Materials Performance

DEC 2018

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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51 MATERIALS PERFORMANCE: VOL. 57, NO. 12 DECEMBER 2018 of 1 in (25.4 mm) and the surface was grounded to a 600-grit silicon carbide (SiC) finish. The sample surface was cleaned using detergent and warm water, followed by ultrasonic cleaning in acetone, after which it was dried thoroughly. In this work, typical sour environments of oil pipeline conditions were used to simulate actual localized UDC activities. Table 1 shows water geochemical analysis for extracted water from an oil pipeline and field gas com- positions; the pH of the water was 5.85. Two types of deposits were used in this study— a collected field sludge deposit and a cleaned sand deposit. The field sludge deposit was collected from an oil pipeline and was used "as is." The sand deposit was filtered and cleaned three times with distilled water to remove impurities and then dried using an oven at 100 °C for 24 h. The results were compared with no-deposit test results. Corrosion Testing The coupled multi-electrode array sys- tem (CMAS) was used to measure localized corrosion under field sludge deposit for CS. The CMAS probe used in this work con- sisted of 24 CS rods with a diameter of 1.5 mm and an exposed area of each electrode of 0.018 cm 2 . A photograph of the test setup is shown in Figure 1. An internally coated stainless steel autoclave vessel was used. Two CMAS probes were installed in the top and bottom of the vessel . The top was immersed in the brine and the bottom one was inserted underneath the field sludge deposit to measure localized corrosion. Then, 200 mL of the brine were transferred into the vessel and purged with N 2 gas for 30 min to get rid of oxygen gas. After that, the deposit was added at a thickness of ~6 mm. The vessel was heated up to a temperature of 48 °C. Blended gas was prepared in a cylinder to match exactly the partial pressure of field H 2 S and CO 2 gases. After the deaerating process, the blended gas was purged and maintained through the experi- ments by bubbling the appropriate gas rate through the vessel. Three coupon discs were also inserted underneath deposits for each test to mea- sure the corrosion rate and to evaluate local- ized corrosion using visual inspection. Expo- sure time for the experiments was 100 h. Results and Discussion The average general corrosion rate of 24 electrodes was also measured using the CMAS technique (Figure 2). The results showed that the field deposit increases the corrosion rate to 27 mpy, which is the maxi- mum rate measured at 18 h of immersion time. Then, the corrosion rate decreased to stabilize around 5 mpy at 120 h of immer- sion time. For the no-deposit and sand deposit tests, the corrosion rate was <2 mpy. These results agree with those measured by weight-loss coupons (Figure 3). It was also found that sand inhibits corrosion and no UDC was observed on the coupon or elec- trode surface compared to other tests. A maximum localized corrosion rate and maximum localized depth are two of the few simple parameters usually mea- sured on the 24 electrodes using the CMAS instrument. The maximum localized corro- sion rate is derived from the electrode that h a s th e m a ximum an o di c cur rent by assuming the corrosion on this electrode is uniform. Figure 4 shows the average maxi- mum localized corrosion rates measured against immersion time for the no-deposit test, the sand deposit test, and field sludge deposit t ests. Th e lo cali zed corrosion depth of the i th electrode is derived from the c u m u l a t i v e a n o d i c c h a rg e , w h i c h i s obtained by integrating the corrosion cur- rent (anodic current only ; cathodic current TABLE 1. COMPOSITION OF FORMATION WATER AND PIPELINE OPERATION CONDITIONS Formation Water Pressure (psia) Gas %Mole Temperature (°C) Element mg/L 84.7 H 2 S 0.9% (PH 2 S = 0.85 psia) 48 Na 70,991 Ca 19,080 Mg 2,561 Cl 150,165 CO 2 0.39% (PCO 2 = 0.33 psia) SO 4 588 HCO 3 186 TDS 243,571 pH 5.85 FIGURE 1 Experimental setup for CMAS probes; top probe was immersed in liquid and the bottom probe was inserted under deposit.

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