HIGH-STRENGTH CORROSION RESISTANT NICKEL-BASE ALLOY 725HS FOR SEVERE SOUR OIL AND GAS FIELD APPLICATIONS

HIGH-STRENGTH CORROSION RESISTANT NICKEL-BASE ALLOY 725HS FOR SEVERE SOUR OIL AND GAS FIELD APPLICATIONS Edward L Hibner and Brett Puckett Special Metals Corp. Huntington, WV 25705 ABSTRACT Today s global exploration of oil and gas leads to a range of highly corrosive environments that, in turn, require corrosion resistant high nickel content alloys for subsurface and wellhead equipment. Materials have to meet criteria for corrosion resistance and mechanical properties in severe environments for the required service life. This paper presents the latest development of a higher strength grade of alloy 725 (UNS N07725), alloy 725HS. This new age-hardenable nickel base alloy 725HS offers many advantages such as higher-strength, toughness and excellent corrosion resistance. The standard alloy 725 grade is typically specified with 120 ksi (827 MPa ) minimum yield strength compared to alloy 725HS with 140 ksi (965 MPa ) minimum yield strength. The grades are approved to NACE MR0175 up to hardness levels of 40 HRC and 43 HRC maximum for the alloys 725 and alloy 725HS, respectively. The higher strength level of alloy 725HS offers higher strength for sour well service compared with the traditionally used alloy 718 (UNS N07718). Corrosion data is also presented showing the performance of the age-hardenable nickel alloys in seawater, resisting chloride stress corrosion cracking, and in sour well environments using the slow strain rate and C-ring test methods. Alloys 725 and 725HS are also shown to be suitable candidate materials in severely corrosive high temperature oil patch environments. Sour environments in which alloys 725 and 725HS have met the criteria for corrosion resistance are defined with and without free sulfur. Keywords: nickel-base alloys, sour environments, corrosion resistance, age-hardened, high-strength, bar, oil field INTRODUCTION In selecting materials for corrosive sour oil field environments, the materials of choice must be reliable and cost-effective. Materials have to meet criteria for corrosion resistance and mechanical

properties in service environments for the required service life. Age-hardened nickel-base alloys offer many advantages such as high-strength, toughness and excellent corrosion resistance. The primary CRA machining quality age-hardened bar products used with alloys G-3 (UNS N06985), 50 (UNS N06950) and C-276 (UNS N10276) Oil Country Tubular Goods (OCTG's) for wellhead and subsurface completions of gas wells are INCONEL alloy 725 (UNS N07725), typically specified with 120 ksi (827 MPa) minimum yield strength, and INCONEL alloy 725HS, the 140 ksi (965 MPa) minimum yield strength grade. In review, Table 1 of NACE Material Requirement MR0175-2001 1 lists seven environmental Test Levels, Level I being the least severe and Level VII the most severe. The 120 ksi (827 MPa) minimum yield strength grade of alloy 725 has been used successfully for two decades in severe sour oil field environments 2-12. Alloy 725 is qualified to MR0175 Test Level VII in the presence of elemental sulfur. Recently developed 140 ksi (965 MPa) minimum yield strength grade alloy 725HS offers both superior corrosion resistance and higher strength than alloy 718 (UNS N07718). Alloy 718 is one of the primary machining quality age-hardenable bar products used with alloys 028 (UNS N08028) and 825 (UNS N08825) OCTG s for completions. Alloys 725HS qualifies to NACE Test Level VII, compared to alloy 718 which only qualifies to NACE Test Level V. DISCUSSION In general, resistance to stress corrosion cracking (SCC), hydrogen embrittlement sulfide stress cracking, (SSC), increases with increasing alloy nickel, chromium, molybdenum, tungsten and niobium content. These materials are cold worked or age-hardened to specified levels in order to obtain the strength needed to support the weight of several thousand meters of tubing and withstand the intense pressure. The effect of alloy molybdenum content on corrosion resistance ranking is shown in Figure 1, a plot of temperature versus maximum environmental H 2 S content for use of nickel-base alloys. Alloy 718 contains 3% Mo, compared to the 9% Mo content of alloy 725. Material selection for down-hole and wellhead equipment such as hangers, valves, pumps, packers, and wire lines is very important. For many of these components age-hardenable alloys are used to obtain the needed strength in heavier cross-sections which cannot be strengthened by cold work. Nickel alloys commonly used for these applications include: alloys 925 (UNS N09925), 718, and 725, alloy K-500 (UNS N05500) and alloy X-750 (UNS N07750). These components must resist SCC. The potential for SCC becomes greater with higher temperature and concentrations of H2S and the presence of chloride ions and elemental sulfur. Lower temperature hydrogen embrittlement and sulfide stress cracking (SSC) are also potential failure mechanisms, which are promoted by galvanic corrosion, acidizing operations, or dissolved H2S. Alloy strength is another factor. As strength increases, environmental cracking susceptibility also increases. In order to obtain the optimum level of strength, ductility, and toughness, and cracking resistance, maximum hardness levels are specified for each alloy in NACE Materials Requirement MR0175. The chemical composition for alloys 725 and 725HS is displayed in Table 1 and the typical mechanical properties for both grades, alloys 725 and 725HS, are shown in Table 2. INCONEL is a registered trademark of the Special Metals Group of companies.

