Abstract
This study investigates the development of a sustainable cashew nut shell liquid (CNSL)-based polymer coating for corrosion protection in harsh marine environments. CNSL-formaldehyde resin was synthesized under different formulation conditions to optimize its physicochemical properties, followed by the incorporation of zinc chromate–red oxide pigment and cobalt naphthenate drier to enhance coating performance. The optimum resin formulation exhibited a high iodine value (199.66 mg I₂/g), indicating favorable unsaturation for effective film formation. Increasing the cobalt naphthenate content accelerated the curing process, while higher pigment loading significantly improved color uniformity and coating appearance. Corrosion testing demonstrated substantial enhancement in corrosion resistance, with the optimized coating reducing the corrosion rate in seawater to 0.0466 MPY while maintaining excellent adhesion (4B classification according to ASTM D3359–17). Overall, the optimized formulation containing 16–20 wt% pigment and 0.4–0.5 wt% cobalt naphthenate exhibited the best overall performance. These findings demonstrate that CNSL-based polymer coatings are a promising partially bio-based alternative for corrosion mitigation in aggressive marine environments, providing an effective approach to valorize cashew shell waste while reducing reliance on petroleum-derived coating materials.