HIC.pdf

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Hydrogen-Induced Cracking of Linepipe SteelsPart 1–Threshold Hydrogen Concentrationand pH✫(1)

R.W. Revie, V.S. Sastri, G.R. Hoey, R.R. Ramsingh, D.K. Mak, and M.T. Shehata*

ABSTRACT

Threshold hydrogen concentration (CthH) and threshold pH

(pHth) for hydrogen-induced cracking (HIC) were determinedfor commercial sour service linepipe steels exposed to H2S-saturated buffer solutions. Metallographic examination of thesteel samples by optical and electron microscopy showedthat ultrasonic C-scan is an effective method for detectingHIC and for locating cracks in exposed steel coupons. Abanded microstructure was found to be detrimental to theHIC resistance of clean steels. The obtained pHth values canbe used to rank the steels with respect to HIC resistance.

KEY WORDS: acetic acid, hydrogen-induced cracking, pHchanges and effects, pipelines

INTRODUCTION

The susceptibility of sour service linepipe steels tohydrogen-induced cracking (HlC), also known asstepwise cracking (SWC), depends on metallurgicalas well as environmental factors.1 The former includesalloying elements, microstructure, strength,segregation, and shape of nonmetallic inclusions; thelatter includes partial pressures of hydrogen sulfideand carbon dioxide, temperature, pH, and thepresence of aggressive species such as chloride.

CORROSION–Vol. 49, No. 10010-9312/92/000005

National Association of C

✫ Submitted for publication March 1992; in revised form, July 1992.* Energy, Mines, & Resources Canada, CANMET, 555 Booth St., Ottawa,

CANADA K1A 0G1.(1) Permission to publish granted by Minister of Supply and Services

Canada, 1992.

Hydrogen atoms, produced as a result of corrosion ofthe inside wall, diffuse through the pipe wall and aretrapped at heterogeneous sites in the steel, thusleading to blistering and HIC. Permeation of hydrogenthrough the pipe walls can be measured by anelectrochemical device strapped to the outer wall ofthe pipe. Hydrogen concentration at the inside pipesurface (C

OH) can be calculated using permeation

current density, diffusion equations, wall thickness,and the diffusion coefficient.2 For HIC to occur, C

OH

must equal or exceed the threshold hydrogenconcentration (C

thH).

The National Association of Corrosion Engineershas developed a standard test (TM-02-84) forevaluating stepwise cracking of linepipe steels.3 In thistest, standard coupons cut from the pipe (20 mm by100 mm by wall thickness [WT]) are immersed inhydrogen sulfide (H

2S) saturated synthetic seawater

solution for 96 h. The coupons are then sectioned andexamined for HIC. The correlation between thediffusible hydrogen extracted from the coupons andHIC and the existence of C

thH were established by

Ikeda et al.4

To evaluate and compare the relative resistancesof the steels to HIC, two parameters for each steelwere determined experimentally:Threshold Hydrogen Concentration–The concentrationof diffusible hydrogen in the steel above whichcracking occurs, andThreshold pH–The pH below which cracking occurs.The steels were also characterized by chemical and

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spectrochemical analysis along with correlation ofcracking with ultrasonic C-scan data and examinationof samples by optical metallography, scanningelectron microscopy (SEM), and energy dispersivex-ray (EDX) analysis.

EXPERIMENTAL

The experimental procedure for determiningthreshold hydrogen concentrations and threshold pHswas divided into four main parts:—ultrasonic C-scan examination of the coupons forpre-existing cracks;—hydrogen charging for 96 h in solutions of bufferedpH, varying from 1.1 to 5.9;—diffusible hydrogen measurements; and—ultrasonic C-scan examination to establish whethercracking occurred during charging. The proceduresused are described in the following paragraphs.

The steel compositions are listed in Table 1.Coupons 100 mm long by 20 mm wide by wallthickness were cut from parent metal 90° from theweld with the longitudinal direction parallel to therolling direction. The coupons were not fattened.Coupons were milled according to the procedure inNACE Standard TM-02-84.

HIC Test ProcedureCoupons were immersed in H

2S-saturated

buffered solution for 96 h in the apparatus shown inFigure 1, which accommodated nine coupons for all-sided charging. At the end of 96 h, the quantity ofdiffusible hydrogen in the coupons was determined bymeasuring displacement of glycerol at 45°C byhydrogen.5 This apparatus is shown in Figure 2.

