Selection of Electrode Area for Electrochemical Noise Measurements for CO2 Corrosion in Different NaCl Solutions

Abstract

Electrochemical investigation of localized CO2 corrosion of X-65 mild steel in 0.1, 1.0, 10 and 20 wt% NaCl solutions at 80oC has been conducted using linear polarization resistance (LPR) and electrochemical noise (EN) techniques. EN measurements were carried out only on 11.6 cm2 electrodes in 0.1, 10 and 20 wt % NaCl solution. EN was obtained from both 11.6 cm2 and 1 cm2 electrodes in 1 wt % NaCl solution to learn how the surface area affects EN measurements. Current noise and potential noise between two nominally identical electrodes were simultaneously recorded using a ZRA (Zero Resistance Ammeter). Chemical dissolution of corrosion product layer was accomplished by adjusting pH and saturation values with respect to FeCO3. Surface morphologies and pit depths were observed by scanning electron microscopy (SEM) and infinite focus microscopy (IFM) for 3D optical analysis. The results showed that electrode size influenced the EN signal. Transients related to metastable pitting were best observed with 1 cm2 coupons in 1 wt % NaCl solutions, but such transients were not clearly obtained for 11.6 cm2 coupons in the 0.1, 1, 10, 20 wt % NaCl solution. Both general corrosion rate and localized corrosion rate did not significantly change with increasing NaCl concentrations.