acom Hydrogen Sulphide Resistance of Highly-Alloyed Austenitic Stainless Steels

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1 Hydrogen Sulphide Resistance of Highly-Alloyed Austenitic Stainless Steels by C. Wolfe, P-E. Arnvig, W. Wasielewska, Avesta Sheffield AB, Research & Development, SE Avesta, Sweden, and R.F.A. Jargelius-Pettersson, Swedish Institute for Metals Research, Drottning Kristinas väg 48, SE Stockholm, Sweden Abstract Two highly-alloyed austenitic stainless steels: 654 SMO (UNS S32654, 24Cr 22Ni 7.3Mo 0.5N) and 254 SMO (UNS S31254, 20Cr 18Ni 6.2Mo 0.2N) have been evaluated in laboratory environments simulating sour service. No evidence of cracking was observed in specimens cold worked up to 80% in NACE testing at 25 C. Data at elevated temperatures and pressures are compared with published application limits for 22Cr and 25Cr duplex stainless steels. In the most aggressive conditions investigated, 1.4 bar H 2S at 150 C, no cracking was observed for 654 SMO. Background Numerous factors must be taken into account in laboratory testing of corrosion resistant alloys (CRAs) for sour service in oil and gas production. The primary variables which affect cracking propensity are temperature, H 2S partial pressure, ph and chloride concentration, while the partial pressure of CO 2 and the presence of cation species are of secondary importance (1). The most widely-used standardized test method is NACE TM 0177 (2) for which specimens are immersed in acidified aqueous environments saturated with H 2S at ambient temperature and pressure. The test environment recommended for CRAs is 5%NaCl + 0.5% glacial acetic acid, standardized loading methods include tensile, C-ring and double cantilever beam specimens. Materials selection recommendations for sour service based on such laboratory tests and service experience are included in NACE MR (3). Other relevant testing standards include ISO-7539 (4) which standardizes the use of various specimen geometries but does not specify any test environments. The European Federation of Corrosion have recently published guidelines for materials testing for H 2S service (5). This makes a clear differentiation between sulphide stress cracking (SSC), a form of hydrogen embrittlement which is generally most severe around room temperature, and stress corrosion cracking (SCO which is an anodic process and which increases in severity with increasing temperature. The primary recommended tests are constant load testing at 90% of the yield stress or constant strain testing (4-point bend, C-ring) at the yield stress with slow strain rate testing as a supplementary test. Some examples of characteristic environments are given, e.g. 1 g/l NaCl ph 3.5 for gas production with traces of formation water or 165 g/l NaCl (100g/l chloride) ph 4.5 for oil production with high dissolved solids, but the general principle is that materials be evaluated under the most severe environmental and mechanical conditions that are realistically anticipated for the intended service and not under standard "worst case" conditions. The general trend is thus towards standard testing methodologies but service-oriented environments. In the present work, testing has been carried out both in the standard NACE environment at 25 C and 90 C and in an autoclave environment at 150 C. A combination of uniaxial tensile specimens (constant load) four point bend and U-bend specimens (constant total strain) has been used acom AVESTA SHEFFIELD CORROSION MANAGEMENT AND APPLICATION ENGINEERING

