Cashew nut shell liquid – based green polymer coatings for corrosion mitigation in harsh marine conditions

This study explores the development of sustainable cashew nut shell liquid (CNSL)– based anticorrosion coatings for marine applications. The CNSL-formaldehyde resin was synthesized at varied molar ratios (1:1.2:0.14 to 1:2.0:0.14), resulting in increased density (1.0053–1.0498 g/mL), viscosity (449.38–518.83 cP), acid value (88.29–115.92 mg KOH/g), and saponification value (97.25–154.76 mg KOH/g). The optimum iodine value (199.66 mg I2/g) was obtained at a CSNL-formaldehyde: NH4OH ratio of 1:1.6:0.14, providing favorable unsaturation for coating performance. Subsequently, coatings were formulated with zinc chromate–red oxide (4–20 wt%) and cobalt naphthenate (0.1–0.5 wt%). As expected, increasing cobalt naphthenate reduced drying times from 4.25 to 3.08 h (soft-to-touch), 7.00 to 5.92 h (dry-to-touch), and 24.67 to 21.92 h (dry-hard). Color analysis revealed improved uniformity, with ΔE* decreasing from 53.02 to 3.42 at 20 wt% pigment, approaching the RAL 3020 standard. Corrosion testing (ASTM G31) demonstrated reduced corrosion rates in seawater from 0.8270 to 0.4892 MPY (cobalt naphthenate) and from 0.9784 to 0.0466 MPY (pigment). In river water, corrosion rates decreased to 0.1747 and 0.1631 MPY, respectively. Adhesion tests (ASTM D3359-17) exhibited strong performance (4B classification, <5% flaking). These results prove that CNSL-based coatings provide an effective, eco-friendly alternative with optimal performance at 16–20 wt% pigment and 0.4–0.5 wt% drier for marine corrosion protection.