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TABLE 1(Steel Compositio

Component A B C D

C 0.130 0.215 0.157 0.1Mn 1.09 0.77 0.85 0.8P 0.012 0.012 0.020 0.0Si 0.165 0.007 0.28 0.1Ni 0.07 0.015 0.02 0.2Mo 0.01 <0.005 <0.005 <0.0Ca 0.011 0.012 0.0072 0.0Ti 0.02 <0.005 0.005 0.0Sn 0.013 <0.005 0.005 <0.0Al 0.02 0.005 0.035 0.0Nb 0.04 <0.005 0.06 0.0B 0.0002 0.0003 0.0

Cu 0.22 0.015 0.005 0.0Cr 0.06 0..03 0.02 0.0S 0.0084 0.0054 0.001 0.0V <0.005 <0.005 <0.005 <0.0

Collector tubes were placed on a fat poly (methylmethacrylate) (PMMA) stand in a glass container filledwith glycerol, which was immersed in a stirred waterbath at 45°C. The coupons were washed, rinsed,dried, and immediately placed in the collector tubes.The diffusible hydrogen was the volume (converted tostandard temperature and pressure [STP]) collectedper 100 g of sample after 72 h. The test solutions usedto obtain the desired pHs are listed in Table 2.

A binary search method was used to evaluate Cth

H

and pHth. The first solution in a sequence of tests was

at pH 4.3. Three coupons were tested for each steel ateach pH. The pH in the subsequent test depended onwhether cracking occurred at pH 4.3. If cracking wasobserved at pH 4.3, the next test was done at pH 5.3,whereas if no cracking was observed at pH 4.3, thenext test was done at pH 3.0. Additional buffered testsolutions were used as required to determine C

thH and

pHth. All of the tests for HIC were done at room

temperature (25°C).

Crack DetectionAn ultrasonic system, operating at 5 MHz and

equipped with a modified C-scan recorder, was usedto detect and locate the cracks in the HIC testspecimens. Cracking of some steel samples wasstudied by SEM and EDX analysis, and longitudinalsections of some samples were studied byquantitative metallography.

RESULTS AND DISCUSSION

Data on diffusible hydrogen determined at variouspH values along with the results indicating whethercracking occurred are presented for 8 steels in Tables

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a)ns (wt%)

E F G H

0 0.080 0.065 0.200 0.0954 0.80 1.08 0.955 0.7318 0.016 0.010 0.011 0.01575 0.41 0.15 0.18 0.205 0.015 0.02 0.02 0.0805 <0.01 <0.005 <0.005 <0.005034 0.004 0.014 0.0015 0.00131 0.019 <0.005 <0.005 0.02505 0.005 <0.005 0.01415 0.13 0.04 0.06 0.022 0.023 0.02 <0.005 0.005001 0.0001 0.000105 0.011 0.04 0.02 0.233 0.027 0.03 0.03 0.06013 0.0016 0.0054 0.0088 0.001805 <0.005 <0.005 <0.005

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TABLE 1(b)Specifications of Steels

Steel Size (diam) Vintage Grade

A 6 in. 1986 386B 4 in. 1983 290C 16 in. 1979/1980 359/386D 16 in. 1979/1980 359/386E 6 in. ERW 1990 359F 8 in. 1986 359G 6 in. seamless 1984 ?H 8 in. seamless 1970 ?

TABLE 2Test Solutions Saturated With H2S

pH Composition

1.1 5% NaCl, 0.1 N HCl3.1 5% NaCl, 14.12 g/L potassium hydrogen phthalate,

1.125 g/L HCl3.4 5% NaCl, 16.88 g/L potassium hydrogen phthalate,



0.01 g/L HCl4.3 5% NaCl, 0.75% sodium acetate, 0.5% acetic acid4.7 5% NaCl, 16.04 g/L potassium hydrogen phthalate,

0.855 g/L NaOH5.0 5% NaCl, 14.05 g/L potassium hydrogen phthalate,



1.748 g NaOH5.9 5% NaCl, 10.89 g/L potassium hydrogen phthalate,

1.87 g/L NaOH

FIGURE 2. Apparatus for diffusible hydrogen measurements.