C-ring Testing C-ring stress corrosion cracking (SCC) tests of INCONEL alloy 725HS (UNS N07725) bar products were conducted for six months in NACE Materials Requirement MR0175 Level VI and VII sour brine Oil Patch environments relative to severe Mobile Bay applications where elemental sulfur is not present. The NACE Level VI and VII sour brine environments containing (a) deaerated 20% NaCl + 508 psi (34.5 bar) H 2 S + 508 psi (34.5 bar) CO 2 at 347 9 F (175 5 C) and (b) deaerated 25% NaCl + 508 psi (34.5 bar) H 2 S + 508 psi (34.5 bar) CO 2 at 401 9 F (205 5 C), respectively. The maximum hardness of the alloy N07725 bars tested varied from 43 HRC to 47 HRC. The C-rings were deflected to obtain a stress of 100% of the yield strength per NACE Test Method TM0177 Method C. Triplicate specimens of each alloy were tested for six months in the NACE Materials Requirement MR0175 Level VI and VII sour brine environments. No SCC was observed for C-rings of alloy N07725 evaluated during the six-month exposure to the NACE Materials Requirement MR0175 Level VI and VII severe sour brine environments. There was no discernable pitting of the C-rings. The present maximum allowable hardness limit in NACE Material Requirement MR0175 for alloy N07725 is 43 HRC. Three of the heats evaluated in this study exhibited a hardness of 43 HRC, the remaining heats exhibited hardnesses of 44, 45 and 47 HRC. The results of this study clearly show that alloy N07725 is acceptable to NACE Level VII environment at a maximum hardness of 44 HRC. In sulfide stress cracking (SSC) tests, conducted on duplicate specimens in accordance with NACE Test Method TM0177 Method A galvanically coupled to steel for 720 hours, this material easily passed. Slow Strain Rate Testing In a review 13, a common pass/fail criteria for SSR testing is a ratio of Time to Failure (TTF), % Reduction of Area (%RA) and/or % Elongation (%El) measured in a simulated oil patch environment relative to the same parameter in an inert environment (air or nitrogen). Depending on the alloy and the environment, a ratio of 0.80 or greater typically passes. If the ratio is between 0.80 and 0.90, the specimen is examined under the Scanning Electron Microscope (SEM) for evidence of ductile or brittle fracture of the primary fracture surface. A ratio below 0.80 typically fails. All specimens are examined for secondary cracking in the gauge length, away from the primary fracture. The absence of secondary cracking is indicative of good Stress Corrosion Cracking (SCC) resistance and passes. The presence of secondary cracks fails. In most cases, one inert SSR test is conducted along with three environmental SSR tests for each acceptance lot. To account for data scatter, two inert SSR tests are sometimes conducted along with two environmental SSR tests. If all the environmental tests pass the TTF, %RA and/or %El ratios (as required by specification) and exhibit no secondary cracking, the test lot of material passes and is released. Tables 3 and 4 display slow strain rate test data for alloy 725HS evaluated in a severe simulated Mobile Bay type environment containing 100,000 ppm Cl - (as NaCl) + 200 psi (1.4 MPa) H 2 S + 200 psi (1.4 MPa) CO 2 at 350 and 400ºF (177 and 204ºC). Alloy 725HS displayed excellent corrosion resistance, exhibiting critical ratios for Time to Failure, % Elongation and % Reduction of Area from 0.96 to 1.07 with no secondary cracking of the specimen gauge away from the primary fracture.

