Electrochemical Investigation of the Role of Cl- on Localized CO2 Corrosion of Mild Steel

Electrochemical investigation of localized CO₂ corrosion of X-65 mild steel in 0.1, 1.0, 10 and 20 wt% NaCl solutions at 80^oC has been conducted using linear polarization resistance (LPR) and electrochemical noise (ECN) techniques. Current noise and potential noise between two nominally identical electrodes were simultaneously recorded using a ZRA (zero resistance ammeter). Localized corrosion was provoked by a chemical removal of corrosion product layer which was accomplished by adjusting pH and the supersaturation of FeCO₃. Surface and cross-section morphologies were observed by Scanning Electron Microscope (SEM) and Infinite Focus Microscopy (IFM) for 3D optical analysis. The results showed that increasing the Cl^- concentration had only a slight effect on the general corrosion rate, and did not accelerate the initiation of localized corrosion. It was demonstrated that the use of ECN is possible for monitoring localized corrosion for carbon steel in CO₂ corrosion environments. The transients related to metastable pitting were best observed on the small coupons (1cm²).