Conditions Under Which Cracks Occur in Modified 13% Chromium Steel in Wet Hydrogen Sulfide Environments

Transcription

1 Conditions Under Which Cracks Occur in Modified 13% Chromium Steel in Wet Hydrogen Sulfide Environments T. Hara and H. Asahi* ABSTRACT Occurrence of cracks in an API 13% Cr steel, modified 13% Cr steel, and duplex stainless steel were compared in various wet, mild hydrogen sulfide (H 2 S) environments. The conditions under which cracks occurred in the modified 13% Cr steel in oil and gas production environments were made clear. No cracks occurred if ph > depassivation ph (ph d ) and redox potential of sulfur (E S[red/ox] ) < pitting potential (V c ). Hydrogen embrittlement-type cracks occurred if ph > ph d and E S(red/ox) > V c. Pitting occurred if ph > ph d and E S(red/ox) > V c. The ph inside the pit decreased drastically and hydrogen embrittlement occurred. Cracks of the hydrogen embrittlement type occurred if ph < ph d and threshold hydrogen concentration under which cracks occur (H th ) < hydrogen concentration in steel (H 0 ). No cracks occurred if ph < ph d and H th > H 0. KEY WORDS: hydrogen embrittlement, hydrogen sulfide, pitting potential, redox potential, sour environments, stainless steel INTRODUCTION API (1) 13% Cr oil country tubular goods (OCTG) are used mainly in wet carbon dioxide (CO 2 ) environments (sweet environments). Recently, use of the API 13% Cr steels in environments containing little hydrogen sulfide (H 2 S, mild sour environments) also have begun. It is well known that the 13% Cr steels Submitted for publication June 1999; in revised form, December * Nippon Steel Corporation, Steel Research Laboratory, 20-1 Shintomi, Futtsu, Chiba-ken , Japan. (1) American Petroleum Institute (API), 1220 L St. NW, Washington, DC have a high susceptibility to cracking in sour environments. 1 Cracking is thought to occur because of hydrogen embrittlement, 2-4 where cracks occur when the hydrogen concentration in steel (H 0 ) exceeds the threshold hydrogen concentration under which cracks occur (H th ). However, duplex stainless steels, which are more resistant to cracking than the 13% Cr steels, often are used in wet, sour environments. Cracks of duplex stainless steels are thought to be active-path, corrosion-type stress corrosion cracking (APC-SCC), which start from pittings. 5-6 The fittings occur when the pitting potential (V c ) is less noble than the redox potential of sulfur (E S[red/ox] ). 5-6 Recently, modified 13% Cr steels with a corrosion resistance between that of the API 13% Cr steels and that of the duplex stainless steels have been developed by some pipe manufacturers They have low carbon; additions of nickel, molybdenum, copper, and nitrogen; and are more resistant to carbon dioxide (CO 2 ) corrosion and sulfide stress cracking (SSC) than API 13% Cr steels. Kushida and Kudo elucidated that cracks, as a result of hydrogen embrittlement, occur when hydrogen enters into a modified 13% Cr steel in sour environments. 3 However, conditions under which cracks occur in the modified 13% Cr steel in various wet, sour environments have not been made clear. The purpose of this study was to clarify the conditions under which cracks occur in the modified 13% Cr steel in various sour environments existing in oil and gas fields, where the ph mostly ranges from 2.5 to 5.5. The study also compared these conditions with those under which cracks occur in /00/000103/$5.00+$0.50/0 CORROSION Vol. 56, No , NACE International 533

