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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59 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 x-ray spectroscopy (EDS), and the struc- tures of the surface corrosion product scales were characterized by x-ray diffrac- tion (XRD). Results and Discussion Corrosion Rates of 3Cr and L245 Steels Figure 1 shows the corrosion rates of 3Cr and L245 steels with different water cuts (10, 30, and 80%) at 60 °C: carbon diox- ide (CO 2 ) partial pressure of 1 MPa ; and flow velocity of 1 m/s. Below 30% water cut, L245 had better corrosion resistance than 3Cr steel. At 80% water cut, the corrosion rate of 3Cr steel was much lower than L245 steel. The corrosion rate of 3Cr and L245 steel remained at a relatively low value below 30% water cut and decreased slightly from 10 to 30% water cut, while the corro- sion rates conspicuously increased when the water cut reached 80%. Morphologies of Corrosion Product Scale on 3Cr Steel Figures 2 and 3 show the surface and cross-section morphologies of corrosion product scale on 3Cr steel with 10, 30, and 80% water cut. At 10% water cut, the corro- sion product scale was thin but compact, and 3Cr steel primarily suffered general corrosion. As the water cut increased, the thickness of the corrosion product scale increased and localized corrosion was initi- ated, becoming more serious at 80% water cut. EDS analysis indicated that corrosion product scale formed on the 3Cr steel sur- face at 10% water cut consisted almost entirely of iron carbonate (FeCO 3 ), with no Ca element observed. This implied oil wet- ting was the control factor, with crude oil preventing the contact of water with the steel matrix. However, the Ca element was discovered in the inner scale at 30% water cut, indicating the influence of water wet- ting had increased. The corrosion product scales formed at different water cut levels were all divided into two parts—an outer layer that mainly consisted of Fe, C, and O and an inner layer that mainly consisted of Fe, Cr, C, and O. TABLE 1. CHEMICAL COMPOSITIONS OF 3Cr AND L245 STEELS Steel C Si Mn S P Cr Ni Mo Fe 3Cr 0.07 0.2 0.55 — — 2.99 — 0.15 bal. L245 0.26 — 1.15 ≤0.003 ≤0.003 — — — bal. TABLE 2. CHEMICAL COMPOSITIONS OF THE SIMULATED SOLUTION Composition Sodium Chloride (NaCl) Calcium Chloride (CaCl 2 ) Sodium Bicarbonate (NaHCO 3 ) Content (mol/L) 0.1 0.01 0.01 TABLE 3. TEST CONDITIONS OF THE EXPERIMENT T (°C) P CO 2 (MPa) Flow Velocity (m/s) Immersion Period (h) Water Cut (%) 60 1 1 168 10, 30, 80 FIGURE 1 The corrosion rates of 3Cr SS and L245 CS with three different water cuts in oil/water-mixed emulsion. Morphologies of Corrosion Product Scale on L245 Steel Figures 4 and 5 show the surface and cross-section morphologies of corrosion product scales on L245 steel with different water cuts and the compositions of scale formed at 80% water cut. It was obvious that a dense corrosion product scale was obtained below 30% water cut. At 80% water cut, the corrosion product scale was thicker and the outer layer was porous. The corrosion product scales formed at differ-

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