Material Selection Process Individuals and companies choosing a CRA for specific sour service environments use different methods. 14 A recognized selection procedure is to review the literature for corrosion data that applies to the anticipated field conditions. Then a group of candidate alloys is selected that represents a range of alternatives. A test program, simulating the subject field environment, is often initiated. A final CRA selection is made for a specific application based on test results and an economic analysis of the cost effective alternatives. While more detailed testing and analysis is sometimes required, guideline tables and diagrams are often used before extensive efforts are made to make a final alloy selection for a specific oil field application. Corrosion data for age-hardened alloy 725 and 725HS are presented 15. Tables 5 and 6 list environments where alloys 725 and 725HS have either been recommended or where corrosion testing has confirmed their use. Results are generally based on SCC and SSC hydrogen embrittlement data. H2S limits are based on the presence of a significant concentration of chloride salts in the aqueous phase 1, 16. It is recognized that alloys exposed to environments with little or no chloride may be able to tolerate higher H2S partial pressures. Appropriate testing and available test data are necessary to identify these environments. SUMMARY 1. Alloys G-3, 50 and C-276 (N06985, N06950 and N10276) have been successfully used as cold worked OCTG in sour gas wells around the world for many years. Alloy 725 (N07725) is the agehardenable alloy commonly used for subsurface and wellhead equipment for alloys G-3, 050 and C- 276 completions, and has many years of service experience. 2. C-rings of alloy N07725 exhibited excellent corrosion resistance during the six-month exposure to the NACE Level VI and VII environments. i.e., No pitting or cracking of the C-rings occurred. 3. As mentioned, the present maximum allowable hardness limit in NACE Material Requirement MR0175 for alloy N07725 is 43 HRC. The results of this study clearly show that alloy N07725 is acceptable to NACE Level VII environment at a maximum hardness of 44 HRC. 4. The age-hardened alloys ranked by corrosion resistance as follows: 725> 725HS > 718. 5. Ultimately, it is the user's responsibility to determine the acceptability of an alloy for a specific environment. A final CRA selection is made for a specific application based on test results and an economic analysis of cost effective alternatives. The manufacturers of equipment and components will also have a data bank of previous service recommendations which can be an excellent aid in establishing the candidate alloys for particular service conditions. Organizations such as the Nickel Development Institute (NiDI) provide selection guidelines for CRA s for the oil and gas industry.

REFERENCES 1. NACE Standard Test Method MR0175-2001, Sulfide Stress Cracking Resistance Metallic Materials for Oilfield Equipment. 2. E. L. Hibner, "A New Age-Hardenable Corrosion Resistant Alloy for Deep Sour Gas Well Service," CORROSION/90, paper no. 50, (Houston, TX: NACE, 1990). 3. E. L. Hibner, "Corrosion Behavior of a New Age Hardenable Alloy for Oil Field Applications," Proceeding of the First International Symposium on Environmental Effects on Advanced Materials, (Houston, TX: NACE, 1991). 4. E. L. Hibner, "A New Age Hardenable Corrosion Resistant Alloy for Deep Sour Gas well Service," Proceedings of the International Symposium on the Metallurgy and Application of Superalloys 718, 625 and Various Derivatives, pp. 895-903, (Warrendale, PA: TMS, 1991). 5. E. L. Hibner, "Corrosion Behavior of Age-Hardenable Alloy UNS N07725 for Oil Field Applications," CORROSION/91, paper no. 18, (Houston, TX: NACE, 1991). 6. E.L.Hibner and M.N.Maligas, "High Strength Weld Overlay for Oil Patch Applications," CORROSION/93, paper no. 144, (Houston, TX: NACE, 1993). 7. E. L. Hibner and R. H. Moeller, "Corrosion-Resistant Alloys UNS N09925 and N07725 for Oilfield Applications," Offshore Technology Conference, paper no. OTC 7206, (Houston, TX: OTC, 1993). 8. E.L.Hibner, "Corrosion Resistant INCONEL alloy 725 Weld Overlay," Proceedings of the International Symposium on Superalloys 718, 625, 706 and Various Derivatives, pp. 893-901, (Warrendale, PA: TMS, 1994). 9. E. L. Hibner and M. N. Maligas, "High Strength and Corrosion Resistant Alloy Weld Overlays for Oil Patch Applications," CORROSION/95, paper no. 52, (Houston, TX: NACE, 1995). 10. E. L. Hibner, H. W. Sizek and S. K. Mannan, "Elevated Temperature Tensile and Creep Rupture Properties of INCONEL alloy 725," Proceedings of the International Symposium on Superalloys 718, 625 and Various Derivatives, pp. 491-501, (Warrendale, PA: TMS, 1997). 11. E. L. Hibner and B. Puckett, " High-Strength Corrosion Resistant Nickel-Base Alloy 725HS for Severe Sour Oil and Gas Field Applications," CORROSION/2003, paper no. 03125, (Houston, TX: NACE, 2003). 12. S. K. Mannan, E. L. Hibner and B. Puckett, Physical Metallurgy of Alloys 718, 925, 725 and 725HS for Service in Aggressive Corrosive Environments, CORROSION/2003, paper no. 03126, (Houston, TX: NACE, 2003). 13. R. B. Bhavsar and E. L. Hibner, Evaluation of Testing Techniques for Selection of Corrosion Resistant Alloys for Sour Gas Service, CORROSION/96, paper no. 59, NACE International, Houston, TX, USA, 1996. 14. B. D. Craig, Selection Guidelines for Corrosion Resistant Alloys in the Oil and Gas Industry, NiDI Technical Series No. 10 073, Toronto, Ontario, Canada, July, 1995.

