Kinetics and Mechanism of Corrosion Inhibition of Mild Steel in Hydrochloric Acid Using Alkaloid Extracts from Picralima nitida Seeds from Cross River State

The development of plant-derived corrosion inhibitors for mild steel protection in acidic pickling and descaling media addresses both the need for less toxic alternatives to synthetic inhibitors and the opportunity to valorise locally abundant Nigerian phytochemical resources. This study investigates the corrosion inhibition performance of crude alkaloid extracts from Picralima nitida seeds collected in Cross River State, South South Nigeria, for mild steel in one molar hydrochloric acid solution. Alkaloids are isolated by acid-base extraction and partially purified by column chromatography, and their identity is confirmed by Fourier transform infrared spectroscopy and mass spectrometry. Corrosion inhibition efficiency is evaluated by weight loss gravimetric method and electrochemical techniques including open circuit potential measurement, potentiodynamic polarisation (Tafel plots), electrochemical impedance spectroscopy, and linear polarisation resistance. Inhibitor concentration, temperature, and immersion time are systematically varied. Thermodynamic activation parameters including activation energy, enthalpy, entropy, and free energy of adsorption are calculated to characterise the adsorption mechanism. Adsorption behaviour is modelled using Langmuir, Freundlich, and Temkin isotherm equations. Surface morphology of inhibited and uninhibited steel specimens is examined by scanning electron microscopy and atomic force microscopy to confirm inhibitor film formation. Density functional theory calculations are performed for major alkaloid constituents to correlate electronic parameters (HOMO energy, LUMO energy, dipole moment) with inhibition efficiency. Findings contribute mechanistic knowledge applicable to the formulation of eco-friendly corrosion inhibitors for industrial use in Nigeria. Keywords: corrosion inhibition, mild steel, alkaloid extracts, Picralima nitida, electrochemical methods