2 TABLE 1 Chemical Compositions and Mechanical Properties of Tested Steels (mass%) Steel C Si Mn Cr Ni Cu Mo Yield Stress Tensile Stress A MPa 885 MPa (106 ksi) (128 ksi) B MPa 932 MPa (108 ksi) (135 ksi) C MPa 792 MPa (81 ksi) (115 ksi) TABLE 2 Test Conditions of pph 2 S and ph pph 2 S ph MPa 0.01 MPa 0.03 MPa 5.5 Steel A Steel A 5.0 Steel A Steel A 4.5 Steel A Steel A 4.3 Steel B 4.0 Steels A and B Steels A and B 3.5 Steels A and B Steels A and B 3.0 Steels A, B, and C Steels A, B, and C 2.5 Steel C Steel C the API 13% Cr steel and duplex stainless steel, respectively. EXPERIMENTAL PROCEDURES Materials Table 1 shows chemical compositions and mechanical properties of tested steels. Steel A was an API 13% Cr pipe, and Steel B was a modified 13% Cr steel. Steels A and B were heat-treated to obtain the yield strength level of C95 grade for API specification 5CT (655 MPa to 758 MPa) to evaluate susceptibility to cracks. 12 Steel A did not meet API mechanical property requirements. Steel A was normalized at 990 C for 30 min after hot-rolling followed by tempering at 650 C for 30 min. This tempering temperature was lower than customary for typical oilfield use for this steel. Steel B was melted in a 50-kg vacuum induction furnace in a laboratory. The ingot reheated at 1,250 C for 1 h was hot-rolled to a 12-mm thickness. Then, plates were heat-treated (830 C for 30 min air cooling [AC] 680 C for 30 min AC 680 C for 30 min AC). Steel C was a duplex stainless steel pipe. The yield strength of this steel was in L80 grade of API standard (i.e., between 551 MPa to 655 MPa). This steel was quenched from 1,050 C for 30 min after hot-rolling. Test Conditions The test solution was a 1-mol/L acetic buffer solution containing 5% sodium chloride (NaCl) with FIGURE 1. Test apparatus for electrochemical measurement. the ph in the range from 2.5 to 5.5. The test gas was a mixture of H 2 S gas and CO 2 gas. H 2 S partial pressure (pph 2 S) was in the range from MPa to 0.03 MPa. Table 2 shows test conditions of pph 2 S and ph in Steels A, B, and C. The test temperature was kept at 24 ± 3 C. Electrochemical Measurement An anodic polarization was measured to evaluate V c. Figure 1 shows the test apparatus for electrochemical measurement. A calomel reference electrode was used. Specimens (15 mm by 20 mm by 3 mm) were machined from the middle thickness. They were polished with no. 320 emery paper, degreased in acetone (CH 3 COCH 3 ), and coated with silicon resin, leaving 1 cm 2 for measurement. The test solution, which had been deaerated with nitrogen gas for > 24 h, was poured into the test cell. Before the test gas was introduced into the test cell, the test solution in the test cell was deaerated further with 534 CORROSION MAY 2000

3 FIGURE 2. Dimensions of constant load test specimen. nitrogen gas for > 1 h. Just before the specimens were immersed in the test solution, specimens were pickled in 50% sulfuric acid (H 2 SO 4 ) at 60 C until hydrogen gas bubbled from the specimen surface to remove the air-formed film. This means that the initial condition of the specimen was active. Then, the specimen was immersed in the test solution as soon as possible after washing it in water. A potentiodynamic measurement was started at a scan rate of 10 mv/min in the anodic direction from the corrosion potential (E corr ) after the specimen was placed in the test solution for 5 min. V c was defined as the potential at a current density of 100 µa/cm 2. Constant Load SSC Test The constant load SSC test following NACE standard TM A was carried out to evaluate susceptibility to cracks. 13 Figure 2 shows dimensions of a constant load SSC test specimen. Specimens were machined from the middle thickness of tested steels in the rolling direction. Specimens were polished with no. 320 and no. 600 emery paper in the longitudinal direction and degreased in CH 3 COCH 3. The applied stress was 85% of specified minimum yield stress (SMYS) of C95 grade (556 MPa) for Steels A and B and L80 grade (468 MPa) for Steel C. After the specimen was set in the test cell, the test solution, which had been deaerated for > 24 h, was poured into it and deaerated with nitrogen gas for > 1 h. Then, a load was applied after the test gas was bubbled in the test cell for at least 30 min until saturated. The test was started when the load was applied. E corr was measured simultaneously with the constant load SSC test specimen. Measurement of Diffusible H 0 The immersion test was carried out to measure the diffusible H 0. Specimens (3 mm by 15 mm by 20 mm) were cut from the middle thickness, polished with no. 320 emery paper, and degreased in CH 3 COCH 3. Then, specimens were immersed in the deaerated test solution. The test duration was 720 h. Immediately after the specimens were picked up from the test solution, specimens were put into liquid nitrogen. After specimens had been left for 168 h at 45 C, the diffusible H 0 was measured using the gas chromatography method. (c) FIGURE 3. Effects of ph and pph 2 S on corrosion damage morphology: Steel A at σy = 728 MPa (106 ksi), Steel B, and (c) Steel C. Note: 5% NaCl acetic buffer solution applied stress; 85% SMYS. CORROSION Vol. 56, No

