Size depent graphene oxide and nickel cobalt graphene oxide hybrid coatings for corrosion protection / Nurul Huda Abu Bakar

Faculty of Industrial Sciences and Technology

Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang – 2021

Includes bibliographical references

Corrosion is an inevitable phenomenon especially when a metal is exposed to a corrosive environment. Owing to the impermeability, high hardness, and hydrophobicity of graphene, there is a growing demand for the developments of graphene-based coatings. However, it is reported that graphene might induce galvanic corrosion due to its high conductivity. As an alternative, graphene oxide (GO) is used for the coating. Therefore, in this study, corrosion protection coating has been developed from different size GO sheets. Then, the surface of the small-sized GO coating was adapted to the nickel-cobalt (Ni-Co) nanoparticles to form a Ni-Co/GO hybrid coating. The hybrid coatings were made using the second-step electrodeposition method at different periods of deposition (50, 200, 400 and 600 s). It is observed that the size of the GO sheets obtained from ultrasonication at 1, 3, 5, 7, and 10 h varied from 297 to 103 nm as a function of time and the changes in the π-π electronic transition was observed using UV-visible spectrophotometer (UV-Vis). During longer ultrasonication time, the GO sheets fragmented, and some of the oxygen functional groups were removed as showed in the reduction of the d-spacing (measured by X-ray diffraction). It was found that at 10 h of ultrasonication, the GO sheets tend to restack and caused a slight increment in the lateral size as observed using Transmission electron microscopy. The GO suspension with various sheets sizes were further electrophoretically deposited (EPD) onto the copper (Cu) substrates. After EPD, the structural and morphological properties of the coatings were examined using Fouriertransform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. It was found that coating from 7 h suspension (7h- GO/Cu) exhibited lowest oxygen-related band at 0.36, the thin and smooth film with a thickness of 1.52 μm and surface roughness, Ra of 26.41 nm. A higher value of ID/IG ratio, water contact angle, and Rockwell hardness was observed at 1.35, 91.4°, and 50.6, respectively. The corrosion performance of the coatings was measured using Tafel plots and electrochemical impedance spectroscopy (EIS). It was found that the corrosion potential, Ecorr was shifted towards a positive position at -0.154 V, lower corrosion current, Icorr at 0.0788 μA/cm2 and corrosion rate (CR) of 0.91×10-3 mm/yr. Detailed EIS analysis showed that the coating from smaller GO sheets size exhibited lower water permeation into the coating due to higher surface activation energy. Meanwhile, the hybrid coatings exhibited higher nickel (II) hydroxide, Ni(OH)2 (28.8 wt%) and cobalt (II) hydroxide, Co(OH)2 (66.3 wt%) compositions as compared to the pure Ni-Co coating when measured using X-ray photoelectron spectroscopy. The morphological and elemental study using field-emission scanning electron microscopy with energydispersive X-ray spectroscopy confirmed that the Co content increase as the deposition time increased. The hybrid coating formed at 400 s of electrodeposition time shows significant improvement in the Ecorr and Icorr at -0.165 V and 0.039 μA/cm2, respectively. The CR was found to be 0.45×10-3 mm/yr, and it shows 2-times magnitude lower than the 7h-GO/Cu while ~36.9 times lower than the CR for pure Ni-Co coating.