The effect of Ca2+ and Mg2+ ions on the performance of corrosion product scale of NC-55E steel for CO2 corrosion
Ma Li (Materials Science)
Directed by Prefessor Zheng Yugui
Abstact
In this dissertation, the polarization resistance technique, electrochemical impedance spectroscopy (EIS) and SEM observation were used to investigate the effect of calcium and magnesium ions on the formation process and the protective performance of the corrosion product scale of NC-55E steel for CO2 corrosion at 80℃. The erosion corrosion resistance of the above mentioned corrosion product scale formed under static conditions was also investigated by modified rotating disk rig at 3m/s and 5m/s.
1. The effect of Ca2+ ions on the protective performance of the corrosion product scale
The test results in 0.6%、1.5%、2%、3%、4% CaCl2 five solutions showed that under static condition the maximum value of Rp and Rf obtained from EIS increased with the increasing of Ca2+ ion concentration. The corrosion product scale formed in 4%CaCl2 solution had better protective ability. The surface morphologies and cross-section morphologies showed that the corrosion product scale after immersion for about 40h presented better protective ability. But, the corrosion scale was degraded due to pitting corrosion after immersion for 61h. In order to simulate the possible oil field medium containing Ca2+ ions and differentiate the effects of Ca2+ ion and Cl- ion on pitting corrosion, 3%NaCl + 1.5%CaCl2 solution as well as 4.6% NaCl solution, which had the same Cl- concentration as 3%NaCl + 1.5%CaCl2 solution, were chosen to compare with 3%NaCl solution. The results showed that the adding of Ca2+ ion decreased the corrosion rate and prolonged the incubation time of pitting corrosion. Conversely, Cl- ion severely increased the corrosion rate and shortened the incubation time of pitting corrosion.
2. The effect of Mg2+ ions on the protective performance of the corrosion product scale
The test results in 0.6%、1.5%、2%、3%、4% MgCl2 five solutions showed that under static condition the maximum value of Rp and Rf obtained from EIS increased with the increasing of Mg2+ ion concentration. The corrosion product scale formed in 4%MgCl2 solution had better protective ability. The surface morphologies and cross-section morphologies showed that the corrosion product scale after immersion for about 48h presented better protective ability. But, the corrosion scale was degraded due to pitting corrosion after immersion for 62h. In order to simulate the possible oil field medium containing Mg2+ ions and differentiate the effects of Mg2+ ion and Cl- ion on pitting corrosion, 3%NaCl +1.5%MgCl2 solution as well as 4.8% NaCl solution, which had the same Cl- concentration as 3%NaCl + 1.5%MgCl2 solution, were chosen to compare with 3%NaCl solution. The results showed that the adding of Mg2+ ion also decreased the corrosion rate and prolonged the incubation time of pitting corrosion.
3. Erosion corrosion resistance of the corrosion product scale formed under static condition
In order to investigate the erosion corrosion resistance of the corrosion product scale formed under static condition, the samples were firstly immersed in the 4%CaCl2 and 4%MgCl2 for 40h and 48h respectively to form the corrosion product scale, and then eroded at flow velocity of 3m/s and 5m/s in 3%NaCl. The results showed that the Rp and Rf of samples with pre-formed corrosion product scale were higher than the samples without the precorrosion which indicated that the pre-formed corrosion product scales did provide some protection to the steel. But, the Rp of the samples with pre-formed corrosion product scale in 4%CaCl2 deteriorated after erosion corrosion tests for 7h and 6h respectively at 3m/s and 5m/s. Similarly, the Rp of the samples with pre-formed corrosion product scale in 4%MgCl2 deteriorated after erosion corrosion tests for 6h and 4h respectively at 3m/s and 5m/s. The higher the flow velocity was, the shorter the time to deteriorate for the scales. Comparatively, the CaCO3 scale provided a little bit better protection to the steel than the MgCO3 scale under dynamic condition.
Key Words: CO2 corrosion, the concentration of Ca2+ ion, the concentration of Mg2+ ion, corrosion product scale, polarization resistance, electrochemical impedance spectroscopy, pitting corrosion, precorrosion, erosion.
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