2 Experimental procedure The compositions of the superaustenitic alloys investigated are given in Table 1. All the materials were taken from standard production heats and were initially in the solution annealed condition. In order to investigate the effect of cold work (and thus the associated increase in yield strength and maximum attainable applied stress), laboratory-scale cold rolling (for plate) or cold drawing (for wire) was employed. Stress corrosion testing was carried out under three sets of conditions, which are detailed for 654 SMO and 254 SMO in Tables 2 and 3 respectively. Testing at ambient temperature and pressure (Series I) was carried out on four point bend, U-bend and tensile specimens. In the majority of cases specimens were taken in both the longitudinal and transverse directions. The environment was that specified in NACE TM 0177 i.e. 5wt % NaCl acidified by the addition of 0.5wt % glacial acetic acid to give a ph of approximately 3. The solution was first deaerated by purging with nitrogen and then saturated with H 2S by bubbling the gas through the test solution throughout the test period. At the end of the 720 hour test period, the specimens were removed from the environment and examined in a light optical microscope for signs of cracking or other corrosive attack. Testing was also carried out on a more limited range of specimens in the same environment at 90 C (Series II). The third series of tests was performed in a static autoclave environment at 150 C and elevated pressure (30 bar). The specimens were placed in the autoclave, which was closed and purged with nitrogen prior to the introduction of the dearated NaCl solution. The autoclave was pressurized with the test gas, comprising H 2S, CO 2 and N 2, at ambient temperature then sealed and heated to the test temperature. The partial pressures of H 2S at the ambient (i) and test (f) temperatures was calculated using: Table 1. Compositions of the alloys investigated. p H2si P oq H2S = (1 + L ; i) P H2S. f = P H2S. i ( Tf )( 1+Li T i 1+L f ) with corresponding equations for the CO 2 partial pressure, where q H2S is the partial pressure of H 2S in the gas supply cylinder (q H2S +q CO2 + q N2 = 1) UNS C Cr Ni Mo N Other 654 SMO S Cu, Mn 254 SMO S Cu 254 SMO and 654 SMO are trade marks registered by Avesta Sheffield AB. Table 2. Summary of test parameters for 654 SMO 654 SMO test series I II III(c) Temperature ( C) System pressure (bar) Test duration (h) Degree of cold work , 40, 50, 60, 80 Stress ratio (% of y.s.) 90,100 70, Partial pressure H 2S (bar) , 0.1, Partial pressure CO 2(bar) NaCl concentration 5% 5% 2% PH Table 3. Summary of test parameters for 254 SMO. 254 SMO test series I IIa IIb IIIa IIIb IIIc Temperature ( C) System pressure (bar) Test duration (h) Degree of cold work Stress ratio (% of y.s.) , , Partial pressure H 2S (bar) , 0.1, Partial pressure CO 2(bar) NaCl concentration 5% 5% 5% 2% 2% 2% PH T i is the starting temperature = 298K T f is the testing temperature = 423K L and M denote the partitioning of H 2S and CO 2 respectively between the liquid and gas phases: L i = H2S(liq) kh2s. i RTiV1 = H 2S (gas) V g M i = CO2 (liq) kco2. i RTiV1 = CO 2 (gas) V g at ambient temperature, with corresponding equations for the test temperature. 2

3 k H2S and k CO2 are solubility constants for H 2S and CO 2 respectively, taken as k H2S = mol L -1 bar -1 at 25 C, mol L -1 bar -1 at 150 C (from equation in ref. 6) k CO2 = mol L -1 bar -1 at 25 C, mol L -1 bar -1 at 150 C (from equation in ref. 6) R= L.bar K -1 mol -1 V I is the volume of liquid and V g the volume of gas in the autoclave. P o is the theoretical instantaneous pressure (before any gases have dissolved in the test solution) supplied from the gas cylinder to the autoclave, and P eq is the equilibrium pressure supplied from the gas cylinder after dissolution. The total instantaneous and equilibrium pressures are evaluated by addition of the partial pressures of N 2 (1.013 bar) and H 2O (from steam tables) initially present in the autoclave. The relation between P eq and P o is given by: P eq = P o { qh2s + qco2 + q N2 1 + L i 1 + M i } This calculation assumes the effect of NaCl on water vapour pressure and the solubility of N 2 in the test solution to be negligible, the solubilities of H 2S and CO 2 are further assumed to be independent of system pressure and the presence of other gases. Results and discussion The results of testing performed on the superaustenitic steels 254 SMO and 654 SMO are given in Tables 4-7 (pages 4-5). At 25 C there was no evidence of sulphide stress cracking in any annealed or cold worked specimen, nor was any pitting