15. E. L. Hibner and C. S. Tassen, Corrosion Resistant OCTG s and Matching Age-Hardenable Bar Products for a Range of Sour Gas Service Conditions, CORROSION/2001, paper no. 01102, NACE International, Houston, TX, USA, 2001. 16. R. H. Moeller, et. al., Large Diameter Cold-worked C-276 for Downhole Equipment, CORROSION/91, paper no. 30, NACE International, Houston, TX, USA, 1991. FIGURE 1 Temperature vs. H 2 S Content for Use of Nickel-Base Alloys 700 600 H2S Content, MPa 500 400 300 200 3% Mo Alloys 9% Mo Alloys 100 0 100 150 200 250 300 350 400 450 500 Temperature, o C Alloy (UNS No.) 725/ 725HS (N07725) TABLE 1 LIMITING CHEMICAL COMPOSITION (WT%) Ni Cr Mo Cu Co Al Ti Fe Other 55.0 59.0 19.0 22.5 7.0 9.5 - - 0.35 max. 1.00 1.70 Balance Nb, 2.75 4.0

TABLE 2 REPRESENTATIVE MECHANICAL PROPERTIES OF NICKEL ALLOYS FOR OIL-COUNTRY APPLICATIONS* Yield Strength Tensile Strength UNS No. Material Condition ksi MPa ksi MPa % El. Hardness 718 Solution Annealed & Aged 130 896 180 1241 20 40 HRC 725 Solution Annealed & Aged 132.9 916 183.3 1264 28 36 HRC 725HS Annealed & Aged 151.3 1043 199.4 1375 25 42 HRC TABLE 3 SLOW STRAIN RATE TEST DATA FOR ALLOY 725HS, EVALUATED IN THE 100,000 PPM CHLORIDE* ENVIRONMENT AT 350 F (177 C) Test SSR# TTF (h) TTF %RA %RA %EL %EL Inert 21.4-49.6-25.6 - - Environment 22.2 1.04 47.8 0.96 27.3 1.07 No * 100,000 ppm Cl - (as NaCl) + 200 psi (1.4 MPa) H 2 S + 200 psi (1.4 MPa) CO 2, gas pressures at test temperature. Strain rate = 4 x 10-6 sec -1. SC TABLE 4 SLOW STRAIN RATE TEST DATA FOR ALLOY 725HS, EVALUATED IN THE 100,000 PPM CHLORIDE* ENVIRONMENT AT 400 F (204 C) Test SSR# TTF (h) TTF %RA %RA %EL %EL Inert 22.9-45.9-28.2 - - Environment 23.1 1.01 45.0 0.98 28.5 1.01 No * 100,000 ppm Cl - (as NaCl) + 200 psi (1.4 MPa) H 2 S + 200 psi (1.4 MPa) CO 2, gas pressures at test temperature. Strain rate = 4 x 10-6 sec -1. SC

TABLE 5 ENVIRONMENTS IN WHICH 827 MPa (120 KSI) MINIMUM YIELD STRENGTH ALLOY 725 HAS BEEN REPORTED AS ACCEPTABLE Reference # 19 19 19 20 20 20 20 Cl - (ppm) Any Any Any 100,000 250,000 250,000 151,750 ph --- --- --- 3.3 3.0 3.0 3.1 Temperature 230 190 150 220 205 175 175 ( C) H2S (MPa) 1.0 3.5 Any 1.4 4.1 8.3 2.1 CO2 (MPa) Any Any Any 1.4 4.8 4.8 4.8 So 0 0 Yes Yes 0 0 Yes TABLE6 ENVIRONMENTS IN WHICH 965 MPa (140 KSI) MINIMUM YIELD STRENGTH ALLOY 725HS HAS BEEN REPORTED AS ACCEPTABLE Reference # 1 20 Cl - (ppm) 121,400 151,750 ph -- -- Temperature ( C) 175 205 H2S (MPa) 3.5 3.5 CO2 (MPa) 3.5 3.5 S o 0 0