Abstract
Corrosion rates of mild steel were measured in oxygen-free, CO2-saturated brines as a function of NaCl concentration employing electrochemical techniques. Decreased corrosion rates were observed as salt concentration increased. However, at high salt concentration (≥20 wt% NaCl), corrosion rates were independent of the flow rate of CO2-saturated brine. To understand this phenomenon, corrosion surfaces were analyzed by scanning electron microscopy and X-ray diffraction and showed only residual iron carbide for salt concentrations of 0.5–5 wt%. However, at 20 wt% NaCl, a porous corrosion scale with embedded crystals, possibly magnetite, was observed. No iron carbonate was observed and water chemistry showed it was 10,000 times below saturation. A numerical model of corrosion in CO2–NaCl systems was able to predict the reduced corrosion rates with salt concentration increase as a consequence of reduced solubility of CO2 (“salting-out”). However, the model did not predict that corrosion rates were flow-independent at high salt concentration. These results demonstrate that flow-independent corrosion is a consequence of a diffusion barrier created by magnetite scale, present only at high salt concentrations.
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Acknowledgments
The authors thank the Fossil Energy Program of the Department of Energy (FE-10-001) for funding support. The authors would like to acknowledge Michael S. Rearick from GGRL Lab (Geology Geochemistry Research Lab), Earth and Environmental Sciences Division, Los Alamos National Laboratory for his water chemistry analysis. The manuscript was significantly improved by comments from anonymous reviewers.
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Han, J., Carey, J.W. & Zhang, J. Effect of sodium chloride on corrosion of mild steel in CO2-saturated brines. J Appl Electrochem 41, 741–749 (2011). https://doi.org/10.1007/s10800-011-0290-3
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DOI: https://doi.org/10.1007/s10800-011-0290-3