attack observed. At 90 C some superficial pitting was observed on the edges of the specimens of 254 SMO and cold worked 654 SMO but there was no evidence of cracking. In the autoclave testing at 150 C no evidence of stress corrosion cracking was observed in annealed or cold worked 654 SMO tested under the most extreme conditions (1.4 bar H 2S), and only slight pitting attack was present on the edges of cold worked specimens. In the case of 254 SMO no cracking occurred in annealed or cold worked material at a hydrogen sulphide partial pressure of 0.4 bar (Series IIIa). At 0.9 bar H 2S (Series IIIb) there was slight surface attack on annealed specimens but this was not classified as stress corrosion cracking according the NACE TM0177 evaluation procedure (visual observation with the aid of a low power binocular microscope). At 1.4 bar H 2S (Series IIIc) stress corrosion cracking occurred on the annealed specimens. In all three test series some pitting occurred both on the edges of specimens and at the contact between the specimens and the loading fixture. Figure 1 shows the ambient temperature data plotted in terms of the hardness and maximum applied strength of the materials investigated. According to many sources (5, 7, 8) the risk of the hydrogen-related failure termed sulphide stress cracking or SSC is most severe at ambient temperatures and decreases with increasing temperature. The data in Figure 1 indicate therefore that both the superaustenitic steels are suitable candidate materials for SSC environments. A further point is that the high strength levels which can be attained from cold working (by virtue of the high work hardening coefficient imparted by nitrogen alloying) have no measurable adverse effect on cracking resistance. Elevated temperature data is frequently plotted in terms of engineering diagrams delineating the cracking/no cracking boundary in terms of temperature and hydrogen sulphide partial pressure. Schematic illustrations of these limits from Barteri et al. (9) and Ogawa et al. (10) are shown in Figure 2. The fact that no cracking was observed for any specimen of 654 SMO in the present work means that only a lower limit for the boundary line may be obtained for this steel, while the Figure 1. Results of ambient temperature H 2S testing plotted in terms of the hardness and maximum applied strength of the materials investigated. No cracking was observed for any specimen. Figure 2. Engineering diagrams illustrating application criteria for CRAs in sour service. Top diagram from Barteri et al. (9), lower diagram from Ogawa et al. (10). 3

4 Table 4. Results of NACE testing (5%NaCl ph = 3, ph 2S = 1 bar) of 654 SMO and 254 SMO for 720 hours at 25 C. L and T denote specimens taken in the longitudinal and transverse directions respectively. Heat No. Product Cold work Hardness Method Stress SSC 654 SMO [%] [HRC] [MPa] Plate, L+T 0,40, 50,60, PB No cracking Plate, L+T 0,40, 50,60, PB No cracking (100% y.s.) Plate, L+T 0,40, 50,60, PB No cracking (100% y.s) 254 SMO Plate, T 0,20 -, 26 Tensile No cracking Plate, L+T 40,50, 60, PB No cracking (73% y.s.) Plate, L 40, 50, Tensile No cracking Plate, L U-bend >1248 No cracking Wire, L U-bend >1242 No cracking Table 5. Results of NACE testing (5%NaCl ph = 3, ph 2S = 1 bar) of 654 SMO and 254 SMO for 720 hours at 90 C *. Heat No. Product Cold work Hardness Method Stress SSC/SCC [%] [HRC] [MPa] 654 SMO Plate, T PB 489 No cracking 254 SMO (100% y.s.) Wire, L U-bend >1242 No cracking Plate, L U-bend >1248 No cracking Wire, L 52 U-bend >1270 No cracking Plate, T 0 4-PB 285 No cracking (100% y.s.) * extensive cracking was observed for a 22Cr duplex steel tested under similar conditions but at 0.1 bar H 2S 4

