Effect of sodium chloride on corrosion of mild steel in CO2-saturated brines

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.