Modified Epoxy Coatings for Corrosion Inhibition in Reinforcing Bars in Concrete

Abstract

Corrosion of reinforced concrete structures is one of the biggest problems faced by the construction industry with billions of dollars spent annually on corrosion control and mitigation. Epoxy Coated Rebars (ECRs) have been reported to effectively mitigate corrosion in concrete structures. However, their utilization is constrained by the presence of micro-pores and the inherent brittleness of epoxy coatings. This study delves into the mechanisms of corrosion inhibition through the application of nano-modified epoxy-based coatings. Three distinct formulations of modified epoxy-based coatings were precisely evaluated: nano-clay-modified epoxy coatings with polyaniline, graphene-derivatives modified epoxy coatings with carbon nanotubes and self-healing tung-oil microcapsules-based nano-modified epoxy coatings for plain bars directly and when they were further embedded in concrete and exposed to accelerated chloride corrosion. A multifaceted evaluation approach of Non-Destructive Testing (NDT) and Destructive testing techniques were employed to ascertain coating corrosion efficacy comprehensively. NDT monitoring comprises of visual inspection, corrosion current measurements and ultrasonic guided waves monitoring techniques whereas destructive testing techniques involved evaluation of loss in mass, tensile strength and pull-out strength vis-à-vis corrosion exposure. Nano-clay modified epoxy coatings examines the efficacy of nano-modified epoxy coatings using four different silane-treated nano-clays (Montmorillonite-MMT, Organic Montmorillonite-OMMT (Cloisite®15A and Cloisite®30B) and Halloysite-HNT) in a combination with polyaniline (PANI). PANI being good pigment material for anti-corrosion coatings is expected to provide enhanced corrosion resistance due to its unique ability to intercept electrons from the metal surface and transfer them to outside the coating. It was confirmed that nano-clay modified epoxy coatings showed superior performance in comparison with pure epoxy coatings, as a result of enhanced barrier properties by nano-clay and synergetic effect of polyaniline fragments closely stacked with nano-sized nano-clay platelets throughout the coating. Delay in corrosion initiation in pure epoxy is 7-8 days in comparison to 70-75 days for PANI with Cloisite®30B nano-clay. Further, ultrasonic guided waves transmitted signals dropped to zero for PE samples in 40 days whereas in PANI with Cloisite®30B after 90 days of exposure ultrasonic signals fall by only 10%. Notably, formulations featuring Cloisite®30B nano-clay with PANI showcased superior barrier properties, with the intercalation of nano-clay particles facilitating tortuous diffusion pathways, thereby impeding aggressive ion ingress and corrosion propagation. Additionally, hydroxyl group of Cloisite®30B promotes interfacial interaction between nano-clay and polymer matrix resulting in impermeable network. Characterization results of silane-treated clays demonstrated the successful grafting of the silane agent on the nano-clay structure, while FE-SEM of different coatings displays the dispersion of nano-fillers inside the coating matrix. Incorporation of graphene-derivatives in the form of graphene oxide (GO) and reduced graphene oxide (rGO) with carbon nanotubes (CNT) and silane agents was studied further for improved ductility and corrosion inhibition properties. Hybrid mixture of CNTs and rGO/GO enhances the mechanical strength of the coating matrix whereas silane agents improve dispersion and bonding characteristics of nano-fillers. The corrosion initiates earlier in rGO/CNT coatings, occurring within 50 days, while no corrosion was observed in GO/CNT coatings even after 150 days of exposure to corrosion. Additionally, in ultrasonic guided wave monitoring, transmitted signals dropped to zero for rGO/CNT samples in 125 days whereas in GO/CNT coatings after 150 days of exposure ultrasonic signals are completely healthy. GO exhibited superior corrosion inhibition performance over rGO, due to its successful blocking of aggressive larger chloride ions, by providing a dense impenetrable network due to the presence of functional groups on basal planes of GO sheets. Further the nano-clay and graphene derivatives-based coatings were further examined with self-healing tung-oil microcapsules to develop smart self-healing coatings. In smart nano-clay modified epoxy coating studies, Cloisite®15A nano-clay were incorporated with tung-oil microcapsules in epoxy coatings (MNC) and compared with pure epoxy. These coatings were pre-damaged to investigate the self-healing capabilities of coatings. Corrosion initiates in 15–17 days for NC coatings (only nano-clay) and 8–10 days for MC coatings (only tung-oil microencapsulated epoxy coating), whereas it initiates in 30–35 days with MNC coatings. Synergetic effect of nano-filler with self-healing microcapsules enhanced the overall performance of epoxy coatings. Smart graphene-derivatives (GO and rGO) modified epoxy coating was also studied with tung-oil microcapsules (MrGO and MGO). A dual layer coating combination (M+GO) with additional dual coating of MC coating as top layer and GO/CNT coating as base layer was also investigated. All three coatings exhibit encouraging outcomes after 160 days of accelerated corrosion exposure; however, in the case of the M+GO coatings, there is no corrosion initiation at all after 160 days. Smart coatings formulations exhibited pronounced self-healing attributes, facilitated by tung-oil polymerization triggered upon by coating damage or corrosion initiation. By using tung-oil in nano-modified epoxy coatings, significant reductions in mass loss alongside, noteworthy enhancements in residual tensile strength vis-à-vis pure epoxy coatings, with negligible compromise on bond strength was observed. In essence, this research effort brings out the pivotal role of advanced nano-modified epoxy coatings in mitigating corrosion-induced structural degradation, thereby bolstering infrastructure resilience and longevity. The understanding obtained from this research offers invaluable insights into the development of robust, cost-effective, and environmentally sustainable corrosion mitigation strategies, vital for fostering resilient infrastructure in the face of evolving challenges.