5 Table 6. Results of 500 hour autoclave testing of 654 SMO in 2%NaCl overpressured with 1.4 bar H 2S and 7 bar CO 2 (total pressure 30 bar at 150 C). Heat No. Product Cold work Hardness Method Stress SSC [%] [HRC] [MPa] Plate, L+T 0, 40, 50, 60, PB No cracking Plate, L+T 0, 40, 50, 60, PB No cracking Table 7. Results of 500 hour autoclave testing of 254 SMO in 2%NaCl at a total pressure of 30 bar. Heat No. Product Cold work Hardness Method Stress SCC [%] [HRC] [MPa] Series IIIa: ph 2S = 0.4 bar, pco 2 = 2.7 bar Plate, T 0, 10, 15,20 -, 30 4-PB No cracking (100% y.s.) Plate, L+T 40, 50, 60, PB No cracking (83-100% y.s.) Series IIIb: ph 2S = 0.9 bar, pco 2 = 6.7 bar* Plate, T 0 4-PB No cracking, (70-100% y.s.) slight surface attack Series IIIc: ph 2S = 1.4 bar, pco 2 = 7 bar Plate, T 0 4-PB 217 SCC * extensive cracking was observed for a 25Cr superduplex steel tested under the same conditions 5

6 single failure point for 254 SMO gives an indication of the application limits. These results may be placed in perspective by a comparison to the relatively large amount of data which has been published for duplex stainless steels. Figure 3 shows a comparison with data compiled by Ikeda et al. (1) and van Gelder et al. (11) and indicates the performance of the two superaustenitic steels to be measurably superior to that of the duplex steels at lower temperatures. In the autoclave testing at 150 C the "no cracking" points for 654 SMO and 254 SMO also lie above the majority of the lines for the duplex steels. This comparison with duplex steels is confirmed by the two cases in which duplex steels were included in the present test series (footnotes in Tables 5 and 7). At 90 C a 22Cr duplex steel exhibited extensive cracking after testing at 0.1 bar H 2S (compare with the absence of cracking for 654 SMO and 254 SMO at 1 bar H 2S). At 150 C a 25Cr superduplex stainless steel suffered cracking at 0.9 bar H 2S (compare with the occurrence of cracking in 254 SMO but not 654 SMO at 1.4 bar H 2S). Austenitic stainless steels are generally regarded (5, 8) as more sensitive to anodic SCC, which is principally induced by the presence of chlorides and which increases in severity with increasing temperature, than to the hydrogen-induced SSC (For duplex steels synergistic interactions give specific worst cases at intermediate temperatures typically C, Ref. 5, 8). However, the results in Figure 3 indicate that the two superaustenitic steels possess good resistance to anodic SCC at elevated temperatures. This may be related to the observation of Miyasaka (12) that application limits of CRAs are governed by the occurrence of pitting corrosion, and the documented excellent pitting corrosion resistance of 254 SMO and 654 SMO (13). A second comparison of the data for 654 SMO and 254 SMO with published results for duplex steels may be made in terms H 2S resistance at elevated strength levers. Figure 4 Figure 3. Compilation of failure/non-failure lines for 22Cr (broken lines) and 25Cr (solid lines) duplex stainless steels and comparison with data points for 254 SMO and 654 SMO. Ikeda (1) van Gelder (11) Barteri (9). Figure 4. Compilation of failure/non-failure lines for 22Cr and 25Cr stainless steels strengthened by cold work to 970 MPa from Barteri et al. (9) and comparison with data points for 254 SMO and 654 SMO for which no cracking was observed in the present investigation. Room temperature yield strengths of 254 SMO and 654 SMO are given in parentheses. 6

7 shows a compilation of data from Barteri et al. (9) for 22Cr and 25Cr duplex stainless steels which had been strengthened to 970 MPa (140 ksi) by cold work. A comparison indicates that the application limits of the heavily cold worked superaustenites are appreciably higher that those for the duplex steels, even though the strength levels of the austenitic steels are higher. There is a paucity of other literature data for superaustenites in sour environments. One investigation of 654 SMO recently published by Aho-Mantila et al. (14) involved testing according to a proposed NACE standard for slow strain rate testing at elevated temperatures and pressures (15). Testing in 20%NaCl at 177 C with partial pressures of 14 bar H 2S and 14 bar CO 2 gave failure at an elongation of less than 5%, to be compared with 65% in nitrogen gas. This is an extremely harsh environment in combination with a test method which is widely considered to be very aggressive (8, 16). It provides an