Localized Corrosion Propagation of Carbon Steel in Inhibited Environment: A Potentiostatic Approach

The initiation of carbon steel localized corrosion is commonly observed in inhibited environments, both in field and laboratory settings, even though the uniform corrosion rate remains low. While many reasons have been reported to explain pit initiation, little research has considered whether these pits will propagate or not. This work focuses on evaluating the tendency of localized corrosion propagation due to galvanic coupling between the inhibited surface and active pit. Complexity in simulating this phenomenon likely influences the paucity of research addressing this mechanism. Given that its main driving force is the difference in potential between both surfaces, the potentiostatic technique is an interesting methodology to simulate this corrosion mechanism. This work was done considering a primarily imidazolinium-based corrosion inhibitor under produced water conditions (5 wt.% NaCl, pH 4.5, CO₂) at 55 and 80°C. Linear polarization resistance (LPR) and potentiodynamic polarization were used to obtain baseline results and characterize the inhibitor performance. The baseline anodic potentiodynamic sweeps indicated that, at certain critical anodic potential/current conditions, the inhibitor is fully desorbed from the surface. Several potentiostatic experiments were conducted, maintaining the electrode potential in between the open circuit potential and this critical desorption potential – this was meant to simulate, albeit very artificially, different levels of galvanic couple that could exist in case of active localized corrosion and to investigate why active corrosion could still persist inside a pit even though the corrosion inhibitor was still present in the bulk solution. It is acknowledged that this methodology makes this work quite exploratory and that the results should be viewed as preliminary. The potentiostatic experiments indicated that, at certain anodic potentials, the injection of inhibitor did not decrease the current measured to the same levels expected from the baseline potentiodynamic sweeps. Increased inhibitor dosage proved to be necessary at certain conditions to significantly decrease the current. However, at high current levels, further injections were insufficient, indicating that substrate dissolution might undermine the adsorption of the inhibitor.