Green corrosion inhibition of mild steel in acidic media using Wahlenbergia riparia extract: gravimetric via CCD-RSM, electrochemical and surface analyses

Corrosion represents a considerable danger to metals, leading to significant risks for human health, the environment, and economic stability. The research evaluated the extract of Wahlenbergia riparia A.DC (Royal Bluebell) (WRE) as a cost-efficient solution for mitigating corrosion in mild steel, utilizing response surface methodology (RSM), open circuit potential-time measurements (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) across various HCl concentrations. The RSM model demonstrated a high level of accuracy in predicting the corrosion inhibition efficiency (IE), weight loss (WL), and corrosion rate (CR) of mild steel. The operational parameters analysed included temperature (299–333 K), WRE concentration (0.1–0.5 g/L), immersion duration (1–6 h), and acid concentration (0.5–2.5 M), all of which significantly affected the model's predictions regarding the corrosion process. An increase in WRE concentration was associated with a decrease in WL and CR over time, while the IE showed an increase. The analyses from OCP, PDP and EIS indicated shifted less negative as the WRE concentration increases with inhibition efficiencies of 96.67 % and 93.50 % at 0.5 g/L, respectively. The PDP test revealed that WRE acts as a mixed-type inhibitor, whereas EIS suggested that charge transfer plays a crucial role in the inhibition mechanism. The Langmuir isotherm effectively illustrates the electrostatic interaction between the WRE inhibitor and the surface of mild steel. Surface morphology analysis conducted via SEM demonstrated the formation of a protective layer on the steel, and FTIR analysis identified OH, C O, and C H as the functional groups that bonded with the metal surface, providing empirical evidence for the adsorption of WRE molecules onto mild steel. RSM prediction models were utilized and validated through normal plots of residuals and comparisons between predicted and actual values, confirming their effectiveness as modelling tools. This study illustrates that WRE is a potent and economical inhibitor for mild steel, even in environments with high acid concentrations.