Materials Performance

MAY 2015

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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53 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 ~C34 HRC, implying that UNS G41400 CS would be a reasonable analog to API grade mat erial . In addition , SSRTs were p er - formed in air for the CS to ensure that it met the material grade properties of cas- ing material. The ultimate tensile strength (U T S) valu e was 150.4 k si , w hile yield strength (YS) was ~140 ksi, which easily satisfies the UNS G41400 requirements (minimum UTS 135 ksi, minimum YS 125 ksi). The scale dissolver mixture was pro- vided by the vendor. Slow Strain Rate Technique The SSRT method (ASTM G129 11 ) is commonly used for SCC studies in a wide variety of environments including oil and gas production . Th ese t ests were con- ducted on SSRT tensile specimens pre- p ared from UN S G41400 C S an d UN S S41000 SS in a control solution of mineral oi l and th e scal e di ss olver mi xture at 120 °C and 1 atm. First, the test coupon was fitted to the loading cell by screwing both coupon ends to the test machine grips. The test mixture was introduced in the sealed autoclave and deaerated by vig- orous sparging of N 2 for at least 2 h. The temperature was then raised to 120 °C and kept constant. At 120 °C, the specimen was pulled to fracture at a strain rate of 4 × 10 –6 in/in/s. Control tests were done using an inert mineral oil environment. The straight scale dissolver mixture was used for the first test, with the spent solution being used for the replicate run. TABLE 1. SSRT RESULTS IN MINERAL OIL AND SCALE DISSOLVER ENVIRONMENTS FOR UNS G41400 AND UNS S41000 STEELS Material Media Time to Failure (h) Plastic Strain to Failure (%) Reduction in Area (%) Maximum Stress (psi) UTS Ratio Reduction in Area Ratio (%) Plastic Strain-to- Failure Ratio (%) Time-to- Failure Ratio (%) UNS G41400 Mineral oil 13.80 8 25 146,054 Scale dissolver 10.28 6 25 136,176 93 102 70 74 UNS S41000 Mineral oil 18.99 13 52 94,302 Scale dissolver 17.53 12 38 95,132 101 73 90 92 FIGURE 1 Cyclic voltammetry curves for UNS G41400 CS in scale dissolver solution at different scan rates. Electrochemical Testing Electrochemical experiments were con- ducted to evaluate unstable passivity or metastable pitting phenomena that could serve as precursors to cracking processes. The test environment consisted of the actual scale dissolver mixture with contin- uous N 2 purging to keep the environment oxygen-free. A 250-mL cell was used to han- dle a three-electrode setup; the vessel is equipped with a thermal jacket connected to a recirculating bath used to control the temperature. The tests were performed at a constant temperature of 120 °C, using a st and ard saturat ed calom el el e ctro d e (SCE) as a reference. To evaluate unstable passive behavior in both UNS G41400 CS and UNS S41000 SS, open circuit potential (OCP) which is in the range of the Flade potential, was recorded during a period of 16 h, followed by cyclic voltammetry within –250 to 500 mV vs. the OCP. The polarization rate in the cyclic voltammetry part was varied between 0.1 and 10 mV/s to get more information on the electrochemical interactions between scale di ssolver and steel , w hich could explain the presence of cracking suscepti-

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