4 FIGURE 4. Relationship between E corr and test duration in constant load test and anodic polarization curve in Steel A, where E H(red/ox) is the reduction/oxidation potential of hydrogen. FIGURE 5. Relationship between E corr and test duration in constant load test and anodic polarization curve in Steel A. RESULTS Constant Load SSC Test Results Figure 3 shows the constant load SSC test results in various sour environments in which pph 2 S ranged from MPa to 0.1 MPa and ph ranged from 2.5 to 5.5. SSC occurred in low ph with high pph 2 S. In Steel A, SSC occurred in ph 4.0 with pph 2 S of MPa and in ph 4.5 with pph 2 S of MPa (Figure 3[a]). In Steel C, no SSC occurred in all examined environments (Figure 3[c]). In Steel B, no SSC occurred in ph 4.0 with pph 2 S of 0.01 MPa and in ph 3.5 with pph 2 S of MPa (Figure 3[b]). When pph 2 S increased to 0.03 MPa, SSC occurred even in the high ph 4.3. This steel had a SSC resistance between that of Steel A and Steel C in high-strength 13% Cr steels. In common oilfield practice, Steel A is used at a minimum yield strength of 551 MPa (80 ksi), while Steel B is used at a minimum yield strength of 655 MPa (95 ksi). Therefore, it is possible that the SSC resistance of Steel A is comparable to that of Steel B because the 551-MPa 536 CORROSION MAY 2000

5 FIGURE 6. Relationship between E corr and test duration in constant load test and anodic polarization curve in Steel C. FIGURE 7. Relationship between E corr and test duration in constant load test and anodic polarization curve in Steel B. minimum yield strength of Steel A has a similar SSC resistance to the 655-MPa minimum yield strength of Steel B. 1,4,8,10 Electrochemical Measurement Results Steel A Figure 4 shows the change of E corr as time passed during the constant load SSC test in ph 3.0 at pph 2 S of 0.01 MPa. Figure 4 shows the anodic polarization curve of the same environmental condition as that shown in Figure 4. E corr, before the loading existed at 550 mv SCE, is indicated in Figure 4. Immediately after the load was applied, E corr dropped to 700 mv, which corresponded to the active state, and the specimen cracked in a short time. CORROSION Vol. 56, No

6 FIGURE 8. Relationship between E corr and test duration in constant load test and anodic polarization curve in Steel B. FIGURE 9. Relationship between E corr and test duration in constant load test and anodic polarization curve in Steel B. This steel underwent general corrosion in this environment, as indicated by the anodic polarization curve (Figure 4[b]). It is reported that general corrosion occurs on 13% Cr steels in ph < 3.8 with acetic buffer solution Figure 5 shows the case in ph 5.5 at pph 2 S of MPa. Before the loading existed at 330 mv, E corr was close to E S(red/ox) (Appendix). The current density at this potential, as indicated by the anodic polarization curve in Figure 5, was 40 µa/cm 2 and relatively small. As indicated by the anodic polarization curve in Figure 5, E corr dropped to 600 mv, at which point the steel was in a passive state immediately after the load was applied. E corr remained constant, and no SSC occurred in the 720-h duration of the test. The specimen had a metallic color, and no pitting occurred. Steel C Figure 6 shows the change of E corr as time passed during the constant load test in ph 3.0 at pph 2 S of MPa. Figure 6 shows the anodic polarization curve under the same environ- 538 CORROSION MAY 2000

