Materials Performance

AUG 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Need reprints of MP ads, articles, or covers? Reprints are a great investment! Profes- sionally printed reprints and photo copied reprints of all MP ads, articles, and covers are available for purchase. Reprints can be customized with your company's logo, ad- ditional product information, or the maga- zine cover—with no limits on creativity! Order your reprints today; it simply makes good business sense! For reprint informa- tion and rates, call +1 281-228-6287. 53 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 MEA reacts with CO 2 to form a mixture of carbamate, bicarbonate, and carbonate. Cracking is worsened by the presence of heat-stable acid salts, acetate, formate, and glycolate following the oxidation of MEA if the process gas contains traces of oxygen. In shale gas, oxygen can enter through the vapor recovery system. Carbonate cracking is found in refinery fluid catalytic cracking units (FCCUs) when the pH of the liquor goes above 8.0 and the carbonate concentration exceeds 100 ppm. Cracking is reported in equipment and pip- ing operating at 95 °F (35 °C) to 130 °F. External carbonate cracking of buried gas transmission pipelines is well known. Cathodic protection raises the pH of the electrolyte beneath disbonded coating on the outside of a pipeline. CO 2 in the soil from decaying organic matter readily dis- s o lv e s in th e hi g h-pH el e ctro ly t e an d forms a concentrated mixture of carbon- ate and bicarbonate in the pH range of 8 to 11. It is this solution that causes external IGSCC in the potential range of –0.5 to –0.8 V vs. SCE. Cracking is worse close to compressor stations where the tempera- ture is near 150 °F (65 °F) but takes longer at temperatures below 100 °F. The range for cracking is reported to be 50 °F (10 °C) to 140 °F (60 °C). Conclusions • Leaks in shale gas pipelines were caused by IGSCC. • The agent responsible was the spent H 2 S scavenger, which accumulated at low points in the pipeline—initially as MEA plus dithiazine and ultimately as s oli d amor phous dithi azin e plus MEA. • CO 2 from the process gas readily dis- solved in the high-pH liquid , and formed a mixture of carbonate and bicarbonate that caused IGSCC at regions of high residual stress close to girth welds. • Cracking due to the spent H 2 S scav- enger is a variant of carbonate crack- ing reported in refinery amine units and FCCUs, and external carbonate cracking in buried pipelines. Mitigation • Change the H 2 S scavenger or prevent the spent scavenger from entering the flowline by using a knockout sys- tem. • Limit the pH to <8. • Per form a PWH T on scav en g in g equipment. • Frequently pig and f lush existing pipelines. • Risk-rank lines based on severity of consequences at the location of low points, and shut in those where the risk exceeds risk tolerability criteria. • Apply fitness-for-service assessment according to API 579-1, Paragraph 9.5.2—Leak before break analysis. 8 • Implement a leak detection program. • Apply mitigation for leaks and check valves to reduce lost inventory. • The use of fracture mechanics to determine critical crack size for rup- ture and remaining life was limited because present crack size, crack growth rate, and interaction between adjacent cracks were all unknown. • Determine if the general corrosion rate will increase if dithiazine (a good corrosion inhibitor) is absent. References 1 API 5L, "Specification for Line Pipe" (Wash- ington, DC: API, 2013). 2 API 1104, "Standard for Welding Pipelines and Related Facilities" (Washington, D C: API, 2013). 3 ANSI/NACE MR0175/ISO 15156, "Petroleum and natural gas industries—Materials for use in H 2 S-containing environments in oil and gas production" (Houston, TX: NACE Inter- national, 2015). 4 J. Gutzeit, J.M. Johnson, "Stress Corrosion Cracking of Carbon Steel Welds in Amine Ser vice," CORRO SION/86, paper no. 206 (Houston, TX: NACE International, 1986). 5 P.G. Hughes, "Stress Corrosion Cracking in an MEA Unit Case Study," UK National Con- ference (1982), p. 87. 6 API 945, "Avoiding Environmental Cracking in Amine Units" (Washington, DC: API, 2003). 7 R .N. Parkins, R .R . Foster, "Line Pipe Stress Corrosion Cracking," CORROSION/86, paper no. 320 (Houston, TX: NACE, 1986). 8 API 579-1/ASME FFS-1, "Fitness-For-Service, Third Edition" (Washington, DC: API, 2016). Bibliography Bagdasarian, A.J., C.A. Shargay, and J. Coombes. Oil Gas J. 1, 13 (1992): p. 42. Beavers, J.A., N.G. Thomson, and R .N. Parkins. "Stress Corrosion Cracking of Low Strength Carbon Steels in Candidate High Level Waste Repositor y Environments." Environmental Effects Nuclear and Chemical Waste Manage- ment 5 (1985): pp. 279-296. NACE Publication 34108. "Review and Survey of Alkaline Carbonate Stress Corrosion Crack- ing in Refinery Sour Waters." Houston, TX: NACE International. Richert, J.P., A.J. Bagdasarian, and C.A. Shargay. "E xtent of Stress Corrosion Cracking in Amine Plants Revealed by Survey." Oil Gas J. 6, 5 (1989): p. 45. Rivera , M., S. Bolinger, and C. Wollenweber. "Carbonate Cracking Risk Assessment for a FCCU Gas Plant." CORROSION 2004, paper no. 04639 (Houston, TX: NACE, 2004). Treseder, R .S. "Inf luence of Yield Stress on Anodic Stress Corrosion Cracking Resistance of Weldable Carbon and Low Alloy Steel with Yield Strength Below 100 ksi." Welding Re- search Council, WRC Bulletin 243, Part 2. 1978. EDWARD HEAVER obtained a B.Sc. (Hons) degree in metallurgy from Aston University followed by four years of conducting metallurgical failure analysis of mining e q u i p m e n t f o r R C M / N C C M M i n i n g Industry Technical Services, Zambia. Joining SASOL, he obtained a Ph.D. from a study of CO-CO 2 -H 2 O SCC in tail and reformed gas circuits. After working for 15 years in the downstream petrochemical Industry, he moved to BHP Petroleum and has worked on upstream oil and gas materials performance for the past 20 years. Internal Stress Corrosion Cracking of Shale Gas Flowlines

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