CORROSION MECHANISM DISCUSSION OF X65 STEEL IN NaCl SOLUTION SATURATED WITH SUPERCRITICAL CO2
Liang WEI,Xiaolu PANG,Kewei GAO()
Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083
Cite this article:
Liang WEI, Xiaolu PANG, Kewei GAO. CORROSION MECHANISM DISCUSSION OF X65 STEEL IN NaCl SOLUTION SATURATED WITH SUPERCRITICAL CO2. Acta Metall Sin, 2015, 51(6): 701-712.
In recent years, the corrosion problem of steels under supercritical CO2/H2O system in oil/gas production has got more and more attention. The temperature and pressure of some oil wells in China usually exceed 120 ℃ and 100 MPa, where CO2 is in supercritical state. To transportation easier and cost reduction, the oil/gas in pipelines is usually pressured to a high pressure, normally causes CO2 in supercritical state. The supercritical CO2 corrosion environment includes CO2-saturated water and H2O-saturated CO2 phases. Moreover, corrosive ions such as Cl- usually exists in CO2 corrosion environment, however the influence of Cl- on corrosion of carbon steel in supercritical CO2-saturated NaCl solution and NaCl solution-saturated supercritical CO2 are investigated limited. The corrosion behaviors and corrosion rates of X65 carbon steel exposed in supercritical CO2-saturated 3.5%NaCl solution, supercritical CO2-saturated deionized water and NaCl solution-saturated supercritical CO2 systems were investigated. SEM, EDS and XRD were used to analyze the morphology and characteristic of corrosion product scale on the steel surface. The results show that the addition of Cl- in supercritical CO2-satureated water significantly increased the corrosion rate of X65 steel, and modified the FeCO3 grain morphology. The average corrosion rate of X65 steel in NaCl solution-saturated supercritical CO2 was much lower than in supercritical CO2-saturated NaCl solution, but in supercritical CO2 phase X65 steel suffered serious localized corrosion. The corrosion process of X65 steel in supercritical CO2-saturated NaCl solution could be divided into three stages: the first was the active dissolution stage, the surface of X65 steel was corroded inhomogeneous due to the competitive adsorption between Cl- and H2CO3, HCO3-, and Fe3C as well as some lumpish matrix were residued on steel surface; the second was the initiation stage of FeCO3 precipitation, Cl- postponed the precipitation of FeCO3, the FeCO3 scale formed in this period was incomplete, and increased the area of cathodic reaction subsequently the corrosion rate; the last was the protective stage of FeCO3 corrosion scale, the corrosion product scale formed in this period was denser and provided better protectiveness to X65 steel matrix, however Cl- could pass this scale and reach the scale/matrix interface, resulted in the corrosion rate of X65 steel keeping at a higher value than in deionized water environment. The corrosion model of normal pipelines was developed to better understand the corrosion mechanism in supercritical CO2-saturated Cl--containing solution.
Table 1 Corrosion rates of X65 steel immersed in deionized water and NaCl solution at 80 ℃, 9.5 and 1.0 MPa for 0.5 and 96 h
Fig.1 Microstructure of X65 steel
Fig.2 Surface (a~c) and cross-section (d) morphologies of corrosion product scale on X65 steel immersed in deionized water for 0.5 and 96 h immersion at 80 ℃, 1.0 and 9.5 MPa
(a) 1.0 MPa, 0.5 h[9] (b) 9.5 MPa, 0.5 h (c, d) 1.0 MPa, 96 h
Fig.3 Surface (a, c, e, g) and cross-section (b, d, f, h) morphologies of X65 steel immersed in NaCl solution at 80 ℃, 1.0 MPa for 0.5 h (a, b), 9.5 MPa for 0.5 h (c, d), 1.0 MPa for 96 h (e, f) and 9.5 MPa for 96 h (g, h)
Fig.4 XRD spectra of corrosion product scale formed on X65 steel for 96 h with different immersion conditions
Fig.5 Surface (a) and cross-section (b) morphologies of corrosion product scale on X65 steel in water-saturated SC CO2 phase after 96 h immersion
Fig.6 Surface morphologies of X65 steel immersed in SC CO2 phase (a) and NaCl solution (b) for 96 h after removing corrosion product scale
Fig.7 Surface morphologies of corrosion product scale on X65 steel immersed in NaCl solution saturated with SC CO2 at 9.5 MPa and 80 ℃ for 12 h (a), 24 h (b), 50 h (c) and 168 h (d)
Fig.8 Cross-section morphologies of corrosion product scale on X65 steel immersed in NaCl solution saturated with SC CO2 at 9.5 MPa and 80 ℃ for 12 h (a), 24 h (b), 50 h (c) and 168 h (d)
Fig.9 XRD spectrum of corrosion product scale on X65 steel immersed in NaCl solution for 12 h at 9.5 MPa and 80 ℃
Fig.10 Changes of elastic modulus E of corrosion product scale with immersion time
Fig.11 EDS analyses (a) of the Cl--rich area at scale/matrix interface on X65 steel after immersed in NaCl solution saturated with SC CO2 for various time and the change of Cl- content with immersion time (b)
Fig.12 Changes of roughness Ra of X65 matrix with immersion time after scale-removing (a) and the outline plots of X65 steel after 0.5 and 96 h immersions (b)
Fig.13 Changes of corrosion rate of X65 steel in NaCl solution and deionized water[9] saturated with SC CO2 for various immersion time at 9.5 MPa and 80 ℃
Fig.14 Corrosion mechanism schematic of X65 steel immersed in NaCl solution saturated with SC CO2
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