Performance benchmarking of nano-clay and carbon-based nanofillers in epoxy coatings for corrosion mitigation

Abstract

Steel corrosion is commonly controlled using epoxy coatings, but their performance is often limited by microdefects that allow the ingress of corrosive ions. In this work, the epoxy matrix was modified using two categories of nanofillers to overcome these issues. The first category included nano-clay such as montmorillonite (MMT) and halloysite nanotubes (HNT), while the second category consisted of carbon-based fillers—carbon nanotubes (CNTs), graphene oxide (GO), and reduced graphene oxide (rGO). Nano-clays were used at 2 wt%, whereas the carbon-based fillers were added at 0.4 wt%. To enable a performance comparison between the two filler categories, all coated steel samples were exposed to accelerated corrosion conditions. Their performance was monitored using ultrasonic guided-wave tests, impressed-current accelerated corrosion tests, surface visual inspection, and destructive techniques, including mass loss and tensile testing. Quantitative results indicated that all nanofillers enhanced the barrier properties of the modified epoxy compared with the neat epoxy matrix. Coatings containing MMT and HNT sustained in a corrosive environment for only 70–80 days, whereas CNT- and rGO-filled systems retained some signal strength and showed less corrosion even after 90 days. The GO-based coatings exhibited the best performance, retaining high ultrasonic signal amplitude (80%), very low impressed current (<0.05 A), minimal mass loss (6–7%), and a moderate reduction in tensile strength (∼35%). Durability differences arise from filler geometry and surface chemistry. The diffusion barriers provided by plate-like and tubular nano-clay structures are time-limited. However, carbon-based fillers deliver extended protection at lower loadings, and GO traps corrosive ions through its oxygenated groups. The study demonstrated that carbon-based fillers provided better protection than nano-clays even at lower loadings. This study evaluates nano-clay and carbon-based fillers at literature-reported optimal loadings under accelerated corrosion conditions. The comparison reflects practically optimized formulation levels rather than equal mass-fraction normalization to guide the development of high-performance, sustainable epoxy coatings.