Electrochemical behavior of low carbon steel in slightly acidic brines

Low carbon steel exhibits active–passive transition in buffered slightly acidic (pH 4–6) brines at various temperatures. The pH, the temperature and the concentration of chloride ions have significant effects on the active–passive behavior and only mild effects on its anodic dissolution in the active region and on the free corrosion potential. An increase in the temperature or a decrease in the pH of the medium shifts the Flade potential by hundreds of millivolts in the noble direction and increases the passive current. The activation energy of the metal dissolution in the active region was estimated at 47.5 kJ mol−1. Electrochemical impedance measurements at potentials in the passive region reveal a significant increase in the polarization resistance and decrease in the double layer capacitance of the interface with increase of pH.