Biopolymer-modified layered cerium carbonate hydroxide coatings on magnesium with a surfactant interface for enhanced corrosion protection

Magnesium and its alloys are highly prone to corrosion. In this study, a multi-layered coating comprising layered cerium carbonate hydroxide CeCO 3 OH, (LH), modified with a surfactant (sodium dodecyl benzene sulfonate (SDBS)), and topped with an epoxy biopolymer, (T), was developed for the corrosion protection of magnesium. This epoxy topcoat was formulated from tannic acid to provide a sustainable, environmentally acceptable coating system. The optimal concentration of SDBS was 0.05 M, ensuring effective interactions between the LH and the epoxy topcoat. Using scanning electron microscopy, energy-dispersive X-ray analysis, and X-ray diffraction, the CeCO 3 OH layer was confirmed to contain a high concentration of Ce with high crystallinity. To evaluate the protective properties of the coatings, potentiodynamic polarisation and electrochemical impedance spectroscopy were performed in a 3.5 wt% NaCl solution across a wide pH range from 2.0 to 10.0. Excellent corrosion protection was achieved with a coating formulation combining the LH modified with a layer of 0.05 M SDBS and a final topcoat of epoxy (Mg/LH/S 0.05 /T). The corrosion current measured for the optimal sample was very low at 5.2 nA, with a corrosion potential of –1.58 V. Excellent corrosion protection was achieved from a pH of 2.0 (5.5 nA) to a pH 10.0 (2.0 nA). EIS analysis further demonstrated a substantial increase in the overall impedance compared to the unmodified coated samples. The coated sample exhibited a low capacitance of 4.5 × 10 -11 F and a polarisation resistance of 3.6 × 10 5 Ω. Good stability was evident over a 40 day period, with evidence of loss in the corrosion protection properties following 40 to 50 days of immersion.