Sustainable Corrosion Protection in Concrete Using Henna Coatings: An Environmentally Friendly Alternative to Zinc Coatings

Corrosion of reinforcement bars in concrete compromises structural integrity and increases maintenance costs. This study investigates the effectiveness of organic (henna powder) and inorganic (zinc powder) corrosion inhibitor coatings in enhancing both corrosion resistance and bond strength retention of coated reinforcement bars embedded in concrete. The significance of the study lies in its approach to measuring the bond strength retention of coated reinforcement bars in a chloride-induced environment. To evaluate the corrosion mitigation and bond strength retention of the coated bars, cylindrical specimens of M20 grade of concrete were cast, having embedded coated and non-coated reinforcement bars having varying coating layers. Accelerated corrosion tests using a 3.5% NaCl solution were applied to cast specimens to simulate aggressive environmental conditions. Bond strength retention was assessed through pull-out tests in accordance with IS 2770: Part 1 (1967). Results showed that reinforcement bars with four coats of henna delayed corrosion initiation by up to 14,525 minutes (~10 days), compared to 6,132 minutes (~5 days) for uncoated bars, representing a 137% improvement in corrosion resistance at 20% corrosion levels. Zinc coatings improved corrosion resistance by up to 65% with four coats at 20% corrosion levels. In bond strength tests, uncoated samples exhibited a 42% reduction in bond strength at 20% corrosion, while henna-coated samples retained up to 90% of their original bond strength, significantly outperforming zinc-coated samples, which retained approximately 84%. The superior performance of henna coatings is attributed to the formation of a protective passive layer containing organic tannins and polyphenolic compounds such as lawsone. Unlike conventional admixture-based or epoxy-based corrosion inhibitors, which are either dispersed within the concrete matrix or applied externally to hardened surfaces, this study pioneers the direct application of henna as a coating on reinforcement bars-targeting corrosion mitigation precisely at the steel–concrete interface without compromising bond strength. These findings highlight the potential of organic inhibitors as cost-effective solutions and present a viable alternative to traditional epoxy-coating-based prevention methods for mitigating reinforcement corrosion while preserving bond strength, offering a promising approach for enhancing the durability of reinforced concrete structures. The study promotes the use of environmentally safe inhibitors to reduce the ecological footprint of reinforced concrete structures and supports the transition toward green and sustainable construction practices.