Henna Extract in Styrene-Acrylic water-based Paint of Mild Steel as an Eco-Friendly Corrosion and Microbial Inhibitor

Corrosion of mild steel in extreme environments is a major challenge, requiring sustainable protective coatings. This study investigates the potential of Lawsonia inermis L. (henna) extract as a sustainable, bio-based additive to enhance corrosion resistance and antimicrobial properties in styrene-acrylic water-based paint. Fourier Transform Infrared (FTIR) spectroscopy identified key functional groups responsible for bioactivity. A broad peak at 3360.07 cm⁻¹ indicated hydroxyl (–OH) groups, linked to phenolic compounds with antimicrobial properties. A peak at 2926.13 cm⁻¹ represented C–H stretching from aliphatic structures, while the peak at 1632.07 cm⁻¹ confirmed the presence of carbonyl (C=O) groups, associated with anticorrosive activity. Aromatic C=C stretching at 1513.21 cm⁻¹, along with peaks for C–O and C–N stretching, confirmed the presence of esters, ethers, and amine groups contributing to bioactivity. Elemental analysis (EDAX) revealed a high carbon (42.65%–43.99%) and oxygen (42.32%–54.31%) content, indicating the presence of elements of bioactive compounds like phenolics, flavonoids, and organic acids. Significant calcium content (up to 35.81%) supports strong adhesion properties, while trace chlorine and iron indicate antimicrobial and anticorrosive potential. Elemental mapping confirmed uniform distribution of key elements, enhancing bioactivity and surface protection. SEM analysis revealed rough, porous morphology, promoting better adhesion and forming a physical barrier to corrosive agents. Antimicrobial testing showed significant activity, with inhibition zones ranging from 9.3 to 15.5 mm against bacteria such as Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, and 11.4 to 13.1 mm against fungi Aspergillus fumigatus and Cryptococcus neoformans. Corrosion tests using immersion in 35 ppt artificial seawater salt solution for 28 days indicated that a 1.5% henna extract formulation (TP3) improved corrosion resistance, reducing pit formation and tarnishing. TP3 also maintained optimal viscosity (4400 cP), strong adhesion (5B), and high hardness (H), demonstrating resistance to acid, alkali, and water. These findings confirm L. inermis extract as an effective, eco-friendly additive for antimicrobial and anticorrosive paint applications, with potential for industrial scalability and enhanced durability in extreme environments.