Modified-polyaspartic acid derivatives as effective corrosion inhibitor for C1018 steel in 3.5% NaCl saturated CO₂ brine solution

Background: Corrosion challenges in the oil transportation pipelines operations worldwide is a serious problem and can lead to accelerated corrosion rates, loss of mechanical integrity and catastrophic failure. The survey of the literature reveals that few polymers haven been investigated as sweet corrosion inhibitors when compared to other small nitrogen containing heterocyclic organic molecules. Previous studies indicate that polyaspartic acid has been used as corrosion inhibitor but are required in high concentrations to be effective. To improve the performance of polyaspartic acid and to enable it usage at low concentrations, two modified-polyaspartic acid derivatives poly(cysteaminoaspartamide (7) and poly(methionenoaspartamide (8) were successfully synthesized, characterized, and demonstrated as effective corrosion inhibitor for C1018 steel in 3.5% NaCl saturated CO₂ brine solution. Methods: Electrochemical techniques such as open circuit potentials (OCP), linear polarization measurements (LPR), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) were used to obtained kinetics and mechanistic data of the corrosion inhibition process in 3.5% NaCl saturated CO₂ brine. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images confirmed the adsorption of the molecules on the steel surface. Significant findings: Results obtained indicate that the two synthesized polymers are effective CO₂ corrosion inhibitors. The inhibition performance of 7 (92.7%) was more pronounced at low concentrations (25 ppm) than 8 (53.6%) as obtained from EIS results. The effect of longer immersion time (24 h) strengthens further the performance of both corrosion inhibitors to 99. 25% for 7 and 96.84% for 8, respectively. Surface characterization using SEM and AFM provided more evidence for the steel surface protection with the two polymers. Molecular modeling using density functional theory (DFT) and Monte Carlo simulations (MC) supported the experimental results. The implication of this study is the possibility of replacing conventional toxic small nitrogen containing heterocyclic organic molecules with non-toxic and cost-effective polymeric counterparts based on modified- polyaspartic acid as corrosion inhibitors for use in oil and gas industry.