upper limit for the applicability of 654 SMO in sour environments, although further work is required to define to boundary conditions. Conclusions Laboratory testing of 654 SMO (UNS S32654, 24Cr 22Ni 7.3Mo 0.5N) and 254 SMO (UNS S31254, 20Cr 18Ni 6.2Mo 0.2N) has been carried out in H 2Scontaining environments simulating sour service. Both alloys are resistant to cracking in the standard NACE environment (5%NaCl + 0.5% glacial acetic acid, 25 C, 1 bar H 2S) in the annealed and cold worked conditions. The maximum applied stresses were 1314 MPa (654 SMO) and 1248 MPa (254 SMO). No cracking was observed in NACE solution at 90 C for annealed or cold worked 254 SMO or for 654 SMO in the annealed condition. Testing in a static autoclave environment at 150 C was performed in 2% NaCl overpressured with H 2S and CO 2. No cracking was observed for 654 SMO at an H 2S partial pressure of 1.4 bar, while cracking occurred in 254 SMO at 1.4 bar H 2S but not at 0.9 bar. A comparison with published application limits for 22Cr and 25Cr duplex stainless steels in sour environments shows the two superaustenitic steels to exhibit superior performance over a wide range of temperatures. References 1. A. Ikeda, Y. Morita, N. Matsuki: "Recent progress in tubular products for oil and gas production" The Sumitomo Search, 37 (Nov 1988) NACE standard TM-0177 "Laboratory testing of metals for resistance to sulfide stress cracking in H 2S environments" 3. NACE standard MR "Standard Material Requirements-Sulphide stress cracking resistant metallic materials for oilfield equipment" 4. International Standard ISO-7539 "Corrosion of metals and alloys-stress corrosion cracking". Parts European Federation of Corrosion Publication No. 17 "Corrosion resistant alloys for oil and gas production: guidance on general requirements and test methods for H 2S service". The Institute of Materials, A. Miyasaka: "Thermodynamic estimation of ph of sour and sweet environments as influenced by the effects of anions and cations" Corrosion '92, Paper No D.R. McIntyre, R.D. Kane, S.M. Wilhelm: "Slow strain rate testing for materials evaluation in high pressure H 2S environments" Corrosion 44:12 (1988) MJ. Schofield, S.M. Wilhelm, J.W. Oldfield: "Application for various stress corrosion cracking test techniques: validity and relevance to practice" Proc. Duplex stainless steels ' M. Barteri, A. Tamba: "Conservative criteria for application of stainless alloys in gas and oil sour wells" Int. Conf. Innovation Stainless Steel, Florence Oct H. Ogawa, K. Denpoh, K. Ina: "Development of high-alloy tubular goods for sour wells' Nippon Steels Tech. Rep. 38 (1988) K. van Gelder, J.G. Erlings, J.W.M. Damen, M.M. Festen: "Limiting conditions for the application of duplex stainless steel and corrosion-resistant clad materials in sour service" Int. Conf. Pipe Technology, Rome, Nov A. Miyasaka, H. Ogawa: "Corrosion performance and application limits of corrosion-resistant alloys in oilfield service" Corrosion 51:3 (1995) P-E. Arnvig, A.D. Bisgård: "Determining the potential independent critical pitting temperature by a potentiostatic method using the Avesta Cell" Corrosion '96, paper No I. Aho-Mantila, C. Christensen, B. Espelid, M. Holmgren, K. Saarinen, I. Thorbjörnsson: "Sulphide stress cracking tests for carbon and highly alloyed steels" Proc. Eurocorr '96, paper VII-OR3 15. Proposed NACE standard test method "Slow strain rate test method for screening corrosion resistant alloys ICRAs) for sour oilfield service" H. Eriksson, S. Bernhardsson: "The applicability of duplex stainless steels in sour environments" Corrosion 47:8 ( This paper was first presented at Materials Technology for the Petroleum Industry. Norske Sivilingeniørers Forening, Trondheim, Norway, January 9-10, (In Norwegian) Although Avesta Sheffield has mode every effort to ensure the accuracy of this publication, neither it nor any contributor can accept any legal responsibility whatsoever for errors or omissions or information found to be misleading or any opinions or advice given. 7

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