Graphene-Enhanced Epoxy Coatings Boost Magnesium Alloy Protection
Magnesium alloys, prized for their low density and high strength-to-weight ratio, have become integral in aerospace, automotive, and other lightweight engineering applications. However, their high chemical reactivity, reflected in a standard electrode potential of ?2.37 V, makes them particularly susceptible to corrosion. The naturally formed MgO/Mg(OH)? layer is porous and loosely adherent, offering limited protection against aggressive environments, especially chloride-rich solutions.
One of the most cost-effective industrial approaches to mitigating this vulnerability is the application of epoxy resin coatings. Oily epoxy systems, in particular, offer strong water resistance, excellent film-forming capability, and robust adhesion. Yet, during curing, defects such as pores and cracks can develop due to bubble rupture and incomplete cross-linking, compromising their protective function. To address these shortcomings, researchers have explored the incorporation of corrosion-inhibiting fillers, with graphene emerging as a standout candidate due to its impermeability to oxygen and water, high chemical stability, and exceptional mechanical properties.
In a recent study, oily bisphenol A epoxy resin was modified with varying amounts of graphene—0 wt%, 0.1 wt%, 0.3 wt%, and 0.6 wt%—and applied to AZ31B magnesium alloy substrates. The graphene, sourced from Changzhou Sixth Element Material Technology Co., Ltd., exhibited predominantly single-layer and few-layer flake morphologies under SEM observation, with lateral dimensions below 20 ?m and minimal surface defects. This morphology is advantageous for dispersion within the resin matrix.
Preparation involved high-speed mechanical stirring of graphene with epoxy paint, followed by mixing with a phenolic amine curing agent at a 3:1 ratio. Coatings were applied via brush and cured at ambient temperature for 14 days, yielding films of 600 ± 20 ?m thickness.
FTIR analysis confirmed the presence of characteristic epoxy resin peaks—methyl and methylene vibrations near 2,900 cm?¹ and 2,800 cm?¹, epoxy group absorption around 1,010 cm?¹, and benzene ring C=C stretching near 1,250 cm?¹—alongside peaks from the phenalkamine curing agent. Graphene spectra displayed peaks at 1,523 cm?¹ (C–O bond vibration in epoxy groups), 1,678 cm?¹ (C=C in aromatic rings), and a broad O–H stretch at 3,506 cm?¹.
Surface morphology of the coatings revealed a clear trend: the unmodified epoxy (0 wt% graphene) exhibited numerous surface defects and high roughness. At 0.1 wt% graphene, defects and roughness decreased markedly as the two-dimensional flakes filled voids formed during curing. With 0.3 wt% graphene, the surface appeared smooth and defect-free, indicating optimal dispersion and coverage. At 0.6 wt%, however, ?–? interactions between graphene sheets led to agglomeration, slightly increasing surface roughness despite maintaining structural integrity.
Electrochemical testing in 3.5 wt% NaCl solution, using a three-electrode setup, showed dramatic improvements in corrosion resistance with graphene incorporation. The bare AZ31B alloy exhibited a corrosion current density of 6.20 × 10?? A/cm². The unmodified epoxy reduced this value, but the 0.6 wt% graphene-modified coating achieved the lowest current density at 6.96 × 10?¹² A/cm²—five orders of magnitude lower than the substrate. This reduction reflects graphene’s role in sealing microdefects, increasing coating density, and enhancing hydrophobicity, thereby extending the diffusion path for corrosive species.
Post-corrosion morphology analysis supported these findings. The unmodified coating showed significant pitting and hole formation, while coatings with 0.1 wt% graphene displayed fewer defects. At 0.3 wt% and 0.6 wt%, surfaces remained largely intact after electrochemical exposure, with the highest graphene content offering the most complete protection.
Mechanistically, low graphene content increases the tortuosity of diffusion paths for water, oxygen, and chloride ions, slowing their ingress. Higher contents (0.3–0.6 wt%) reduce surface porosity and further boost hydrophobicity, making it difficult for corrosive media to reach the coating–substrate interface. Even with some agglomeration at 0.6 wt%, the shielding effect remains strong, preserving the coating’s integrity.
These results demonstrate that graphene-modified oily epoxy coatings, applied via straightforward brush techniques, can significantly enhance the corrosion resistance of magnesium alloys. The optimal balance between dispersion and filler content is critical, with 0.6 wt% graphene delivering the lowest corrosion current density and the most robust barrier performance in chloride-rich environments.
