Embrittlement Of High-Strength, High-Alloy Tubular Materials In Sour Environments
Authors: Kane R.D. 
Published: December 2007

The resistance of several high-strength nickel and cobalt-base alloys to embrittlement in aqueous H2S-containing environments was examined. The alloys included HASTELLOY C-276 and C-4, INCONEL 625 and 718, and MP35N. These materials were received as tubing in the cold-reduced condition and were subjected to various aging treatments ranging from 300-150 degrees F (148–816 degrees C) for times of 10 minutes to over 500 hrs. The effects of (1) cold work, (2) aging time and temperature, (3) applied stress level, (4) sample orientation, (5) galvanic interactions with steel, and (6) environment temperature and composition were determined. Aging for very short periods at high temperatures [900–1100 degrees F (483–593 degrees C)/10 minutes] or for longer times at relatively low temperatures [300–400 degrees F (148–204 degrees C)/~500 hrs] reduced the resistance of these materials to hydrogen embrittlement. This behavior was dependent on galvanic interaction with steel and sample orientation. Failures were limited to samples that were coupled to steel and stressed in the transverse direction. No failures were observed in longitudinally stressed specimens, even under severe testing conditions, or transversely stressed specimens without steel couples. The susceptibility to cracking decreased as the temperature of the test environment increased. Elevated-temperature heat treatments [1400–1500 degrees F (760–816 degrees C)] substantially improve the resistance of MP35N to hydrogen embrittlement in sour environments.

Introduction

In the presence of H2S, steels with strengths generally above about 80 ksi (559 MPa) often exhibit catastrophic, brittle failure [sulfide stress cracking (SSC) which is the result of hydrogen embrittlement. This problem has seriously limited the use of high-strength steels in sour environments. However, as the demand for gas and oil continues to increase, deeper, higher-pressure wells, which may contain H2S, must be drilled, completed, and produced. For safe and economical operations under these conditions, high-strength materials resistant to hydrogen embrittlement are needed.

As an alternative to steels and other ferrous alloys, nickel and cobalt-base alloys offer very high strength in addition to outstanding corrosion resistance. Unfortunately, much of the published work pertaining to hydrogen embrittlement of these alloys has been restricted to relatively low-strength materials. Consequently, only limited meaningful data are available concerning the effect of hydrogenating environments on the performance of higher strength alloys. One such study by Watkins and Greer dealt specifically with a new family of tubular goods produced from alloys containing large amounts of nickel, cobalt, chromium, and molybdenum. These alloys are virtually inert to corrosion and have yield strengths in the range of 150–250 ksi [1034–1724 MPa]. This new development was brought about by (1) the need to safely produce gas from deep [20,000 ft (6096 m)], high-pressure [20,000 psi (138 MPa)], high-temperature [400–500 degrees F (204–260 degrees C)] reservoirs containing high percentages of H2S and CO2 and salt water, and (2) problems associated with effective inhibition of conventional steel tubing from corrosion at these conditions.

Watkins and Greer evaluated more than 30 alloys for susceptibility to stress corrosion cracking by sulfides and chlorides, weight-loss corrosion, and pitting and crevice corrosion. They found that pitting and crevice corrosion. They found that several high-strength, nickel and cobalt-base alloys seem to have exceptional resistance to corrosion and embrittlement. These alloys included HASTELLOY C276, MP35N, and INCONEL 625. All possessed yield strengths of approximately 200 ksi and achieved a major portion of this strength through high degrees of cold working. But, a point to keep in mind is that Watkins and Greer's environmental embrittlement tests were conducted on specimens that were (1) stressed longitudinally with respect to the direction of cold working or (2) taken from cross-rolled materials.