FIGURE 1. HIC test apparatus.

3 through 10. Cracking was observed in the case ofsteel C below pH 5.6; hence, the pH

th value is 5.6. No

cracking was observed in steels D and E, even at apH as low as 1.1; hence, no pH

th can be defined for

these steels.The mean volume of hydrogen evolved in glycerol

from the three samples exposed at pHth is defined as

Cth

H for cracking. The experimental values of Cth

H andpH

th obtained for 8 steels are presented in Table 11.The steels studied exhibited a wide range of

susceptibility to HIC. Steels D and E did not crackeven at pH 1.1, i.e., pH

th < 1.1, whereas steel F

showed cracking at all pH values including pH 5.9, i.e.,pH

th> 5.9. Steels C, D, and E were claimed to be HIC-

resistant steels, but the present HIC tests showed thatsteels D and E were superior to steel C.

A wide range in Cth

H values was observed for thesteels. Steel G had the highest C

thH value at 2.0

mL/100 g steel, whereas a steel with poor HICresistance, steel H, had a C

thH value of 0.2 mL/100 g

steel, which is close to the lowest limit of detection ofhydrogen by the glycerol displacement method. UsingpH

th as a criterion for HIC resistance, it is possible to

rank the steels with respect to HIC resistance. Valuesof C

thH equal to or greater than 2.0 mL/100 g steel

indicated good resistance to HIC.Although the suitability of a particular steel for

sour service depends on the corrosiveness of the sourgas, steels D and E would be the best choices amongthe steels studied.

Ultrasonic C-Scan and MetallographyUltrasonic C-scan used in these studies is a

sensitive technique for detecting cracks in couponsexposed to H

2S-saturated saline solutions. Since

C-scans were conducted before and after exposure tothe test solutions, and those obtained before exposurehad no defect indications, all defects indicated in thepost-exposure C-scans were due to HIC. Quantitativedata to characterize the extent of cracking, such ascrack length ratios, were not obtained, but C-scan hasbeen established as a viable crack detectiontechnique in the evaluation of C

thH and pH

th for sour

service linepipe steels.

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TABLE 3Cracking and Hydrogen Analysis Results for Steel A

Volume H 2

Evolved mL(STP)/100 g Steel

pH Sample # Actual Cracking

4.3 A-1 6.9 YesA-2 7.8 YesA-3 6.4 Yes

5.0 A-7 4.1 YesA-8 4.4 YesA-9 3.7 Yes

5.3 A-4 0.4 NoA-5 0.4 NoA-6 0.9 No

TABLE 4Cracking and Hydrogen Analysis Results for Steel B

Volume H 2



4.3 B-1 3.7 YesB-2 3.5 YesB-3 3.2 Yes

5.0 B-7 4.7 YesB-8 3.6 YesB-9 3.4 Yes

5.3 B-4 0.1 NoB-5 0.8 NoB-6 0.1 No

TABLE 5Cracking and Hydrogen Analysis Results for Steel C

Volume H 2



4.3 C-1 1.8 YesC-2 1.7 YesC-3 2.0 Yes

5.0 C-4 0.9 YesC-5 1.3 YesC-6 1.4 Yes

5.3 C-7 0.2 NoC-8 0.9 NoC-9 0.9 No

TABLE 6Cracking and Hydrogen Analysis Results for Steel D

Volume H 2



1.1 D-13 1.2 NoD-14 1.4 NoD-15 1.3 NoD-16 1.6 NoD-17 1.7 NoD-18 1.7 No

3.1 D-10 1.9 NoD-11 1.6 NoD-12 1.4 No

3.4 D-7 2.0 NoD-8 2.0 NoD-9 1.8 No

3.7 D-4 1.9 NoD-5 1.8 NoD-6 1.2 No

4.3 D-1 1.4 NoD-2 1.5 NoD-3 1.4 No

C-scan indications were used to identify locationsof cracking in a number of coupons before sectioningfor metallographic examination. The cracks observedmetallographically were found at the locationsidentified from the C-scans. Examples of cracks areillustrated in Figure 3 for coupons A-2, B-9, and C-4along with the C-scans for these coupons. Because ofthe localized nature of the cracking in coupons B-9and C-4, the locations where the sections formetallographic examination were taken are indicatedby arrows for these two coupons. The locations of theobserved cracks agreed with the locations predictedfrom the C-scan indications.