7 FIGURE 10. H 2 S vs ph diagram with S redox potential, V c, and E corr in Steel A. FIGURE 11. H 2 S vs ph diagram including S redox potential, V c, and E corr in Steel C. mental condition as in Figure 6. Before the loading existed at 175 mv, E corr was close to the E S(red/ox). The current density of this potential was 0.2 µa/cm 2 and very small. E corr remained constant after the load was applied, and no SSC occurred in the 720-h duration of the test. The specimen had a metallic color, and pitting did not occur. The specimen was repassivated immediately when it was immersed in the test solution after being immersed in H 2 SO 4 solution, and E corr shifted close to E S(red/ox). Judging from the anodic polarization curve, this steel had a stable passive film because the current density was ~ 10 µa/cm 2 when a potential of 400 mv was applied. Steel B Figure 7 shows the change of E corr as time passed during the constant load test in ph 3.0 at pph 2 S of 0.01 MPa. Figure 7 shows the anodic polarization curve under the same environmental condition as in Figure 7. Before the loading existed at 215 mv, E corr was close to E S(red/ox). Immediately after the load was applied, E corr dropped to 600 mv, as indicated by the anodic polarization curve in Figure 7, at which point the steel was in an active state, and the specimen cracked in a short time. As mentioned, general corrosion occurs on 13% Cr steels in ph < 3.8 with acetic buffer solution Before the loading existed at 215 mv, E corr was close to E S(red/ox) in ph 3.5 at pph 2 S of MPa (Figure 8[a]). After the load was applied, E corr existed in the range from 500 mv to 550 mv, at which point the specimen was in a passive state up to 350 h. E corr then was lowered to 600 mv, at which point the specimen was in an active state, indicated by the anodic polarization curve (Figure 8[b]). However, no SSC occurred in the 720-h duration of the test. FIGURE 12. H 2 S vs ph diagram with S redox potential, V c, and E corr in Steel B. ph TABLE 3 Content of Diffusible H 0 in Steel B Before loading existed, E corr was close to E S(red/ox) in ph 4.0 at pph 2 S of MPa (Figure 9[a]). Judging from the anodic polarization curve indicated in Figure 9, the current density of E S(red/ox) was H 0 (ppm) avg: avg: avg: 0 Note: pph 2 S: 0.01 MPa; immersion time: 720 h; gas chromatograph. CORROSION Vol. 56, No

8 FIGURE 13. Scanning electron micrograph of fracture surface of Steel B in a constant load test. (H2S; 0.01 MPa, ph: 3.5) 6 µa/cm2 and relatively small. Ecorr dropped immediately after the load was applied and shifted toward the noble side, thereafter remaining constant. No SSC occurred in the 720-h duration of the test. The specimen had a metallic color, and no pitting occurred. DISCUSSION Comparison of Conditions Under Which Cracks Occurred Figures 10 through 12 show the limit of crack occurrence, ES(red/ox), Vc, and Ecorr, when the constant load SSC test was finished in the relationship between pph2s and ph. The relationship between these electrochemical characteristic values and SSC was considered. Steel A Figure 10 indicates Ecorr located at the potential of the active state in ph 5.0 as indicated by the anodic polarization curve (Figure 4[b]). Because a cathodic reaction is the reduction of hydrogen, SSC could occur as a result of hydrogen embrittlement. The cracks in Steel A were of the hydrogen embrittlement type in ph 5.0, and hydrogen embrittlement occurs when H0 > Hth.2-4 However, when ph increased to 5.5, Ecorr shifted to a passive state (Figure 5). Hence, cracks did not occur because hydrogen did not enter into the specimen in this environment. 540 Steel C Figure 11 indicates that depassivation ph (phd) was 2.5. Ecorr existed in a passive state in all present cases, and ES(red/ox) was much less noble than Vc. Miyasaka and coworkers found that no cracking or pitting occurred when ES(red/ox) was less noble than Vc.5-6 Steel B Figure 12 indicates that Ecorr existed in a passive state in ph 4.0 at pph2s 0.01 MPa. In this case, no cracks occurred because hydrogen did not enter into this steel. However, Ecorr existed in an active state in ph 3.5. Table 3 shows the diffusible H0 when pph2s is 0.01 MPa in ph 2.5 to 4.0. H0 increased with a decrease in ph. Steel B was passive in ph 4.0 at pph2s of 0.01 MPa. No cracks occurred because hydrogen did not enter into this steel. Cracks caused by hydrogen embrittlement in Steel B were thought to occur in ph 3.5 and the same as cracks in Steel A. Cracks were determined by the relative value of H0 and Hth. Hth was supposed to be < 2.6 ppm. Figure 13 shows a scanning electron micrograph of a fracture surface of Steel B in ph 3.5 at pph2s of 0.01 MPa in a constant load test. The fracture surface showed a quasi cleavage, so the cracks caused by hydrogen embrittlement were thought to occur in this environment. SSC occurred when pph2s was high in ph 4.0. Pitting was thought to occur because Vc was less noble than ES(red/ox) in ph 4.3 at pph2s of 0.01 MPa. Actually, pitting occurred when the specimen was CORROSION MAY 2000

