Lignin from Coconut Pith as a Corrosion Inhibitor: Comparative Evaluation on Yield, Purity and Physicochemical Characteristics Across Different Extraction Treatments

About half of coconut pith (CP) contains a renewable bio-polymer called lignin, which has great promise as a corrosion inhibitor; nonetheless, its performance depends on the source, the extraction process, and the conditions. Therefore, this study focuses on exploring different treatment methods, including organosolv, alkaline, deep eutectic solvent (DES), and ionic liquid (IL) treatments, to extract CP lignin and investigate their yield, purity, and physicochemical properties, with the aim of enhancing lignin’s effectiveness as a corrosion inhibitor. This study revealed that CP lignin extraction by ionic liquid treatment (CPL-IT) achieved the highest lignin yield at 96.82% with purity levels of 90.95%, making it the most efficient extraction treatment for extracting lignin as a corrosion inhibitor. FTIR analysis highlighted that CPL-IT has many hydroxyl groups and aromatic skeletal vibrations, indicating the dominance of guaiacyl units. SEM analysis revealed that CPL-IT forms a smooth, uniform protective layer, providing superior corrosion resistance compared to other lignins. Furthermore, TGA results illustrated that CPL-IT demonstrates high thermal stability (664°C), ensuring durability under harsh conditions. XRD analysis revealed that CPL-AT had the lowest crystallinity index (0.6%), which means it is amorphous lignin, whereas CPL-OT is the most crystalline lignin with the highest (14.14%) crystallinity index. Molecular weight distribution analysis indicated that CPL-DT had the highest molecular weight, forming a dense protective film. However, CPL-OT exhibited the lowest polydispersity index (PDI), indicating a more uniform molecular size distribution, which optimizes its coating properties. Overall, CPL-IT offered the best balance of surface chemistry, surface morphology structure, thermal stability, crystalline structure, and molecular weight properties, making it the most effective corrosion inhibitor. These screening and characterization results have enhanced the viability of CPL-IT as a potential choice for corrosion inhibitor use in coating industries.