The cracks in samples B-9 and C-4 (Figures 3[b]and [c]) were studied by SEM and EDX. Theinclusions numbered 1, 2 and 3 in Figure 3(b) werepredominantly manganese sulfide. The hydrogen-induced crack in coupon C-4 (labelled 1 in Figure 3[c])developed along a stringer of elongated manganesesulfide inclusions (labelled 3 in Figure 3[c]). Aluminumoxide particles, one of which is labelled 2 in Figure

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3(c), were also present. Figure 4 shows an SEMmicrograph of the etched microstructure aroundcracks in sample B-9. The cracks followed a paththrough bainite band or at the bainite-ferrite interfaceand did not enter the ferrite phase. Thus, a model forformation of this crack would involve initiation atmanganese sulfide inclusions and propagation alongthe bainite band in the microstructure.

Quantitative image analysis of the inclusions insteels C and D (Table 12) showed these steels to beclean, as expected from their compositions in Table 1.The observed difference in HIC resistance cannot be


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TABLE 11Threshold Hydrogen Concentration

and pH for All-Sided Charging of Steels

Threshold Threshold H 2 Conc.Steel pH mL (STP)/100 g Steel

A 5.3 0.9B 5.3 0.8C 5.6 0.9D <1.1(A) >2.0(A)

E <1.1(A) >2.0(A)

F >5.9(A) <0.1(A)

G 5.3 2.0H 5.9 0.2

(A)Threshold values were not observed.

TABLE 10Cracking and Hydrogen Analysis Results for Steel H

Volume H 2



4.3 H-1 5.9 YesH-2 6.4 YesH-3 6.1 Yes

5.3 H-4 1.0 YesH-5 2.3 YesH-6 2.0 Yes

5.6 H-7 0.2 NoH-8 4.8 YesH-9 4.8 Yes

5.9 H-10 0.2 NoH-11 0.1 NoH-12 0.1 No

TABLE 9Cracking and Hydrogen Analysis Results for Steel G

Volume H 2



4.3 G-1 2.7 YesG-2 2.5 YesG-3 2.8 Yes

5.0 G-7 3.0 NoG-8 2.9 YesG-9 2.1 Yes

5.3 G-4 1.4 NoG-5 2.0 NoG-6 2.0 No

TABLE 8Cracking and Hydrogen Analysis Results for Steel F

Volume H 2



4.3 F-1 1.0 YesF-2 3.0 YesF-3 3.6 Yes

5.3 F-4 1.4 YesF-5 2.8 YesF-6 3.0 Yes

5.6 F-7 3.0 YesF-8 3.1 YesF-9 2.7 Yes

5.9 F-10 0.1 YesF-11 0.4 YesF-12 0.5 Yes

TABLE 7Cracking and Hydrogen Analysis Results for Steel E

Volume H 2



1.1 E-13 2.0 NoE-15 1.7 NoE-16 1.8 No

3.1 E-7 1.0 NoE-8 1.2 NoE-9 2.0 No

3.7 E-4 1.2 NoE-5 1.6 NoE-6 1.6 No

4.3 E-1 1.2 NoE-2 1.7 NoE-3 1.6 No

attributed to inclusion properties.6 The niobium contentof steel C (0.06%) was higher than that of steel D(0.02%), and a niobium-rich phase was identified(Figure 5) in the rolling plane of steel C. Packwood’sstudies7 indicated the formation of Fe-NbC eutectic insteels containing 0.035% or more Nb.Photomicrographs of steel C (Figure 6) and steel D(Figure 7) showed that these steels had banded anduniform microstructures, respectively. Previous studiesin the authors’ laboratories showed that a bandedmicrostructure provides a path of low fractureresistance to HIC.8 Thus, cracking in steel C is mostlikely due to its banded microstructure.

CONCLUSIONS

❖ A method of measuring the level of diffusiblehydrogen evolved from steel samples exposed to H

2S-

21CORROSION–Vol. 49, No. 1

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(b) (c)

FIGURE 3. C–scan traces of 3 coupons and photomicrographs of sections from these coupons: (a) A-2, (b) B-9, and (c) C-4.