9 FIGURE 15. Potential vs ph diagram in H2S environments. FIGURE 14. Scanning electron micrograph of specimen surface in Steel B after immersion test. (H2S: MPa, ph: 4.3) observed by scanning electron microscopy (SEM) after the constant load test (Figure 14). It is known that the ph inside the pit is lowered when pitting starts. It is possible for cracking caused by hydrogen embrittlement to occur when the ph inside the pit is lower than phd.16 Figure 15 shows the results of EH(red/ox) and ES(red/ox) calculated by the Nernst equation in these environments (Appendix). For example, the ph inside the pit was lowered to 1.0 in ph 4.3 at pph2s of 0.01 MPa. The cathodic reaction inside the pit was the reduction reaction of hydrogen because ph 1.0 < phd (= 4.0) in Steel B. Therefore, if the pitting occurred and the ph inside the pit dropped drastically, hydrogen entered into the steel. Then, the cracking caused by hydrogen embrittlement occurred when H0 > Hth. Conditions Under Which Cracks Occur in Oil and Gas Fields Figure 16 shows conditions under which cracks occur on Steels A, B, and C in oil and gas production environments in which ph, phd, ES(red/ox), and Vc are CORROSION Vol. 56, No. 5 compared. Hydrogen entered into the steels because Ecorr existed in an active state when the ph < phd. Cracks caused by hydrogen embrittlement were thought to occur if H0 > Hth, as in the high pph2s (Figure 16, Region 2). Ecorr exists in a passive state when ph > phd. No cracks occurred if ES(red/ox) < Vc, as in the low pph2s (Figure 16, Region 3). However, pitting started when pph2s was high and Vc was less noble than ES(red/ox). APC-SCC occurred when the ph inside the pit was higher than phd (Figure 16, Region 5). Hydrogen embrittlement occurred when the ph inside the pit was lower than phd (Figure 16, Region 4). Most ph in oil and gas fields is in the range from 3 to 5. Hydrogen embrittlement occurred because most of the Ecorr in Steel A existed in an active state. Ecorr in Steel C existed in a passive state. No cracks occurred if ES(red/ox) was much less noble than Vc. The occurrence of cracks in Steel B was accompanied by a complicated change because phd ranged from 3 to 5. The mechanism of the occurrence of cracks varied according to ph and pph2s. CONCLUSIONS Occurrence of cracks in an API 13% Cr steel, modified 13% Cr steel, and duplex stainless steel were compared in various wet, mild, sour environments. Conditions under which cracks occurred in the modified 13% Cr steel were made clear. No cracks occurred if ph > phd and Vc > ES(red/ox). Cracks caused by hydrogen embrittlement occurred if ph > phd and Vc < ES(red/ox). Pitting occurred if ph > phd and Vc < ES(red/ox). The ph inside the pit decreased drastically and hydrogen embrittlement occurred. Cracks of the hydrogen embrittlement type occurred if ph < phd and Hth < H0. No cracks occurred if ph < phd and Hth > H0. 541

