Lignin-based corrosion barriers have gained interest among researchers due to the abundance, low-cost and film-forming ability of lignin. However, previous studies have used petroleum-based synthetic polymer composites, thereby introducing environmental concerns. Herein, microwave irradiation was utilized to functionalize rice husk-derived lignin with furanic groups for corrosion protection of SUS304L stainless steel. Lignin was extracted using a deep eutectic solvent (DES) choline chloride-p-toluenesulfonic acid mixture, before further modification. Results showed the successful introduction of furan moieties into the lignin structure, which is substantiated by the increase in oxygen-rich functional groups in the pyrolysis data. From the electrochemical tests in 3.5% NaCl, the functionalized lignin coatings (fRH-L) exhibited significantly lower current density of 2.7 nA·cm⁻², leading to 99% inhibition efficiency as compared to 190.2 nA·cm⁻² for bare SUS304L stainless steel. Furthermore, impedance modeling revealed enhanced dielectric properties and reduced capacitance. These results strongly suggest that the prepared functionalized DES-extracted lignin exhibits a noteworthy improvement in corrosion resistance compared to pure lignin (RH-L) and bare SUS304L stainless steel. Although degradation occurred after 24 hours, fRH-L coatings retained superior protection compared to unmodified lignin. This study highlights the potential of rice husk lignin as a sustainable, corrosion barrier through biomass valorization using DES and microwave irradiation techniques.