(a)

FIGURE 4. SEM micrograph of etched crack in coupon of steel B-9.

saturated buffer solutions, and the detection of cracksby ultrasonic C-scans was used in the evaluation ofC

thH and pH

th for linepipe steels.

❖ After charging coupons of the steels discussed inthis paper, the level of diffusible hydrogen varied from7.8 to 0.1 mL of hydrogen/100 g steel in the pH rangefrom 1.1 to 5.9. High C

thH values (>2.0 mL [STP]

H2/100 g steel) were associated with high resistance

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to cracking. pHth is a useful parameter in ranking

linepipe steels with respect to cracking resistance.❖ Steels D and E were the most crack-resistantsteels, whereas steel F was the least resistant of thesteels discussed in this paper.❖ HIC of samples was correlated with C-scanpatterns by optical metallography and SEM. Althoughsteels C, D, and E were all claimed to be HIC


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FIGURE 5. Photomicrograph of steel C showing Nb-richphase.

TABLE 12Quantitative Image Analysis of Inclusions

Steel Vol% Mean Length, µm Aspect Ratio

C 0.0421 2.67 1.26D 0.0417 2.29 1.13

FIGURE 7. Photomicrograph of steel D.

FIGURE 6. Photomicrograph of steel C.

resistant, only steels D and E were completelyresistant to HIC at all of the pH values studied. Thedifference in HIC susceptibility was probably due tothe banded microstructure and the presence of NbCprecipitates in steel C, whereas steel D had a uniformmicrostructure devoid of NbC precipitates.

ACKNOWLEDGMENTS

The authors acknowledge helpful discussions withcolleagues at the Metals Technology Laboratories andthe Canadian Standards Association Sour ServiceTask Force, which provided partial financial supportfor the project. The authors acknowledge J.J. Ferryand the late I.R. Somerville for ultrasonic C-scantesting and B.R. Casault for metallographicexamination of samples.

REFERENCES

1. G.J. Biefer, Materials Performance 21, 6 (1982): pp. 19-34.2. M.C. Hay, “An Electrochemical Device for Monitoring Hydrogen

diffusing Through Steel,’’ Presented at CIM Conference of Metallurgists,Hydrogen Sulfide Symposium, Edmonton, Alberta, 1983.

3. NACE Standard TM-02-84, “Test Method - Evaluation of Pipeline Steelsfor Resistance to Stepwise Cracking” (Houston, TX: NACE, 1984).

4. A. Ikeda, T. Kaneka, I. Hashimoto, M. Takeyama, Y. Sumitomo, T.Yamura, “Development of Hydrogen Induced Cracking (HIC) ResistantSteels and HIC Test Methods for Hydrogen Sulfide Service,” Proc. ofSymp. on Effect of Hydrogen Sulfide on Steel, 22nd Annual Conf.Metallurg. (Edmonton, Canada: Can. Inst. Mining Metallurg., 1983),pp. 1-71.

5. Japanese Standards Association, “Method for Measurement ofHydrogen Evolved from Deposited Metal,’’ Japanese IndustrialStandard JIS Z 3113 - 1975 (Tokyo, Japan: Japanese StandardsAssociation, 1975).

6. K. Van Gelder, J.G. Erlings, H.W. de Groot, M.J.J. Simon-Thomas, J.Nanta, “Cracking of Materials in Aqueous H

2S-Containing

Environments,” 2nd Int. Conf. on Sour Service in the Oil, Gas and

CORROSION–Vol. 49, No. 1

Petrochemical Industries (Manchester, United Kingdom: University ofManchester Institute of Science & Technology, 1987).

7. V.K. Heikkinen, R.H. Packwood, “On the Occurrence of Fe-NbCEutectic in Niobium-Bearing Mild Steel,” Scandinavian J. of Metallurgy,6 (1977): pp. 170-175.

8. M.J. Godden, G.J. Biefer, M.T. Shehata, J.D. Boyd, “Effects ofInclusions and Segregation on Stepwise Cracking of Linepipe Steels,”Proc. of Conf. on Inclusions and Residuals in Steels: Effects onFabrication and Service Behavior, J.D. Boyd, C.S. Champion, eds.(Ottawa, Canada: CANMET, 1985), pp. 93-208.

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