Mixture of Rubia cordifolia and KI as an efficient corrosion inhibitor for steel in 3.5% NaCl: electrochemical and surface studies of steel

This study evaluates the corrosion inhibition performance of Rubia cordifolia extract on mild steel in a 3.5% NaCl solution, with and without the addition of potassium iodide (KI). The extract contains active phytoconstituents such as alizarin, mollugin, naphthoquinone, ruberythric acid, saponins, and triterpenoids, which contribute to its anticorrosive behavior through adsorption on the steel surface. Potentiodynamic polarization (PDP) results revealed that both anodic and cathodic reactions were effectively suppressed, achieving maximum inhibition efficiencies of 88% without KI and 95.2% with KI at 400 mg/L concentration. Electrochemical impedance spectroscopy (EIS) confirmed this trend, showing an increase in charge transfer resistance (Rct) from 15.78 Ω·cm² (blank) to 139.21 Ω·cm² (without KI) and 260.03 Ω·cm² (with KI), indicating the formation of a compact and stable protective film. UV–Visible analysis further validated the adsorption of phytochemical components on the steel surface, as reflected by decreased absorbance after corrosion testing. These results demonstrate that Rubia cordifolia extract acts as an efficient, eco-friendly corrosion inhibitor for steel in saline environments, and its synergistic combination with KI enhances inhibition performance through improved film stability and surface coverage.