10 FIGURE 16. Schematic showing conditions under which cracks occur in oil and gas environments. APPENDIX Calculation of Redox Potential of Hydrogen and Sulfur Redox potential of hydrogen and sulfur is calculated by using the Nernst equation. 17 Calculation of redox potential of sulfur: + S+ 2H + 2e H2S( aq) (1) Es( red/ ox) = 0 E ( RT / F) ph ( RT / 2F) log ah S s 2 (2) Es ( SHE ) = ph log ah2 S (3) Es ( SCE ) = ph log ah2 S (4) Calculation of redox potential of hydrogen: 2H + + 2e H2( g) (5) EH( red/ ox) = E 0 H ( RT/ F) ph (6) LIST OF SYMBOLS EH ( SHE ) = ph (7) EH ( SCE ) = ph (8) E i0 : Standard electrode potential of i species a: Activity, a = K H P assuming that activity obeys Henry s law P: Pressure K H : Solubility constant K H,H2 S 3221/T log T pph 2 S: H 2 S partial pressure (MPa) R: Gas constant (5.314 J/mol) T: Temperature (K) F: Faraday constant (96,485 C/mol) V c : Pitting potential E corr : E i(red/ox) ph d : H 0 : H th : REFERENCES Corrosion potential Redox potential of i species Depassivation ph Hydrogen concentration in steel Threshold hydrogen concentration in steel 1. H. Kurahashi, T. Kurisu, Y. Sone, K. Wada, Y. Nakai, Corrosion 41, 4 (1985): p T. Kushida, T. Kudo, Evaluation of Crack Sensitivity of Steel Material in Wet H 2 S Environments, in Symp. Committee on Stress Corrosion of Steels (Toyko, Japan: The Iron and Steel Institute of Japan [ISIJ], 1991), p T. Kushida, T. Kudo, Zairyo to Kankyo 41 (1992): p S. Sakamoto, A. Kawakami, H. Asahi, A. Nakamura, Effect of Environmental Factors and Yield Strength on SSC Property of API Stainless Steel, CORROSION/95, paper no. 82 (Houston, TX: NACE International, 1995). 5. A. Miyasaka, K. Denpo, H. Ogawa, ISIJ Int. 31, 2 (1991): p K. Denpo, A. Miyasaka, H. Ogawa, Corrosion Prevention in the Process Industries (1988): p S. Hashizume, K. Masamura, Y. Ishizawa, Corrosion Resistance and Mechanical Properties of High-Strength 15% Cr Stainless Steel for OCTG, CORROSION/95, paper no. 78 (Houston, TX: NACE, 1995). 8. H. Asahi, T. Hara, A. Kawakami, A. Takahashi, Development of Sour-Resistant Modified 13% Cr OCTG, CORROSION/95, paper no. 79 (Houston, TX: NACE, 1995). 9. M. Ueda, T. Kushida, T. Moli, Evaluation of SSC Resistance on Super 13% Cr Stainless Steel in Sour Applications, CORRO- SION/95, paper no. 80 (Houston, TX: NACE, 1995). 10. Y. Miyata, Y. Yamane, O. Furukimi, Corrosion of a New 13% Cr Martensitic Stainless Steel OCTG in Severe CO 2 Environment, CORROSION/95, paper no. 83 (Houston, TX: NACE, 1995). 11. H. Asahi, T. Hara, M. Sugiyama, Corrosion Performance of Modified 13% Cr OCTG, CORROSION/96, paper no. 61 (Houston, TX: NACE, 1996). 12. API Specification 5CT, Specification for Casing and Tubing, 3rd ed. (Washington, DC: API, 1990). 13. NACE Standard TM , Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking in H 2 S Environments, (Houston, TX: NACE, 1990), p H. Amaya, M. Ueda, Effect of Test Solution Compositions on Corrosion Resistance of 13% Cr Materials in a Little Amount of H 2 S Environment, CORROSION/99, paper no. 585 (Houston, TX: NACE, 1999). 15. J. Drugli, T. Rogne, M. Svenning, S. Axelsen, Effect of Using Buffered Solutions in Corrosion Testing of Alloyed 13% Cr Martensitic Stainless Steels for Mildly Sour Applications, CORROSION/99, paper no. 586 (Houston, TX: NACE, 1999). 16. K. Denpo, H. Ogawa, Corrosion 47, 8 (1991): p H.H. Uhlig, Corrosion and Corrosion Control, 2nd ed. (Tokyo: Sangyotosyo, 1974), p CORROSION MAY 2000