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

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 (