ~olution ~eticI0.12. ~s ~~ ~~d ~ItitI tie SCC initiation on micro alloyed sreel in coficoj

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7. RRFesaler, T.P.Greenevel~ A.R.Elaeq in Stress Corrosion Cracking and Hydrogen Embrittlemerrt of Iron Base Alloy$ NAC33S (1%7) p. 135. 8. F,BoIzorri, M, Cabrini, M.Cacci~ M.Tarcnsi, “Hydrogen embrittlement of pipeline steels under cathadic protection: mmpariaon of various teat methods”, Progress in the Underatarrdirrg end Prevention of CorresioU The Inatitrrts ef Matedls University Free& Cambridge, 1993, VO1.2,P.1500. 9. KVidem arrd A.Drrgatad, “E&d of flow rate, pm Few concentration and steel quaMy on tbe @ corrosion of carbon steels”, CORROSION/87, paper no. 42, (Hmatou TXNACE International, 1987). 10. A.Wieckowaki, E.Glrali, Electrechirrdca Acts 28, (1983): p.1619. 11.KVide.m and A.M.Koren, Corrosion 49, (1993) p.746. 12. C.DeWaard arrd D.E.Milliama, Corrosion 31, (1975): p. 177. 13.RN.Parkirra rmd B.S. Dekmty, “The initiation and early atages efgrowtb of SCC cracks in pipeline steel exposed to a dilute, near neutral pH solution”, 9th Symposimrr on Pi@ne Research, Houstoq1996.

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7. RR.Feaaler, T.P.Greenoveld, A.RElaea, in Stress Corrosion Cracking and Hydrngen Embrittkenent of Iron Base AUey&NACE 5 (1967) p. 135. 8. F.Bokm@ Mlabrioi, M. Cacc@ M.Tarenzi, “Hydrogen embrifflement of pipeline steels onder catlmdic protectiorx cnmpariaen of various test methcda”, Progress in the Understanding and Prevention of Corrosio% The Instituts ef hdaterial~ University PrOSS,Cambridge, 1993, VO1.2,p.1500. 9. K.Videm and A.Duga@ “EtTect of flow rate, PH. Fez+ cencentratinn and steel quality on the C@ corrosion of carbon steels”, CORROSION/87, paper no. 42, (Houatorr, ‘fXNACE International, 1987). 10. A.Wiecko~ E.Glmli, Electrechinrica Acts 28, (1983): p.1619. 11.K.Videm and A.M.Koron, Corrosion 49, (1993): p.746. 12. C.DeWeard end D.E.Milliama, Corrosion 31, (1975} p.177. 13.RN.Psrkirrs and B.S. Delanty, “The initiation and early stages efgrowth of SCC cracks in pipeline steel expesed tn a dilute, near neutxal pH solution”, 9th Sympnsirmr on P@ine Research, H0ust0n,1996.

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FIGURE 8- SEM photo of the fmcture au$ace of the X65a steel after SRRT in the NS4 solution- a) saturated wiIh N2; b) saturated with CO, at free corrosion potential

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g ; J 1. .-.--------------.

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b) a) FIGURE 10- SEM photoof thefrsctumsmfweof theX65bsteelsfterSRRTin theNS4 solutionat pH = 6.8,8= 1.1.10