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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54 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 CHEMICAL TREATMENT bility. If unstable passivity is found within ±50 mV of the OCP, susceptibility is esti- mat ed from th e magnitude of current within the passive region and peak at Flade potential. To validate the presence of pit- ting associated with the likelihood of crack- ing, a staircase potentiostatic test was also performed at OCP +5 mV and OCP +50 mV, with the potential being kept constant for 2 h in each step. To verify the presence of a passive layer on the steel surface under the conditions tested, EIS was performed using the same conditions as those used in cyclic polariza- tion, with a potential range of ±10 mV from the recorded OCP and frequency between 10 KHz and 0.1 mHz. Weight Loss Testing To better simulate the field conditions related to a gas lift system, seven-day corro- TABLE 2. pH, CORROSION RATES FROM WEIGHT LOSS TESTING IN GLASS CELL REACTORS Cell Solution Coupon ID GCR (mpy) LCR (mpy) No. Pits Depth (mils) pH (Before) pH (After) A 50:50 scale dissolver: seawater CS 4 630.40 2,271.86 128 43.6 11.00 9.03 CS 3 635.70 B 50:50 scale dissolver: seawater SS 4 111.29 0 0 0 11.00 8.62 SS 3 112.07 C 100% scale dissolver CS 2 1464.85 5,099.57 288 97.8 11.60 9.87 CS 1 1457.42 D 100% scale dissolver SS 2 232.55 0 0 0 11.60 9.27 SS 1 225.92 FIGURE 2 Cyclic voltammetry curves for UNS S41000 SS in scale dissolver solution at different scan rates. sion testing was done in modified glass cell reactors. Assuming a total pressure of 2,000 psi (~14 MPa) and 3 to 4% carbon dioxide (CO 2 ) in the gas, a partial pressure of 75 psi CO 2 (517 KPa) was used at a temperature of 120 °C. Rectangular weight loss coupons for both UNS G41400 CS and UNS S41000 SS were prepared and the surfaces were pol- ished to 600 grit. Glass sample hangers were placed at the cell bottom. Two coupons were installed on each hanger with ~500 mL liquid added to completely immerse the coupons. Two types of test solutions were used: 100% scale dissolver and 50% scale dissolver with 50% simulated seawater. The vessels were pressurized to 61 psi CO 2 (420 kPa) at room temperature, and then the solution was heated to 120 °C to achieve the target pressure of 75 psi (517 kPa). Solution pH was measured before and after testing (after degassing and cooling down). Coupon weight was also measured before and after testing (after cleaning them in inhibited acid), followed by pitting assessment using white light profilometry. Results and Discussion Effect of Scale Dissolver on Cracking of Casing and Tubing Materials Table 1 shows the results of replicate SSRTs. Ratios were obtained by dividing the parameters determined in scale dissolver testing with the values from the mineral oil tests. These mechanical properties were obtain ed from stress-strain plots (not shown here). The UNS S41000 tests dis- played higher values for three of the four calculated ratios, compared to values for UNS G41400 CS. All ratios were >70% for both steels. This implies some reduction in properties for tests in scale dissolver vs. mineral oil, but the change was not drastic. Fractography of the SSRT samples sub- stantiated these findings. UNS G41400 CS displayed more ductility and strength than UNS S41000 SS, but no indications of SCC or immediate susceptibility to cracking were observed in either case. Also, no evi- dence of secondar y cracking was found along the gauge length of either material. Electrochemical Testing using Casing and Tubing Materials Figure 1 shows the resulting cyclic voltammetry curves for UNS G41400 CS. Th e current resp onse at th e dif ferent potential scan rates applied indicates a c l e a r, sh a r p a c t iv e - p a ssiv e tra n sit i o n observed at –1,200 mV vs. SCE. However, since this potential range is above the OCP

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