Corrosion barrier and flame retardancy properties of epoxy coating modified with nanofillers

Abstract

Epoxy resin is widely used as coatings for corrosion protection of metals due to many advantages including good corrosion resistance, high mechanical strength and low shrinkage. Nevertheless, one of the major drawbacks of epoxy is it flammability which restricts the use in certain applications. Nanofillers has received great attention to increase the anticorrosive properties of the epoxy coating due to the ability to enhance the barrier properties. This study focused on the use of several types of nanofillers; graphene nanoplatelets (GNP), graphene oxide (GO), halloysite (HNT) and montmorillonite (MMT) in epoxy nanocomposite coating of mild steel plate. The effect of nanofillers content on corrosion resistance, flame retardancy, adhesion strength and thermal properties of the epoxy coating were studied. The effects of hybrid nanofillers on anticorrosion performance and flame retardancy of epoxy coatings were also evaluated. Electrochemical impedance spectroscopy, Tafel polarization and salt spray test were conducted to reveal the effect of nanofillers on corrosion protection performance. Besides that limiting oxygen index (LOI) and thermogravimetry analysis (TGA) were performed to evaluate the flame retardancy and thermal stability of epoxy filled coatings. Adhesion of the coating to its substrate and water absorption behavior were used to determine the time associated with the loss of adhesion and water permeability through the epoxy coating. The dispersion and interaction of the nanofillers with the matrix were characterized by transmission electron microscopy (TEM) and Fourier transform infrared (FTIR). The best anticorrosion performance of epoxy coating was exhibited by the incorporation of nanofillers into the epoxy matrix with 0.8 phr (for GNP and GO) and 1.5 phr (for MMT and HNT). For the hybrid nanofillers coatings some increment were observed at the ratio 0.6/0.3 phr of GO/HNT and was the best when compared to the other contents. The incorporation of nanofillers had increased the LOI values of epoxy coatings and the value was further improved in the presence of hybrid nanofiller. The maximum LOI value of 26 was observed for the epoxy coating with 1 phr ammonium polyphosphate in the intumescent flame retardant (IFR) formulation. TGA analysis showed that the incorporation of nanofillers and IFR formulation slightly enhanced the thermal stabilities and char formations of the coated samples. The water absorption results were fairly consistent with the data obtained from the corrosion studies. The water resistance had improved by 48.7 % and 52.4 % for EGO0.8 and EH1.5 coatings, respectively. The hybrid coating of EGO0.6H0.3 was more effective in repelling the water and durable. Adhesion strength for all coating samples decreased after 200 hours of exposure to 5 wt. % NaCl solution and more noticeable for neat epoxy coating. The positive effect of the nanofillers on the epoxy coating properties was attributed to the interaction between nanofillers and the epoxy matrix which was confirmed by FTIR and TEM, producing epoxy coatings with well-dispersed and great barrier properties. Therefore, the combination of hybrid GO/HNT as an additive may be considered as an efficient method to obtain good anticorrosion performance and flame retardancy properties for epoxy coating. The results demonstrated that the incorporation of hybrid GO/HNT nanofillers with IFR had further enhanced the flame retardancy with a slight decrease in anticorrosion properties.