Anti-corrosion studies on cardanol epoxy coatings cured with redox-active aromatic trimer and tetramer oligoanilines

Abstract

In this article, structurally feasible aniline oligomers such as amine-capped trimer (DCTA) and tetramer (DCTAni) were synthesized and used as curing agents for coatings derived from epoxidized cardanol (ECD) and DGEBA (diglycidyl ether of bisphenol A). These two epoxies were optimized to an 80:20 (ECD: DGEBA) ratio to ensure that the coating system has a higher ECD content. The coatings (DCTA 20, DCTA 10, DCTA 5, DCTAni 20, DCTAni 10, and DCTAni 5) were formulated with different ratios of DCTA, and DCTAni with 20, 10, and 5 wt% ratios with respect to BDA (1,4-butane diamine). To evaluate the corrosion resistance of these coatings, steel panels were coated and exposed to a standard 3.5 % NaCl saline solution. Tafel polarization studies revealed that 20 wt% of DCTA/DCTAni provided excellent anti-corrosion properties, with a corrosion rate (C_R) of 3.162 × 10⁻⁶ and substantial improvement in polarization resistance (Rp) up to 128.32 MΩ. Furthermore, EIS analysis demonstrated that the coatings exhibited a maximum charge transfer resistance (R_ct) of 72 MΩ, and 94 MΩ for DCTA-20, and DCTAni -20, respectively. The coatings sustained for 500 h of salt-spray test demonstrated the high performance of these coatings. Mechanical strength and storage modulus were assessed using UTM and DMTA, revealing significantly improved results. Thermal analysis showed improved thermal stability of the epoxy coatings when the BDA percentage was reduced with DCTA/DCTAni and improved glass transition (T_g) temperature. Epoxy coatings on MS (mild steel) panels exhibited excellent abrasion resistance and adhesion to the substrate.