3 Table 1. Chemical composition of X-52 pipeline steel (wt. %) Elements X-52 Elements X-52 C (Carbon) 0.261 Cr (Chromium) 0.027 Mn (Manganese) 1.150 Mo (Molybdenum) 0.013 P (Phosphorus) 0.008 Al (Aluminum) 0.002 S (Sulfur) 0.019 V (Vanadium) 0.003 Si (Silicon) 0.036 Nb (Niobium) 0.003 Cu (Copper) 0.033 Ti (Titanium) 0.002 Sn (Tin) 0.002 Co (Cobalt) 0.006 Ni (Nickel) 0.032 Fe (Iron) Balance Dog-bone specimens with dimension of 36×7×3.5 mm in reduced section were machined from pipe with the length of the specimens along the transverse direction of the pipe so that the crack-advance plane (radial-axial) was the same as for NNpHSCC in the field (see Figure 1 for a schematic of the orientations). After the sample surfaces had been ground to 1200 grit in accordance with the standard metallographic procedure, they were degreased and cleaned using acetone and ethanol. Figure 1. Schematic of the sample with orientations in the pipe. 2.2. PITTING THE SURFACES After the surfaces were masked using PTFE tape leaving the gauge section exposed, the samples were put in concentrated H2SO4 solution to passivate the surfaces and then exposed to dilute HCl solution (the ratio of HCl to distilled water was 1:10) to grow pits, as reported by Fang et al. [15]. After 135 hours, the pitted samples were taken out and washed using tap water. Then they were cleaned using EDTA and then ethanol in an ultrasonic cleaner to ensure that the passive films on the surfaces had been almost removed. 2.3. SCC TESTS The pitted specimens were cyclically loaded under a balanced triangular waveform as shown in Table 2 in a soil solution (ratio of soil to water is 1:5) and a NNpH inorganic test solution identified as C2 solution, both sparged with 5% CO2 /balance N2 gas mixture. C2 solution is made with the following ingredients 0.0274 g/L MgSO4•7H2O, 0.0255 g/L CaCl2, 0.0035 g/L KCl, 0.0195 g/L NaHCO3, and 0.0606 g/L CaCO3 dissolved in high purity water. The pH after sparging was close to 6.2 for the C2 solution and near 7.0 in the soil solution.
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